Gas exchange and carbon isotope discrimination of Juniperus osteosperma and Juniperus occidentalis across environmental gradients in the Great Basin of western North America

We determined how ecophysiological characteristics of two juniper species, Juniperus occidentalis Hook. (western juniper) and Juniperus osteosperma (T orr.) Little (Utah juniper), changed along altitudinal and regional environ mental gradients in the Great Basin of western North America. We obtained d iurnal measurements of leaf gas exchange and xylem water potential (Psi) fr om plants at a low and a high altitude site within each of six mountain ran ges during fall 1994, spring, summer, and fall 1995, and summer 1996, We al so determined carbon isotope composition (delta(13)C) of leaf cellulose pro duced during the 1995 growing season. Overall, leaf gas exchange, Psi and d elta(13)C did not differ significantly between species. Differences in dail y (A(d)) and season-long (A(s)) carbon assimilation among mountain ranges s uggested two groupings-a group of northern ranges and a group of southern r anges. Each group contained one mountain range with J. occidentalis and two with J. osteosperma. Differences in carbon assimilation based on this grou ping were associated with two findings: (1) conductance of CO2 from substom atal cavities to the site of carboxylation (g(m)) for junipers in the north ern ranges averaged almost twice that of junipers in the southern ranges; a nd (2) physiological shifts occurred such that A(d) of junipers in the nort hern ranges was influenced more by Psi(pd), whereas A(d) of junipers in the southern ranges was influenced more by leaf temperature. Mean delta(13)C o ver all trees at a site was significantly correlated with annual precipitat ion. Significant differences in A(d) occurred between altitudes, but these differences were associated with differences in the timing of optimum leaf temperature for photosynthesis rather than with physiological acclimation t o temperature, irradiance, or Psi. Most gas exchange parameters (e,g., assi milation, transpiration, stomatal conductance, and water use efficiency) va ried seasonally, and the seasonal differences were strongly influenced by w ater stress.