© 1996 Heron Publishing—Victoria, Canada
Simulations of the effects of shoot structure and orientation on vertical gradients in intercepted light by conifer canopies
Pauline Stenberg
Department of Forest Ecology, P.O. Box 24, FIN-00014 University of Helsinki, Finland / Received March 2, 1995
Summary
Coniferous tree canopies typically carry more leaf area than is necessary to intercept most of the incoming light. I postulated
that an excessively large leaf area will reduce net productivity at tree level, unless the net photosynthetic production of
the most shaded shoots in the canopy remains positive. The hypothesis tested was that a coniferous tree canopy maintains a
large productive leaf area by increasing the efficiency of light capture as the available light decreases.
The light interception efficiency of a shoot was quantified by the ratio of shoot silhouette area to total needle surface
area (STAR). The STAR depends on shoot geometry and varies with shoot orientation relative to the direction of light. Shade
shoots have a larger STAR and, in particular, higher values of STARmax than sun shoots. In addition, shade shoots tend to be horizontally inclined, which may increase the advantage of a large
STARmax in the lower canopy, where radiation is incident from angles closer to the zenith. Adaptation to shade (changes in STAR and
shoot orientation) was described on the basis of empirical data for several coniferous species, and the vertical gradient
of seasonal light interception by shoots was simulated assuming different adaptive strategies. Simulations were performed
at two latitudes, to account for differences in the amount and directional distribution of light during the growing season.
Results support the hypothesis that increases in STAR, shoot zenith angle and shoot asymmetry (flatness) with shading increase
the efficiency of light interception by deeply shaded shoots. However, because competition for light among shoots increases
progressively as soon as shade acclimation occurs, there cannot exist a deep layer of shade shoots, such that the net productivity
of each shoot remains positive (i.e., irradiance is above the compensation point). Therefore, if maximization of productive
leaf area is the goal, the optimal strategy is to maintain an inefficient deep canopy and to increase light interception efficiency
only when shading becomes severe.
Keywords:
leaf area, productivity, shade acclimation, STAR.
