Citation:

Clark, C., J. Phelan, J. Ash, J. Buckley, J. Cajka, K. Horn, R. Thomas, AND Robert D. Sabo. Future climate change effects on U.S. forest composition may offset benefits of reduced atmospheric deposition of N and S. GLOBAL CHANGE BIOLOGY. Blackwell Publishing, Malden, MA, 29(17):4793-4810, (2023). https://doi.org/10.1111/gcb.16817

Impact/Purpose:

The purpose of the study is to estimate the changes in forest composition at the species level and the community level for the CONUS from 2010-2100 across various potential future scenarios of atmospheric deposition (nitrogen and sulfur) and changes in climate. This study builds off the "Horn curves" (Horn et al. 2018, https://doi.org/10.1371/journal.pone.0205296) that were developed under the StRAP3, and uses those curves to better understand how forest composition may change. The abstract of the current paper is below: Climate change and atmospheric deposition of nitrogen (N) and sulfur (S) are important drivers of forest demography. Here we apply previously-derived growth and survival responses for 94 tree species to 2.6 million trees across the CONUS to project how changes in temperature, precipitation, and N and S deposition from 20 different future scenarios may affect forest composition to 2100. We find that negative effects from future projected increases in mean annual temperature may ultimately overwhelm positive effects from reductions in N and S deposition for the majority of tree species. These broad trends underlie wide variation among species. We found the relative abundance of 60 and 15 species were projected to decrease more than 5% with elevated temperature and reduction of N and S deposition, respectively, while the biomass of 40 and 20 species were projected to increase more than 5%. The net responses of species to reduced atmospheric deposition stem mostly from beneficial effects from reduced S deposition (less acidification) offsetting negative or idiosyncratic effects from reduced N. Climate effects were mostly from impacts from elevated temperature. These results may underestimate future changes in forest composition as recruitment and other factors were not included, and may be highly uncertain especially for 0-20% of trees pushed outside the empirical record used in our simulations.

Description:

Climate change and atmospheric deposition of nitrogen (N) and sulfur (S) are important drivers of forest demography. Here we apply previously derived growth and survival responses for 94 tree species, representing >90% of the contiguous US forest basal area, to project how changes in mean annual temperature, precipitation, and N and S deposition from 20 different future scenarios may affect forest composition to 2100. We find that under the low climate change scenario (RCP 4.5), reductions in aboveground tree biomass from higher temperatures are roughly offset by increases in aboveground tree biomass from reductions in N and S deposition. However, under the higher climate change scenario (RCP 8.5) the decreases from climate change overwhelm increases from reductions in N and S deposition. These broad trends underlie wide variation among species. We found averaged across temperature scenarios the relative abundance of 60 species were projected to decrease more than 5% and 20 species were projected to increase more than 5%; and reductions of N and S deposition led to a decrease for 13 species and an increase for 40 species. This suggests large shifts in the composition of US forests in the future. Negative climate effects were mostly from elevated temperature and were not offset by scenarios with wetter conditions. We found that by 2100 an estimated 1 billion trees under the RCP 4.5 scenario and 20 billion trees under the RCP 8.5 scenario may be pushed outside the temperature record upon which these relationships were derived. These results may not fully capture future changes in forest composition as several other factors were not included. Overall efforts to reduce atmospheric deposition of N and S will likely be insufficient to overcome climate change impacts on forest demography across much of the United States unless we adhere to the low climate change scenario.

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