This study employed an intensive sampling regime in which leaf gas exchange and tissue-water relations were measured simultaneously on the same leaf at midday on 19 tree species from three distinct forest communities during wet (1990) and dry (1991) growing seasons. The study sites were located on a xeric barrens, a misic valley floor, and a wet-mesic floodplain in central Pennsylvania, United States. The xeric, mesic, and wetmesic sties had drought-related decreases in gravimetric soil moisture of 53, 34 and 27%, respectively. During the wet year, xeric and mesic communities had high seasonal mean photosynthetic rates (A) and stomatal conductance of water vapor (g wv) and low midday leaf water potential (ψ), whereas the wet-mesic community had low A and g wv and high midday ψ. The mesic and wet-mesic communities had dry year decreases in predawn ψ, g wv and A with the greatest drought effect occurring in the mesic community. Regression analysis indicated that species from each site that exhibited high wet-year A and g wv tended to have low midday ψ. This trend was reversed only in the mesic community in the drought year. Despite differences in midday ψ, all three communities had similar midday leaf turgor pressure (ψp) in the wet year attributable to lower osmotic potential at zero turgor (ψ π0 ) with increasing site droughtiness. Lower wet year ψ π0 in the xeric community was due to low symplast volume rather than high solute content. Species with the lowest ψ π0 in the wet year often did not have the lowest ψ π100 possibly related to differences in tissue elasticity. Moreover, increased elasticity during drought may have masked osmotic adjustment in ψ π100 but not in ψ π0 , via dilution of solutes at full hydration in some species. Despite the sampling regime used, there were no relationships between gas exchange and osmotic and elastic parameters that were consistently significant among communities or years. This result questions the universal, direct effect of osmotic and elastic adjustments in the maintenance of photosynthesis during drought. By including a large number of species, this study provided new insight to the ecophysiology of contrasting forest communities, and the community-wide impact of drought on contrasting sites.
Keywords: Drought; Microenvironment; Pennsylvania; Tissue-water relations Gas exchange.