Projected changes in forest biomass to 2100 by county and species for 20 future scenarios

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 U.S. 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 that averaged across temperature
scenarios, the relative abundance of 60 species was projected to decrease
by more than 5%, 20 species were projected to increase by 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 U.S. 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.