Interactions among fires, vegetation and aeolian processes in changing landscapes: from shrub encroachment to exotic grass invasions

Abstract

<p>Dryland degradation associated with rapid shifts in vegetation composition is of critical concern due to impacts on climate, water cycle, environmental quality, and food security. These abrupt transitions are often sustained by positive feedbacks between the state of the system and environmental conditions or disturbances, which can act from patch to landscape scales. In the case of shrub encroachment in the Chihuahuan desert, fire suppression and overgrazing resulted in the formation of a heterogeneous shrub-dominated landscape with concentration of resources beneath the shrub canopies (islands of fertility). Aeolian processes, which are a dominant mechanism for sediment transport in these arid landscapes, enhance and maintain the local spatial heterogeneities in nutrient and vegetation distribution through removal of nutrient-rich soil from interspaces and the subsequent deposition onto shrub-vegetated areas. Here, from 7 years of monitoring at a shrub-grass ecotone in the Chihuahuan desert, we demonstrate the internal feedback mechanisms that reinforce the shrub-dominated state at the patch scale and show how these feedbacks are altered by prescribed fires. Tracer-based studies (using rare earth elements and magnetic tracers) to quantify soil erosion, as well as microtopographic change detection from lidar remote sensing, demonstrated that the shrub-vegetated microsites, which are sinks of aeolian sediments, became active sediment sources post-fire, resulting in the spatial homogenization of microtopographic surface roughness, sediment, and nutrient distribution in the landscape (nutrient feedback). Sensor-based field observations showed that post-fire shrub microsites experienced lower soil moisture (moisture feedback) and temperature (microclimate feedback), which can negatively impact shrub reestablishment and enhance grass regrowth. Seven years following prescribed fire, the impact of fire on these feedbacks diminished, as the shrubs recovered. Our results indicate that the role of fire is biophysical, and the fertility islands are dynamic features in these landscapes. We hypothesize that similar feedbacks which create post-fire resource spatial homogeneity in the Chihuahuan desert may be involved in the transition of native shrubland to invasive grasslands in the Sonoran desert, though with differential impacts on ecosystem degradation. Thus, the process of land degradation can be facilitated by spatial heterogeneity (e.g., shrub encroachment) and homogeneity (e.g., exotic annual grass invasion) of soil resources, depending on the plant functional type inducing the change in resource distribution in a desert ecosystem.<br></p>