Disturbance and climate change in United States/Mexico borderland plant communities

This review evaluates the effects and importance of disturbance and climate change on plant community dynamics in the United States/Mexico borderlands region. Our primary focus is on knowledge of physiognomic-level change in grasslands and woodlands of southeastern Arizona and southwestern New Mexico. Changes in vegetation physiognomy have broad implications for management and land use in the borderlands. Historically, livestock grazing and fire were the dominant disturbances in borderland ecosystems. Livestock grazing accelerates the rate of establishment and growth of woody plants (notably mesquite) in the borderlands region. However, the current role of livestock grazing in this region may be considerably less important than the historic role. Excluding livestock from grasslands now would not prevent, and may not delay, a substantial increase in woody plant abundance. In general, frequent fires are detrimental to woody plants and beneficial to Lehmann lovegrass.^ Although fires favor herbaceous plants at the expense of woody plants, woody plant dominance on many sites precludes the deliberate application of fire without first using chemical or mechanical treatments. Climate change is perhaps the most critical factor facing the current generation of land managers who are concerned about the future condition of borderlands ecosystems. Rising concentrations of CO₂ and other trace gases have the potential to replace grazing and fire suppression as important regulators of vegetation change. Increased atmospheric CO₂ concentrations may contribute to increased abundance of woody plants in borderlands grasslands, particularly if these increases are accompanied by shifts in seasonal precipitation or other climatic factors. Much of the knowledge garnered about borderlands plant communities is derived from descriptive research such as comparative studies and "natural" experiments.^ This research has been valuable for documenting changes in vegetation and identifying candidate explanations for observed changes. However, because descriptive research is generally inappropriate for testing hypotheses, we outline and describe a strategy for hypothesis-testing and provide recommendations for future research in the borderlands. Implications for management are also outlined in this paper.