Microclimate at Arctic Tree Line 1. Radiation Balance of Tundra and Forest

Components of the radiation balance were measured or calculated for upland tundra and adjacent open spruce forest for the period from April to September 1979 at Churchill, Manitoba, Canada, which is positioned near the tree line and which is underlain by continuous permafrost. Because of deep winter snow accumulations, snow melt in the forest lags behind that of the tundra by almost a month. The tundra undergoes a stepwise change in albedo from 0.78 to 0.09 over a period of a few days of final snow melt. Its albedo is constant over the summer. On clear days for both surfaces there is a strong diurnal pattern of raised albedos in morning and evening due to specular reflection. On cloudy days there is almost no daily change. The tundra radiative surface temperatures are almost always larger than those of the forest, but differences decrease systematically as the summer progresses. In late winter, on clear days, the forest canopy averages 6.6°C warmer than the surrounding air and 12.6°C warmer than the snow beneath. This results from an absorption of incident and reflected solar radiation by the nontranspiring trees. In winter the forest canopy serves as a convective and radiative heat source to the snow beneath. This hastens snow melt in later winter. The forest has a larger net radiation than the tundra at all times, but this is particularly pronounced with the large solar radiation input of late winter before any snow melt takes place. Destruction of the forest will lead to a greatly modified radiation and surface temperature regime.