Canopy Photosynthesis: History, Measurements, and Models

A plant canopy consists of an assemblage of plants with leaves that possess a particular spatial distribution and assortment of angle orientations (de Wit, 1965; Monsi et al., 1973). How a collection of leaves intercepts sunlight and uses light energy to assimilate CO 2 is the basis of canopy photosynthesis. The major factors affecting canopy photosynthesis, through light interception, include the angular relationship between leaves and Earth-sun geometry and the leaves' vertical and horizontal positions. Other factors affecting canopy photosynthesis include environmental conditions (temperature, wind speed, humidity, and CO 2 concentration), the life history of leaves, the availability of soil moisture and nutrients, stomatal conductance, and specific photosynthetic pathways (Schulze, 1986; Gutschick, 1991; Stenberg et al., 1995). The complexity of canopy photosynthesis can be illustrated with a closed canopy on a fair day during the growing season. Inside the canopy some leaves are fully sunlit, others are exposed to frequent sunflecks, and the remainder exist in deep, but occasionally punctuated, shade (Chazdon, 1988; Oker-Blom et al., 1991). Many sunlit leaves of C~ spetzies will experience saturated rates of carboxylation if CO 2 supply is limited (Farquhar and von Caemmerer, 1982). Furthermore, sunlit leaves are often warmer than shaded leaves. This situation promotes dark respiration (Amthor, 1994), decreases the solubility of CO 2 relative to O 2 (Farquhar et al., 1980), and decreases the CO 2 specificity factor of ribulose, bisphosphate, and carboxylase/oxygenase (rubisco) (Harley and Tenhunen, 1991). The photosynthetic response of leaves to sunflecks does not have the same relationship observed during steady light conditions. As the light exposure