Abiotic conditions, biotic interactions, and species traits affect plant distribution across topographic microclimates in the Klamath-Siskiyou Mountains

Rugged topography affects species distributions and community composition by creating fine-scale differences in microclimate. Topographic complexity has the capacity to affect plant distribution with climate change by creating refugia - locations that are mesic compared to the surrounding landscape --and within short dispersal distance for many plant species. However, biotic interactions with neighboring plant species or symbiotic mycorrhizae could compete or facilitate plant occupancy of different abiotic environments. Furthermore, abiotic features such as soil type or elevation band also alter climatic conditions and potentially the strength and effects of biotic interactions. Finally, species traits related to stress tolerance could influence their response to topographic microclimates.;We addressed the capacity for biotic interactions, abiotic conditions, and species characteristics to affect species topographic microclimate distribution in three studies: a field experiment, a widespread set of observational plots, and an analysis of a database of species occurrences and traits. The three studies were located in the Klamath-Siskiyou Mountains, a topographically and geologically complex region containing a high diversity of plant species of broad geographic origin (Whittaker 1960).;The results for the database study suggest that among multiple species traits, the characteristics most related to species distribution with respect to topography are biogeographic origin and the species global range with respect to macroclimate. The experimental study indicated that elevation band and topographic contrasts interact with the effects of neighbor removal, such that fitness decreased with greater neighbor cover in warm microclimates and was lower at warmer microclimates at high elevations. Finally, the observational study suggested that biotic factors have strong effects on plant performance and distribution relative to the effects of abiotic conditions - such as soil fertility and climate. Furthermore, biotic factors frequently mediated the effects of abiotic conditions on plants with abiotic variables having few direct effects.