Relationships between transmission of solar radiation and coniferous forest stand characteristics

Abstract

Transmission of global, direct, diffuse, and diffuse photosynthetically active solar radiation through coniferous forest canopies was studied on 12 plots during sunny days in summer. The proportions of radiation transmitted were a function of forest stand characteristics and differed among radiation components. The relationship between the proportion of diffuse radiation transmitted and stand characteristics differed among stand structures, but transmission of direct radiation did not differ. When direct beam radiation was scattered by open crowns of short trees and crowns extended within the height of radiation sensors, diffuse radiation within the canopy was higher than outside the stand. The extent to which different forest stand characteristics predict transmission and approximate a model analogous to Beer's law was explored. Sum of tree diameters and Reineke's stand density index were the better predictors of tranmission of global ( i 2=0.80−0.95) and direct radiation ( i 2=0.70−0.98). Overstory cover predicted transmission of diffuse radiation best ( i 2=0.74) when bases of tree crowns were above sensors. The same forest stand characteristics obtained by different sampling methods gave different predictabilities of the components transmitted. Differences were found between hemispherical photograph estimates of transmission of diffuse radiation and measured transmission of diffuse solar and diffuse photosynthetically active radiation. Photographic estimates of transmission of direct radiation differed from measured transmission of direct radiation.