Experimental Warming and Increased Precipitation Alter Early Successional Forest Community Assembly

Background/Question/Methods

Community assembly following a disturbance has strong influence on future forest composition and may be altered by climate change. Climate can directly influence tree regeneration, but climate sensitivity of non-tree vegetation can also affect tree species establishment. From May 2008 through August 2010, we performed a field climate manipulation experiment to test the impacts of 2°C warming and 20% increased precipitation on community composition of a recently harvested forest in central Pennsylvania.  Forest assembly was analyzed in two ways: 1) species composition and 2) cover of functional groups through time.  For functional group analyses, all species were classified into one of the following five groups: short forbs, tall forbs, grass, trees, and shrubs/small trees/vines.  Additionally, we used random forest modeling to determine which biotic and abiotic factors were associated with functional group dominance over the course of the experiment.

Results/Conclusions

Of the five functional groups, only trees displayed a shift in species composition in response to warming and increased precipitation (p=0.03).  Other functional groups contained individual species that were significantly associated with specific treatments, but overall composition was similar among treatments.  However, changes in composition were not associated with changes in plant cover or biomass during the experiment.  Only grass and total cover increased with warming and precipitation treatments while short forb cover decreased with added precipitation (p < 0.05).  Random forests models indicated that influence from competing functional groups had the greatest influence on functional group cover during our experiment.  Surprisingly, direct effects of temperature and precipitation were consistently the least important environmental factors driving functional group cover, indicating that climate influences on forest community assembly are often indirect.  Our results suggest that forest community assembly during temperate forest regeneration may be resilient to climate change due to a diverse array of biotic and environmental factors that constrain community development.