Light absorption and use efficiency in forests: Why patterns differ for trees and stands

The production of stem wood by trees and stands depends on the absorption of light, and the efficiency of converting light into stem wood. Larger trees within a stand tend to absorb more light and use it more efficiently in growing wood; greater growth of large trees typically results from a combination of increased light absorption (about three-fourths of the effect) and increased efficiency of light use (about one-fourth of the effect). Similarly, more productive forests commonly show greater light absorption and higher efficiency of light use; differences of 50-80% are commonly reported for both light absorption and for light use efficiency in comparisons of forests that differ in species composition, site fertility, and silvicultural treatments. These quantitative assessments of production ecology require estimation of light absorption at the appropriate scale, because patterns of light absorption in relation to leaf area differ fundamentally for trees and stands. Three types of relationships between light absorption and leaf area are important, and the typical patterns differ among types. The absorption of light through the crown of an individual tree (Type I) typically follows a logarithmic trend where each successive layer of leaves absorbs a consistent proportion of incident light. This pattern is often related to Beer's Law for absorption of light. The second type of relationship considers light absorption in relation to leaf area index within a set of stands; Type II comparisons of light absorption across ranges of stand leaf area indexes do not usually show a logarithmic trend, and expectations for the pattern among stands should not be based on Beer's Law. The third type of relationship focuses on total light absorption as a function of a tree's total leaf area; Type III comparisons for sets of trees within a stand generally show linear (or even exponential) increases in light absorbance with increasing leaf area, again deviating from an (inappropriate) application of Beer's Law. This distinction in patterns between light absorption and leaf area of trees and stands is particularly important when hypotheses are tested about the stem growth/leaf area (often termed growth efficiency, or leaf area efficiency). At the stand level (Type I), the logarithmic (or flat) relationship between light absorption and leaf area index may lead to different outcomes for hypotheses tested as a function of light absorption and those tested in relation to leaf area. At the level of individual trees (Type III), hypothesis tests based on light absorption or leaf area will provide similar answers, owing to the linear (or slightly exponential) relationship between light absorption and leaf area. (C) 2011 Elsevier B.V. All rights reserved.