An analysis of demographic rates shows that the biomass and turnover of forests depend on which tree demographic strategies are present.
Plants take up carbon from the atmosphere via photosynthesis and store it in their tissues. The growth and survival of trees determine how much, and for how long, carbon is stored by forests. A recent analysis of the growth and survival rates of thousands of tree species explored how the number of species in a forest plot is related to the range of tree growth and survival rates (demographic diversity), and how that influences carbon cycling dynamics. The study reveals that demographic diversity plateaus as the numbers of species increases. Further, the presence of species with particular demographic rates, rather than demographic diversity, govern carbon dynamics.
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 the frequency and intensity of disturbances which are both likely to alter the species composition of forests globally. It is therefore essential that we 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.
The growth and survival of individual trees determine the physical structure of a forest with important consequences for forest function. The authors of this study calculated growth and survival rates of 1,961 tree species from temperate and tropical forests and explored how the range of demographic rates, and the presence or absence of distinct demographic strategies differ across forests, and how these differences in demography relate to the number of species in the forest, and carbon storage. The authors found wide variation in demographic rates across forest plots, which could not be explained by the number of species or climate variables alone. There is 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 the precision and accuracy of predictions of future forest dynamics.
Figure. 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. Image courtesy of Jessica Needham, Earth and Environmental Sciences Area, Lawrence Berkeley National Laboratory.
Contact: Jessica Needham; Earth and Environmental Sciences Area, Lawrence Berkeley National Laboratory; firstname.lastname@example.org
This project began and was developed at ForestGEO workshops in 2016, 2017 and 2018 (NSF DEB-1046113 to S. J. Davies). M. McMahon was partially funded by the USA National Science Foundation (NSF 640261 to S. M. McMahon). This research was supported as part of the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research. 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 SI of this published manuscript.
Needham, J.F., et al. “Demographic composition, not demographic diversity, predicts biomass and turnover across temperate and tropical forests.” Glob Change Biol. (2022) https://doi.org/10.1111/gcb.16100