Species composition determines resistance to drought in dry forests of the Great Lakes - St. Lawrence forest region of central Ontario

Question: Predicted changes in the frequency of short raise concerns about potential effects of climate change Few experiments have investigated die effects of tree spec;rat drought events forest ecosystems. s.richness and composition in :forest assemblages exposed to drought, and general conclusions for these systems arc currently lacking. We tested the hypothesis that different species richness and composition affects community-level stability during a severe short-term drought event across a gradient of tree species assemblages typical of temperate forest ecosystems on dry substrates. Location: Great Lakes St. Lawrence forest region, central Ontario, Canad.a. Methods: The study assemblages spanned a gradient of 15 different combinations of species richness and composition of lour tree species. The drought occurred in 2005 and was characterized by high growing season temperature and low precipitation. Plots (n = 63) representing replicate assernblages were selected to collect increment cores (n = 1193) and examine growth responses to past drought at Elie community and population levels. Cross-dated tree ring measurements were used to reconstruct basal area increment (BM) and calculate indices of resistance, resilience and productivity over the drought period. We used hierarchical analysis of variance models to estimate the effects of species richness and composition on drought responses. Results: We identified a significant effect of species composition on con:tin-unity-level resistance and productivity. White pine (Pious strobus) abundance was associated with lower community resistance and white birch (Betula papyrifera) abundance with higher resistance. Assemblages with greater productivity were often characterized by the high abundance of trembling aspen (Populus tionutoides). There was no overall effect of the assemblage gradient on the population-level stability; however, red pine (Pious resinosa) productivity was higher in combination with trembling aspen and white birch. Conclusion: Our study demonstrated that species richness had no effect on the community-level stability of growth during a drought in temperate forests on dry substrates. Instead, there were important compositional effects that deter 'nine some aspects of stability during drought events. If pine forests were 'nap aged to maintain a component of deciduous species, the capacity community-level effects of more :frequent droughtwould increase.