Water Availability Influences Species Distributions on Serpentine Soils

Abstract

Serpentine soils are highly variable environments, but how this variability contributes to maintaining distinct distributions of species at small scales is unclear. We studied parapatric populations of Layia jonesii A. Gray, a rare serpentine endemic, and L. platyglossa (Fisch. & C. A. Mey) A. Gray, a wide-spread species, to understand how edaphic variability and competitive interactions maintain co-occurring, somewhat interfertile, relatives within serpentine environments. We analyzed soil characteristics along a serpentine hillside where L. jonesii and L. platyglossa are restricted to the top and bottom of the hillside, respectively. We used reciprocal transplants to investigate fitness differences on the hillside and a competition experiment to determine if species interactions also restrict the distribution of L. jonesii. The soil analyses showed the top of the hill is drier than the bottom, yet both locations are chemically indistinguishable. Layia platyglossa is better adapted to the edaphic conditions at the bottom of the hillside and L. jonesii is sensitive to competition in the bottom region. Differences in soil water availability, and potentially its effect on the competitive environment, likely contributes to maintaining the distribution of these species. Ionic stress is often cited as a driver of species' distributions within serpentine environments, but our research suggests soil water availability can be an important factor influencing the distribution of interfertile species on serpentine at small scales. Annual variation in precipitation may drive local shifts in species distributions through alteration of the competitive environment.