Low water availability decreases niche overlap of soil water resources in woody perennials

Background/Question/Methods

In semi-arid environments, co-existing plant species may vary in rooting depth, reflecting niche partitioning and functional differences in water source.  In mountains of the southwestern U.S., moisture availability increases with elevation, and winter and summer precipitation inputs differ isotopically.  Examining variation in stem water isotopes among plant communities and seasons over elevational gradients is important for understanding how niche partitioning may respond to the predicted warming and drying of the Southwest.  The goal of this study was to assess the soil water use of woody plant communities along an elevational moisture gradient using stem and soil water isotopes to estimate rooting depth.  We hypothesized that niche partitioning in functional rooting depth would be greater at more arid sites because of greater interspecific competition for water.  We collected soil samples from three depths and stem samples of dominant woody perennials during the late spring dry season and the summer monsoon season from five biotic communities located over a gradient of elevation and aridity.  Water was extracted using a cryo-vacuum line, and δ¹⁸O was determined by off-axis cavity ringdown spectroscopy.  Niche partitioning was determined by calculating niche overlap, using functional rooting depth estimated from logarithmic regressions of soil depth and soil δ¹⁸O.

Results/Conclusions

Soil moisture content increased with elevation and after the onset of monsoon rains across all sites.  Soil δ¹⁸O generally decreased with increasing depth. Plant δ¹⁸O were significantly different among species at all sites except the high-elevation mixed-conifer forest.  June and August plant δ¹⁸O differed significantly in both the pinyon-juniper and ponderosa forests.   Niche overlap increased as the moisture index (the ratio of precipitation to potential evapotranspiration) increased; the moisture index explained 18.2% of the variation in niche overlap among sites.  This was primarily driven by low niche overlap among species at the desert site.  In August, desert tree species had greater functional rooting depth than desert shrubs, indicating that trees and shrubs use different water sources.  Surprisingly, functional rooting depth did not differ consistently between trees and shrubs in the pinyon-juniper community in either season.  Our findings support the hypothesis that niche overlap in functional rooting depth is lower at more arid sites, and we present a simple metric for quantifying functional rooting depth and niche overlap based on soil and stem water isotopes.