Environmental influences on photosynthesis within the crown of a white oak

Photosynthesis, leaf conductance, xylem pressure potential as well as six environmental factors were measured within the crown of a dominant, 18.9 m, white oak tree (Quercus alba L.) during the summers of 1974 and 1975. Photosynthesis was measured with a ¹⁴CO₂ apparatus which enabled a description of the within crown and with leaf variations to be made. Photosynthesis averaged 7.0, 5.6 and 4.6 mgCO₂ dm^-2 h^-1 for top, middle and bottom crown positions. High leaf temperatures and mid-day stomatal closure appeared to limit photosynthesis in the upper crown while light appeared limiting in the lower. Under conditions of a moderate drought, the lower crown's photosynthethic activity approached that of the upper crown. Seasonal shifts in photosynthesis were noted and these were related to shifts in leaf conductance associated with temperature or drought or both pre-conditioning. For example, the xylem pressure potential threshold for stomatal closure varied from-18.9 to-26.1 bars depending upon drought history. With the ¹⁴CO₂ technique, photosynthetic activity was statistically uniform over the entire leaf surface; thus sampling location within a white oak leaf was not a matter of concern. Similarly, no statistically significant within-leaf variation was found with respect to leaf conductance.