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.
Background Woody plant encroachment into grasslands is a global phenomenon that results from a variety of global change drivers. Over the last 150 years the southwestern United States has undergone dramatic changes in the composition and structure of vegetation due to shrub invasion in southwestern deserts. Concern about the encroachment of woody vegetation is motivated by economical losses associated with the conversion of rangelands into woodlands, regional carbon dynamics and the occurrence of erosion-driven loss of nutrient-rich soil particles in response to the decrease in grass cover, with consequent loss of ecosystem services and functioning. Intellectual Merit The relatively abrupt character of grassland-to-shrubland transitions suggests that these transitions may be sustained by positive interactions between vegetation and the physical environment. This project has provided experimental evidence that creosote bush, the major native shrub species of North American hot deserts, is able to modify its microclimate by increasing the minimum nighttime temperatures. Because the encroachment of desert shrubs is known to be limited by low winter temperatures, a self-sustained sequence of interactions between shrub vegetation and microclimate conditions may enhance the process of shrub encroachment. Despite their important effect on land surface cover, soil productivity, and near-surface climate conditions over large areas of the US, these interactions have been always poorly investigated. This research project has used a combination of empirical and theoretical approaches to assess the significance of these vegetation-microclimate interactions and their role in the process of shrub encroachment in the southwestern U.S. It has also provided a theoretical framework explaining the physical mechanisms underlying this warming effect. This understanding of the interactions between changes in land cover and surface air temperatures has been used in the parameterization and testing of coupled land surface-climate models that provide a better prediction of the spatial extent of areas expected to be prone to shrub encroachment under the current climate conditions. Broader Impacts The project has contributed to the development of new knowledge on vegetation-atmospheric interactions in landscapes affected by shrub encroachment and has provided a quantitative evaluation of the impact of this change in vegetation cover on the regime of minimum temperatures. The results of this study provide useful information on processes controlling shrub encroachment in the desert southwest. As part of the academic training, one Ph.D. student has developed her dissertation research. A new course on Dryland Ecohydrology was developed and offered at the University of Virginia (Fall 2009) presenting ideas and some of the data from this project. This research was also presented in seminars at the Charlottesville High School. Moreover, information from this project was included in a GK12 program that connects research at the Sevilleta LTER to middle school science classes in nearby Belen and Socorro, New Mexico.
