Does canopy microclimate structure lichen epiphyte communities? Evidence from intra- versus interspecific variation in functional traits

Background/Question/Methods

Forest canopies are heterogeneous environments where changes in microclimate create the opportunity for niche-based filtering of epiphyte communities over relatively small spatial scales. Research primarily in evergreen conifer-dominated forests has demonstrated marked vertical gradients in lichen epiphyte abundance and species turnover, which are presumed to arise from differential species’ responses to decreasing light, temperature, and wind exposure and increasing humidity from the upper to lower canopy. Few studies have explicitly correlated lichen community change with measured microenvironmental variation and none have related species turnover to interspecific differences in physiologically relevant traits. Our goals were to determine 1) whether microclimatic gradients in a warm temperate deciduous forest canopy influence the taxonomic and functional composition of lichen epiphyte communities and 2) the extent to which community-level trait variation derives from intraspecific plasticity versus environmental filters causing turnover of  species with fixed traits. We measured light, temperature and eight functional traits related to resource acquisition, photoprotection, water-balance, and reproductive strategy on macrolichens occurring in 72 samples in twelve Northern Red Oaks in the Piedmont of North Carolina. Hierarchical models were used to assess relationships between individual traits and environmental covariates as well as partition trait variation into intra- and interspecific components.

Results/Conclusions

Mean light intensity and the frequency of full-sun exposure decreased with distance from the top of the canopy and were also strongly related to branch surface aspect. Variation in temperature primarily reflected variation in light intensity. Although some species exhibited different peaks in abundance along the vertical canopy gradient, differences in species and functional composition were not strongly related to measured environmental variation. A preliminary analysis of data from three trees showed that most traits exhibited large intra-specific variation which was unrelated to changes in light and temperature or to changes in species composition. A positive correlation between chlorophyll content and light intensity was the only trait-environment relationship with significant support. If these results hold for all 12 trees, it would imply that microclimatic variation does not impose an environmental filter on lichen epiphyte communities in oaks that is strong enough to affect community composition. This suggests that either warm temperate deciduous oak canopies do not create physiologically relevant environmental gradients or that dispersal across these small scales alleviates any signature of environmental limitation on community structure. Thus, environmental effects on epiphyte communities are likely scale-dependent and moderated by the dispersal potential of their constituent species.