Soil Water Repellency: a Method of Soil Moisture Sequestration in Pinyon-Juniper Woodland

Encroachment of pinyon–juniper woodland into rangeland ecosystems is prevalent across the western United States. Mechanisms associated with this successful encroachment are speculative, but probably, in part, involve the effective use of water resources. We explored the ecohydrological characteristics of a two-needle pinyon pine (Pinus edulis Engelm.)–Utah juniper [Juniperus osteosperma (Torr.) Little] woodland on the Colorado Plateau in Utah. We have discovered that a high level of natural soil water repellency or hydrophobicity exists under the canopies of both pinyon and juniper species. We found, following summer precipitation events, that soil water repellency under trees concentrated the soil water below the surface through finger flow or bypass infiltration, contrasting 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. In addition, we speculate that soil water repellency may be a major contributing factor to the "nutrient islands" observed to persist under juniper canopies even after the trees have been cut down and removed.