Local variation in intergrading Abies grandis-Abies concolor populations in the central Oregon Cascades. 2: Stomatal reaction to moisture stress

Seasonal and daily patterns of xylem pressure potential (P) (measured with a pressure chamber) andstomatal opening (measured by an infiltration technique) were determined for four sapling populations, oneof Abies grandis and three intermediate to A. concolor, in contrasting habitats in the central Oregon Cascades.Unexpectedly, populations east of the Cascade crest show higher P values (i.e., less moisture stress) thanthose from the west slope, where precipitation is greater and vegetation more mesic. Differences in daily andseasonal patterns of stomatal closure also occur among the populations studied. East-side trees show littledaily variation in stomatal opening during daylight, but close somewhat in late season, even with littleseasonal change in P. In contrast, stomata of low-elevation west-side saplings close considerably during eachday, and the maximal daily opening decreases as P decreases. A high-elevation west-side population shiftsfrom "constant-daily-opening" to "daily-closure" behavior as the summer progresses. On the west side,stomata of a low-elevation population close more rapidly as P decreases than those of high-elevation trees.Determination of leaf-resistance equivalents of infiltration pressures allows calculation of the magnitude ofthe possible effect of different stomatal patterns on water loss and on photosynthesis predicted from amathematical model. Assumption of an east-side (constant-daily-opening) pattern by a low-elevation west-side tree would increase calculated transpiration and photosynthesis by 1.6 to 1.7 times. Such phenotypicdifferences in stomata] behavior of populations should be considered in modeling plant processes