26 - CO2 Elevation and Canopy Development in Stands of Herbaceous Plants

Abstract

This chapter describes a model designed to analyze the canopy structure of plant communities in many species. The model assumes that photosynthetic photon flux density (PPFD) attenuates exponentially through the canopy and that PPFD is intercepted by constituent species in proportion to their leaf area. One of the direct consequences of growth in dense stands is the increased intensity of competition for light. Under such conditions, the effects of changing CO2 concentrations—or levels of other resources—may be critically mediated by alterations in canopy structure and the costs and benefits of leaf placement in different strata of the community. The analysis of a model of optimal leaf area index (LAI) and nitrogen allocation, in relation to stand-level carbon gain, indicates that elevated CO2 levels should lead to only slight enhancements in LAI, compared to stands in ambient CO2 with similar total foliar nitrogen. However, the carbon gain of these stands is greatly enhanced because of the large enhancement in assimilation rates and nitrogen-use efficiency. CO2 responses of productive plant communities with high LAIs and high biological diversity play a particularly important role in the global carbon budget.