EARLY-SEASON CUTICULAR CONDUCTANCE AND GAS-EXCHANGE IN 2 OAKS NEAR THE WESTERN EDGE OF THEIR RANGE

Seasonal changes in minimum leaf conductance to water vapor (g(min)), an estimate of cuticular conductance, and photosynthetic gas exchange in two co-occurring oak species in north-east Kansas (USA) were examin ed to determine if leaf gas exchange characteristics correlated with d ifferences in tree distribution. Bur oak (Quercus macrocarpa Michx.) i s more abundant in mesic gallery forest sites, whereas chinquapin oak (Quercus muehlenbergii Englm.) is more abundant in xeric sites. Early, during leaf expansion, g(min) was significantly lower in chinquapin o ak than in bur oak, though midday water potentials were similar. After leaves had fully expanded, g(min) decreased to seasonal minimum value s of 4.57 (+/- 0.274) mmol m(-2) s(-1) in bur oak, and 2.66 (+/- 0.156 ) mmol m(-2) s(-1) in chinquapin oak. Water potentials at these times were significantly higher in chinquapin oak. As leaves were expanding, photosynthesis (A(net)) was significantly higher in chinquapin oak th an in bur oak. Later in the growing season, A(net) and g(leaf) increas ed dramatically in both species, and were significantly higher in bur oak relative to chinquapin oak. We concluded that bur and chinquapin o ak have a number of leaf gas exchange characteristics that minimize se asonal water loss. These characteristics are distinct from trees from more mesic sites, and are consistent with the distribution patterns of these tries in tall-grass prairie gallery forests.