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。灌木入侵是指木本植物取代禾本科植物的过程。灌木入侵会使牧场变成不生产的林地，并移走富含养分的土壤颗粒，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。相对突然的特点，草地到灌木过渡表明，这些过渡可能是持续的植被覆盖和上覆大气之间的微妙平衡。特别是，我们假设，近地面的大气在夜间变得温暖，响应于植被覆盖从草地到灌木丛的变化。地面附近较温暖的大气反过来又可能促进灌木入侵的过程，因为灌木的生长受到较高的冬季温度的促进。本研究结合了经验和理论方法，以确定这些植被-气候相互作用的意义和它们在美国西南部灌木入侵过程中的作用。实地研究将揭示灌木入侵如何改变周围环境。生态学研究将提供关键知识，以发现植被如何响应区域小气候条件的变化。实地研究的结果将纳入植被和大气的数值模型。这些模型将能够预测灌木入侵在当前和未来的气候条件下。研究结果和模型将通过为决策者提供更好地了解灌木入侵来改善西南部生态系统的管理。

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.