Encroachment of pinyon-juniper woodland into rangeland ecosystems is prevalent across the western US. Mechanisms associated with this successful encroachment are speculative, but likely in part, involve effective use of water resources.  We explored the ecohydrological characteristics of pinyon-juniper woodland on the Colorado Plateau in Utah, especially the role of soil water repellency found to occur under the trees. Using soil water content measurements along a transect, an infiltration experiment and 2-D soil moisture simulation we were able to demonstrate the impact of soil water repellency on soil moisture measured in this ecosystem.

Results/Conclusions

We have discovered that a high level of natural soil-water-repellency / hydrophobicity exists under the canopies of both pinyon and juniper species. We found, following summer precipitation events that soil-water-repellency under trees concentrated soil water below the surface through finger-flow / bypass-infiltration and contrasted with piston flow, and much more uniform soil wetting in intercanopy locations. We propose that the trees ‘engineer’ their environment, creating water repellency as a way of providing an ecohydrological advantage, and reducing potential water uptake by shallow-rooted herbaceous species.