Feedback between Shrub Encroachment and Microclimate in the Southwestern United States

Shrub encroachment is a world-wide ecological phenomenon which is associated with abrupt transition from grassland to shrubland. Such a change of vegetation cover has the potential of impacting the local and regional climate and may contribute to further shift in vegetation cover in many ecosystems. It has been recently proposed that such a vegetation-microclimate feedback may exist in the southwestern U.S. deserts, in which a cold sensitive shrub species, Larrea tridentata, could be favored by the feedback. This dissertation uses both observational and numerical modeling approaches to investigate the interaction between shrub encroachment and microclimate in the northern Chihuahuan desert, and to assess the role of the vegetation-microclimate feedback in favoring further shrub encroachment. In particular, field observations reveal that the shrubland has a significantly higher nighttime temperature (of about 2K on average) than the adjacent grassland, demonstrating the effect of different vegetation covers on local microclimate. Such a temperature difference only extends to a low height level (<20m above around), which establishes itself shortly after sunset and then persists throughout the night. The observed warmer nocturnal condition over the shrubland is interpreted as the consequence of the increased bare soil fraction caused by shrub encroachment: the less insulated ground surface enhances the diurnal soil heating and the nocturnal release of longwave radiation, which results in a higher nighttime temperature. Such a shrub-induced warming is found to be overall important, because it is comparable to a regional climate warming over a timescale of one century. To better understand the underlying mechanisms, both an idealized single column configuration and an idealized two-dimensional configuration of the Weather Research and Forecast (WRF) model coupled with the Noah land surface model are used to simulate the land-surface interactions which result in the observed temperature difference. It is found that the green vegetation fraction is the key parameter that drives the temperature difference, based on the sensitivity tests of various parameterizations. Then, with the knowledge learnt from the idealized simulations, three-dimensional realistic simulations using the WRF model with multiple grassland and/or shrubland vegetation cover scenarios are carried out to investigate the role of the vegetation-microclimate feedback in promoting the shrub encroachment. The simulation results show that the effect of feedback could result in a microclimate condition that is generally more favorable for the survival of juvenile shrubs. This can induce bi-stability to the shrubland-grassland system in extreme cold winters, and contribute to further the shrub encroachment.