Aboveground production efficiency and canopy nutrient contents of mixed-hardwood forest communities along a moisture gradient in the central United States

Abstract

The significance of canopy nutrients in regulating aboveground production efficiency of mixed-hardwood forest communities (ecological land type phases, ELTPs) was examined along a moisture gradient in southern Indiana, U.S.A. A total of 39 plots were established in six ELTPs in which canopy specific leaf area, canopy nutrient contents (N, P, K, Ca, and Mg on unit leaf area basis and on unit leaf weight basis), light transmittance, aboveground net primary productivity, and production efficiency (on unit leaf area basis, i.e., Earea; and on unit leaf weight basis, i.e., Emass) were quantified. ELTPs exhibited significant differences in canopy specific leaf area and aboveground net primary productivity. Although Earea varied significantly among ELTPs, Emass exhibited no significant differences. Variation in canopy specific leaf area was significantly correlated with ELTP site water balance (R2 = 0.93, p = 0.0083). Along a decreasing moisture gradient, canopy specific leaf area decreased, which resulted in an increase in canopy nutrient content per unit leaf area. Canopy nutrient contents (N, P, and K) in turn exhibited strong positive correlations with Earea (no strong correlations with Emass), which increased along a decreasing moisture gradient. Higher canopy nutrient contents coupled with higher light transmittance through the canopy probably results in a higher canopy photosynthetic efficiency (area basis) in dry ELTPs. This is suggested as one of the reasons for higher Earea in dry ELTPs than in mesic ELTPs.