A method is described which estimates the photosynthetic products which are available for the growth of a plant within a canopy. The basic assumptions are that a plant can be treated as an integrator of the curvilinear relationship of photosynthetic rate on light, and that variation in photosynthetic rate of the leaves of community members can be reduced to the variation in the light intensity. Equipment is described which integrates light intensity, simulating the curvilinear relationship between photosynthesis and light intensity. A strong correlation was found between actual dry matter production and the photosynthetic production estimated by the present method.