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Darrin J. Moore, Robert S. Nowak, Robin J. Tausch, Gas exchange and carbon isotope discrimination of Juniperus osteosperma and Juniperus occidentalis across environmental gradients in the Great Basin of western North America, Tree Physiology, Volume 19, Issue 7, June 1999, Pages 421–433, https://doi.org/10.1093/treephys/19.7.421
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Abstract
We determined how ecophysiological characteristics of two juniper species, Juniperus occidentalis Hook. (western juniper) and Juniperus osteosperma (Torr.) Little (Utah juniper), changed along altitudinal and regional environmental gradients in the Great Basin of western North America. We obtained diurnal measurements of leaf gas exchange and xylem water potential (Ψ) from plants at a low and a high altitude site within each of six mountain ranges during fall 1994, spring, summer, and fall 1995, and summer 1996. We also determined carbon isotope composition (δ13C) of leaf cellulose produced during the 1995 growing season. Overall, leaf gas exchange, Ψ and δ13C did not differ significantly between species. Differences in daily (Ad) and season-long (As) carbon assimilation among mountain ranges suggested two groupings—a group of northern ranges and a group of southern ranges. Each group contained one mountain range with J. occidentalis and two with J. osteosperma. Differences in carbon assimilation based on this grouping were associated with two findings: (1) conductance of CO2 from substomatal cavities to the site of carboxylation (gm) for junipers in the northern ranges averaged almost twice that of junipers in the southern ranges; and (2) physiological shifts occurred such that Ad of junipers in the northern ranges was influenced more by Ψpd, whereas Ad of junipers in the southern ranges was influenced more by leaf temperature. Mean δ13C over all trees at a site was significantly correlated with annual precipitation. Significant differences in Ad occurred between altitudes, but these differences were associated with differences in the timing of optimum leaf temperature for photosynthesis rather than with physiological acclimation to temperature, irradiance, or Ψ. Most gas exchange parameters (e.g., assimilation, transpiration, stomatal conductance, and water use efficiency) varied seasonally, and the seasonal differences were strongly influenced by water stress.
