BER Research Highlights

Search Date: October 19, 2017

9 Records match the search term(s):


November 16, 2011

Deforestation Drives Cooling at Mid- to High Latitudes

Deforestation in mid- to high latitudes is hypothesized to have the potential to cool the Earth's surface by altering biophysical processes. When continental-scale land clearing is included in climate models, cooling is triggered by increases in surface albedo and is reinforced by a land albedo-sea ice feedback. This feedback is a key component of the model predictions; without it other processes overwhelm the albedo effect to generate warming. Ongoing activities, such as land management for climate mitigation, are occurring at local scales (hectares) presumably too small to generate the feedback. It is not known if the intrinsic biophysical mechanism on its own can consistently change surface temperatures. The effect of deforestation on climate has also not been demonstrated over large areas from direct observations. Now, DOE researchers show that surface air temperature is lower in open land than in nearby forested land. The effect is 0.85°±0.44K (mean ± one standard deviation) north of 45°N (essentially north of the U.S.-Canadian border) and 0.21°±0.53K southwards. Below 35°N (south of Tennessee, all of Texas and New Mexico, and southern California), there is weak evidence that deforestation leads to warming. Results are based on temperature comparisons at forested eddy covariance towers in the United States and Canada and, as a proxy for small areas of cleared land, nearby surface weather stations. Night-time temperature changes unrelated to changes in surface albedo are also an important contributor to the overall cooling effect. The observed latitudinal dependence is consistent with theoretical expectations of changes in energy loss from convection and radiation across latitudes in both the daytime and night-time phase of the diurnal cycle, the latter of which remains uncertain in climate models.

Reference: Lee, X., M. L. Goulden, D. Y. Hollinger, A. Barr, T. A. Black, G. Bohrer, R. Bracho, B. Drake, A. Goldstein, L. Gu, G. Katul, T. Kolb, B. E. Law, H. Margolis, T. Meyers, R. Monson, W. Munger, R. Oren, K. T. Paw, A. D. Richardson, H. P. Schmid, R. Stabler, S. Wofsy, and L. Zhao. 2011. “Observed Increase in Local Cooling Effect of Deforestation at Higher Latitudes,” Nature 479, 384-87. DOI: 10.1038/nature10588. (Reference link)

Contact: 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


November 02, 2011

Forest Soil Carbon Lost at a Greater Rate in Warmer Climates

Understanding and predicting the impacts of climate change and the stability of carbon stored in terrestrial ecosystems is an important part of planning future energy strategies. This Oak Ridge National Laboratory-led study compared the turnover time of labile soil carbon, in relation to temperature and soil texture, in several forest ecosystems that are representative of large areas of North America. Carbon (C) and nitrogen (N) stocks and C:N ratios were measured in the forest floor, mineral soil, and two mineral soil fractions (particulate and mineral-associated organic matter) at five AmeriFlux sites (a network that provides continuous observations of ecosystem-level exchanges of CO2, water, and energy across the Americas) along a latitudinal gradient in the eastern United States. With one exception, forest floor and mineral soil carbon stocks increased from warm, southern sites (with fine-textured soils) to cool, northern sites (with more coarse-textured soils). The exception was a northern site, with less than 10% silt-clay content, that had a soil organic carbon stock similar to the southern sites. Moving from south to north, the turnover time of labile soil organic C increased from approximately 5 to 14 years. Consistent with its role in stabilization of soil organic carbon, silt-clay content was positively correlated with stable C at each site. Latitudinal differences in the storage and turnover of soil C were related to mean annual temperature, but soil texture superseded temperature when there was too little silt and clay to stabilize labile soil C and protect it from decomposition. Overall, this study suggests that large labile pools of forest soil C are at risk of decomposition in a warming climate, especially in coarse textured forest soils.

Reference: Garten, C. T., Jr. 2011. "Comparison of Forest Soil Carbon Dynamics at Five Sites along a Latitudinal Gradient," Geoderma 167-168, 30-40, DOI: 10.1016/j.geoderma.2011.08.007. (Reference link)

Contact: 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


September 29, 2011

Global Rates of Photosynthesis Greater than Previously Assumed

Estimates of global carbon sinks have large uncertainties that complicate estimates of Earth's capacity to buffer rising atmospheric carbon dioxide (CO2). Photosynthesis is a major contributor to these carbon sinks. A DOE-funded team led by Ralph Keeling at the Scripps Institution of Oceanography followed the path of oxygen atoms on CO2 molecules during photosynthesis to create a new way to measure the efficiency of the world's plants. The ratio of two natural isotopes of oxygen in CO2 told researchers how long the CO2 had been in the atmosphere and how fast it had passed through plants. From this, they estimated that the global rate of photosynthesis is about 25 percent faster than thought. This new approach linked the changes in oxygen isotopes to El Niño, the global climate phenomenon associated with a variety of unusual weather patterns including low rainfall in tropical regions of Asia and South America. The naturally occurring isotopes of oxygen, 18O and 16O, are present in different proportions in the water inside leaves during dry, El Niño periods in the tropics. This oxygen ratio in leaf waters is passed along to CO2 when CO2 mixes with water inside leaves. This exchange of oxygen between CO2 and plant water also occurs in regions outside of the tropics that are not as affected by El Niño and where the 18O/16O ratio is more "normal." The team measured the time it took for the global 18O/16O ratio to return to normal following an El Niño event to infer the speed at which photosynthesis is taking place. They discovered that the ratio returned to normal faster than expected indicating that global photosynthesis occurs at a greater rate than previously assumed. The rate, expressed in terms of how much carbon is processed by plants in a year, has now been revised upward from the previous estimate of 120 Pg of carbon a year to a new annual rate between 150-175 Pg. These results suggest that the uncertainty in estimating global carbon sinks is even greater than previously thought.

Reference: Welp, L. R., R. F. Keeling, H. A. J. Meijer, A. F. Bollenbacher, S. C. Piper, K. Yoshimura, R. J. Francey, C. E. Allison, and M. Wahlen. 2011. "Interannual Variability in the Oxygen Isotopes of Atmospheric CO2 Driven by El Niño," Nature 477, 579-82. DOI:10.1038/nature10421. (Reference link)

Contact: 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


July 01, 2011

Elevated CO2 and O3 Alter Soil Organic Matter Cycling in Northern Deciduous Forests

Over time, changes in plant growth and litter production caused by rising CO2 and O3 concentrations could impact the storage and cycling of carbon in soil organic matter. DOE’s decade-long investment in a multi-factor Free-Air CO2 Enrichment (FACE) experiment in Rhinelander, Wisconsin, enabled scientists from Argonne National Laboratory and two Midwest universities to observe that elevated CO2 changed the trajectories of three soil organic matter pools characterized by extent of decomposition. As the experiment progressed, relatively undecomposed particulate organic matter fragments built up more rapidly in the soil of plots exposed to elevated CO2, while the amount of carbon found in more highly processed mineral-associated organic matter pools declined under elevated CO2 but not in ambient soils. Thus, elevated CO2 appears to have increased the cycling of carbon and nitrogen in the soil organic matter of the sandy soils at this site. In contrast, elevated O3 tended to have the opposite effect, reducing both detritus inputs and the cycling of soil carbon and nitrogen. The effects of O3 occurred regardless of atmospheric CO2 concentration. Although forest community composition altered the magnitude of the responses, enhanced turnover of soil organic matter could limit the potential for long-term soil carbon sequestration in the northern deciduous forests of an elevated CO2 world.

References: Hofmockel, K. S., D. R. Zak, K. K. Moran, and J. D. Jastrow. 2011. "Changes in Forest Soil Organic Matter Pools After a Decade of Elevated CO2 and O3," Soil Biology and Biochemistry 43, 1518-1527.

Contact: 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


April 28, 2011

Declining Soil Nitrogen in a Free Air CO2-Enrichment Experiment (FACE)

The sustainability of higher ecosystem production under elevated atmospheric carbon dioxide (CO,2) is unknown. Nitrogen (N) is often limited in ecosystems as a result of N incorporation into long-lived biomass and soil organic matter. As a result, N limitation could eventually limit or nullify increasing forest productivity under elevated CO2 (i.e., “Progressive N Limitation”). In the first six years of the Oak Ridge National Laboratory (ORNL) FACE experiment, there was no apparent evidence that N limitation was exacerbated by elevated CO2 or that N limitation reduced sweetgum tree growth. However, the CO2 stimulation of sweetgum tree growth has more recently declined and was tentatively attributed to N limitation. Using stable N isotopes, temporal trends in sweetgum leaf litterfall 15N abundance provided strong evidence that N availability in the ORNL FACE plots has in fact declined over time, and declined faster in plots exposed to elevated CO2, providing evidence for progressive N limitation. Although these results cannot be generalized for other FACE sites, examination of leaf litterfall d15N may provide an accurate indicator of soil N availability and progressive N limitation.

References: Garten Jr., C. T., C. M. Iversen, and R. J. Norby. 2011. “Litterfall 15N Abundance Indicates Declining Soil Nitrogen Availability in a Free Air CO2 Enrichment Experiment,” Ecology 92, 133–39 [doi:10.1890/10-0293.1].

Contact: 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


April 18, 2011

Changing Water Balance in Forests Exposed to Elevated CO2

Plants influence ecosystem water balance through responses to environmental conditions, and their sensitivity to climate change could alter the ecohydrology of future forests. DOE scientists at Oak Ridge National Laboratory used a combination of measurements, synthesis of existing literature, and modeling to study the consequences of elevated CO2 on ecohydrologic processes in forests. Data from five of DOE’s free-air CO2 enrichment (FACE) sites reveal that elevated CO2 reduced the passage of water vapor through the stomata, or small pores of the plant, leading to declines in canopy transpiration and water use for three closed-canopy forest sites. At the sweetgum FACE experiment in Oak Ridge, Tennessee, elevated CO2 reduced seasonal transpiration by 10–16%. Model simulations also predicted reduced demand for water in response to elevated CO2. The direct effect of elevated CO2 on forest water balance through reductions in transpiration could be considerable, especially following canopy closure and development of maximal leaf area index. Complementary, indirect effects of elevated CO2 include potential increases in root or leaf litter and soil organic matter, shifts in root distribution and altered patterns of water extraction.

References: Warren, J. M., E. Pötzelsberger, S. D. Wullschleger, P. E. Thornton, H. Hasenauer, and R. J. Norby. 2011. “Ecohydrologic Impact of Reduced Stomatal Conductance in Forests Exposed to Elevated CO2,” Ecohydrology 4, 196–210. DOI: 10.1002/eco.173.

Contact: 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


March 21, 2011

Carbon Release from Roots to Microbes Prevents Nitrogen Limitation Under CO2 Enrichment

A forest’s ability to store carbon depends on resource limitations, such as nitrogen. The Progressive Nitrogen Limitation (PNL) theory suggests that under elevated CO2, a forest will immobilize nitrogen in biomass, limiting nitrogen needed for enhanced growth. DOE scientists show, for the first time, that mature trees exposed to CO2 enrichment increase the release of soluble carbon from roots to soil, and that such increases are coupled to the accelerated turnover of nitrogen pools in the rhizosphere. Over the course of three years, the team measured in situ rates of root exudation from intact loblolly pine (Pinus taeda L.) roots at the Duke Forest, near Chapel Hill, North Carolina. Trees fumigated with elevated CO2 increased exudation rates by 55% during the primary growing season, leading to a 50% annual increase in dissolved organic inputs to fumigated forest soils. These increases in root-derived carbon were positively correlated with microbial release of extracellular enzymes involved in breakdown of organic nitrogen in the rhizosphere, indicating that exudation stimulated microbial activity and accelerated the rate of soil organic matter turnover. Trees exposed to both elevated CO2 and nitrogen fertilization did not increase exudation rates and had reduced enzyme activities in the rhizosphere. These results provide field-based empirical support suggesting that sustained growth responses of forests to elevated CO2 in low fertility soils are maintained by enhanced rates of microbial activity and nitrogen cycling fuelled by inputs of root-derived carbon. However, the decomposition of soil organic matter by the stimulated microbes may prevent a large soil carbon pool from accumulating in forest soils.

References: Phillips, R. P, A. C. Finzi, and E. S. Bernhardt. 2011. “Enhanced Root Exudation Induces Microbial Feedbacks to N Cycling in a Pine Forest Under Long-Term CO2 Fumigation,” Ecology Letters 14, 187–94.

Drake, J. E, A. Gallet-Budynek, K. S. Hofmockel, E. S. Bernhardt, S. A. Billings, R. B. Jackson, K. S. Johnsen, J. Lichter, H. R. McCarthy, M. L. McCormack, D. J. P. Moore, R. Oren, S. Palmroth, R. P. Phillips, J. S. Pippen, S. G. Pritchard, K. K. Treseder, W. H. Schlesinger, E. H. DeLucia, and A. C. Finzi. 2011. “Increases in the Flux of Carbon Belowground Stimulate Nitrogen Uptake and Sustain the Long-Term Enhancement of Forest Productivity Under Elevated CO2,” Ecology Letters 14, 349–57.

Contact: 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


February 22, 2011

Improving Our Understanding of Carbon Fluxes in Diverse Ecosystems

AmeriFlux is a long-term carbon dioxide measuring and monitoring network to help define the global carbon dioxide budget, improve predictions of future carbon dioxide concentrations, and enhance understanding of net ecosystem productivity and carbon sequestration of the terrestrial biosphere. DOE scientists studied key environmental and meteorological drivers from different vegetation types at 56 AmeriFlux sites that influence their ability to measure the fluxes of carbon dioxide. Using 305 site years worth of data and a statistical analysis of the cluster differences, the authors identified light intensity, vegetation type, and water vapor as key factors that impact the pattern and magnitude of the turbulent exchange. These results will improve our ability to measure and model carbon dioxide fluxes in diverse ecosystems.

Reference: Schmidt, A., C. Hanson, J. Kathilankal, and B. E. Law. 2011. “Classification and Assessment of Turbulent Fluxes above Ecosystems in North America with Self-Organizing Feature Map Networks,” Agricultural and Forest Meteorology 151, 508–20.

Contact: Daniel Stover, SC-23.1, (301) 903-0289, Mike Kuperberg, SC-23.1, (301) 903-3281
Topic Areas:

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


February 22, 2011

New Approach for Predicting Ecosystem Responses to Global Change

Long-term ecological responses to global change are strongly regulated by slow processes, such as changes in species composition, carbon dynamics in soil and long-lived plants, and accumulation of nutrient capitals. Understanding and predicting these processes require experiments on decadal time scales. But even decadal experiments may not be adequate because many of the slow processes have time scales much longer than those experiments. DOE-funded scientists have proposed a new, coordinated research approach that combines long-term, large-scale global change experiments with process studies and modeling. They propose that long-term global change manipulative experiments, especially in high-priority ecosystems such as tropical forests and high-latitude regions, be conducted in tandem with complementary process studies (e.g., using model ecosystems, species replacements, laboratory incubations, isotope tracers, and greenhouse facilities) to best inform long- and short-term responses. This new, coordinated approach that combines long-term experiments, process studies, and modeling has the potential to be the most effective strategy for gaining information on long-term ecosystem dynamics in response to global change.

Reference: Luo, Y., J. Melillo, S. Niu, C. Beier, J. S. Clarks, A. T. Classen, E. Davidson, J. S. Dukes, R. D. Evans, C. B. Field, C. J. Czimczik, M. Keller, B. A. Kimball, L. M. Kueppers, R. J. Norby, S. L. Pelini, E. Pendall, E. Rastetter, J. Six, M Smith, M. G. Tjoelker, and M. S. Torn. 2011. “Coordinated Approaches To Quantify Long-Term Ecosystem Dynamics in Response to Global Change,” Global Change Biology 17, 843–54.

Contact: 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