Collaborative Research: Do Vegetation-microclimate Feedbacks Promote Shrub Encroachment in the Southwestern United States?

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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.