Early season cuticular conductance and gas exchange in two 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 examined to determine if leaf gas exchange characteristics correlated with differences in tree distribution. Bur oak (Quercus macrocarpa Michx.) is 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 oak than in bur oak, though midday water potentials were similar. After leaves had fully expanded, g_min decreased to seasonal minimum values 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 than in bur oak. Later in the growing season, A_net and gleaf increased dramatically in both species, and were significantly higher in bur oak relative to chinquapin oak. We concluded that bur and chinquapin oak have a number of leaf gas exchange characteristics that minimize seasonal water loss. These characteristics are distinct from trees from more mesic sites, and are consistent with the distribution patterns of these trees in tall-grass prairie gallery forests.