Grass decline in the Chihuahuan Desert: The role of plant functional diversity and herbivore selectivity (New Mexico)

Grass decline is the earliest and most generalized symptom of and land degradation and desertification. The aim of this work was to introduce and test three key assumptions of a conceptual model of grass decline in semiarid ecosystems of southern New Mexico that formalizes the interactions between the effects of drought and cattle overgrazing. The model proposes that perennial grasses are functionally diverse, and that both drought and grazing have a proportionally larger negative effect on the relatively mesic than on the relatively xeric species coexisting within a community. In both controlled-environment and field measurements, I found eight perennial warm-season (C4) grasses of the area to encompass a five-fold range of growth capacity under well-watered conditions. In a growth-chamber experiment with all these species, I showed this capacity to be unrelated with their capacity to withstand drought---in open contrast with expectations. In a more detailed growth analysis conducted with two of these species, I found that the effects of water shortage on growth are mediated by morphological---rather than physiological---changes in response to early soil dessication. The same kind of correlation generally holds irrespective of the ambient partial pressure of CO2. Combining soil-moisture records during three seasons with simulation modeling, I was able to quantify the effects of these populations upon soil water dynamics, and to conclude that the proportion of the water balance attributable to grass uptake and transpiration can be significant (15--40%), and for a given site equally influenced by past and current management conditions than by weather conditions. I was not able, however, to discern a differential effect of particular species on soil water dynamics, although the modeling work suggests that it is likely to occur. I propose a self-imposed drought mechanism that would explain why species of different potential growth rate exhibit different sensitivity to drought in the field. The mechanism is based on plant effects on soil water and its feedback on plant growth. I show that this mechanism has the potential to influence temporal patterns of selective grazing and, therefore, the chances of recovery of these communities from past overuse.