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

在过去的150年里，由于灌木的入侵，美国西南部的植被组成发生了巨大的变化。灌木入侵是禾本科植物被木本植物取代的过程。灌木的入侵会把牧场变成贫瘠的林地，并带走富含营养的土壤颗粒，从而造成经济损失。灌木入侵还可以改变生态系统吸收大气二氧化碳的能力，从而影响区域碳收支。从草地到灌木林转变的相对突兀特征表明，这些转变可能是由植被覆盖和上覆大气之间的微妙平衡维持的。特别是，我们假设地面附近的大气在夜间变暖，以响应植被覆盖从草地到灌木地的变化。接近地面的温暖的大气反过来又可能加强灌木入侵的过程，因为较高的冬季温度促进了灌木的生长。本研究结合实证和理论方法来确定这些植被-气候相互作用的意义及其在美国西南部灌木入侵过程中的作用。野外研究将揭示灌木入侵如何改变其周围环境。生态学研究将为发现植被如何响应区域小气候条件的变化提供关键知识。实地研究的结果将纳入植被和大气的数值模式。这些模型将能够预测当前和未来气候条件下的灌木入侵。研究结果和模型将有助于决策者更好地了解灌木入侵，从而改善西南生态系统的管理。

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.