Leaf nutrition and photosynthetic performance of sugar maple (Acer saccharum) in stands with contrasting health conditions

Leaf nurition and photosynthetic performance of sugar maple (Acer saccharum Marsh.) were compared between two sugar maple stands in northwestern Vermont with contrasting health conditions as indicated by annual basal area growth, degree of crown dieback, and foliar appearance. Observations made during the diurnal cycle of both stands showed no apparent leaf water stress. In both stands, leaves had similar concentrations of major non-structural carbohydrates (starch and sucrose). Over two consecutive growing seasons (1991 and 1992), we consistently observed lower leaf Ca and Mg concentrations in the declining stand than in the healthy stand. Compared with the healthy stand, lower leaf chlorophyll concentrations and apparent leaf chlorosis were observed in the declining stand, and some trees had very low foliar Ca and Mg concentrations (0.31 ± 0.03% and 0.09 ± 0.01%, respectively). Trees in the declining stand had lower light-saturated net photosynthetic rates on a dry mass basis at both ambient CO₂ (P(n,amb)) and saturating CO₂ (P(n,sat)) than trees in the healthy stand. There were significant linear correlations between P(n,amb) and leaf mass per unit area (LMA) and between P(n,sat) per unit leaf area and LMA. There were also linear correlations between both P (n,amb) and P(n,sat) and leaf N when expressed on an area basis in both stands, indicating that variation in LMA may have been largely responsible for the observed photosynthesis- nitrogen relationship. The values of P(n,amb) and P(n,sat) were not significantly correlated with leaf N on a mass basis but were weakly correlaled with leaf Ca and Mg on a mass basis. We conclude that low leaf Ca or Mg concentrations may limit leaf CO₂ assimilation and tree carbohydrate status in the declining sland.