Canopy Albedo And Transm1ittance In A Boreal Forest

Abstract

The shortwave radiation balance of a forest stand of predominately spruce and hemlock in central Maine was investigated. Pyranomder measurements of insolation, reflected solar radiation from the top of the canopy, radiation incident on the forest floor, and radiation reftect­ ed from the forest floor were taken under both cloudless sky condi­tions and overcast cloud conditions. Measurements were made on consecutive days; insuring minimal change in forest canopy character­istics. The canopy albedo computed from half-day radiation totals, thus representative of a daily value, was 10.8 % on the clear day and 9. 8 % on the overcast day. The instantaneous albedo on the clear day ranged from 8.7 % at 43° solar zenith angle (solar noon) to 20.1% at 78° solar zenith angle, while on the overcast day the albedo was relatively constant due to the diffuse irradiance condition. Canopy transmittance for the daily period was 2.9% on the clear day and 4.2% on the overcast day. The canopy transmittance was nearly constant throughout the day on the overcast day at about 4.2% while sunfleck penetration on the clear day resulted in a higher transmittance of 4.5% from 43-47° solar zenith angles, that dropped to 1.8% at 68-72° solar zenith angles. Thus over the entire day, for the range of solar zenith angles measured, diffuse radiation on the overcast day penetrated further down into the forest canopy than the predominately direct radiation on the clear day. Keywords: albedo, transmittance, forest, pyranometer.