Seasonal Tissue Water Relations of Four Successional Pennsylvania Barrens Species in Open and Understory Environments

Seasonal tissue water relations were measured in co-occurring saplings of Quercus velutina Lam., Quercus prinus L., Sassafras albidum (Nutt.) Nees, and Acer rubrum L. from adjacent open and understory sites in the central Pennsylvania barrens. Open-growing plants exhibited greater and earlier phenological shifts in osmotic potentials under moist conditions, whereas understory plants had greater osmotic adjustment during a mild, late-season drought. Sassafras albidum was an exception, exhibiting steadily declining osmotic potentials at full and zero turgor over the course of the season on both sites. Elastic modulus (ε) steadily increased for all species on the open site, while A. rubrum showed a decrease and S. albidum and Q. velutina showed an increase in ε in the understory. Relative water content at zero turgor (RWC0) was similar in the understory and open sites except during the drought period when understory plants had lower values. Quercus prinus generally exhibited the lowest RWC0 values, although A. rubrum, a later successional species, had a seasonal decrease in RWC0 at both sites. Thus, each species exhibited somewhat unique combinations of seasonal osmotic and elastic adjustment, which acted in concert to balance tissue water loss with turgor maintenance under changing environmental conditions. These results indicate that a variety of ecophysiological mechanisms operate to allow species of different successional rank to tolerate open and understory barrens environments.