Positive Feedback between Shrub Encroachment and Nocturnal Air Temperature over the Northern Chihuahuan Desert

Many arid grasslands around the world are affected by the encroachment of woody plants. A number of drivers have been invoked to explain these changes in plant community composition, including climate change, increase in atmospheric CO2 concentrations, nitrogen deposition, or internal feedbacks involving soil erosion or fire dynamics. An overlooked aspect of this shift in vegetation cover is its possible feedback on microclimate conditions. In this study we investigate how in the northern Chihuahuan Desert these changes in vegetation composition and structure influence near surface climate conditions and what feedbacks these conditions have on vegetation dynamics. To this end, the impact of shrub encroachment on the thermal structure of the near surface boundary layer and on the surface energy budget was analyzed using concurrent micrometeorological observations at two adjacent sites dominated respectively by Larrea tridentata shrubs and native grass species at the Sevilleta Wildlife Refuge (northern Chihuahuan Desert, NM). The nighttime air temperature was found to be substantially higher (> 2 degrees Celsius) in the shrubland than in the grassland, especially during calm winter nights. Low temperatures are considered to be the limiting factor of the northward migration of Larrea tridentata. Thus, a positive feedback mechanism seems to exist, where shrub encroachment leads to warmer near-ground nighttime conditions, particularly during winter, which in turn favor woody species encroachment. Our analysis shows that these differences in surface air temperature are accompanied by differences in longwave radiation, and surface sensible and ground heat fluxes. These differences in surface fluxes are interpreted as an effect of the larger fraction of bare soil that typically exists in the shrubland sites. Therefore, the ground surface remains less insulated and more energy flows into the ground at the shrubland site than in the grassland during daytime. This energy is then released at night mainly as longwave radiation, which causes the differences in the nocturnal air temperatures between the two land covers.