Globally, trees are increasingly dying from extreme droughts and heatwaves, a trend that is expected to increase with climate change. Droughts impact tree species unequally and therefore have the potential to alter the species composition of forests. As droughts intensify under climate change, models that can account for both changing environmental conditions as well as joint species-specific responses are needed to forecast shifts in species ranges, composition, abundance, and mortality. We used a Generalized Joint Attribute Model, GJAM, to simultaneously model species-specific live and dead basal area for 76 species across Texas. We used tree abundance data from Forest Inventory and Analysis plots from 2001-2015 and covariates related to climate, topography and soils. We then projected shifts in species abundance into the 21st century under multiple climate warming scenarios. Unlike traditional species distribution models, as a multivariate model, GJAM, can explain covariation among species that remains after accounting for the environment. Also, to account for species-specific tolerances to extreme events, we used covariates related to historical average climate as well as climate variability.
Results/Conclusions
Our model estimates of dead and live tree abundance had high spatial congruence with an independent dataset of remotely sensed canopy loss observations from a severe drought in 2011. Of the 30 most dominant species in Texas, loss in basal area was greatest among the eastern hardwoods, including Quercus stellata, Q. nigra, Pinus echinata, Q. falcata, and Q. phellos, which saw 10-17% loss over the past 15 years. We found that the historical variability in annual climatic water deficit (e.g., standard precipitation minus evapotranspiration) was an important predictor of species-specific dead basal area. This emphasizes the importance of including climate variability when forecasting future shifts in species compositions. Using climate projections for representative concentration scenarios RCP 4.5 and RCP 8.5, we found that many of the eastern hardwood species are projected to shift eastward throughout the 21st century. However, many of the western woodland species (e.g., Prosopis glandulosa and Juniperus pinchotii) are projected to become increasingly dominant throughout Texas. By modeling species abundance and mortality simultaneously and by using covariates related to climate variability, we can improve upon projections of forest responses to continuing climate change.