Comparative water relations of three successional hardwood species in central Wisconsin.

Water relations of co-occurring understory saplings of Quercus ellipsoidalis E.J. Hill, an early successional, xeric species, Populus tremuloides Michx., an early successional, mesic species, and Acer rubrum L., a late successional species that occurs on both wet and dry sites, were evaluated on four dates during the 1986 growing season. The understory was characterized by high soil water content, low irradiance and low vapor pressure deficit throughout the growing season. Stomatal conductance and calculated transpiration flux were lowest for A. rubrum and highest for P. tremuloides and Q. ellipsoidalis. Except early in the growing season, leaf water potentials were lower in P. tremuloides than in the other species. Populus tremuloides had the highest bulk modulus of elasticity, Q. ellipsoidalis the lowest. Over the growing season, Populus tremuloides and Q. ellipsoidalis, but not A. rubrum, exhibited a decrease in osmotic potential at both full and zero turgor. Of the three species, Populus tremuloides exhibited the sharpest decrease in leaf water potential and turgor pressure with decreasing relative water content.