Annual and seasonal variations in photosynthetic capacity of Fagus crenata along an elevation gradient in the Naeba Mountains, Japan

Canopy photosynthetic capacity, measured as leaf maximum carboxylation rate (V-cmax), is a key factor in ecosystem gas exchange models applied at different scales. We report seasonal and interannual variations in V-cmax of natural beech stands (Fagus crenata Blume) along an altitudinal gradient in the temperate climate zone of Japan. Estimates are based on 6 years of gas exchange measurements. Pronounced seasonal and interannual variations in V-cmax normalized to 25 degrees C (V-c,V-25) were found for sun leaves. The seasonal pattern of V-c,V-25 generally followed an inverse parabolic curve, with an increase in spring, peak values in the middle of the growth period and a decline in autumn. Leaf nitrogen concentration (N-1) and leaf mass per area were significantly related to V-c,V-25 during spring and summer, but were unrelated in autumn when V-c,V-25 declined. Annual peak V-c,V-25 ranged from 40.1 to 97.0 mu mol m(-2) s(-1) and varied over as much as a twofold range at a particular site. Annual peak V-c,V-25 occurred about 28 days before annual peak N-1, with which it was poorly related. Our results show that it can be inappropriate to include constant values of photosynthetic parameters in ecosystem gas exchange models.