© 1998 Heron Publishing—Victoria, Canada
Osmotic potential of several hardwood species as affected by manipulation of throughfall precipitation in an upland oak forest
during a dry year
Timothy J. Tschaplinski (1), G. Michael Gebre (1) and Terri L. Shirshac (2)
1. Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6422, USA / 2. Water Resources Center, University of Minnesota, St. Paul, MN 55108, USA / Received February 13, 1997
Summary
Components of dehydration tolerance, including osmotic potential at full turgor (Ψπo) and osmotic adjustment (lowering of Ψπo), of several deciduous species were investigated in a mature, upland oak forest in eastern Tennessee. Beginning July 1993,
the trees were subjected 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 species declined to
between –2.5 and –3.1 MPa in the dry treatment. There was considerable variation in Ψπo among species (–1.0 to –2.0 MPa). Based on Ψπo values, American beech (Fagus grandifolia Ehrh.), dogwood (Cornus florida L.), and sugar maple (Acer saccharum Marsh.) were least dehydration tolerant, red maple (A. rubrum L.) was intermediate in tolerance, and white oak (Quercus alba L.) and chestnut oak (Quercus prinus L.) were most tolerant. During severe drought, overstory chestnut oak and understory dogwood, red maple and chestnut oak
displayed osmotic adjustment (–0.12 to –0.20 MPa) in the dry treatment relative to the wet treatment. (No osmotic adjustment
was evident in understory red maple and chestnut oak during the previous wet year.) Osmotic potential at full turgor was generally
correlated with leaf water potential, with both declining over the growing season, especially in species that displayed osmotic
adjustment. However, osmotic adjustment was not restricted to species considered dehydration tolerant; for example, dogwood
typically maintained high Ψπo and displayed osmotic adjustment to drought, but had the highest mortality rates of the species studied. Understory saplings
tended to have higher Ψπo than overstory trees when water availability was high, but Ψπo of understory trees declined to values observed for overstory trees during severe drought. We conclude that Ψπo varies among deciduous hardwood species and is dependent on canopy position and soil water potential in the rooting zone.
Keywords:
Acer rubrum, Acer saccharum, American beech, chestnut oak, Cornus florida, dehydration tolerance, dogwood, Fagus grandifolia, leaf water potential, osmotic adjustment, Quercus alba, Quercus prinus, red maple, sugar maple, white oak.
