PHOTOSYNTHESIS, WATER RELATIONS AND DROUGHT ADAPTATION OF SEVERAL OAK-HICKORY FOREST SPECIES

Only a limited number of studies on how photosynthesis, water relations and the drought resistances of the dominant oak-hickory forest species relate to their successional position had been reported. Research in this area could provide better insight into the pattern of plant distribution and plant performance relative to water conditions.;The ('14)carbon technique was used to measure gross photosynthesis; other related plant, soil and environmental factors also were measured in the field. The drought avoidance, drought tolerance and tissue-water relations of leaves were measured in the laboratory using atmospheric desiccation, electrical conductivity and pressure-volume techniques, respectively.;The mean seasonal photosynthetic rate for sugar maple was significantly lower than those of all other species. The oaks had higher light-saturated rates of photosynthesis than did sugar maple and dogwood. Values of osmotic potential at the turgor loss point of the oaks and eastern redcedar were significantly lower than those of dogwood and sugar maple which allowed gas exchange to continue to lower values of leaf water potential. Osmotic adjustments in response to increasing drought stress were clearly expressed in white, northern red, black oak and dogwood. White and black oak exhibited highest bulk moduli of elasticity which, in combination with osmotic adjustment, would enhance water absorption by roots.;Leaves of eastern redcedar, sugar maple and dogwood exhibited greater drought avoidance than the oak species. White, northern red and black oak exhibited greater protoplasmic drought tolerance than did dogwood and sugar maple.;The objectives of this study were to relate field observations of the drought resistance and photosynthesis of white oak (Quercus alba L.), northern red oak (Quercus rubra L.), black oak (Quercus velutina Lam.), flowering dogwood (Cornus florida L.), sugar maple (Acer saccharum Marsh.) and eastern redcedar (Juniperus virginiana L.) to their successional position.;The three oak species and eastern redcedar were better adapted to xeric conditions and old-field environments. The photosynthetic characteristics in high-light environments and the drought resistances of black, white, northern red oak and eastern redcedar were in agreement with the distribution of these species under conditions of high light and limiting water.