BER Research Highlights

Search Date: October 19, 2017

38 Records match the search term(s):


December 22, 2015

Groundwater Increases Carbon Emissions from a Tropical Rainforest Stream

Discharges of old groundwater can greatly increase carbon dioxide emissions from streams and other surface waters.

The Science
Streams and rivers are increasingly recognized as an important component in the carbon cycle, from local to global scales. A recent study measured carbon dioxide (CO2) and methane (CH4) emissions from tropical rainforest streams in both the wet and dry seasons. Measurements were made in a stream receiving inputs of very old (about 3,000 years) groundwater and in other streams without such inputs.

The Impact
Measuring elevated stream CO2 degassing rates might suggest that an ecosystem has elevated respiration and is a net source (rather than sink) with respect to atmospheric CO2. In ecosystems with inputs of old, high-carbon groundwater, however, knowing that elevated stream CO2 degassing is supported and driven by a large input of nonbiogenic CO2 from old groundwater helps to avoid an overestimation of ecosystem respiration and provides a more accurate picture of the ecosystem’s carbon source and sink status.

Summary
CO2 and CH4 degassing was measured in two rainforest streams at La Selva, Costa Rica: one stream fed only by young (<10 years old) local groundwater recharged within the watershed, and another fed by about two-thirds young groundwater and one-third older groundwater (about 3,000 years old) from a large regional aquifer system. Regional groundwater inputs had no measurable effect on stream gas exchange velocity, stream water CH4 concentration, or stream CH4 emissions, but it significantly increased stream water CO2 concentration and degassing. CO2 emissions from the stream receiving regional groundwater averaged 5.5 moles of carbon per m2 of stream surface per day, about 7.5 times higher than the average from the stream with no regional groundwater input. Carbon emissions from both streams were dominated by CO2, with CH4 accounting for only 0.06 percent to 1.70 percent of the total (average CH4 degassing rate from both streams was 0.005 moles of carbon per m2 of stream surface per day). Annual stream degassing fluxes normalized by watershed area were 299 and 48 moles of carbon per m2 of watershed surface in the watersheds with and without inputs of old regional groundwater, respectively. Stream degassing of CO2 is a major carbon flux in the watershed receiving inputs of old regional groundwater, and is similar in magnitude to the average net ecosystem exchange estimated by eddy covariance. Examining the effects of watershed connections to underlying hydrogeological systems can help avoid overestimation of ecosystem respiration and advance understanding of the carbon source and sink status and overall carbon budgets of terrestrial ecosystems.

Contacts
(BER PM)

Daniel Stover, SC-23.1, daniel.stover@science.doe.gov, 301-903-0289; and Jared DeForest, SC-23.1, jared.deforest@science.doe.gov, 301-903-1678

(PI Contact)
David Genereux
North Carolina State University
genereux@ncsu.edu, 919-515-6017

Funding
This work was funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, Terrestrial Ecosystem Science program (award DE-SC0006703).

Publications
Oviedo-Vargas, D., D. P. Genereux, D. Dierick, and S. F. Oberbauer. 2015. “The Effect of Regional Groundwater on Carbon Dioxide and Methane Emissions from a Lowland Rainforest Stream in Costa Rica,” Journal of Geophysical Research Biogeosciences 120(12), 2579–95. DOI: 10.1002/2015JG003009. (Reference link)

Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


December 11, 2015

Physiologically-Linked Indices of Rainfall Variation Predict Water Stress For Central U.S. Tree Species

Multiyear measurements enable researchers to predict impacts of precipitation regimes on central U.S. deciduous forest trees.

The Science
Long-term measurements at an AmeriFlux site in Missouri have enabled researchers to understand and predict how precipitation regimes affect water stress levels for key plant species in a central U.S. deciduous forest.

The Impact
How precipitation regimes affect water stress levels for plant species with contrasting water use strategies is not well understood. This study establishes a simple approach to quantifying plant physiological drought and the ecological impacts of precipitation regimes. This approach will be useful in predictions of forest response to climate change.

Summary
Variations in precipitation regimes can shift ecosystem structure and function by altering frequency, severity, and timing of plant water stress. Being able to predictively understand impacts of precipitation regimes on plant water stress is crucial in a changing climate. The research team, led by Oak Ridge National Laboratory (ORNL), formulated complementary, physiologically-linked indices of precipitation variability (PV) and related them to continuous measurements of predawn leaf water potential—a fundamental indicator of plant water status—in six tree species with different water use strategies in a central U.S. forest. These indices explained nearly all interannual variations in water stress levels for all species. These species differed in sensitivities to variations in precipitation regimes with the differences more pronounced in response to PV than to amount. Further, they exhibited stress tradeoffs between low and high PV, suggesting that how different plant species respond to PV is part of species-specific water use strategies in a plant community facing the uncertainty of fluctuating precipitation regimes. The new indices provide simple ways to quantify physiological drought and the ecological impacts of precipitation regimes in a changing climate.

Contacts
(BER PM)

Daniel Stover, SC-23.1, daniel.stover@science.doe.gov, 301-903-0289; and Jared DeForest, SC-23.1, jared.deforest@science.doe.gov, 301-903-1678

(PI Contact)
Lianhong Gu
Environmental Sciences Division and Climate Change Science Institute, ORNL
lianhong-gu@ornl.gov, 865-241-5925

Funding
This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Biological and Environmental Research, Climate and Environmental Sciences Division. ORNL is managed by the University of Tennessee (UT)-Battelle, LLC, for DOE under contract DE-AC05-00OR22725.

Publications
Gu, L., S. G. Pallardy, K. P. Hosman, and Y. Sun. 2016. “Impacts of Precipitation Variability on Plant Species and Community Water Stress in a Temperate Deciduous Forest in the Central US,” Agricultural and Forest Meteorology 217, 120–36. DOI: 10.1016/j.agrformet.2015.11.014. (Reference link)

Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


December 07, 2015

Large Divergence of Satellite and Earth System Model Estimates of Global Terrestrial CO2 Fertilization

Atmospheric mass balance analyses suggest that terrestrial carbon storage is increasing, partially abating the atmospheric carbon dioxide (CO2) growth rate, although the continued strength of this ecosystem service remains uncertain. This research presents a new, satellite-derived global terrestrial Net Primary Production (NNP) dataset, which shows a significant increase in NPP from 1982 to 2011. However, comparison against Earth system model (ESM) estimates reveals a significant divergence, with satellite-derived increases (2.8 ± 1.5%) less than half of ESM-derived increases (7.60 ± 1.67%) over the 30-year period. By isolating the CO2 fertilization effect and comparing against a synthesis of available free-air CO2 enrichment data, the researchers provide evidence that much of the discrepancy may be due to an over-sensitivity of ESMs to atmospheric CO2, potentially reflecting an under-representation of climatic feedbacks and a lack of representation of nutrient constraints. Understanding of CO2 fertilization effects on NPP needs rapid improvement to enable more accurate projections of future carbon cycle-climate feedbacks. The study suggests that better integration of modeling, satellite, and experimental approaches offers a promising way forward.

Reference: Smith, W. K., S. C. Reed, C. C. Cleveland, A. P. Ballantyne,W. R. L. Anderegg, W. R.Wieder, Y. Y. Liu, and S. W. Running. 2015 “Large Divergence of Satellite and Earth System Model Estimates of Global Terrestrial CO2 Fertilization,” Nature Climate Change 6, 306-10. DOI: 10.1038/nclimate2879. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


November 26, 2015

Warming Increases Carbon Losses in Biocrust Soils

Many arid and semiarid ecosystems have soils covered with well-developed biological soil crust communities (biocrusts) made up of mosses, lichens, cyanobacteria, and heterotrophs living at the soil surface. These communities are a fundamental component of dryland ecosystems and are critical to dryland carbon cycling. To examine the effects of warming temperatures on soil carbon balance in a dryland ecosystem, a recent study used infrared heaters to warm biocrust-dominated soils to 2°C above control conditions at a field site on the Colorado Plateau. The researchers monitored net soil exchange (NSE) of carbon dioxide (CO2) every hour for 21 months using automated flux chambers (5 control and 5 warmed chambers), which included the CO2 fluxes of the biocrusts and the soil beneath them. They observed measurable photosynthesis in biocrust soils on 12 percent of measurement days, which correlated well with precipitation events and soil wet-up. These days included several snow events, providing what is believed to be the first evidence of substantial photosynthesis underneath snow by biocrust organisms in drylands. Overall, biocrust soils in both the control and warmed plots were net CO2 sources to the atmosphere, with control plots losing 62 ± 8 g carbon m-2 (mean ± SE) over the first year of measurement and warmed plots losing 74 ± 9 g carbon m-2. Between the control and warmed plots, the difference in soil carbon loss was uncertain over the course of the entire year due to large and variable rates in spring, but on days during which soils were wet and crusts were actively photosynthesizing, biocrusts that were warmed by 2 oC had a substantially more negative carbon balance (i.e., biocrust soils took up less carbon and/or lost more carbon in warmed plots). Taken together, these data suggest a substantial risk of increased carbon loss from biocrust soils with higher future temperatures, and highlight a robust capacity to predict CO2 exchange in biocrust soils using easily measured environmental parameters.

Reference: Darrouzet-Nardi, A., S. C. Reed, E. E. Grote, and J. Belnap. 2015. “Observations of Net Soil Exchange of CO2 in a Dryland Show Experimental Warming Increases Carbon Losses in Biocrust Soils,” Biogeochemistry 126, 366-78. DOI: 10.1007/s10533-015-0163-7. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


November 23, 2015

Geochemical Analysis of Permafrost Soils Reveals Factors Controlling Methane Emissions from Arctic Tundra

A recent study measured the changes in dissolved organic carbon compounds during anoxic incubations of low-centered polygon soils from the Barrow Environmental Observatory in Alaska (Herndon et al. 2015a). Analyses used Fourier transform infrared and ultraviolet-visible spectroscopies to identify an initial increase in soluble carbohydrate and organic acid pools, followed by a decline in organic acids. These results describe the upstream microbial processes of soil organic matter decomposition that feed anaerobic microbial fermentation, methanogenesis, and iron reduction, which are highly temperature-sensitive processes and thus likely to control rate and magnitude of methane emissions from thawing permafrost. In a companion study, samples from mineral and organic soils were analyzed at the Stanford Synchrotron Radiation Lightsource to further characterize the geochemistry of active layer soils and permafrost (Herndon et al. 2015b). From those results, the researchers infer that geochemical differences induced by water saturation dictate microbial products of soil organic matter decomposition, and that iron geochemistry is an important factor regulating methanogenesis in anoxic tundra soils. Together, these coordinated datasets provided a conceptual framework from which to parameterize and enhance fine-scale biogeochemical models from the Next-Generation Ecosystem Experiments-Arctic project that specifically represent these anaerobic processes. The datasets are being used to assess the effects of newly represented iron-reduction processes on simulations of carbon dioxide, methane, and pH production in one-dimensional models.

References:
Herndon, E. M., B. F. Mann, T. R. Chowdhury, Z. Yang, D. E. Graham, S. D. Wullschleger, L. Liang, and B. Gu. 2015a. “Pathways of Anaerobic Organic Matter Decomposition in Tundra Soils from Barrow, Alaska,” Journal of Geophysical Research Biogeosciences 120, 2345-59. DOI: 10.1002/2015JG003147. (Reference link)

Herndon, E. M., Z. Yang, J. Bargar, N. Janot, T. Z. Regier, D. E. Graham, S. D. Wullschleger, B. Gu, and L. Liang. 2015b. “Geochemical Drivers of Organic Matter Decomposition in the Active Layer of Arctic Tundra Soils,” Biogeochemistry 126(3), 397-414. DOI: 10.1007/s10533-015-0165-5. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


November 18, 2015

Elevated CO2 Levels Alter Forest Succession and Carbon Cycling

Regenerating forests influence the global carbon cycle, and understanding how climate change will affect patterns of regeneration and carbon storage is necessary to predict the rate of atmospheric carbon dioxide (CO2) increase in future decades. While experimental CO2 elevation has revealed that young forests respond with increased productivity, there remains considerable uncertainty as to how the long-term dynamics of forest regrowth are shaped by elevated CO2 (eCO2). In a recent study, researchers used the mechanistic size- and age-structured Ecosystem Demography model to investigate the effects of CO2 enrichment on forest regeneration, using data from the Duke Forest Free-Air Carbon Dioxide Enrichment (FACE) experiment, a forest, and an eddy-covariance tower for model parameterization and evaluation. They found that the dynamics of forest regeneration are accelerated, and stands consistently hit a variety of developmental benchmarks earlier under eCO2. Because responses to eCO2 varied by plant functional type, successional pathways and mature forest composition differed under eCO2, with mid- and late-successional hardwood functional types experiencing greater increases in biomass compared to early-successional functional types and the pine canopy. Over the simulation period, eCO2 led to an increase in total ecosystem carbon storage of 9.7 Mg carbon/ha. Model predictions of mature forest biomass and ecosystem-atmosphere exchange of CO2 and water were sensitive to assumptions about nitrogen limitation; both the magnitude and persistence of the ecosystem response to eCO2 were reduced under nitrogen limitation. These simulations demonstrate that eCO2 can result in a general acceleration of forest regeneration, while altering the course of successional change and having a lasting impact on forest ecosystems.

Reference: Miller, A. D., M. C. Dietze, E. H. DeLucia, and K. J. Anderson-Teixeira. 20156. “Alteration of Forest Succession and Carbon Cycling Under Elevated CO2,” Global Change Biology 22(1), 351-63. DOI: 10.1111/gcb.13077. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


November 12, 2015

Representing Northern Peatland Microtopography and Hydrology Within the Community Land Model

Predictive understanding of northern peatland hydrology is a necessary precursor to understanding the fate of massive carbon stores in these systems under the influence of present and future climate change. Current models have begun to address microtopographic controls on peatland hydrology, but none have included a prognostic calculation of peatland water table depth for a vegetated wetland, independent of prescribed regional water tables. A recent study introduces a new configuration of the Community Land Model (CLM), which includes a fully prognostic water table calculation for a vegetated peatland. The structural and process changes to CLM focus on modifications needed to represent the hydrologic cycle of the bog environment with perched water tables, as well as distinct hydrologic dynamics and vegetation communities of the raised hummock and sunken hollow microtopography characteristic of peatland bogs. The modified model was parameterized and independently evaluated against observations from an ombrotrophic raised-dome bog in northern Minnesota (S1-Bog), the site for the Spruce and Peatland Responses Under Climatic and Environmental Change experiment (SPRUCE). Simulated water table levels compared well with site-level observations. The new model predicts hydrologic changes in response to planned warming at the SPRUCE site. At present, standing water is commonly observed in bog hollows after large rainfall events during the growing season, but simulations suggest a sharp decrease in water table levels due to increased evapotranspiration under the most extreme warming level, nearly eliminating the occurrence of standing water in the growing season. Simulated soil energy balance was strongly influenced by reduced winter snowpack under warming simulations, with the warming influence on soil temperature partly offset by the loss of insulating snowpack in early and late winter. The new model provides improved predictive capacity for seasonal hydrological dynamics in northern peatlands and a useful foundation for investigating northern peatland carbon exchange.

Reference: Shi, X., P. E. Thornton, D. M. Ricciuto, P. J. Hanson, J. Mao, S. D. Sebestyen, N. A. Griffiths, and G. Bisht. 2015. “Representing Northern Peatland Microtopography and Hydrology Within the Community Land Model,” Biogeosciences 12(21), 6463–77. DOI: 10.5194/bg-12-6463-2015. (Reference link)

Contact: Dorothy Koch, SC-23.1, (301) 903-0105, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


November 09, 2015

Comprehensive Data Acquisition and Management System for Ecosystem-Scale Warming and Elevated CO2 Experiment

Ecosystem-scale manipulation experiments represent large science investments that require well-designed data acquisition and management systems to provide reliable, accurate information to project participants and third party users. The Spruce and Peatland Responses Under Climatic and Environmental Change (SPRUCE) project is such an experiment funded by the Department of Energy’s Terrestrial Ecosystem Science program. The SPRUCE experimental mission is to assess ecosystem-level biological responses of vulnerable, high-carbon terrestrial ecosystems to a range of climate warming manipulations and an elevated carbon dioxide (CO2) atmosphere. SPRUCE provides a platform for testing mechanisms controlling the vulnerability of organisms, biogeochemical processes, and ecosystems to climatic change (e.g., thresholds for organism decline or mortality, limitations to regeneration, biogeochemical limitations to productivity, and cycling and release of CO2 and methane to the atmosphere). As a result, the SPRUCE experiment will generate a wide range of continuous and discrete measurements. In a recent publication, project researchers lay out their approach to meeting the challenges of designing and constructing an efficient data system for managing high volume sources of in situ observations in a remote and harsh environmental location. The approach covers data flow starting from the sensors and ending at the archival and distribution points, discusses types of hardware and software used, examines design considerations that were used to choose them, and describes the data management practices chosen to control and enhance the data’s value.

Reference:Krassovski, M. B., J. S. Riggs, L. A. Hook, W. R. Nettles, P. J. Hanson, and T. A. Boden. 2015. “A Comprehensive Data Acquisition and Management System for an Ecosystem-Scale Peatland Warming and Elevated CO2 Experiment,” Geoscientific Instrumentation, Methods, and Data Systems 4, 203–13. DOI: 10.5194/gi-4-203-2015. (Reference link)
See also SPRUCE project website at http://mnspruce.ornl.gov.

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


November 02, 2015

Toward Improved Model structures for Analyzing Priming Effect

Rising atmospheric carbon dioxide (CO2) concentrations are projected to increase plant inputs to soil, which may stimulate soil carbon decomposition. Many studies attempting to quantify this priming effect use a simple analytical framework that is inappropriate for inferring complex dynamics. Using a multipool soil carbon model, a recent study shows that changes in carbon flows that would be attributed to priming in a one-pool model (using overall respiration and carbon stocks) can be explained without a change in decomposition rate constants of individual pools. Furthermore, a sensitivity analysis demonstrates the potential range of “false priming” responses inferred from simple, first-order models. The researchers argue that, in addition to standard measurements of carbon stocks and CO2 fluxes, quantifying the fate of new plant inputs requires isotopic tracers and microbial measurements. They discuss the pitfalls of using simple model structures to infer complex dynamics and suggest appropriate model structures and necessary observational constraints for projections of carbon feedbacks.

Reference: Georgiou, K., C. D. Koven, W. J. Riley, and M. S. Torn. 2015. “Toward Improved Model Structures for Analyzing Priming: Potential Pitfalls of Using Bulk Turnover Time,” Global Change Biology 21(12), 4298–4302. DOI:10.1111/gcb.13039. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


October 17, 2015

Vertical Transport of Greenhouse Gases Through the Nocturnal Atmospheric Boundary Layer

Results from a tracer release study were applied to estimate a tower footprint.

The Science
At night, can an upper-level carbon dioxide sensor be overly influenced by gas released from nearby vegetation, reducing researchers’ confidence in its ability to provide information on continental-scale surface fluxes? The vertical dispersion of a gas released at night was studied with a field project in South Carolina comprising (1) the release of five perfluorocarbons (inert airborne “tracer” gases) from multiple surface locations, and (2) downwind detection of the tracers at four elevations on a tall television transmitter tower.

The Impact
A simulation of the tracer release reproduced the motion of tracer from its source to the detectors, but also indicated that the uppermost detector (at 329 m above ground) was mainly sampling air from far beyond 25 km, with a minor contribution from areas within that range. Therefore, for nocturnal conditions, the researchers are confident that the tower is sampling air from over a regional-scale area (25 km to 150 km), and is only weakly influenced by nearby emissions.

Summary
On two nights characterized by moderate to strong vertical stability, tracer gases were released at the surface from locations upwind of a South Carolina tower equipped with sensors at 34 m, 68 m, and 329 m. The uppermost sensor was able to detect the tracer gas released from the ground at a distance of about 25 km—evidence for some vertical transport despite the weak vertical mixing on the nights it was released. Simulations of the experiment, validated against the field project data, were conducted to estimate the tower “footprint,” or total area from which tracer released at the surface will be detected by the 329 m sensor. These simulations indicate that most of the air reaching the highest tower level came from surface locations much more distant than the domain of the tracer release, with the sensor footprint extending well beyond 25 km. The low-level nocturnal jet (located at 100 m to 1000 m above ground, and at 8-20 m/sec speed) was an important reason for the dominant role of distant upwind sources.

Contact (BER PM)
Dan Stover
SC-23.1
daniel.stover@science.doe.gov, 301-903-0289

(PI Contact)
David Werth
Savannah River National Laboratory
David.Werth@srnl.doe.gov, 803-725-3717

Funding
Funding was provided by the U.S Department of Energy, Office of Science, Office of Biological and Environmental Research, Terrestrial Ecosystem Science program.

Publication
Werth, D., R. Buckley, G. Zhang, R. Kurzeja, M. Leclerc, H. Duarte, M. Parker, and T. Watson. 2015. “Quantifying the Local Influence at a Tall Tower Site in Nocturnal Conditions,”Theoretical and Applied Climatology, DOI:10.1007/s00704-015-1648-y. (Reference link)

Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


September 29, 2015

Climate Change and Physical Disturbance Cause Similar Community Shifts in Biological Soil Crusts

Biological soil crusts (biocrusts)—communities of mosses, lichens, cyanobacteria, and heterotrophs living at the soil surface—are fundamental components of drylands worldwide, and their destruction dramatically alters biogeochemical processes, hydrology, surface energy balance, and vegetation cover. Impacts of physical disturbances on biocrusts (e.g., trampling by livestock and damage from vehicles) have been a long-standing concern, and concern is also increasing over the potential for climate change to alter biocrust community structure. Using long-term data from the Colorado Plateau, a recent study examined the effects of 10 years of experimental warming and altered precipitation on biocrust communities and compared the effects of altered climate with those of long-term physical disturbance (more than 10 years of replicated human trampling). Surprisingly, altered climate and physical disturbance treatments had similar effects on biocrust community structure. Warming, altered precipitation, and physical disturbance from trampling all promoted early successional community states. Although the pace of biocrust community change varied significantly among treatments, these results suggest that multiple aspects of climate change will affect biocrusts to the same degree as physical disturbance. This finding is particularly disconcerting in the context of warming, as temperatures for drylands are projected to increase beyond those imposed as treatments in this study.

Reference: Ferrenberg, S., S. C. Reed, and J. Belnap. 2015. “Climate Change and Physical Disturbance Cause Similar Community Shifts in Biological Soil Crust,” Proceedings of the National Academy of Sciences (USA) 112(39), 12116-121. DOI: 10.1073/pnas.1509150112. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


September 15, 2015

Fog and Rain in the Amazon

The diurnal and seasonal water cycles in the Amazon remain poorly simulated in general circulation models. Simulations using existing models exhibit peak evapotranspiration during the wrong season and rain occurring too early in the day. A team of researchers supported by the Terrestrial Ecosystem Science and Atmospheric System Research programs and using data from the GOAmazon campaign show that those biases are not present in an approach opposite to that taken by general circulation models, in which they resolve convection and parameterize large-scale circulation as a function of the resolved convection.

The ability to simulate the seasonality of the hydrologic cycle in the Amazon using this approach is attributed to (1) the representation of the morning fog layer, and (2) more accurate characterization of convection and its coupling with large-scale circulation. The morning fog layer, present during the wet season, but absent in the dry season, dramatically increases cloud albedo, which reduces evapotranspiration through its modulation of the surface energy budget. These results highlight the importance of the coupling between the energy and hydrological cycles and the key role of cloud albedo feedback for climates over tropical continents. The study indicates understanding of tropical climates over land can be considerably advanced by using coupled land–atmosphere models with explicit convection and parameterized large-scale dynamics.

Reference: Anber, U., P. Gentine, S. Wang, and A. H. Sobel. 2015. “Fog and Rain in the Amazon,” Proceedings of the National Academy of Sciences (USA) 112(37), 11,473–477. DOI: 10.1073/pnas.1505077112. (Reference link)

Contact: Sally McFarlane, SC-23.1, (301) 903-0943, Daniel Stover, SC-23.1, (301) 903-0289, Ashley Williamson, SC-23.1, (301) 903-3120
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


September 02, 2015

Links Between Ecosystem Multifunctionality and Above- and Belowground Biodiversity Mediated by Climate

Plant biodiversity is often correlated with ecosystem functioning in terrestrial ecosystems. However, little is known about the relative and combined effects of above- and belowground biodiversity on multiple ecosystem functions [e.g., ecosystem multifunctionality (EMF)] or how climate might mediate those relationships. A recent study teases apart the effects of biotic and abiotic factors, both above- and belowground, on EMF on the Tibetan Plateau in China. The researchers found that a suite of biotic and abiotic variables account for up to 86% of the EMF variation, with the combined effects of above- and belowground biodiversity accounting for 45% of the EMF variation. These results have two important implications: (1) including belowground biodiversity in models can improve the ability to explain and predict EMF, and (2) regional-scale variation in climate, and perhaps climate change, can determine, or at least modify, the effects of biodiversity on EMF in natural ecosystems.

Reference: Jing, X., N. J. Sanders, Y. Shi, H. Chu, A. T. Classen, K. Zhao, L. Chen, Y. Shi, Y. Jiang, and J.-S. He. 2015. “The Links Between Ecosystem Multifunctionality and Above- and Belowground Biodiversity are Mediated by Climate,” Nature Communications 6(8159), DOI: 10.1038/ncomms9159. (Reference link)

Media:

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


August 24, 2015

Call for Expansion of International Soil Experiment Networks

Researchers are calling for an expansion of international networks of deep soil manipulation experiments in the field, with coordination, common variables, integration, and collaboration. Siting along environmental and land-use gradients will accelerate understanding of soil organic carbon (SOC) cycling. Data are lacking to unravel the importance of various mechanisms controlling deep SOC cycling in different soils under different environmental conditions. Field manipulation experiments will overcome limitations of laboratory studies, enabling testing for cause and effect and isolating direct response function in real ecosystems. Reduced uncertainty of the role of soils as positive or negative feedbacks to global climate change will improve climate projections. Also, mitigation strategies and solutions for ecological and agricultural challenges can be developed and tested at the networks’ facilities.

Reference: Torn, M. S., A. Chabbi, P. Crill, P. J. Hanson, I. A. Janssens, Y. Luo, C. H. Pries, C. Rumpel, M. W. I. Schmidt, J. Six, M. Schrumpf, and B. Zhu. 2015. “A Call for International Soil Experiment Networks for Studying, Predicting, and Managing Global Change Impacts,” SOIL 1, 575–82. DOI:10.5194/soil-1-575-2015. (Reference link)

Contact: Daniel Stover, SC-23.1, (301) 903-0289, Jared DeForest, SC-23, (301) 903-3251
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


July 28, 2015

Foliar Age and Season Affect Photosynthetic Temperature Response in Black Spruce

Black spruce trees at the southern edge of the vast boreal forest are being exposed to progressive increases in temperature due to climate change. Temperature increases could change the balance between photosynthetic uptake of carbon dioxide (CO2) and respiratory release of CO2, which could further affect climate change. Since black spruce trees retain their needles for several years, the different age classes may have different responses to temperature increases. Thus, to understand and model how the boreal forest will function in the future, seasonal- and age-specific photosynthetic and respiratory temperature response functions must be measured. From 2011 to 2014, research was undertaken in a nutrient-limited black spruce and Sphagnum bog forest in northern Minnesota in the United States. Measurements were collected seasonally on different needle age classes from mature trees and included photosynthetic capacity, foliar respiration (Rd), and leaf biochemistry. Scientists from Oak Ridge National Laboratory used the results to model the predicted total annual carbon uptake by the trees under normal and elevated temperature scenarios. Temperature responses of key photosynthetic parameters were dependent on season and less responsive in the developing new needles (Y0) as compared with 1-year-old (Y1) or 2-year-old (Y2) needles. Each process initially increased with temperature, peaking between 19 °C and 38 °C, then declined at higher temperatures. Different age classes differed in their leaf structure and photosynthetic capacity, which resulted in 64% of modeled total annual carbon uptake from the older Y1 and Y2 needles (56% of the tree leaf area), and just 36% from Y0 cohorts (44% of tree leaf area). Under warmer climate change scenarios, the contribution of young needles was even less, just 31% of annual carbon uptake for a modeled 9 °C rise in summer temperature. Results suggest that net annual carbon uptake by black spruce could increase under elevated temperature and become more dependent on the older needle age classes. This study illustrates the physiological and ecological significance of different leaf ages, and indicates the need for seasonal- and leaf age-specific model parameterization when estimating carbon uptake capacity of boreal forests under current or future temperatures.

Reference: Jensen, A. M., J. M. Warren, P. J. Hanson, J. Childs, and S. D. Wullschleger. 2015. “Needle Age and Season Influence Photosynthetic Temperature Response in Mature Picea mariana Trees,” Annals of Botany 116, 821–32. DOI: 10.1093/aob/mcv115. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


July 04, 2015

Sustained Carbon Uptake and Storage Following Moderate Disturbance in a Great Lakes Forest

Carbon uptake rates in many forests are sustained, or decline only briefly, following disturbances that partially defoliate the canopy. The mechanisms supporting such functional resistance to moderate forest disturbance are largely unknown. Researchers used a large-scale experiment to identify mechanisms sustaining carbon uptake through partial canopy defoliation. The Forest Accelerated Succession Experiment in northern Michigan employs a suite of carbon-cycling measurements within paired treatment and control meteorological flux tower footprints. They found that enhancement of canopy light-use efficiency and maintenance of light absorption maintained net ecosystem production and aboveground wood net primary production (NPP) when leaf-area index (LAI) of the treatment forest temporarily declined by nearly half its maximum value. In the year following peak defoliation, redistribution of nitrogen in the treatment forest from senescent early successional aspen and birch to nongirdled later successional species facilitated the recovery of total LAI to predisturbance levels. Sustained canopy physiological competency following disturbance coincided with a downward shift in maximum canopy height, indicating that compensatory photosynthetic carbon uptake by undisturbed, later successional subdominant and subcanopy vegetation supported carbon-uptake resistance to disturbance. These findings have implications for ecosystem management and modeling, demonstrating that forests may tolerate considerable leaf-area losses without diminishing rates of carbon uptake. They conclude that the resistance of carbon uptake to moderate disturbance depends not only on replacement of lost leaf area, but also on rapid compensatory photosynthetic carbon uptake during defoliation by emerging later successional species.

Reference: Gough, C. M., B. S. Hardiman, L. E. Nave, G. Bohrer, K. D. Maurer, C. S. Vogel, K.J. Nadelhoffer, and P. S. Curtis. 2013. “Sustained Carbon Uptake and Storage Following Moderate Disturbance in a Great Lakes Forest,” Ecological Applications 23(5), 1202–15. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


May 21, 2015

Using Ecosystem Experiments to Improve Vegetation Models

Ecosystem responses to rising carbon dioxide (CO2) concentrations are a major source of uncertainty in climate change projections. Data from ecosystem-scale Free-Air CO2 Enrichment (FACE) experiments provide a unique opportunity to reduce this uncertainty. The recent FACE Model–Data Synthesis project aimed to use information gathered in two forest FACE experiments to assess and improve land ecosystem models. A new ‘assumption-centred’ model intercomparison approach was used, in which participating models were evaluated against experimental data based on the ways in which they represent key ecological processes. By identifying and evaluating the main assumptions causing differences among models, the assumption-centred approach produced a clear roadmap for reducing model uncertainty. In a recent paper, researchers explained this approach and summarized the resulting research agenda. They encourage the application of this approach in other model intercomparison projects to fundamentally improve predictive understanding of the Earth system.

Reference: Medlyn, B. E., S. Zaehle, M. G. De Kauwe, A. P. Walker, M. C. Dietze, P. J. Hanson, T. Hickler, A. K. Jain, Y. Luo, W. Parton, I. C. Prentice, P. E. Thornton, S. Wang, Y.-P. Wang, E. Weng, C. M. Iversen, H. R. McCarthy, J. M. Warren, R. Oren, and R. J. Norby. 2015. “Using Ecosystem Experiments to Improve Vegetation Models,” Nature Climate Change 5, 528–34. DOI: 10.1038/nclimate2621. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


May 19, 2015

Global Model Simulation for 3-D Radiative Transfer Impact on Surface Hydrology

Orographic forcing is an efficient and dominant mechanism for harnessing water vapor into consumable fresh water in the form of precipitation, snowpack, and runoff. Mountain water resources not only support human activities, but are also vital to diverse terrestrial and aquatic ecosystems. To study the long-term effect of solar radiation effect over three-dimensional (3-D) mountains and snow on surface energy and hydrology, the 3-D radiative transfer parameterization developed for the computation of surface solar fluxes has been incorporated into the Community Climate System Model version 4 [(CCSM4); Community Atmosphere Model version 4 (CAM4)/Community Land Model version 4 (CLM4)] global model and applied at a resolution of 0.23°x0.31° over the Rocky Mountains and Sierra Nevada areas in the western United States. In the 3-D radiative transfer parameterization, the surface topography data have been updated from a resolution of 1 km to 90 meters to improve parameterization accuracy. In addition, the upward-flux deviation [3D–plane-parallel (PP)] adjustment has also been modified to ensure that energy balance at the surface is conserved in global climate simulations based on 3-D radiation parameterization. Findings show that deviations of the net surface fluxes are not only affected by 3-D mountains, but also influenced by feedbacks of clouds and snow in conjunction with long-term simulations. Deviations in the sensible heat and surface temperature generally follow the patterns of net surface solar flux. Including 3D-mountain effects significantly increases (decreases) solar radiation at higher (lower) elevations, leading to increased (reduced) snowmelt. Combined with precipitation changes influenced by changes in the surface fluxes, runoff is significantly reduced in mountainous regions after the snow accumulation peaks in April. The 3-D mountain effects could have an important impact on vegetation by changing the energy and water available to plants. With the larger differences in solar radiation, soil moisture, and soil temperature developing in late spring and early summer, changes in photosynthetic rate and plant phenology may affect leaf area index and gross primary production. These findings will be further investigated in the future using longer simulations to quantify the 3-D mountain effects on radiation and the impacts on water and carbon cycles and vegetation globally.

Reference: Lee, W.-L., Y. Gu, K. N. Liou, L. R. Leung, and H.-H. Hsu. 2015. “A Global Model Simulation for 3-D Radiative Transfer Impact on Surface Hydrology over the Sierra Nevada and Rocky Mountains,” Atmospheric Chemistry and Physics 15, 5405-13. DOI: 10.5194/acp-15-5405-2015. (Reference link)

Contact: Dorothy Koch, SC-23.1, (301) 903-0105
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


May 18, 2015

Darcy’s Law Predicts Widespread Forest Mortality Under Climate Warming

Drought and heat-induced tree mortality is accelerating in many forest biomes as a consequence of a warming climate, resulting in a threat to global forests unlike any in recorded history. Forests store the majority of terrestrial carbon, thus their loss may have significant and sustained impacts on the global carbon cycle. In a recent paper, researchers used a hydraulic corollary to Darcy’s law, a core principle of vascular plant physiology, to predict characteristics of plants that will survive and die during drought under warmer future climates. Plants that are tall with isohydric stomatal regulation, low hydraulic conductance, and high leaf area are most likely to die from future drought stress. Thus, tall trees of old-growth forests are at the greatest risk of loss, which has ominous implications for terrestrial carbon storage. This application of Darcy’s law indicates today’s forests generally should be replaced by shorter and more xeric plants, owing to future warmer droughts and associated wildfires and pest attacks. The Darcy’s corollary also provides a simple, robust framework for informing forest management interventions needed to promote the survival of current forests. Given the robustness of Darcy’s law for predictions of vascular plant function, the researchers conclude with high certainty that today’s forests will be subject to continued increases in mortality rates that result in substantial reorganization of their structure and carbon storage.

Reference: McDowell, N. G., and C. D. Allen. 2015. “Darcy's Law Predicts Widespread Forest Mortality Under Climate Warming,” Nature Climate Change 5, 669–72. DOI: 10.1038/nclimate2641. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


May 18, 2015

Tall Trees Most Susceptible to Drought Stress

A significant portion of the carbon emitted from fossil fuel combustion is taken up by ocean and terrestrial systems. However, drought and heat-induced tree mortality is accelerating in many forest biomes, resulting in a threat to global forests unlike any in recorded history. Forests store the majority of terrestrial carbon, thus their loss may have significant and sustained impacts on the global carbon cycle. Researchers from Los Alamos National Laboratory have used a hydraulic corollary to Darcy’s law, a core principle of vascular plant physiology, to predict characteristics of plants that will survive and die during drought under warmer future climates. They find that plants that are tall are most likely to die from future drought stress. Thus, tall trees of old-growth forests are at the greatest risk of loss, which has ominous implications for terrestrial carbon storage. This application of Darcy’s law indicates today’s forests generally should be replaced by shorter and more xeric plants, owing to future warmer droughts and associated wildfires and pest attacks. The Darcy’s corollary also provides a simple, robust framework for informing forest management interventions needed to promote the survival of current forests. Given the robustness of Darcy’s law for predictions of vascular plant function, they conclude with high certainty that today’s forests are going to be subject to continued increases in mortality rates that will result in substantial reorganization of their structure and carbon storage.

Reference: McDowell, N. G., and C. D. Allen. 2015. “Darcy's Law Predicts Widespread Forest Mortality Under Climate Warming,” Nature Climate Change 5, 669–72. DOI: 10.1038/nclimate2641. (Reference link)

Contact: Renu Joseph, SC-23.1, (301) 903-9237, Dorothy Koch, SC-23.1, (301) 903-0105, Mike Kuperberg, SC-23.1, (301) 903-3281, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


May 11, 2015

Dual Controls on Carbon Loss During Drought in Peatlands

Peatlands store a third of global soil carbon. Drought and drainage coupled with climate warming present the main threat to these stores. Hence, understanding drought effects and inherent feedbacks related to peat decomposition has been a primary global challenge. However, widely divergent results in recent studies concerning drought effects challenge the accepted paradigm that waterlogging and associated anoxia are the overarching controls locking up carbon stored in peat. By linking field and microcosm experiments, a recent study shows how previously unrecognized mechanisms regulate the buildup of phenolics, which protects stored carbon directly by reducing phenol oxidase activity during short-term drought and, indirectly, through a shift from low-phenolic Sphagnum and herbs to high-phenolic shrubs after long-term moderate drought. The study demonstrates that shrub expansion induced by drought and warming in boreal peatlands might be a long-term, self-adaptive mechanism not only increasing carbon sequestration but also potentially protecting historic soil carbon. The researchers propose that the projected “positive feedback loop” between carbon emissions and drought in peatlands may not occur in the long term.

Reference: Wang, H., C. J. Richardson, and M. Ho. 2015. “Dual Controls on Carbon Loss During Drought in Peatlands,” Nature Climate Change 5(6), 584–87. DOI: 10.1038/nclimate2643. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


May 02, 2015

Does Day and Night Sampling Reduce Spurious Correlation Between Canopy Photosynthesis and Ecosystem Respiration?

Eddy covariance measurements of carbon dioxide (CO2) exchange have to be partitioned into offsetting gross fluxes, canopy photosynthesis, and ecosystem respiration to understand biophysical controls on the net fluxes. Additionally, independent estimates of canopy photosynthesis (G) and ecosystem respiration (R) are needed to validate and parametrize carbon cycle models that are coupled with climate and ecosystem dynamics models. Carbon flux partitioning methods, however, may suffer from spurious correlation, because derived values of canopy photosynthesis and ecosystem respiration both contain common information on net carbon fluxes at annual time scales.

Researchers hypothesized that spurious correlation between canopy photosynthesis and ecosystem respiration can be minimized using day–night conditional sampling of CO2 exchange, with daytime fluxes dominated by photosynthesis and nighttime fluxes dominated by respiration. To test this hypothesis, the research team derived explicit equations that quantify the degree of spurious correlation between photosynthesis and respiration. Theoretically, day and night samples of net carbon exchange share a different common variable, daytime ecosystem respiration, and the degree of spurious correlation depends upon the variance of this shared variable. This theory was applied to ideal measurements of carbon exchange over a vigorous, irrigated, and frequently harvested alfalfa field in the sunny and windy region of the Sacramento-San Joaquin Delta of California, where soil CO2 efflux is strong. Results showed a correlation coefficient between canopy photosynthesis and ecosystem respiration of -0.79. This relatively high correlation between canopy photosynthesis and respiration was mostly real as the degree of spurious correlation was only -0.32.

This analysis was expanded to the FLUXNET database, which spans a spectrum of climate and plant functional types. On average, the correlation between gross photosynthesis and ecosystem respiration, using day–night sampling, was close to minus one (-0.828 ± 0.130). For perspective, a large fraction of this correlation was real, as the degree of spurious correlation (Eq. (22)) was -0.526. Consequently, the potential for spurious correlation between canopy photosynthesis and ecosystem respiration across the FLUXNET database was moderate. Looking across the database, the researchers found that the least negative spurious correlation coefficients (>-0.3) were associated with seasonal deciduous forests. The most negative spurious correlations coefficients (<-0.7) were associated with evergreen forests found in most boreal climates.

Reference: Baldocchi, D., C. Sturtevant, and FLUXNET contributors. 2015. “Does Day and Night Sampling Reduce Spurious Correlation Between Canopy Photosynthesis and Ecosystem Respiration?” Agricultural and Forest Meteorology 207, 117–26. DOI: 10.1016/j.agrformet.2015.03.010. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


April 30, 2015

Global Carbon Budget Audit

Over the last 5 decades, monitoring systems have been developed to detect changes in carbon (C) accumulations in the atmosphere and oceans, but the ability to detect changes in the behavior of the global carbon cycle is still hindered by measurement and estimate errors. In a recent study, researchers developed a rigorous and flexible framework for assessing the temporal and spatial components of estimate errors and their impact on uncertainty in net carbon uptake by the biosphere. They present a novel approach for incorporating temporally correlated random error into the error structure of emission estimates. Based on this approach, they conclude that the 2σ uncertainties of the atmospheric growth rate have decreased from 1.2 Pg C yr-1 in the 1960s to 0.3 Pg C yr-1 in the 2000s due to an expansion of the atmospheric observation network. The 2σ uncertainties in fossil fuel emissions have increased from 0.3 Pg C yr-1 in the 1960s to almost 1.0 Pg C yr-1 during the 2000s due to differences in national reporting errors and differences in energy inventories. Lastly, while land use emissions have remained fairly constant, their errors still remain high and thus their global carbon uptake uncertainty is not trivial. Currently, the absolute errors in fossil fuel emissions rival the total emissions from land use, highlighting the extent to which fossil fuels dominate the global carbon budget. Because errors in the atmospheric growth rate have decreased faster than errors in total emissions have increased, a 20% reduction in the overall uncertainty of net carbon global uptake has occurred. Given all the major sources of error in the global carbon budget that could be identified, the results are 93% confident that terrestrial carbon uptake has increased and 97% confident that ocean carbon uptake has increased over the last 5 decades. Thus, arguably one of the most vital ecosystem services that the biosphere currently provides is the continued removal of approximately half of atmospheric carbon dioxide emissions from the atmosphere, although there are certain environmental costs associated with this service, such as the acidification of ocean waters.

Reference: Ballantyne, A. P., R. Andres, R. Houghton, B. D. Stocker, R. Wanninkhof, W. Anderegg, L. A. Cooper, M. DeGrandpre, P. P. Tans, J. B. Miller, C. Alden, and J. W. C. White. 2015. “Audit of the Global Carbon Budget: Estimate Errors and Their Impact on Uptake Uncertainty,” Biogeosciences 12(8), 2565–84. DOI: 10.5194/bg-12-2565-2015. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


April 27, 2015

Predicting Long-Term Carbon Sequestration in Response to CO2 Enrichment

Large uncertainty exists in model projections of the land carbon sink response to increasing atmospheric carbon dioxide (CO2). Free-Air CO2 Enrichment (FACE) experiments lasting a decade or more have investigated ecosystem responses to a step change in atmospheric CO2 concentration. To interpret FACE results in the context of gradual increases in atmospheric CO2 over decades to centuries, a recent study used a suite of seven models to simulate the Duke Forest and Oak Ridge FACE experiments extended for 300 years of CO2 enrichment. It also determined key modeling assumptions that drive divergent projections of terrestrial carbon uptake and evaluated whether these assumptions can be constrained by experimental evidence. All models simulated increased terrestrial carbon pools resulting from CO2 enrichment, though there was substantial variability in quasi-equilibrium carbon sequestration and rates of change. In two of two models that assume that plant nitrogen uptake is solely a function of soil nitrogen supply, the net primary production response to elevated CO2 became progressively nitrogen limited. In four of five models assuming that nitrogen uptake is a function of both soil nitrogen supply and plant nitrogen demand, elevated CO2 led to reduced ecosystem nitrogen losses and thus progressively relaxed nitrogen limitation. Many allocation assumptions resulted in increased wood allocation relative to leaves and roots, which reduced the vegetation turnover rate and increased carbon sequestration. In addition, self-thinning assumptions had a substantial impact on carbon sequestration in two models. Accurate representation of nitrogen process dynamics (in particular nitrogen uptake), allocation, and forest self-thinning is key to minimizing uncertainty in projections of future carbon sequestration in response to elevated atmospheric CO2.

Reference: Walker, A. P., S. Zaehle, B. E. Medlyn, M. G. De Kauwe, S. Asao, T. Hickler, W. Parton, D. M. Ricciuto, Y.-P. Wang, D. Wårlind, and R. J. Norby. 2015. “Predicting Long–Term Carbon Sequestration in Response to CO2 Enrichment: How and Why Do Current Ecosystem Models Differ?” Global Biogeochemical Cycles 29(4), 476–95. DOI: 10.1002/2014GB004995. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


April 17, 2015

Differences in Organic Matter from a Range of Soil Types and Ecosystems

Organic matter in soils is a key reservoir for carbon and plays a significant role in nutrient biogeochemical cycling. Because of limited understanding of the molecular composition of soil organic matter (SOM), scientists are challenged to decipher the range of chemical processes in soils and to predict how terrestrial carbon fluxes will respond to changing climatic conditions and land use. To address this need, a team of scientists from the University of Idaho and Department of Energy’s Environmental Molecular Sciences Laboratory (EMSL) extracted SOM from multiple ecosystems using a variety of organic solvents, and then analyzed the SOM using EMSL’s ultra-high resolution mass spectrometry capabilities. The team found different solvents extracted different types of compounds from soils, significantly expanding the ability to sensitively detect and identify the vast suite of diverse organic molecules that compose SOM. These findings enable targeted extraction approaches to elucidate differences in organic matter among soils from different ecosystems. These findings also demonstrate that by using multiple solvents on the same soil material, scientists will be able to obtain a more complete characterization of the organic matter in a specific soil sample. Increased understanding of SOM composition in soils from multiple ecosystems is expected to improve predictions of how terrestrial carbon fluxes will respond to future climate change.

References: Tfaily, M., R. K. Chu, N. Tolic, K. M. Roscioli, C. R. Anderton, L. Paša-Tolic, E. W. Robinson, and N. J. Hess. 2015. “Advanced Solvent Based Methods for Molecular Characterization of Soil Organic Matter by High Resolution Mass Spectrometry,” Analytical Chemistry 87(10), 5206-15. DOI: 10.1021/acs.analchem.5b00116. (Reference link)
(See also)

Contact: Paul E. Bayer, SC-23.1, (301) 903-5324, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


April 09, 2015

Climate Change and Permafrost Carbon Feedback

Large quantities of organic carbon are stored in frozen soils (permafrost) within Arctic and sub-Arctic regions. A warming climate can induce environmental changes that accelerate the microbial breakdown of organic carbon and the release of the greenhouse gases carbon dioxide and methane. This feedback can accelerate climate change, but the magnitude and timing of greenhouse gas emissions from these regions and their impact on climate change remain uncertain. In a recent study, researchers find that current evidence suggests a gradual and prolonged release of greenhouse gas emissions in a warming climate and present a research strategy with which to target poorly understood aspects of permafrost carbon dynamics.

Reference: Schuur, E. A. G., A. D. McGuire, C. Schädel, G. Grosse, J. W. Harden, D. J. Hayes, G. Hugelius. C. D. Koven, P. Kuhry, D. M. Lawrence, S. M. Natali, D. Olefeldt, V. E. Romanovsky, K. Schaefer, M. R. Turetsky, C. C. Treat, and J. E. Vonk. “Climate Change and the Permafrost Carbon Feedback,” Nature 520, 171–79. DOI: 10.1038/nature14338. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


April 09, 2015

Effect of Temperature on Rate, Affinity, and 15N Fractionation of NO3- During Biological Denitrification in Soils

Soil isotopes are commonly used in environmental, agricultural, and biogeochemical studies to track sources and fate of labeled compounds, and also because they facilitate quantification of the intensity of a process relative to others. In a recent study, researchers worked to (1) elucidate the linear and nonlinear contributions of temperature to the reaction rate of isotopically labeled reactants, (2) highlight whether effects arise in other parameters, and (3) provide a comprehensive sensitivity analysis of kinetic isotopic effects over the concentration-temperature space using mathematical modeling of the effects in (1) and (2). To accomplish this, nine independent experiments of nitrate (NO3-) denitrification were analyzed using the Arrhenius law and the Eyring’s transition-state theory to highlight how temperature affects reaction rate constants, affinities, and kinetic isotopic effects. For temperatures between 20 and 35 °C, the Arrhenius law and the transition-state theory described equally well observed temperature increases in 14NO3- and 15NO3- denitrification rates. These increases were partly caused by an increase in frequency factor and a slight decrease in activation energy (enthalpy and entropy). Parametric analysis also showed that the affinity of 14NO3- and 15NO3- toward a microbial enzyme increased exponentially with temperature and a strong correlation with the rate constants was found. Experimental time and temperature-averaged fractionation factor αP/S showed only a slight increase with increasing temperature (i.e., lower isotopic effects); however, a comprehensive sensitivity analysis in the concentration temperature domain using average thermodynamic quantities estimated here showed a more complex response; αP/S was relatively constant for initial bulk concentrations [NO3-]0 ≤ 0.01 mol kg-1, while substantial nonlinearities developed for [NO3-]0 ≥ 0.01 mol kg-1 and appeared to be strongly correlated with microbial biomass, whose concentration and activity varied primarily as a function of temperature and available substrate. Values of αP/S ranging between 0.9 and 0.98 for the tested temperatures suggested that interpretations of environmental isotopic signatures should include a sensitivity analysis to the temperature as this affects directly the rate constants and affinities in biochemical reactions and may hide process- and source-related isotopic effects.

Reference: Maggi, F., and W. J. Riley. 2015. “The Effect of Temperature on the Rate, Affinity, and 15N Fractionation of NO3- During Biological Denitrification in Soils,” Biogeochemistry 124(1), 235–53. DOI 10.1007/s10533-015-0095-2. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


April 07, 2015

Phenolic Profile Highlights Disconnect in Root Tissue Quality Predicted by Elemental- and Molecular-Level Carbon Composition

Fine roots constitute a significant source of plant productivity and litter turnover across terrestrial ecosystems, but less is known about the quantitative and qualitative profile of phenolic compounds within the fine-root architecture, which could regulate the potential contribution of plant roots to the soil organic matter pool. To understand the linkage between traditional macro-elemental and morphological traits of roots and their molecular-level carbon chemistry, researchers analyzed seasonal variations in monomeric yields of the free, bound, and lignin phenols in fine roots (distal five orders) and leaves of Ardisia quinquegona. Fine roots contained two-fold higher concentrations of bound phenols and three-fold higher concentrations of lignin phenols than leaves. Within fine roots, the concentrations of free and bound phenols decreased with increasing root order, and seasonal variation in the phenolic profile was more evident in lower-order than in higher-order roots. The morphological and macro-elemental root traits were decoupled from the quantity, composition, and tissue association of phenolic compounds, revealing the potential inability of these traditional parameters to capture the molecular identity of phenolic carbon within the fine-root architecture and between fine roots and leaves. These results highlight the molecular-level heterogeneity in phenolic carbon composition within the fine-root architecture, and imply that traits that capture the molecular identity of the root construct might better predict the decomposition dynamics within fine-root orders.

Reference: Wang, J.-J., N. Tharayil, A. T. Chow, V. Suseela, and H. Zeng. 2015. “Phenolic Profile Within the Fine-Root Branching Orders of an Evergreen Species Highlights a Disconnect in Root Tissue Quality Predicted by Elemental- and Molecular-Level Carbon Composition,” New Phytologist 206(4), 1261–73. DOI: 10.1111/nph.13385. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


March 24, 2015

Stability of Carbon in Permafrost Soils

Permafrost soils contain enormous amounts of organic carbon whose stability is contingent on remaining frozen. With future warming, these soils may release carbon to the atmosphere and act as a positive feedback to climate change. Significant uncertainty remains on the post-thaw carbon dynamics of permafrost-affected ecosystems, in particular since most of the carbon resides at depth where decomposition dynamics may differ from surface soils, and since nitrogen mineralized by decomposition may enhance plant growth. Using a carbon–nitrogen model that includes permafrost processes forced in an unmitigated warming scenario, researchers show that the permafrost region’s future carbon balance is highly sensitive to the decomposability of deeper carbon, with the net balance ranging from 21 Pg of carbon to 164 Pg carbon losses by 2300. Increased soil nitrogen mineralization reduces nutrient limitations, but the impact of deep nitrogen on the carbon budget is small due to enhanced nitrogen availability from warming surface soils and seasonal asynchrony between deeper nitrogen availability and plant nitrogen demands. Although nitrogen dynamics are highly uncertain, the future carbon balance of this region is projected to hinge more on the rate and extent of permafrost thaw and soil decomposition than on enhanced nitrogen availability for vegetation growth resulting from permafrost thaw.

Reference: Koven, C. D., D. M. Lawrence, and W. J. Riley. 2015. “Permafrost Carbon– Climate Feedback is Sensitive to Deep Soil Carbon Decomposability but not Deep Soil Nitrogen Dynamics,” Proceedings of the National Academy of Sciences (USA) 112(12), 3752–57. DOI: 10.1073/pnas.1415123112. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


March 06, 2015

Decomposition by Ectomycorrhizal Fungi Alters Soil Carbon Storage in Simulation Model

Carbon cycle models often lack explicit belowground organism activity, yet belowground organisms regulate carbon storage and release in soil. Ectomycorrhizal fungi are important players in the carbon cycle because they are a conduit into soil for carbon assimilated by the plant. It is hypothesized that ectomycorrhizal fungi can also be active decomposers when plant carbon allocation to fungi is low. In this study, researchers developed a simulation model of the plant-mycorrhizae interaction where a reduction in plant productivity stimulates ectomycorrhizal fungi to decompose soil organic matter. The model output suggests that ectomycorrhizal activity accounts for a portion of carbon decomposed in soil, but this portion varied with plant productivity and the mycorrhizal carbon uptake strategy simulated. Lower organic matter inputs to soil were largely responsible for reduced soil carbon storage. Using mathematical theory, the researchers demonstrated that biotic interactions affect predictions of ecosystem functions. Specifically, they developed a simple function to model the mycorrhizal switch in function from plant symbiont to decomposer. The study shows that including mycorrhizal fungi with the flexibility of mutualistic and saprotrophic lifestyles alters predictions of ecosystem function.

Reference: Moore, J. A. M., J. Jiang, W. M. Post, and A. T. Classen. 2015. “Decomposition by Ectomycorrhizal Fungi Alters Soil Carbon Storage in a Simulation Model,” Ecosphere 6(3), 29. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


March 06, 2015

Monoterpenes Play Important Antioxidant Roles, Serve as Sources of Secondary Organic Aerosol Precursors

Despite orders of magnitude difference in atmospheric reactivity and great diversity in biological functioning, little is known about monoterpene speciation in tropical forests. In a recent study, researchers report vertically resolved ambient air mixing ratios for 12 monoterpenes in a central Amazon rainforest, including observations of the highly reactive cis-β-ocimene [160 parts per trillion (ppt)], trans-β-ocimene (79 ppt), and terpinolene (32 ppt), which accounted for an estimated 21% of total monoterpene composition, yet 55% of the upper canopy monoterpene ozonolysis rate. All 12 monoterpenes showed a mixing ratio peak in the upper canopy, with three demonstrating subcanopy peaks in seven of 11 profiles. Leaf-level emissions of highly reactive monoterpenes accounted for up to 1.9% of photosynthesis, confirming light-dependent emissions across several Amazon tree genera. These results suggest that highly reactive monoterpenes play important antioxidant roles during photosynthesis in plants and serve as near-canopy sources of secondary organic aerosol precursors through atmospheric photooxidation via ozonolysis.

Reference: Jardine, A. B., K. J. Jardine, J. D. Fuentes, S. T. Martin, G. Martins, F. Durgante, V. Carneiro, N. Higuchi, A. O. Manzi, and J. Q. Chambers. 2015. “Highly Reactive Light-Dependent Monoterpenes in the Amazon,” Geophysical Research Letters 42(5), 1576–83. DOI: 10.1002/2014GL062573. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


March 06, 2015

Urgent Need for Warming Experiments in Tropical Forests

Although tropical forests account for only a fraction of the planet’s terrestrial surface, they exchange more carbon dioxide with the atmosphere than any other biome on Earth and thus play a disproportionate role in the global climate. Over the next 20 years, the tropics will experience unprecedented warming, yet exceedingly high uncertainty persists about their potential responses to this imminent climatic change. In a recent study, researchers investigated overall model uncertainty of tropical latitudes and explored the scientific benefits and inevitable trade-offs inherent in large-scale manipulative field experiments. With a Coupled Model Intercomparison Project Phase 5 analysis, they found that model variability in projected net ecosystem production was nearly three times greater in the tropics than for any other latitude. Through a review of the most current literature, they concluded that manipulative warming experiments are vital to accurately predict future tropical forest carbon balance, and they further recommend establishing a network of comparable studies spanning gradients of precipitation, edaphic qualities, plant types, and land use change. In addition, they provide arguments for long-term, single-factor warming experiments that incorporate warming of the most biogeochemically active ecosystem components (i.e., leaves, roots, and soil microbes). Hypothesis testing of underlying mechanisms should be a priority, along with improving model parameterization and constraints. No single tropical forest is representative of all tropical forests; therefore, logistical feasibility should be the most important consideration for locating largescale manipulative experiments.

Reference: Cavaleri, M. A., S. C. Reed, W. K. Smith, and T. E. Wood. 2015. “Urgent Need for Warming Experiments in Tropical Forests,” Global Change Biology 21 (6), 2111–21. DOI: 10.1111/gcb.12860. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


March 02, 2015

Optimal Stomatal Behavior Around the World

Stomatal conductance (gs) is a key land-surface attribute as it links transpiration, the dominant component of global land evapotranspiration, and photosynthesis, the driving force of the global carbon cycle. Despite the pivotal role of gs in predictions of global water and carbon cycle changes, a global scale database and an associated globally applicable gs model that enable predictions of stomatal behavior are lacking. In a recent study, researchers present a database of globally distributed gs obtained in the field for a wide range of plant functional types (PFTs) and biomes. They found that stomatal behavior differs among PFTs according to their marginal carbon cost of water use, as predicted by the theory underpinning the optimal stomatal model and the leaf and wood economics spectrum. They also demonstrate a global relationship with climate. These findings provide a robust theoretical framework for understanding and predicting gs behavior across biomes and across PFTs that can be applied to regional, continental, and global-scale modeling of ecosystem productivity, energy balance, and ecohydrological processes in a future changing climate.

Reference: Lin, Y.-S., et al. 2015. “Optimal Stomatal Behaviour Around the World,” Nature Climate Change 5, 459–64. DOI: 10.1038/nclimate2550. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


February 05, 2015

Permafrost Soils and Carbon Cycling

Knowledge of soils in the permafrost region has advanced immensely in recent decades, despite the remoteness and inaccessibility of most of the region and the sampling limitations posed by the severe environment. These efforts have significantly increased estimates of the amount of organic carbon stored in permafrost-region soils and improved understanding of how pedogenic processes unique to permafrost environments built enormous organic carbon stocks during the Quaternary. This knowledge also has called attention to the importance of permafrost-affected soils to the global carbon cycle and the potential vulnerability of the region’s soil organic carbon (SOC) stocks to changing climatic conditions. In a recent review, researchers briefly introduce the permafrost characteristics, ice structures, and cryopedogenic processes that shape the development of permafrost-affected soils and discuss their effects on soil structures and organic matter distributions within the soil profile. They examine the quantity of organic carbon stored in permafrost-region soils, as well as the characteristics, intrinsic decomposability, and potential vulnerability of this organic carbon to permafrost thaw under a warming climate. Overall, frozen conditions and cryopedogenic processes, such as cryoturbation, have slowed decomposition and enhanced sequestration of organic carbon in permafrost-affected soils over millennial timescales. Due to the low temperatures, the organic matter in permafrost soils is often less humified than in more temperate soils, making some portion of this stored organic carbon relatively vulnerable to mineralization upon thawing of permafrost.

Reference: Ping, C. L., J. D. Jastrow, M. T. Jorgenson, G. J. Michaelson, and Y. L. Shur. 2015. “Permafrost Soils and Carbon Cycling,” SOIL 1, 147–71. DOI: 10.5194/soil-1-147-2015. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


January 27, 2015

Moderate Forest Disturbance as a Stringent Test for Gap and Big-Leaf Models

Disturbance-induced tree mortality is a key factor regulating the carbon balance of a forest, but tree mortality and its subsequent effects are poorly represented processes in terrestrial ecosystem models. Thus unclear is whether models can robustly simulate moderate (noncatastrophic) disturbances, which tend to increase biological and structural complexity and are increasingly common in aging U.S. forests. Researchers recently tested whether three forest ecosystem models—Biome-BGC (BioGeochemical Cycles), a classic big-leaf model, and the ZELIG and ED (Ecosystem Demography) gap-oriented models—could reproduce the resilience to moderate disturbance observed in an experimentally manipulated forest (Forest Accelerated Succession Experiment in northern Michigan, where 38% of canopy dominants were stem girdled and compared to control plots). Each model was parameterized, spun up, and disturbed following similar protocols and run for 5 years post-disturbance. The models replicated observed declines in aboveground biomass well. Biome-BGC captured the timing and rebound of observed leaf area index (LAI), while ZELIG and ED correctly estimated the magnitude of LAI decline. None of the models fully captured the observed post-disturbance carbon fluxes, in particular gross primary production or net primary production (NPP). Biome-BGC NPP was correctly resilient but for the wrong reasons, and could not match the absolute observational values. ZELIG and ED, in contrast, exhibited large, unobserved drops in NPP and net ecosystem production. The biological mechanisms proposed to explain the observed rapid resilience of the carbon cycle typically are not incorporated by these or other models. Thus, an open question is whether most ecosystem models will simulate correctly the gradual and less extensive tree mortality characteristic of moderate disturbances.

Reference: Bond-Lamberty, B., J. P. Fisk, J. A. Holm, V. Bailey, G. Bohrer, and C. M. Gough. 2015. “Moderate Forest Disturbance as a Stringent Test for Gap and Big-Leaf Models,” Biogeosciences 12(2), 513–26. DOI: 10.5194/bg-12-513-2015. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


January 08, 2015

Dimethyl Sulfide Emissions in the Amazon Rainforest

Surface-to-atmosphere emissions of dimethyl sulfide (DMS) may impact global climate through the formation of gaseous sulfuric acid, which can yield secondary sulfate aerosols and contribute to new particle formation. While oceans are generally considered the dominant sources of DMS, a shortage of ecosystem observations prevents an accurate analysis of terrestrial DMS sources. Using mass spectrometry, researchers recently quantified ambient DMS mixing ratios within and above a primary rainforest ecosystem in the central Amazon Basin in real time (2010–2011) and at high vertical resolution (2013–2014). Elevated, but highly variable DMS mixing ratios were observed within the canopy, showing clear evidence of a net ecosystem source to the atmosphere during both day and night in both the dry and wet seasons. Periods of high DMS mixing ratios lasting up to 8 hours [up to 160 parts per trillion (ppt)] often occurred within the canopy and near the surface during many evenings and nights. Daytime gradients showed mixing ratios (up to 80 ppt) peaking near the top of the canopy as well as near the ground following a rain event. The spatial and temporal DMS distribution suggests that ambient levels and their potential climatic impacts are dominated by local soil and plant emissions. A soil source was confirmed by measurements of DMS emission fluxes from Amazon soils as a function of temperature and soil moisture. Furthermore, light- and temperature-dependent DMS emissions were measured from seven tropical tree species. This study has important implications for understanding terrestrial DMS sources and their role in coupled land-atmosphere climate feedbacks.

Reference: Jardine, K., A. M. Yañez-Serrano, J. Williams, N. Kunert, A. Jardine, T. Taylor, L. Abrell, P. Artaxo, A. Guenther, C. N. Hewitt, E. House, A. P. Florentino, A. Manzi, N. Higuchi, J. Kesselmeier, T. Behrendt, P. R. Veres, B. Derstroff, J. D. Fuentes, S. T. Martin, and M. O. Andreae. 2015. “Dimethyl Sulfide in the Amazon Rainforest,” Global Biogeochemical Cycles 29(1), 19–32. DOI: 10.1002/2014GB004969. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


January 08, 2015

Global Leaf Trait Database Supports Earth System Models

In science, researchers collaborate so that they can complement existing disciplinary expertise, gain access to specialized equipment, or expand the depth and breadth of datasets that can be used to derive new knowledge. Motivated by this latter objective, a research team has compiled a global database (GlobResp) that details rates of leaf dark respiration and associated traits from sites that span Arctic tundra to tropical forests. This database builds on earlier research and was supplemented by recent field campaigns and unpublished data. In keeping with other trait databases, GlobResp provides insights on how physiological traits, especially rates of dark respiration, vary as a function of environment and how that variation can be used to inform terrestrial biosphere models and land surface components of Earth system models. Although an important component of plant and ecosystem carbon budgets, respiration has only limited representation in models. This database gives users a unique perspective of the climatic controls on respiration, thermal acclimation and evolutionary adaptation of dark respiration, and insights into the covariation of respiration with other leaf traits.

References:

  1. Atkin, O. K., et al. 2015. “Global Variability in Leaf Respiration in Relation to Climate, Plant Functional Types, and Leaf Traits,” New Phytologist 206(2), 614–36. DOI: 10.1111/nph.13253. (Reference link)
  2. Wullschleger, S. D., J. M. Warren, and P. E. Thornton. 2015. “Leaf Respiration (GlobResp)–Global Trait Database Supports Earth System Models,” New Phytologist 206(2), 483–85. DOI: 10.1111/nph.13364. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


January 01, 2015

Net Primary Production of Temperate Deciduous Forest Exhibits Threshold Response to Increasing Disturbance Severity

The global carbon balance is vulnerable to disturbances that alter terrestrial carbon storage. Disturbances to forests occur along a continuum of severity, from low-intensity disturbance causing the mortality or defoliation of only a subset of trees to severe stand-replacing disturbance that kills all trees; yet, considerable uncertainty remains in how forest production changes across gradients of disturbance intensity. In a recent study, researchers used a gradient of tree mortality in an upper Great Lakes forest ecosystem to: (1) quantify how aboveground wood net primary production (ANPPw) responds to a range of disturbance severities and 2) identify mechanisms supporting ANPPw resistance or resilience following moderate disturbance. They found that ANPPw declined nonlinearly with rising disturbance severity, remaining stable until > 60 % of the total tree basal area senesced. As upper canopy openness increased from disturbance, greater light availability to the subcanopy enhanced the leaf-level photosynthesis and growth of this formerly light-limited canopy stratum, compensating for upper canopy production losses and a reduction in total leaf area index (LAI). As a result, whole-ecosystem production efficiency (ANPPw/LAI) increased with rising disturbance severity, except in plots beyond the disturbance threshold. These findings provide a mechanistic explanation for a nonlinear relationship between ANPPw and disturbance severity, in which the physiological and growth enhancement of undisturbed vegetation is proportional to the level of disturbance until a threshold is exceeded. These results have important ecological and management implications, demonstrating that in some ecosystems moderate disturbance levels minimally alter forest production.

Reference: Stuart-Haëntjens, E., P. S. Curtis, R. T. Fahey, C. S. Vogel, and C. M. Gough. 2015. “Net Primary Production of a Temperate Deciduous Forest Exhibits a Threshold Response to Increasing Disturbance Severity,” Ecology 96, 2478–87. (Reference link)

Contact: Jared DeForest, SC-23, (301) 903-3251, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER