January 25, 2022

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Using Tree Growth and Survival Rates To Understand Temperate and Tropical Forest Dynamics

An analysis of demographic rates shows that forest biomass and turnover depend on which tree demographic strategies are present.

Tree species can be clustered into demographic types, shown by colors, based on growth, survival, and stature. The relative abundance of these demographic types, rather than a diversity of demographic rates, determines the biomass and turnover of forests.

[Reprinted with permission from Needham, J.F., et al. "Demographic Composition, Not Demographic Diversity, Predicts Biomass and Turnover Across Temperate and Tropical Forests." Global Change Biology 28 (9), 2895–909 (2022). DOI:10.1111/gcb.16100.]

The Science

Plants take up carbon from the atmosphere through photosynthesis and store it in their tissues. Tree growth and survival determine how much, and how long, carbon is stored by forests. Recent growth and survival rate analysis of thousands of tree species explored (1) how the number of species in a forest plot is related to the range of tree growth and survival rates (demographic diversity) and (2) how that influences carbon cycling dynamics. The study revealed that demographic diversity plateaus as numbers of species increases. Further, presence of species with particular demographic rates, rather than demographic diversity, govern carbon dynamics.

The Impact

Forests play a critical role in regulating the world’s climate by cycling large amounts of carbon, water, and energy with the atmosphere. Yet, forests are threatened by changes to climate and an increase in disturbance frequency and intensity, which are both likely to alter the species composition of forests globally. Therefore, scientists must understand how the species composition of forests relate to demographic rates and forest dynamics. This study highlighted the importance of high survival, large statured species for carbon storage.

Summary

Individual tree growth and survival determine a forest’s physical structure, with important consequences for forest function. This study calculated growth and survival rates of 1,961 tree species from temperate and tropical forests and explored (1) how the range of demographic rates and the presence or absence of distinct demographic strategies differ across forests and (2) how these differences in demography relate to the number of species in the forest and carbon storage. Results showed wide variation in demographic rates across forest plots, which could not be explained by the number of species or climate variables alone. Results showed no evidence that a large range of demographic rates lead to higher carbon storage. Rather, the relative abundance of high-survival, large-statured species predicts both biomass and carbon residence time. Linking the demographic composition of forests to resilience or vulnerability to climate change will improve precision and accuracy of predictions of future forest dynamics.

Principal Investigator

Jessica Needham
Lawrence Berkeley National Laboratory
jfneedham@lbl.gov

Program Manager

Brian Benscoter
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
brian.benscoter@science.doe.gov

Funding

This project began and was developed at Forest Global Earth Observatory (ForestGEO) workshops in 2016, 2017, and 2018 (NSF DEB-1046113). S. M. McMahon was partially funded by the U.S. National Science Foundation (NSF 640261). This research was supported as part of the Next-Generation Ecosystem Experiments-Tropics, which is funded by the Biological and Environmental Research (BER) Program, within the U.S. Department of Energy’s (DOE) Office of Science. Lawrence Berkeley National Laboratory (LBNL) is managed and operated by the Regents of the University of California under prime contract number DE-AC02-05CH11231. For individual forest plot funding acknowledgements, see the published manuscript’s Supplementary Information.

References

Needham, J.F., et al. "Demographic Composition, Not Demographic Diversity, Predicts Biomass and Turnover Across Temperate and Tropical Forests." Global Change Biology 28 (9), 2895–909  (2022). https://doi.org/10.1111/gcb.16100.