Photosynthetic characteristics in canopies of Quercus rubra, Quercus prinus and Acer rubrum differ in response to soil water availability

Photosynthesis and related leaf characteristics were measured in canopies of co-occurring Quercus rubra L. (red oak), Quercus prinus L. (chestnut oak) and Acer rubrum L. (red maple) trees. Mature (20+ m tall) trees were investigated at sites of differing soil water availability within a catchment (a drier upper site and a wetter lower site). Leaf photosynthetic characteristics differed significantly between species and in response to site and position in the canopy. Photosynthetic capacity (A_max) was significantly greater at the wetter site in all canopy strata in A. rubrum but not in Q. rubra or Q. prinus. Our findings for A. rubrum are generally consistent with those predicting that species with higher specific leaf area (SLA) will have higher A_max per unit leaf nitrogen (N) and that species with leaves with lower SLA (e.g. Q. rubra and Q. prinus) will have shallower slopes of the A_max-N relationship. Importantly, the relationships between A_max and N_area (and by implication photosynthetic nitrogen-use efficiency, PNUE) differed in A. rubrum between the sites, with PNUE significantly lower at the drier site. The lower photosynthetic capacity and PNUE must substantially reduce carbon acquisition capacity in A. rubrum under these field conditions. Maximum stomatal conductance (g_smax) differed significantly between species, with g_smax greatest in Q. rubra and Q. prinus. In Q. rubra and Q. prinus, g_smax was significantly lower at the upper site than the lower site. There was no significant response of g_smax to site in A. rubrum. These stomatal responses were consistent with the C_i/C_a ratio, which was significantly lower in leaves of Q. rubra and Q. prinus at the upper site, but did not differ between sites in A. rubrum. Leaf '¹³C was significantly lower in A. rubrum than in either Q. rubra or Q. prinus at both sites. These findings indicate differences in stomatal behaviour in A. rubrum which are likely to contribute to lower water use efficiency at both sites. Our results support the hypothesis that the two Quercus species, in contrast to A. rubrum, maintain photosynthetic capacity at the drier site whilst minimising transpirational water loss. They also suggest, based primarily on physiological evidence, that the ability of A. rubrum to compete with other species of these deciduous forests may be limited, particularly in sites of low moisture availability and during low rainfall years.