The albedo of a mature mixed forest and a clear-cut site at Petawawa, Ontario

Abstract

The results of a study of the seasonal variation of albedo (α) of a mature, mixed coniferous-deciduous forest and a clear-cut site, mainly covered with bracken fern, are presented. Diurnal and seasonal patterns of α have been documented, and the principal findings for the forest are: (1) the daily mean α varies from 0.12 to 0.15 when the canopy is in full leaf in the summer, (2) in the winter, the value of α falls to a minimum of 0.10 in the absence of snow, but immediately after a snowfall increases to ∼ 0.50, (3) the value of α decays quickly after a snowfall and averages ∼ 0.20 while snow remains on the ground beneath the canopy, (4) there are well marked shoulders of change in α in the spring and autumn as the radiation balance of the forest adjusts to leaf development and leaf fall. When the clear-cut site is fully vegetated in the summer, α varies from 0.19 to 0.20 depending upon the macroclimatic conditions, but when the bracken fern dies in the autumn, α is reduced to 0.13, which is essentially the same as for the forest at this time. An analysis of hourly data from the summer revealed that α depends upon zenith angle for both sites, and these dependencies are strongly controlled by sky condition and canopy wetness; they are strongest for clear skies and dry canopy states.