The effect of a seasonal change in canopy structure on the photosynthetic efficiency of a salt marsh.

A seasonal decline in the photosynthetic efficiency of a Chesapeake Bay salt marsh occurs concurrently with a seasonal shift in foliage canopy structure from erect- to horizontal-leaved. Infrared gas analysis was used to measure net photosynthesis by isolated sections of marsh before and after the canopy was subjected to experimental manipulations designed to mimiic the natural canopy change. During June and July when the canopy was normally erect, inducing horizontal leaf orientation reduced net photosynthesis by 50 percent. In late August, when the canopy was naturally horizontal, making the leaves erect increased photosynthesis. These results support the hypothesis that the normal seasonal decline in photosynthesis is due, at least in part, to the shift in canopy structure.