Canopy stratification and leaf area efficiency: aconceptualization. For. Ecol. and Management

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For. Ecol. & Management



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Species differences have been observed in the relationship between leaf area efficiency (E) and leaf area (LA) of individual trees within stands. In some species, for example, E decreases with greater LA, suggesting that the smallest trees in the stand are the most efficient. In other species E increases with greater LA, at least across a portion of the range of individual tree LA found in a stand. Rather than being distinct physiological behaviors, we propose a single model to explain these observed differences in E-LA relationships. Leaf area efficiency is affected by two countervailing factors. As tree size, relative to neighboring trees, increases with greater LA, E tends to increase, as a result of a generally improved canopy position, and presumably a more favorable light environment. However, carbon allocation patterns within the tree also change with tree size; increased carbon requirements for crown construction and maintenance, as well as root system development and stemwood respiration, result in less fixed carbon available for stem growth and, therefore, lower E. We examined two species with reported differences in E-LA relationships. In Pinus contorta stands, E decreases with increasing LA, whereas, in Abies lasiocarpa stands, E increases with LA up to some intermediate level of LA, and then declines with furtther increases in LA. Pinus contorta stands are characterized by limited canopy stratification, lower leaf area index, and lower canopy coverage, resulting in few trees found in poor light conditions. Decreases in E with greater LA are presumably associated with changing carbon allocation patterns, primarily greater respiratory requirements per unit of leaf area. Abies lasiocarpa stands have greater canopy stratification, higher leaf area index, and greater canopy coverage, and this results in relatively small trees situated in poor light environments. Increases in E with greater LA are associated with improved canopy position, and continue as long as height also increases. A general model is presented that explains production-leaf area relationships in forest-grown trees based on changes in relative tree size and canopy structural characteristics.