Hydraulic conductivity in a piñon-juniper woodland: Influence of vegetation

In semiarid environments, vegetation affects surface runoff either by altering surface characteristics (e.g., surface roughness, litter absorption) or subsurface characteristics (e.g., hydraulic conductivity). Previous observations of runoff within a piñon-juniper [Pinus edulis Englem. and Juniperus monosperma (Englem.) Sarg.] woodland led us to hypothesize that hydraulic conductivity differs between vegetation types. Using ponded and tension infiltrometers, we measured saturated (K_s) and unsaturated [K(h)] hydraulic conductivity at three levels of a nested hierarchy: the patch (canopy and intercanopy), the unit (juniper canopy, piñon canopy, vegetated intercanopy, and bare intercanopy), and the intercanopy locus (grass, biological soil crust, bare spot). Differences were smaller than expected and generally not significant. Canopy and intercanopy K_s values were comparable with the exception of a small number of exceedingly high readings under the juniper canopy - a difference we attribute to higher surface macroporosity beneath juniper canopies. The unsaturated hydraulic conductivity, K(h), values were higher for canopy soils than for intercanopy soils, although differences were small. At the unit level, the only significant differences were for K(h) between juniper or piñon canopies vs. bare interspaces. Median K values for vegetated intercanopy areas were intermediate between but not significantly different from those for canopies and bare areas. There were no significant differences between grass, biological soil crust, and bare spots within the herbaceous intercanopy area. Overall, the observed differences in K between canopy and intercanopy patches do not account for differences in runoff observed previously.