Photosynthesis and water-use efficiency in pinyon-juniper communities along an elevation gradient in northern New Mexico.

We investigated plant ecophysiological response to fertilization of selected sites along an elevation gradient in a pinyon-juniper woodland. Plant density and species composition followed typical patterns for pinyon-juniper woodlands over this gradient, with a sparse juniper (Juniperus monosperma (Engelm.) Sarg.)-grassland community at the lowest elevation 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 lowest, 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 pinyon, although it maintained positive carbon gain at lower predawn xylem pressure potentials than pinyon. In pinyon, both A max and WUE increased 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 δ13C analysis indicated that WUE increased 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.