Feedbacks between fires and wind erosion in heterogeneous arid lands

Shrub encroachment, a widespread phenomenon in arid landscapes, creates islands of fertility'' in degraded systems as wind erosion removes nutrient-rich soil from intercanopy areas and deposits it in nearby shrub-vegetated patches. These islands of fertility generally are considered to be irreversible. Recently, fire has been observed to alter this pattern of resource heterogeneity through the redistribution of nutrients from the fertile islands of burnt shrubs to the surrounding bare soil areas. Despite the recognized relevance of both fires and wind erosion to the structure and function of arid ecosystems, the interactions between these two processes remains poorly understood. This study tests the hypothesis that fire-induced soil hydrophobicity developing in the soils beneath burned shrubs enhances soil erodibility by weakening the interparticle wet-bonding forces. To test this hypothesis, the effects of grass and shrub fires on changes in soil erodibility and on the intensity of fire-induced soil water repellency are compared at both the field and patch scales in heterogeneous arid landscapes. Higher water repellency was observed in conjunction with a stronger decrease in wind erosion threshold velocity around the shrubs than in grass-dominated patches affected by fire, while neither water repellency nor changes in threshold velocity was noticed in the bare soil interspaces. Thus, fires are found to induce soil hydrophobicity and to consequently enhance soil erodibility in shrub-vegetated islands of fertility. These processes create temporally dynamic islands of fertility and contribute to a decrease in resource heterogeneity in aridland ecosystems following fire.