RELATING WET AND DRY YEAR ECOPHYSIOLOGY TO LEAF STRUCTURE IN CONTRASTING TEMPERATE TREE SPECIES

Leaf structure in 17 temperate, broad-leaved tree species on xeric, mesic, and wet-mesic sites in central Pennsylvania (USA) was compared to leaf gas exchange and tissue-water relations data for these species measured during wet (1990) and dry (1991) growing seasons. We tested the hypothesis that ecophysiological responses across contrasting species, sites, and growing-season conditions can be related to specific aspects of leaf structure. Species guard-cell length (GCL), leaf mass per area (LMA), and leaf thickness were positively correlated and increased as sites became drier. Mesic species had lower stomatal density than xeric and wet-mesic species. During the wet year, xeric and mesic species had higher area-based net photosynthesis (A) and leaf conductance (g(wv)) and lower osmotic potentials (Psi(pi)) than wet-mesic species. Unlike the xeric species, mesic and wet-mesic species had decreased A and g(wv) in the dry vs, the wet year. Thus, gas exchange in mesic species resembled that of xeric species in the wet year and that of wet-mesic species in the dry year, which may be due to differences in leaf structure. Maximum A rates and mean g(wv) were positively related to species LMA, whereas wet and dry year mean A and g(wv) increased with increasing leaf thickness and GCL across the study sites. Minimum and mean midday leaf water potential and Psi(pi) at zero turgor decreased with increasing LMA, GCL, and leaf thickness. Stomatal density and ecophysiology were not significantly related across the study sites, but they were related within sites. Intrasite comparisons indicated that, in general, gas exchange parameters increased and tissue water parameters decreased with increases in leaf parameters, trends that were consistent with intersite comparisons. These results suggest that both gas exchange and tissue-water relations during wet and dry years were related to specific aspects of species leaf morphology, and that leaf structure has an important predictive role in evaluating ecophysiological responses to environmental stress at the community and landscape level.