While the idea that water stress contributes to geographic range limits in arid-land plants is well founded, it remains to be determined how environmental sources of water stress help determine—along with other environmental, demographic, and evolutionary genetic factors—the precise locations of species borders.  In addition to gradients in precipitation, gradients in soil properties that limit water availability (e.g., compaction, texture, and moisture-release properties) might create gradients in water stress and, in this way, contribute to the location of species borders.  We evaluated plant water stress (midday and/or pre-dawn water potential) and its potential sources in over 20 populations across a 30 km west-to-east geographic transect (from west of the range center to the eastern species border) in the California-endemic annual Clarkia xantiana ssp. xantiana.
Results/Conclusions

Shoot water potential declined significantly from west to east, with an approximately 0.5 mPa drop across the range in each of two consecutive years, while mature plant height declined by a factor of two.  Across the transect, growing-season precipitation (estimated from automated weather stations) declined significantly by a factor of five in 2006-2007.  From west to east, soils within C. xantiana ssp. xantiana populations became finer-textured, partly because eastern range-edge populations often occur on soils derived from metamorphic and sedimentary rock outcrops, rather than on granite-derived soils more common to the west (and also to the east beyond the species border).  Soil surface compaction within populations increased three-fold along the west-to-east transect.  Lab-estimated soil moisture release curves revealed a significant gradient in the matric potential of soils toward the border (likely because of variation in particle size); toward the eastern border, soils have increasingly low water potentials for any given water content.  Similar variation in water stress, soil compaction, and texture at the scale of small, local populations suggest soil properties can induce water stress independent of precipitation.  These findings suggest that the gradient in water stress in C. xantiana ssp. xantiana across the species’ range is a function not only of declining water input from precipitation but also of soil properties that make soil water less available to plants.