ECOPHYSIOLOGY OF 3 C-4 PERENNIAL GRASSES IN THE NORTHERN CHIHUAHUAN DESERT

Despite changes in the relative abundance of C, perennial grasses that have occurred in the northern portion of the Chihuahuan desert, no co mparative physiological studies on these grasses have been reported. C oncurrent measurements were made of leaf physiology, gas exchange, chl orophyll and nitrogen content for mature Bouteloua eriopoda, Sporobolu s contractus, and Hilaria mutica during the spring period following an unusually long winter drought. Except for a brief period following ra in, midday photosynthesis (A) in Bouteloua was below 10 mu mol m(-2) s (-1). Sporobolus also had low midday A early in the season but had the greatest increase in A following rain when rates approached 25 mu mol m(-2) s(-1) and maintained the higher rates through the subsequent dr y period, In contrast to the other species, Hilaria exhibited higher A early in the season but a more moderate response to increased soil wa ter. Results for the individual species are discussed in the context o f speculations contained in published literature concerning varying de grees of drought resistance among the species. Future field studies on these grasses must be broader in scope to fully elucidate differences in drought resistance and the importance of any adaptive physiologica l mechanisms to past or present vegetation patterns.