Energy investment in leaves of red maple and co-occurring oaks within a forested watershed.

Despite its recent expansion in eastern US forests, red maple (Acer rubrum L.) generally exhibits a low leaf photosynthetic rate, leaf mass per unit area (LMA) and leaf nitrogen concentration ([N]) relative to co-occurring oaks (Quercus spp.). To evaluate these differences from the perspective of leaf energy investment, we compared leaf construction cost (CC) and leaf maintenance cost (MC) with leaf photosynthetic rate at saturating photon flux density and ambient CO2 partial pressure (Amax) in red maple and co-occurring red oak (Quercus rubra L.) and chestnut oak (Quercus prinus L.). We also examined relationships among leaf physiological, biochemical and structural characteristics of upper-canopy leaves of these three species at lower (wetter) and upper (drier) elevation sites of a watershed in the Black Rock Forest, Cornwall, NY, USA. Although A(max), leaf [N], leaf carbon concentration ([C]) and LMA were significantly less in red maple than in either oak species at both sites, CC per unit leaf area of red maple was 28.2 and 35.4% less than that of red oak at the lower and upper site, respectively, and 38.8 and 32% less than that of chestnut oak at the lower and upper site, respectively. Leaf MC per unit leaf area, which was positively associated with leaf CC (r2 = 0.95), was also significantly lower in red maple than in either oak species at both sites. When expressed per unit leaf area, A(max) was positively correlated with both CC (r2 = 0.65) and MC (r2 = 0.59). The cost/benefit ratio of CC/Amax of red maple was significantly less than that of chestnut oak at the lower site, however, CC/A(max) did not exhibit any significant interspecific differences at the upper site. Expressed per unit leaf area, CC was correlated positively with LMA (r2 = 0.90), leaf [N] (r2 = 0.97), and leaf [C] (r2 = 0.89), and negatively correlated with leaf molar carbon to nitrogen ratio (r2 = 0.92). Combined with red maple's general success in many oak-dominated forests, our findings suggest that reduced leaf-level photosynthetic capacity and related leaf characteristics in red maple are partially balanced by lower energy and resource requirements for leaf biomass construction and maintenance, which could enhance the competitive success of this species.