Land-atmosphere energy exchange in Arctic tundra and boreal forest: available data and feedbacks to climate

This paper summarizes and analyses available data on the surface energy bal ance of Arctic tundra and boreal forest. The complex interactions between e cosystems and their surface energy balance are also examined, including cli matically induced shifts in ecosystem type that might amplify or reduce the effects of potential climatic change. High latitudes are characterized by large annual changes in solar input. Al bedo decreases strongly from winter, when the surface is snow-covered, to s ummer, especially in nonforested regions such as Arctic tundra and boreal w etlands. Evapotranspiration (Q(E)) of high-latitude ecosystems is less than from a freely evaporating surface and decreases late in the season, when s oil moisture declines, indicating stomatal control over Q(E), particularly in evergreen forests. Evergreen conifer forests have a canopy conductance h alf that of deciduous forests and consequently lower Q(E) and higher sensib le heat flux (Q(H)), There is a broad overlap in energy partitioning betwee n Arctic and boreal ecosystems, although Arctic ecosystems and light taiga generally have higher ground heat flux because there is less leaf and stem area to shade the ground surface, and the thermal gradient from the surface to permafrost is steeper. Permafrost creates a strong heat sink in summer that reduces surface temper ature and therefore heat flux to the atmosphere. Loss of permafrost would t herefore amplify climatic warming. If warming caused an increase in product ivity and leaf area, or fire caused a shift from evergreen to deciduous for est, this would increase Q(E) and reduce Q(H). Potential future shifts in v egetation would have varying climate feedbacks, with largest effects caused by shifts from boreal conifer to shrubland or deciduous forest (or vice ve rsa) and from Arctic coastal to wet tundra. An increase of logging activity in the boreal forests appears to reduce Q(E) by roughly 50% with little ch ange in Q(H), while the ground heat flux is strongly enhanced.