GAS-EXCHANGE, LEAF STRUCTURE AND NITROGEN IN CONTRASTING SUCCESSIONALTREE SPECIES GROWING IN OPEN AND UNDERSTORY SITES DURING A DROUGHT

Seasonal ecophysiology, leaf structure and nitrogen were measured in s aplings of early (Populus grandidentata Michx. and Prunus serotina J.F . Ehrh.), middle (Fraxinus americana L. and Carya tomentosa Nutt.) and late (Acer rubrum L. and Cornus florida L.) successional tree species during severe drought on adjacent open and understory sites in centra l Pennsylvania, USA. Area-based net photosynthesis (A) and leaf conduc tance to water vapor diffusion (g(wv)) varied by site and species and were highest in open growing plants and early successional species at both the open and understory sites. In response to the period of maxim um drought, both sunfleck and sun leaves of the early successional spe cies exhibited smaller decreases in A than leaves of the other species . Shaded understory leaves of all species were more susceptible to dro ught than sun leaves and had negative midday A values during the middl e and later growing season. Shaded understory leaves also displayed a reduced photosynthetic light response during the peak drought period. Sun leaves were thicker and had a greater mass per area (LMA) and nitr ogen (N) content than shaded leaves, and early and middle successional species had higher N contents and concentrations than late succession al species. In both sunfleck and sun leaves, seasonal A was positively related to predawn leaf Psi, g(wv), LMA and N, and was negatively rel ated to vapor pressure deficit, midday leaf Psi and internal CO2. Alth ough a significant amount of plasticity occurred in all species for mo st gas exchange and leaf structural parameters, middle successional sp ecies exhibited the largest degree of phenotypic plasticity between op en and understory plants.