Energy Conversion and Use in Forests: An Analysis of Forest Production in Terms of Radiation Utilisation Efficiency (ɛ)

Abstract

The linear relationship between the photosynthetically active solar radiation (PAR) absorbed by forest canopies (APAR) and the production of dry mass by forests provides a simple, robust model with only one parameter for the estimation of forest production. The slope of the relationship is normally denoted epsilon . The epsilon model has been developed from plant production studies and is soundly based physiologically. It has also evolved from remote sensing studies. Although the relationship between APAR and canopy photosynthesis may be highly variable over short periods, it remains constant over longer periods, such as months or seasons. The effectiveness with which canopies convert energy into dry mass depends on environmental conditions such as temperature, water status and nutrition. Values of epsilon derived experimentally, in terms of net primary production (NPP) and APAR, range from 2.8 g MJ-2 for agricultural crops to a low value of 0.2 g MJ-2 for tropical rain forests. Values for other forests tend to lie in the range of 1-2 g MJ-2. Modifying factors with values between 0 and 1, dependent upon the availability of water in the soil, atmospheric vapour pressure deficits or nutrition, are used to account for the effects of these variables on epsilon . If the modifiers are applied to epsilon , the result is an estimate of potential epsilon . Analysis of experimental results, therefore, should lead to the identification of optimum, unconstrained values of epsilon , which may vary considerably less than the current experimental values. Since the proportion of incident energy absorbed by a plant canopy depends on the leaf area index (L*) of the canopy, good estimates of L* provide a sound basis for estimating APAR. Strong statistical relationships exist between reflectance rations in the infrared, newer-infrared and visible spectra, monitored from space, and L*, but these vary with vegetation type and surface conditions. More important are recent developments showing that the fraction of incoming energy absorbed by a canopy can be calculated directly from canopy reflectance properties measured from space. These allow direct application of the epsilon model. As knowledge of the factors causing variation in radiation utilization by canopies improves, along with the capacity to estimate environmental conditions and their modifying effects on radiation utilisation, it should be possible to use the epsilon model with remotely sensed data to estimate forest productivity over large areas