© 1991 Heron Publishing—Victoria, Canada
Carbon gain and water use in pinyon pine-juniper woodlands of northern New Mexico: field versus phytotron chamber measurements
Kate Lajtha (1) and Fairley J. Barnes (2)
1. Department of Biology, Boston University, Boston, MA 02215, USA / 2. Environmental Science Research, MS-J495, Los Alamos National Laboratory, Los Alamos, NM 87545, USA /
Summary
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 (Ψ) and minimum Ψ 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
Ψ than juniper, although juniper was able to continue photosynthetic activity at lower values of Ψ 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 Ψ 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, δ13C 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 Ψ 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.
