Feedbacks Between Fires and Soil Erosion Processes at the Desert Margins

Shrub encroachment at the desert margins, a worldwide phenomenon, results in a heterogeneous landscape characterized by a mosaic of nutrient-depleted barren soil bordered by nutrient-enriched shrubby areas known as "fertile islands". Even though shrub encroachment is considered as a major contributor to the desertification of several regions around the world, little is known about mechanisms favoring the reversibility of the early stages of this process. In this dissertation, it is shown that fires interact with soil erosion processes to encourage a more homogeneous distribution of soil resources. To this end, I used a combination of replicated field scale techniques for soil erosion monitoring, microtopography measurements, infiltration experiments and isotope tracer studies conducted at a shrub-grass transition zone in the northern Chihuahuan desert (New Mexico, USA). The results indicate that fires tend to counteract the heterogeneityforming dynamics of land degradation associated with shrub encroachment, thereby enhancing the reversibility of the early stages of this process. The enhancement of post fire redistribution of soil resources is attributed to the enhancement of post fire soil erodibility, in particular, erosion by wind, which dominates in these landscapes. Further, it is hypothesized that the mechanisms causing the enhancement of post-fire soil erodibility are induced by post-fire soil hydrophobicity. These mechanisms (as well as their ecological implications) are a unique finding of this dissertation, in that, to my knowledge, they had never been reported before in the case of wind erosion. The effect of fire induced water repellency on soil susceptibility to wind erosion is demonstrated by wind tunnel experiments using laboratory treated clean sands and natural soils sampled after wild fires and prescribed burns. A theoretical model is developed to explain the III observed effects. The possible impacts of fire-wind erosion interactions on vegetation composition and structure are investigated using a spatially explicit model of vegetation dynamics. The modeling study shows the possible long-term effect of fires on the stability and resilience of fertility island systems. In particular, it is found that the fertility islands are dynamic rather than static features of these landscapes.

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