Provenance variation in carbon isotope discrimination of mature ponderosa pine trees at two locations in the Great Plains

We analyzed genotypic variation in carbon isotope discrimination (delta), photosynthetic gas exchange, and needle morphology of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) trees from four seed sources growing in two 26-year-old provenance plantings near Plattsmouth, Neb., and Norman, Okla. The populations studied were from South Dakota, New Mexico, Wyoming, and Nebraska. Net photosynthesis (A) and needle conductance to water vapor (gwv), were measured during the growing season of 1994. Specific leaf area, stomatal density, and delta were analyzed in needles grown from 1991 to 1994. The southernmost source (New Mexico) had the highest intrinsic water-use efficiency (A/gwv) among the sources studied. Carbon isotope discrimination was correlated with growth (r = -0.81, P < 0.05), A/gwv (r < 0.54, P < 0.001), and gwv (r > 0.46, P < 0.05) but not A. Variation in delta was significant among seed sources and years (P < 0.001). We observed a strong genotype × environment interaction in delta resulting from geographic location but not moisture availability within locations. We hypothesize that the genotype × environment interaction is related to variation in growth phenology among the seed sources. Improving water-use efficiency or growth of ponderosa pine via delta will require an understanding in genotypic variation in growth rhythms.