Shade-shoot structure, photosynthetic performance in the field, and photosynthetic capacity of evergreen conifers.

Shoot structure can be quantified as the ratio of maximum shoot silhouette area to maximum leaf silhouette (projected) area (R(max)). I have used published studies on the effects of shade-shoot structure on photosynthetic production of evergreen conifer stands to test the hypothesis that the lower crown of stands of species with a high R(max) contributes significantly to photosynthetic production. Pruning studies were found inadequate to test this hypothesis rigorously. Eight studies that used cuvettes to measure photosynthetic production in different crown layers are reviewed. Of six studies on species with R(max) values larger than 0.7, five found significant rates of photosynthesis in the lower half of the crowns. In contrast, pines, which have a low R(max) and a low leaf area index (>/= 3.5), had low rates of photosynthesis in the lowest crown zone. In field-grown shoots of all species examined, photosynthetic capacity was negatively related to R(max).