LAI as the Driving Force of Canopy Exchanges

LAI describes a fundamental property of the plant canopy in its interaction with the atmosphere, especially concerning radiation, energy, momentum, and gas exchange. Stand function includes: (1) the rate of biological energy flux through the ecosystem, that is, rates of production and respiration; (2) the rate of material and nutrient cycling, that is, the biogeochemical cycles; and

(3) biological and ecological regulation, including prevention of soil erosion and regulation of water uptake, or radiation interception and conversion. LAI is the favored canopy variable because it is required for estimating many process rates, from canopy gas exchange to nutrient return in litterfall, including understorey microclimate control and competition for light, water, and mineral nutrients.

LAI acts as the canopy-atmosphere interface where water and carbon gas exchange occurs and is, therefore, a core parameter of biogeochemical cycles in ecosystems. Any change in canopy LAI, as a result of frost, storm, defoliation, grazing, drought, or management practice, is accompanied by modifications in stand productivity. Process-based ecosystem simulations require LAI as a key input parameter to produce quantitative analyses of productivity. When LAI of a community is low (<4), which is usually in arid environments or during the establishment of a crop, the transpiration rate (T/PET) is linearly related to LAI. Beyond this point, transpiration rate increases more slowly due to: (1) the saturation of canopy radiation interception and (2) soil water availability limitation (Figure 4). Growth rates are also dependent on LAI, but as LAI increases, the growth rate reaches a maximum value. Thereafter, it may decline. The existence of an optimum LAI was first observed for herbaceous plants. At the slope inflexion, called critical LAI, an increase in LAI and its associated CO2 uptake will not counterbalance the reduction of CO2 uptake in the existing leaf area because of self-shading. The community might still continue to gain in biomass, but at a lower rate. Plant growth and life form strongly affects optimum LAI depending on leaf angle, clustering, and vertical distribution of leaves resulting in differences in the self-shading and greater or lesser depths of penetration of light into the canopy.

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