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PI-Submitted Research Highlights for
Terrestrial Ecosystem Science Program

Climate Sensitive Size-Dependent Survival In Tropical Trees

Chonggang Xu

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14 August 2018

Important links indicated between evolutionary strategies, climate, and carbon cycling. 

The Science
Tropical forests have very high species diversity that poses a significant challenge to predictive understanding of tropical forest dynamics. Scientists from the Next-Generation Ecosystem Experiments (NGEE)–Tropics project found that tropical species could be classified into four "survival modes," which explain life-history variation in survival that shapes carbon cycling under different climate conditions as measured by annual temperature and cumulative water deficit.

The Impact
The consistent survival modes identified across different tropical forests allowed researchers to simulate the forest survival in a relatively small number of trackable functional types for hyper-diverse tropical forests within Earth System Models (ESMs). Frequently collected functional traits, such as wood density, leaf mass per area, and seed mass, were not generally predictive of the survival modes of species. Mean annual temperature and cumulative water deficit predicted the proportion of biomass of survival modes, indicating important links between evolutionary strategies, climate, and carbon cycling. As tree survival plays a key role in regulating vegetation dynamics, researchers expect that this analysis can provide insights to better simulate vegetation dynamics in ESMs.

Summary
Survival rates of large trees determine forest biomass dynamics. Survival rates of small trees have been linked to mechanisms that maintain biodiversity across tropical forests. How species survival rates change with size offers insight into the links between biodiversity and ecosystem function across tropical forests. Scientists from the NGEE-Tropics study tested patterns of size-dependent tree survival across the tropics using data from 1,781 species and over 2 million individuals to assess whether tropical forests can be characterized by size-dependent, life-history survival strategies. They found that species were classifiable into four "survival modes" that explain life-history variation that shapes carbon cycling and the relative abundance within forests. Frequently collected functional traits, such as wood density, leaf mass per area, and seed mass, were not generally predictive of the survival modes of species. Mean annual temperature and cumulative water deficit predicted the proportion of biomass of survival modes, indicating important links between evolutionary strategies, climate, and carbon cycling. Project results reveal globally identifiable size-dependent survival strategies that differ across diverse systems in a consistent way.

Contacts
BER Program Managers
Daniel Stover
Terrestrial Ecosystem Science, SC-23.1
Daniel.Stover@science.doe.gov (301-903-0289)

Dorothy Koch
SC-23.1
Dorothy.Koch@science.doe.gov (301-903-0105)

Principal Investigators
Chonggang Xu
Los Alamos National Laboratory
Los Alamos, NM 87545
cxu@lanl.gov

Nate McDowell
Pacific Northwest National Laboratory
Richland, WA 99352
nate.mcdowell@pnnl.gov

Funding
The first author, Daniel Johnson, was supported by Los Alamos National Laboratory (LANL; Director’s Post-Doctoral Fellowship). Contributions by Chonggang Xu, Jeff Chamber, Stuart David, and Nate McDowell were supported by the Next-Generation Ecosystem Experiments (NGEE)–Tropics) project, funded by the Office of Biological and Environmental Research within the U.S. Department of  Energy Office of Science. Sean McMahon was partially funded by the National Science Foundation (NSF-EF1137366).

Publications
Johnson, D. J. et al. "Climate sensitive size-dependent survival in tropical trees." Nature Ecology & Evolution 2, 1436–1442 (2018). [DOI:10.1038/s41559-018-0626-z]

Related Links
BEHIND THE PAPER — Tropical tree survival: the large and the small of it

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