Abstract
Identifying the drivers of community assembly has long been a central goal in ecology, and the development of functional diversity indices has provided a new way of detecting the influence of environmental gradients on biotic communities. For an old-growth Appalachian forest, we used path analysis to understand how patterns of tree functional diversity relate to topography and soil gradients and to determine whether topographic effects are mediated through soil chemistry. All of our path models supported the idea of environmental filtering: stressful areas (high elevation, low soil moisture, low soil nutrients) were occupied by communities of low functional diversity, which suggests a selective effect for species with traits adapted to such harsh conditions. The effects of topography (slope, aspect, elevation) on functional diversity were often indirect and moderated through soil moisture and fertility. Soil moisture was a key component of our models and was featured consistently in each one, having either strong direct effects on functional diversity or indirect effects via soil fertility. Our results provide a comprehensive view of the interplay among functional trait assemblages, topography, and edaphic conditions and contribute to the baseline understanding of the role of environmental filtering in temperate forest community assembly.
Highlights
Understanding patterns and processes of community assembly has long been a central goal in plant ecology with important implications for both ecosystem management and advancement of ecological theory
Variation in functional richness (FRic) was best explained through a combination of a direct effect of elevation and indirect effects of topography moderated through soil moisture and soil fertility (Ca, Mg, K, pH, base cation saturation; Figure 1)
Functional richness had a significant inverse relationship with elevation (−0.338, p = 0.001) where FRic tended to be higher at low elevations
Summary
Understanding patterns and processes of community assembly has long been a central goal in plant ecology with important implications for both ecosystem management and advancement of ecological theory. The role of topographic variation in shaping plant communities and species distributions was well-recognized by early ecologists (e.g., [1,2,3]) and has since been linked to other drivers such as soil fertility and texture, soil moisture availability, soil temperature, and annual solar radiation [4,5,6] These abiotic gradients are thought to drive community assembly by a number of mechanisms, of which the two most commonly invoked are competitive interactions that limit the morphological and physiological similarity of species (limiting similarity principle) and environmental filtering [7,8]. The segregation of community types across abiotic environmental gradients has historically been a major theme in forest ecology research [4,11] and, in recent decades, has been met with renewed interest due to theoretical and analytical advances in studying patterns of biodiversity
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