CANOPY STRATIFICATION AND LEAF-AREA EFFICIENCY - A CONCEPTUALIZATION

Species differences have been observed in the relationship between lea f area efficiency (E) and leaf area (LA) of individual trees within st ands. In some species, for example, E decreases with greater LA, sugge sting that the smallest trees in the stand are the most efficient. In other species E increases with greater LA, at least across a portion o f 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 efficienc y 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 mo re favorable light environment. However, carbon allocation patterns wi thin the tree also change with tree size; increased carbon requirement s for crown construction and maintenance, as well as root system devel opment 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 decl ines with further increases in LA. Pinus contorta stands are character ized by limited canopy stratification, lower leaf area index, and lowe r canopy coverage, resulting in few trees found in poor light conditio ns. Decreases in E with greater LA are presumably associated with chan ging carbon allocation patterns, primarily greater respiratory require ments per unit of leaf area. Abies lasiocarpa stands have greater cano py stratification, higher leaf area index, and greater canopy coverage , and this results in relatively small trees situated in poor light en vironments. Increases in E with greater LA are associated with improve d canopy position, and continue as long as height also increases. A ge neral model is presented that explains production-leaf area relationsh ips in forest-grown trees based on changes in relative tree size and c anopy structural characteristics.