Comparative tree growth efficiency in Sierra Nevada mixed-conifer forests

Tree growth efficiency, or volume growth increment per unit leaf area, was calculated in multiaged mixed-conifer stands in the Sierra Nevada, California. Five conifer species with a range of tolerance to shade were sampled over a wide range of canopy positions and ages to detect species-specific patterns of growth efficiency (GE). All species showed non-linear, sigmoidal trends of volume increment with increasing leaf area. This pattern indicates slow increase in GE in shaded understory conditions and a peak in GE at intermediate tree size in mid-canopy positions. Larger trees showed lower GE than intermediate size trees. Species with lower shade tolerance had greater GE than species with greater tolerance. To test the effect of local light environment on tree volume growth, we used a light model to calculate the growing season average of absorbed light for individual trees. Models of volume increment as a function of weighted leaf area showed increased GE for small trees, indicating that the slow increase in volume increment in smaller understory trees was an effect of limited light conditions. Patterns of growth efficiency in multiaged mixed-species stands can be used to predict growth patterns of sub-canopy trees and judge relative shade tolerance.