WATER RELATIONS OF SEVERAL HARDWOOD SPECIES IN RESPONSE TO THROUGHFALL MANIPULATION IN AN UPLAND OAK FOREST DURING A WET YEAR

We investigated the effects of altered precipitation on leaf osmotic p otential at full turgor (Psi(pi o)) of several species in an upland oa k forest during the 1994 growing season as part of a Throughfall Displ acement Experiment at the Walker Branch Watershed near Oak Ridge, Tenn essee. The main species sampled included overstory chestnut oak (Querc us prinus L.), white oak (Q. alba L.), red maple (Acer rubrum L.); int ermediates sugar maple (A. saccharum L,) and blackgum (Nyssa sylvatica Marsh,); and understory dogwood (Cornus florida L.) and red maple. Th e precipitation treatments were: ambient precipitation; ambient minus 33% of throughfall (dry); and ambient plus 33% of throughfall (wet), E xcept in late September, midday leaf water potentials (Psi(1)) were ge nerally high in all species in all treatments, ranging from-031 to -1. 34 MPa for C. florida, -0.58 to -1.51 MPa for A. rubrum, and -0.78 to -1.86 MPa for e. prinus. Both treatment and species differences in Psi (pi o) were evident, with oak species generally exhibiting lower Psi(p i o) than A. saccharum, A. rubrum, C. florida, and N. sylvatica. The P si(pi o) of C. florida saplings declined in the dry treatment, and Q. prinus, Q. alba, and A. saccharum all exhibited a declining trend of P si(pi o) in the dry treatment, although Psi(pi o) of Q. prinus leaves increased in late August, corresponding to a recovery in soil water po tential. Cornus florida exhibited osmotic adjustment with the largest adjustment coinciding with the period of lowest soil water potential i n June. The only other species to exhibit osmotic adjustment was e. pr inus, which also maintained a lower baseline Psi(pi o) than the other species. We conclude that a 33% reduction of throughfall is sufficient both to alter the water relations of some species in the upland oak f orest and to enable the identification of those species capable of osm otic adjustment to a short-term drought during a wet year.