Adaptive physiological changes were investigated in seven populations of Sitka (Picea sitchensis (Bong.) Carr.) x interior spruce (P. glauca (Moench) Voss x P. engelmannii Parry ex Engelm.) spanning the Nass-Skeena transition zone in British Columbia, Canada. Each population was represented by an Si rDNA index that was calculated from the relative optical densities on a gel autoradiogram of five ribosomal DNA bands characteristic of Sitka spruce and interior spruce. This index estimates the proportion of the genome contributed by interior spruce. Physiological adaptations were assessed by gas exchange parameters measured under both well-watered and drought conditions. Under well-watered conditions, Sitka spruce populations had higher maximal photosynthesis at saturating light and ambient CO(2), higher quantum yield at the light compensation point, and higher dark respiration than interior spruce populations. Sitka spruce populations also reached maximal photosynthesis at lower photosynthetically active radiation and higher CO(2) concentrations, and had higher stomatal densities that resulted in lower stomatal limitations to photosynthesis than interior spruce populations. In contrast, interior spruce populations exhibited greater drought tolerance than Sitka spruce populations. Their gas exchange rates declined at a slower rate in response to drought. They maintained higher gas exchange rates in response to moderate to severe drought (predawn plant water potentials = -1.5 MPa), and their photosynthetic rates recovered faster when they were rewatered after exposure to drought. Comparison of the seven populations indicated that physiological parameters were significantly related to the Si rDNA index. An increase in Si rDNA index was associated with proportional changes in physiological measurements, suggesting that genetic interchange among species with contrasting ecological adaptations can enhance the environmental adaptation of natural populations.