Interpreting Woody Plant Richness from Seasonal Ratios of Photosynthesis

In forested portions of Oregon, species richness of woody plants on plots 400–500 m2 ranged from 1 to .20. We investigated what might account for this variation using a satellite-driven process model that predicts gross photosynthesis and establishes, on a monthly time step, the most constraining environmental variable. Independent satellite and ground-based data confirmed that the highest species richness occurs on sites of intermediate productivity, where 60–70% of the light is intercepted by vegetation. We demonstrated that most photosynthesis takes place during the spring and summer months for both evergreen and deciduous species. We estimated that the spring to summer ratio of gross photosynthesis (DPG) varies from ,1 to .5 across the state. Both the most productive coastal rainforests and least productive arid woodlands exhibited the lowest values of DPG near 1, and had lowest species richness. Plots with highest species richness were located in areas with mild, moist, spring weather conditions, followed by a summer drought, with DPG averaging above 3. Satellite-derived estimates of gross photosynthesis are available for more extensive analysis.