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Components of dehydration tolerance, including osmotic potential at full turgor (Ψ πo ) and osmotic adjustment (lowering of Ψ πo ), 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 Ψ πo  among species (–1.0 to –2.0 MPa). Based on Ψ πo  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 Ψ πo  and displayed osmotic adjustment to drought, but had the highest mortality rates of the species studied. Understory saplings tended to have higher Ψ πo  than overstory trees when water availability was high, but Ψ πo  of understory trees declined to values observed for overstory trees during severe drought. We conclude that Ψ πo  varies among deciduous hardwood species and is dependent on canopy position and soil water potential in the rooting zone.

Osmotic potential of several hardwood species as affected by manipulation of throughfall precipitation in an upland oak forest during a dry year