Energy utilization and carbon metabolism in mediterranean scrub vegetation of Chile and California : II. The relationship between photosynthesis and cover in chaparral evergreen shrubs.

Two concepts of vegetation function and organization are examined in this paper. The first concerns the possible relationship between energy acquisition and species dominance in a given growth form and vegetation type. The second questions whether homeostatic mechanisms operate at sufficient levels to yield similar and positive photosynthetic performance of evergreen shrubs across a marked environmental and geographical gradient in southern California. This gradient runs from the coast at 135 m to 1,435 m in the mountains and on to 1,150 m at the margin of the desert transition.Total photosynthesis in several species was estimated at each site from late spring through summer. Within a common growth form, species success correlated with photosynthetic capacity. The species with the greatest carbon uptake were best represented in the vegetation. This relationship would not be expected to hold either between widely differing growth forms or where other factors exclude or decrease the abundance or productivity of certain of the species.Across the gradient examined, species displayed a marked degree of photosynthetic homeostasis despite marked environmental changes. For example, photosynthetic uptake in Adenostoma fasciculatum varied by only 32% between the greatest and least productive areas despite large differences in precipitation and temperature. In most cases there was a strong correspondence between cover and photosynthesis across the transect studied.From the available data, the transitions from chaparral to coastal sage, montane forests, or desert transition vegetation do not appear to result from sharp suppression of photosynthetic uptake at these borders. Other factors such as those affecting germination and establishment may be responsible for these transitions. However, further investigations are required at the edges of species distribution to adequately describe the pattern that does exist and to ascertain the relationship between energy capture and species success in these areas.