LEAF-LEVEL RESPONSES TO LIGHT AND TEMPERATURE IN 2 COOCCURRING QUERCUS (FAGACEAE) SPECIES - IMPLICATIONS FOR TREE DISTRIBUTION PATTERNS

Leaf-level responses to light environment (sun vs. shade) and high temperature were compared in two gallery forest oaks, bur oak (Quercus macrocarpa) and chinquapin oak (Quercus muehlenbergii). Our goal was to determine if species-specific differences in response to light or temperature could explain the distribution of these oaks in tallgrass prairie gallery forests. On the Konza Prairie Research Natural Area in NE Kansas (USA), bur oak is more abundant than chinquapin oak along lower reaches of streams where the forests are most productive and canopy closure is greatest. We hypothesized that bur oak is better able to acclimate physiologically to reduced light availability than chinquapin oak. Leaf level acclimation to low light in both oaks occurred through reduced photosynthetic light compensation points (PLC) (bur oak, 60.77+/-3.02 mu mol photons m(-2) s(-1) in sun leaves vs. 29.25+/-0.29 photons m(-2) s(-1) in shade leaves; chinquapin oak, 49.47+/-2.82 photons m(-2) s(-1) in sun leaves vs. 26.48+/-1.74 photons m(-2) s(-1) in shade leaves), resulting from nearly 50% reductions in dark respiration and specific leaf mass in both species. Apparent quantum requirement (Q(req); mol photons mol(-1) O-2) did not differ between sun and shade leaves in either species, and only bur oak showed differences in sun and shade leaf shape. We also hypothesized that chinquapin oak, which is more abundant in exposed, upper stream reaches, has greater photosynthetic tolerance to the high temperatures characteristic of this region. The maximum temperature tolerances (T-max) of these oaks were compared by measuring increases in chlorophyll fluorescence. For most of the growing season, T-max in chinquapin oak was 2.0 degrees C higher than in bur oak. Field measurements of leaf vs. air temperatures in seedlings and adult trees indicated that T-max was more likely to be exceeded in bur than in chinquapin oak, particularly in seedlings exposed to high solar radiation loads. We concluded that differences in thermal tolerance, in addition to previously documented differences in water relations are important determinants of distributional patterns of these oaks within gallery forests.