Carbon gain and water use in pinyon pine-juniper woodlands of northern New Mexico: field versus phytotron chamber measurements.

We compared phytotron chamber- and field-obtained gas exchange parameters from a semiarid pinyon pine-juniper (Pinus edulis Engelm.-Juniperus monosperma (Engelm.) Sarg.) community in northern New Mexico, an ecosystem where carbon gain could be limited by both water and nitrogen availability. In both species, variations in rates of photosynthesis with xylem pressure potential (Psi) and minimum Psi for positive carbon gain were similar in the field and phytotron chamber studies. Rates of maximum photosynthesis were greater in pinyon pine than in juniper in both the field and phytotron chamber studies. Pinyon pine showed a greater response to increases in Psi than juniper, although juniper was able to continue photosynthetic activity at lower values of Psi than pinyon pine. There were significant differences in instantaneous water use efficiency (WUE) measured in the field and the phytotron chamber, although relative rankings of the species and their responses to Psi were similar. In the field experiments, WUE of N-fertilized plants was marginally greater in pinyon pine than juniper during the wet season, and in both the field and phytotron chamber experiments instantaneous values of WUE were greater in juniper during dry seasons or during drying cycles. However, delta(13)C values suggested that pinyon pine had greater seasonally integrated values of WUE, perhaps because this species ceases stomatal activity during dry seasons. Field data showed significant interactions between N and WUE, as well as between Psi and the degree of response to N, however, there were insufficient data to derive predictive equations because of the limited range of environmental conditions encountered in the field.