Seasonal Patterns of Growth, Water Potential and Gas Exchange of Red and White Fir Saplings across a Montane Ecotone

Abstract

-Saplings of Abies magnifica (red fir) and A. concolor (white fir) were measured for leaf gas exchange characteristics, xylem water potential and growth during 1 growing season. Eight saplings of each species, 22-34 yr old, were located along a 170-m elevational transect. Although white and red fir replace each other as dominants along the transect, ecophysiological traits of saplings were statistically identical. Net photosynthesis rates, leaf conductance to water vapor, water-use efficiency, xylem water potential and patterns of interactions or change through time were all similar for the two species. Height growth of white fir began earlier, peaked earlier and was greater than that of red fir. Diameter growth of white fir was also greater. Differences and similarities in physiological responses between the species were unaffected by position along the transect. We suggest that environmental factors which favor red fir at high elevation and white fir at low elevation are either not integrated by gas exchange behavior, not operative in summer, not differentially important this particular drought year or not differentially effective at this stage of the life cycle.