OSMOTIC POTENTIAL OF SEVERAL HARDWOOD SPECIES AS AFFECTED BY MANIPULATION OF THROUGHFALL PRECIPITATION IN AN UPLAND OAK FOREST DURING A DRYYEAR

Components of dehydration tolerance, including osmotic potential at fu ll turgor (Psi(pi o)) and osmotic adjustment (lowering of (Psi(pi o)), of several deciduous species were investigated in a mature, upland oa k forest in eastern Tennessee. Beginning July 1993, the trees were sub jected to one of three throughfall precipitation treatments: ambient, ambient minus 33% (dry treatment), and ambient plus 33% (wet treatment ). During the dry 1995 growing season, leaf water potentials of all sp ecies declined to between -2.5 and -3.1 MPa in the dry treatment. Ther e was considerable variation in Psi(pi o) among species (-1.0 to -2.0 MPa). Based on Psi(pi o) values, American beech (Fagus grandifolia Ehr h,), dogwood (Cornus florida L.), and sugar maple (Acer saccharum Mars h.) were least dehydration tolerant, red maple (A. rubrum L.) was inte rmediate in tolerance, and white oak (Quercus alba L.) and chestnut oa k (Quercus prinus L.) were most tolerant. During severe drought, overs tory chestnut oak and understory dogwood, red maple and chestnut oak d isplayed osmotic adjustment (-0.12 to -0.20 MPa) in the dry treatment relative to the wet treatment. (No osmotic adjustment was evident in u nderstory red maple and chestnut oak during the previous wet year.) Os motic potential at full turgor was generally correlated with leaf wate r potential, with both declining over the growing season, especially i n species that displayed osmotic adjustment. However, osmotic adjustme nt was not restricted to species considered dehydration tolerant; for example, dogwood typically maintained high Psi(pi o) and displayed osm otic adjustment to drought, but had the highest mortality rates of the species studied. Understory saplings tended to have higher Psi(pi o) than overstory trees when water availability was high, but Psi(pi o) o f understory trees declined to values observed for overstory trees dur ing severe drought. We conclude that Psi(pi o) varies among deciduous hardwood species and is dependent on canopy position and soil water po tential in the rooting zone.