On the impact of shrub encroachment on microclimate conditions in the northern Chihuahuan desert

Abstract

[1] Changes in vegetation cover are known for their ability to modify the surface energy balance and near-surface microclimate conditions. A major change in vegetation composition that has been occurring in many dryland regions around the world is associated with the replacement of arid grasslands by desert shrublands. The impact of shrub encroachment on regional climate conditions remains poorly investigated, and, to date, it is unclear how this shift in plant community composition may affect the microclimate. Here we used concurrent meteorological observations at two adjacent sites dominated by Larrea tridentata shrubs and native grass species, respectively, in the northern Chihuahuan desert to investigate differences in nighttime air temperatures between the shrubland and grassland vegetation covers. The nighttime air temperature was found to be substantially higher (>2°C) in the shrubland than in the grassland, especially during calm winter nights. These differences in surface air temperature were accompanied by differences in longwave radiation and sensible and ground heat fluxes. We developed a one-dimensional model to show how longwave radiation emitted by the ground at night can explain the higher nighttime air temperature over the shrubland. Because of the larger fraction of bare soil typically existing in the shrub cover, 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 nighttime air temperatures between the two land covers.

Positive feedback between microclimate and shrub encroachment in the northern Chihuahuan desert

“…Similarly, landscape-scale warming at the shrubland site cannot be explained by its lower latitude with respect to the grassland, as the two sites are only a few kilometers apart. Rather, the difference in nighttime temperatures between grass-and shrubdominated sites is the result of a higher fraction of bare soil in the shrubland (He et al 2010). Bare soil exhibits larger ground heat fluxes and soil heating during the day.…”

“…During the day, the large areas of bare soil in shrubland absorb greater amounts of solar radiation, which is transformed into thermal energy and stored in the soil. Therefore, at night shrublands experience greater transfer of thermal energy and irradiance from the soil to the overlying atmosphere than the adjacent grasslands resulting in higher nighttime, near-surface air temperature over the shrubland (He et al 2010). Because the establishment and survival of Larrea tridentata (creosote bush), a dominant native shrub species in the southwestern U.S.A., is strongly influenced by cold wintertime conditions (Pockman and Sperry 1997;Medeiros and Pockman, in press), the nocturnal warming associated with the shift to shrub dominance has a positive feedback on shrub encroachment.…”

“…Major changes in land cover are also known for their ability to modify the regional climate, as reported in the case of tropical deforestation, ecosystem succession, and desertification (e.g., Bonan 2002). The climatic implications of woody plant encroachment in arid grasslands, however, remains poorly investigated (Beltran-Przekurat et al 2008) and it has seldom been recognized as one of the drivers of the shift from grass to shrub dominance (He et al 2010). Mechanisms commonly invoked to explain these regional shifts in plant community composition include overgrazing, fire suppression, atmospheric carbon dioxide enrichment, nitrogen deposition, and regional climate change (e.g., van Auken 2000).…”

Dryland ecohydrology and climate change: critical issues and technical advances

“…Shrub encroachment can affect the land surface albedo, emissivity, and roughness with important impacts on the near surface climate (Beltran-Przekurat et al, 2008). Even though in some cases changes in albedo are negligible, the increase in soil energy storage at encroached area can modify the microclimate with a positive feedback on vegetation (D'Odorico et al, 2010c;He et al, 2010). Recent regional climate modeling activities that seek to change the boundary conditions of the surface state may provide some insight into the influence and strength of land-atmosphere couplings as a response to changing surface conditions.…”

Hydrologic variability in dryland regions: impacts on ecosystem dynamics and food security

“…This bistability could be induced by positive feedbacks operating at the local scale, whereby two plant communities could-for example-coexist in the same landscapes because one of them (or both) modifies its physical environment to create its own habitat. Known as 'ecosystem engineering' [85], this phenomenon is ubiquitous in dryland ecosystems, where organisms have been reported to enhance their access to resources, reduce disturbance intensity or create an exceedingly unfavourable habitat for their competitors [55,59,86,87]. In the absence of switches, one might design some experiments to assess the existence of a positive feedback.…”