Responses of forests to changes in environmental conditions reflect the integrated behavior of their constituent species. We investigated sap flux-scaled transpiration responses of two species prevalent in upland eastern hardwood forests, Quercus alba in the upper canopy and Acer rubrum in the low to mid canopy, to changes in photosynthetically active radiation above the canopy (Qo), vapor pressure deficit within the canopy (D), and soil moisture depletion during an entire growing season. Water loss before bud break (presumably through the bark) increased linearly with D, reaching 8% of daily stand transpiration (EC) as measured when leaf area index was at maximum, and accounting for 5% of annual water loss. After leaves were completely expanded and when soil moisture was high, sap flux-scaled daily EC increased linearly with the daily sum of Qo. Species differences in this response were observed. Q. alba reached a maximum transpiration at low Qo, while A. rubrum showed increasing transpiration with Qo at all light levels. Daily EC increased in response to daily average D, with an asymptotic response due to the behavior of Q. alba. Transpiration of A. rubrum showed a greater response to soil moisture depletion than did that of Q. alba. When evaluated at a half-hourly scale under high Qo, mean canopy stomatal conductance (GS) of individuals decreased with D. The sensitivity of GS to D was greater in species with higher intrinsic GS. Regardless of position in the canopy, diffuse-porous species in this and an additional, more mesic stand showed higher GS and greater stomatal sensitivity to environmental variation than do ring-porous species.
