Shading and litter mediate the effects of soil fertility on the performance of an understorey herb.
Soil fertility and topographic microclimate are common determinants of plant species distributions. However, biotic conditions also vary along these abiotic gradients, and may mediate their effects on plants. In this study, we investigated whether soils and topographic microclimate acted directly on the performance of a focal understorey plant, or indirectly via changing biotic conditions.



We examined direct and indirect relationships between abiotic variables (soil fertility and topographic microclimate) and biotic factors (overstorey and understorey cover, litter depth and mycorrhizal colonization) and the occurrence, density and flowering of a common understorey herb, Trientalis latifolia, in the Klamath-Siskiyou Mountains, Oregon, USA.



We found that the positive effects of soil fertility on Trientalis occurrence were mediated by greater overstorey shading and deeper litter. However, we did not find any effects of topographic microclimate on Trientalis distribution that were mediated by the biotic variables we measured. The predictive success of Trientalis species distribution models with soils and topographic microclimate increased by 12 % with the addition of the biotic variables.



Our results reinforce the idea that species distributions are the outcome of interrelated abiotic gradients and biotic interactions, and suggest that biotic conditions, such as overstorey density, should be included in species distribution models if data are available.
Environmental Science and Policy, University of California, Davis, CA, USA scopeland@ucdavis.edu.
Stella M
SM
Copeland
Environmental Science and Policy, University of California, Davis, CA, USA.
Susan P
SP
Harrison
D004463:Ecology; D038821:Mycorrhizae / Q000378:metabolism; D029623:Primulaceae / Q000254:growth & development / Q000502:physiology*; D012098:Reproduction / Q000502:physiology; D012987:Soil; D012988:Soil Microbiology; D013472:Sunlight
D012987:Soil
Abiotic factors; Trientalis latifolia; biotic conditions; distribution; mycorrhizae; overstorey shading; soil fertility; topographic microclimate