© 1995 Heron Publishing—Victoria, Canada
Gas exchange, leaf structure and nitrogen in contrasting successional tree species growing in open and understory sites during
a drought
Marc D. Abrams and Scott A. Mostoller
School of Forest Resources, The Pennsylvania State University, 4 Ferguson Building, University Park, PA 16802, USA / Received April 14, 1994
Summary
Seasonal ecophysiology, leaf structure and nitrogen were measured in saplings 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 central Pennsylvania, USA. Area-based
net photosynthesis (A) and leaf conductance to water vapor diffusion (gwv) 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 maximum drought, both sunfleck and sun leaves of the early successional species exhibited
smaller decreases in A than leaves of the other species. Shaded understory leaves of all species were more susceptible to drought than sun leaves
and had negative midday A values during the middle 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 nitrogen (N) content
than shaded leaves, and early and middle successional species had higher N contents and concentrations than late successional
species. In both sunfleck and sun leaves, seasonal A was positively related to predawn leaf Ψ, gwv, LMA and N, and was negatively related to vapor pressure deficit, midday leaf Ψ and internal CO2. Although a significant amount of plasticity occurred in all species for most gas exchange and leaf structural parameters,
middle successional species exhibited the largest degree of phenotypic plasticity between open and understory plants.
Keywords:
net photosynthesis, phenotypic plasticity, shading, sunflecks, understory, water potential.
