GENETIC DIFFERENTIATION IN CARBON ISOTOPE DISCRIMINATION AND GAS-EXCHANGE IN PSEUDOTSUGA-MENZIESII - A COMMON-GARDEN EXPERIMENT

Patterns of genetic variation in gas-exchange physiology were analyzed in a 15-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation that contains 25 populations grown from seed collected from across the natural distribution of the species. Seed was collected from 33-degrees 30' to 53-degrees 12' north latitude and from 170 m to 2930 m above sea level, and from the coastal and interior (Rocky Mountain) varieties of the species. Carbon isotope discrimination (DELTA) ranged from 19.70(parts per thousand) to 22.43(parts per thousand) and was closely related to geographic location of the seed source. The coastal variety (20.50 (SE = 0.21)parts per thousand) was not significantly different from the interior variety (20.91 (0.15)parts per thousand). Instead, most variation was found within the interior variety; populations from the southern Rockies had the highest discrimination (21.53 (0.20)parts per thousand) (lowest water-use efficiency). Carbon isotope discrimination (DELTA), stomatal conductance to water vapor (g), the ratio of intercellular to ambient CO2 concentration (c(i)/c(a)), and intrinsic water-use efficiency (A/g) were all correlated with altitude of origin (r = 0.76, 0.73, 0.74, and -0.63 respectively); all were statistically significant at the 0.01 level. The same variables were correlated with both height and diameter at age 15 (all at P less-than-or-equal-to 0.0005). Observed patterns in the common garden did not conform to our expectation of higher WUE, measured by both A/g and DELTA, in trees from the drier habitats of the interior, nor did they agree with published in situ observations of decreasing g and DELTA with altitude. The genetic effect opposes the altitudinal one, leading to some degree of homeostasis in physiological characteritics in situ.