The purpose of this study was to determine physiological differences in drought response among Garry oak (Quercus garryana Douglas ex Hook. [Fagaceae]) seedlings from acorns collected at seven acorn collection sites in Washington state. Our objective was to aid in Garry oak restoration efforts by investigating potential differences in drought tolerance for seedlings grown from acorns collected at different sites. Acorns were collected from six sites east of the Cascades, and one site on Whidbey Island (Oak Harbor) west of the Cascades. Differences in morphological and photosynthetic characteristics observed for seedlings grown from acorns collected at these different sites became more pronounced after drought stress was induced by withholding water. Seedlings from acorns collected at the northernmost east-side site (Swauk Creek) were most susceptible to drought and had photosynthetic rates that were 35% lower under drought conditions than when well-watered. Seedlings from acorns collected at the site west of the cascades (Oak Harbor) were the least susceptible to drought and had assimilation rates under drought conditions that were similar to those when well-watered. Differences were also observed among seedlings from different sites in the occurrence of drought-induced loss of stem hydraulic conductivity and changes in leaf water potential and relative water content in response to drought. We conclude that seedlings grown from Garry oak trees at different sites across Washington State have distinct physiological responses to drought that may lead to differing survivorship when used in reforestation efforts and exposed to drought stress in the field.
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Vol. 91 • No. 2