Linking understory evergreen herbaceous distributions and niche differentiation using habitat-specific demography and experimental common gardens
Warren, Robert J.
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The assumption that a 1:1 correspondence exists between a species’ distribution and “suitable habitat,” the ecological niche, is undermined by current theory - metapopulation, source/sink, dispersal limitation and remnant populations - and empirical evidence. Here I summarize dynamics where organisms are found in unsuitable habitat and are absent from suitable habitat. I then use evergreen understory herbaceous surveys across north- and south-facing aspects to investigate potential habitat suitability. The results, based on Bayesian generalized linear hierarchical models, indicate evergreen understory herbs occur less and perform worse on south-facing than north-facing slopes, and they likely are limited on south-facing slopes by low soil moisture combined with high temperatures in summer and high light combined with low temperatures in winter. I use habitat-specific demography for further insight into the suitable habitat of two understory evergreen herbs, Hexastylis arifolia and Hepatica nobilis, and the relationship between morphological trait differences and niche using Bayesian hierarchical models and simulations. The results suggest that H. arifolia and H. nobilis occur in declining populations in habitat made unsuitable habitat by declining levels of soil moisture. I also find that leaf traits have potential in predicting niche characteristics only when paired with demographic analysis. Since both survey and demography data of natural understory evergreen populations suggest that seasonal light, soil moisture and temperature explain a great deal of plant distribution and performance, I use experimental gardens with light and soil moisture manipulation to test these results. Experimental common gardens were established on north- and south-facing slopes in North Carolina and Georgia, and three understory evergreen species were transplanted into plots that included nested water augmentation and light suppression treatments. The plants responded to the environmental variables in a manner consistent with a guild that limited on south-facing slopes. Increased temperatures and light exposure and lower soil moisture generally led to decreased survival and growth and increased photoinhibition, and the responses varied among the three species. I conclude that the understory evergreen herbs often occur in unsuitable habitat, both at landscape and microsite scales, and performance and physiological responses to environmental variables provide better estimations of the species’ niche than presence.