Over the last 150 years the southwestern United States has undergone dramatic changes in the composition of vegetation due to shrub invasion. Shrub invasion is a process by which grasses are replaced by woody plants. Shrub invasion can cause economic losses by transforming rangelands into unproductive woodlands and by removing nutrient-rich soil particles. Shrub invasion can also alter the capacity of the ecosystem to assimilate atmospheric carbon dioxide, and therefore it can affect the regional carbon budget. The relatively abrupt character of grassland-to-shrubland transitions suggests that these transitions may be sustained by a delicate balance between vegetation cover and the overlying atmosphere. In particular, we hypothesize that the atmosphere near the ground becomes warmer at night in response to a change in vegetation cover from grassland to shrubland. A warmer atmosphere near the ground, in turn, may enhance the process of shrub invasion since the growth of shrubs is promoted by higher winter temperatures.
This research combines empirical and theoretical approaches to determine the significance of these vegetation-climate interactions and their role in the process of shrub invasion in the southwestern U.S. Field studies will reveal how the invasion of shrubs can alter their surrounding environments. Ecological studies will provide critical knowledge to discover how the vegetation responds to changes in regional microclimatic conditions. Results of the field studies will be integrated in numerical models of the vegetation and the atmosphere. These models will enable the prediction of shrub invasion under current and future climatic conditions. The results and models will lead to improved management of southwestern ecosystems by providing decision-makers with a better understanding of shrub invasion.
The objectives of this research were to advance our understanding of the how the structure and function of grasslands in the southwestern United States respond to a rapid increase in woody plant density, and how these changes, in turn, alter local and regional climate. We focused in particular on how creosote (Larrea tridentata) shrubs modified energy flows between the land surface and the atmosphere by making concurrent micrometeorological observations from 2009-2012 at two adjacent sites dominated respectively by creosote and native grass species in the Northern Chihuahuan desert. Nighttime air temperature was found to be substantially higher (2-5 °C) in the shrubland compared to the grassland, particularly during calm winter nights. These differences in surface air temperature are associated with differences in thermal energy emission and sensible and ground heat fluxes associated with structural differences between the two land covers. Shrub establishment is associated with more bare soil patches as the native grasses typically die-off. Because of the larger fraction of bare soil typically observed in the shrub cover, the ground surface remains less insulated and more energy flows into the ground and is stored during the day at the shrubland site than in the grassland. Using the principle of energy conservation, we demonstrated that the higher thermal energy emitted by the less insulated ground in the shrubland at night explains the observed differences in nighttime air temperature over the shrubland. This process is the main mechanism behind higher nighttime air temperatures observed above creosote landscapes compared to adjacent grasslands. An analysis of long-term temperature records from a grass-dominated and shrub-dominated ecosystems in NM confirmed our measurements by showing that the minimum temperature was nearly always lower at the grassland sites. Finally, we used mesoscale model simulations to confirm that the green vegetation fraction is the key factor that causes higher nighttime temperature in shrubland than in adjacent grassland, mainly due to its effects on soil surface insulation, soil thermal diffusivity and thus ground heat fluxes. Our results are significant in that they demonstrate that the effect of vegetation change in the southwestern US aridlands (i.e. shrub encroachment) on near surface temperatures is as important as climate change, in that it is comparable to the regional climate warming that is expected to take place in this region in the next century. In addition, this research fills an important gap existing between ecosystem ecology and micrometeorology by increasing our understanding of how and why the distribution of creosote has changed and is likely to change in arid grasslands in the southwestern US over the next century. We have directly observed how this prevalent change in land cover associated with shrub encroachment affects surface air temperature, and by combining our understanding of the physiology of creosote with the differences in energy flows and surface temperature for grass and woody landscapes, we see that shrubs, in effect, create microclimate conditions that are more favorable for their own survival by reducing seedling mortality to low temperatures. Our research overall will provide a process based understanding of how shrub encroachment affects microclimate and will lead to a better representation of these processes in mesoscale climate models.
