Chapter 5 - The Carbon Balance of Trees and Stands

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Radiation interception and biomass production by forest stands are the fundamental plant ecosystem processes. Radiation interception depends on the leaf area index and canopy structure, while canopy photosynthesis depends on the photosynthetic characteristics of the foliage and is modified by stomatal behavior. Moreover, the photosynthetic characteristics of leaves are determined by the nitrogen distribution in the canopy, and there is strong observational and theoretical evidence that this distribution tends to optimize whole-canopy production. This chapter considers how canopy structure, leaf area, and photosynthetic properties can be coupled using commonly used and useful radiation interception models to give estimates of tree and canopy biomass production. Canopy production is a complex process because foliage is distributed throughout a canopy, and the main factors that influence the photosynthetic production–radiation, temperature, vapor pressure deficit, leaf nutrient status–vary temporally and spatially. Predicting canopy production thus requires integration of the equations describing photosynthesis over time and space, and the determination of how these environmental factors vary within the canopy in response to transpiration. Complete energy balance and photosynthesis modeling has been accomplished, and is outlined later. However, more simplified models suffice for most practical purposes, and especially when production is required for a stand over an extended period of time. These are the main subject of this chapter. Various factors impose constraints on canopy productivity, operating through both light interception and photosynthesis. This chapter considers how these processes can be coupled through commonly used and useful radiation interception models to give estimates of tree and canopy biomass production. In particular, the extent and distribution of foliage in a canopy clearly affect the amount of light intercepted by the canopy, with shading by the upper canopy and by neighboring trees playing an important role. But despite shading effects, some leaves deep in the canopy experience full sunlight some of the time, and the difference between the sun and shade leaves must be taken into account.