Leaf Conductance and Photosynthesis in Four Species of the Oak-Hickory Forest Type

Forest grown saplings of sugar maple, northern red oak, white oak, and black oak were transplanted to a greenhouse. Measurements of leaf conductance, xylem pressure potential, osmotic potential, photosynthesis, leaf temperature, photosynthetically active radiation, vapor pressure gradient, and soil water potential were recorded on days which represented various stages in a dehydration-rehydration cycle. Under light sufficient conditions, leaf conductance in sugar maple, white oak, and northern red oak reacted to changes in turgor pressure while stomata in black oak responded only to changes in vapor pressure gradient. Photosynthesis was significantly related to leaf conductance in all species, and photosynthesis decreased with decreasing photosynthetically active radiation and/or decreasing soil water potential. After reaching a species specific optimum, photosynthesis decreased with increasing leaf temperature. Appreciable rates of photosynthesis (between 18 and 33 percent of maximum) were observed at leaf temperatures as high as 40°C. These physiological observations are discussed in terms of known ecological patterns of behavior for these species. Forest Sci. 24:73-84.