Carbon allocation, gas exchange, and needle morphology of Pinus ponderosa genotypes known to differ in growth and survival under imposed drought.

Seedlings from 27 open-pollinated families of ponderosa pine representing nine geographically diverse origins were screened for drought tolerance based on survival and growth under imposed drought. Seedlings that had been preconditioned to drought survived 14 days longer than seedlings that had been well watered before being subjected to drought. Seed sources varied in their ability to survive drought and this variation was accentuated by drought preconditioning. Seedlings from a South Dakota source and a Nebraska source generally survived the longest under drought. Seedlings from a Montana source and a New Mexico source succumbed the fastest after water was withheld. Significant family within source variation in drought survival was observed for some sources. In general, drought survival was poorly correlated to climate indices of the seed sources. Allocation of biomass to roots, stems, and needles varied significantly among the seed sources with the most drought-sensitive sources (Montana and New Mexico) showing the most divergent allocation patterns. The relation between drought survival and shoot/root ratio suggested that there is an optimum pattern of allocation for drought survival. A comparison of the most and least drought-tolerant sources indicated that needle gas exchange (net photosynthesis and needle conductance to water vapor) and predawn needle water potential were similar among the sources regardless of their relative ability to survive drought. Needle morphology traits often associated with variation in drought tolerance, such as stomatal density and specific leaf area, did not differ among the seed sources. However, seedlings from the drought-tolerant sources had shorter needles, less surface area per needle, and fewer stomata per needle than seedlings from the drought-sensitive sources. The results suggest that drought tolerance of ponderosa pine may be improved through seed source selection and, within certain sources, family selection. Allocation patterns and needle morphology appear to play a larger role than needle gas exchange patterns in determining drought tolerance in this species.