PHOTOSYNTHESIS AND WATER-USE EFFICIENCY IN PINYON-JUNIPER COMMUNITIESALONG AN ELEVATION GRADIENT IN NORTHERN NEW-MEXICO

We investigated plant ecophysiological response to fertilization of se lected sites along an elevation gradient in a pinyon-juniper woodland. Plant density and species composition followed typical patterns for p inyon-juniper woodlands over this gradient, with a sparse juniper (Jun iperus monosperma (Engelm.) Sarg.)-grassland community at the lowest e levation and gradually increasing total canopy cover and pinyon (Pinus edulis Engelm.) cover with elevation. Carbon isotope analysis showed that both tree species had higher water-use efficiency (WUE) at the lo west, and presumably driest, sites. Over most of the gradient, however , it appeared that changes in stand density compensated for changes in water availability. Contrary to initial hypotheses, the more drought- tolerant juniper did not demonstrate significantly greater WUE than pi nyon, although it maintained positive carbon gain at lower predawn xyl em pressure potentials than pinyon, In pinyon, both A(max) and WUE inc reased with increasing N concentration in tissues. Pinyon needles also demonstrated declining nitrogen-use efficiency with age. There was no relationship between tissue N and either A(max) or WUE measured at A( max) in juniper, although deltaC-13 analysis indicated that WUE increa sed in juniper with increased N availability. Results from this study suggest that plasticity in plant physiological processes could result in nonlinear responses of organic matter production to climate change, and therefore must be accounted for in ecosystem models.