Print PageThe encroachment of woody plants into grasslands is a vegetation change that has been observed worldwide and explained by invoking a variety of drivers related to land use and global environmental change. In the southwestern U.S., shrub encroachment can be associated with fire management, climate warming, increased levels of atmospheric CO2, soil erosion, and the formation of heterogeneous distributions of soil resources. The relatively abrupt character of grassland-to-shrubland transitions observed in this region both in time and in space indicates that arid rangelands may exhibit two alternative stable states dominated by either grass or shrub cover. Bistable ecosystem dynamics are often induced by positive feedbacks between the biota and the physical environment. Here we show that in the southwestern USA positive feedbacks between shrub encroachment and microclimate conditions may enhance shrub dominance. We investigate changes in air temperature and surface energy balance resulting from grass-to-shrub transitions in the northern Chiuhuahuan desert.
Results/Conclusions
We find that shrub encroachment into desert grasslands modifies the surface energy balance with the overall net effect of increasing near-surface air temperatures at night, thereby favoring the establishment and growth of freeze-sensitive Larrea plants. By using mesoscale models of land-atmosphere interactions and data analyses we explain this warming effect as the result of an increase in the bare soil fraction, which results in an increase in diurnal ground heat fluxes and soil temperature in areas affected by shrub encroachment. At night, this difference in surface temperature results in an increase in longwave radiation and near surface air temperature. A comparison with regional patterns of climate change shows that the warming induced by the shift from grass to shrub dominance is comparable to the effect of regional climate warming over the past 100 years.
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