Photosynthesis and canopy nutrition of four sugar maple forests on acid soils in northern Vermont

Abstract

Canopy nutrition, leaf chlorophyll concentration, and leaf CO2 assimilation capacity (Amax) were examined in sugar maple (Acersaccharum Marsh.) trees exhibiting symptoms of crown dieback in four stands on acid soils (pH ≈ 4.0) in northern Vermont. Leaf CO2 assimilation capacity was measured on foliage from detached and rehydrated branches harvested from the upper portion of the canopy. Leaf calcium (Ca) and magnesium (Mg) concentrations were among the lowest reported for sugar maple in its natural range. Total leaf chlorophyll concentrations of canopy leaves were lowest on the sites exhibiting the lowest leaf nitrogen (N) and Ca, and CO2 assimilation capacity was correlated with chlorophyll concentration among canopy leaves from all sites. Strong linear relationships were observed between leaf CO2 assimilation capacity per unit leaf mass and leaf N (r2 = 0.60) as well as leaf Ca (r2 = 0.51) among the four sites. On the basis of the observed strong correlation between leaf Ca and leaf N (r2 = 0.64) and the lack of clear enhancement of leaf CO2 assimilation capacity in trees fertilized with base cations (K, Ca, and Mg), it appears that leaf CO2 assimilation capacity and leaf Ca may not necessarily be functionally related. However, since low leaf CO2 assimilation capacity and photosynthetic N-use efficiency were common in unfertilized trees with low Ca (Ca < 0.6%), CO2 assimilation processes in sugar maple on acid soils may be limited by N and Ca × Mg interactions. The strongly acidic nature of the soils in these stands and the magnitude of acidic deposition in the region may precondition sugar maple trees on some sites to levels of cation deficiency that may be associated with low CO2 assimilation in the forest canopy.