SEED-SOURCE VARIATION IN WATER RELATIONS, GAS-EXCHANGE, AND NEEDLE MORPHOLOGY OF MATURE PONDEROSA PINE TREES

Xylem pressure potential (psi(s)), net photosynthesis (A), needle conductance (g(n)), and transpiration (E) were measured periodically throughout the 1991 growing season on 16 ponderosa pine (Pinus ponderosa Dougl. ex Laws.) trees growing in a 23-year-old provenance planting in eastern Nebraska. The trees studied were from four diverse sources: western Montana, northwest South Dakota, southern New Mexico, and central Arizona. In addition to water relations and gas exchange, specific leaf area, stomatal density, and surface to volume ratios were determined on 1-year-old foliage of each tree. Compared with the other seed sources, gas exchange of the South Dakota source was lowest early in the summer, when psi(x) was generally high, and highest in the late summer, when psi(x) declined. However, the relation Of g(n) to psi(x) did not appear to differ among the seed sources. The South Dakota source had lower stomatal density and needle length than the other sources tested. No differences in specific leaf area or surface to volume ratio were observed. Overall, variation in physiology and needle morphology among seed sources of mature ponderosa pine appears to be more subtle than intraspecific variation of seedlings of other species reported in the literature.