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

Abstract

Summary This paper summarizes and analyses available data on the surface energy balance of Arctic tundra and boreal forest. The complex interactions between ecosystems and their surface energy balance are also examined, including climatically 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. Albedo decreases strongly from winter, when the surface is snow‐covered, to summer, especially in nonforested regions such as Arctic tundra and boreal wetlands. Evapotranspiration (QE) of high‐latitude ecosystems is less than from a freely evaporating surface and decreases late in the season, when soil moisture declines, indicating stomatal control over QE, particularly in evergreen forests. Evergreen conifer forests have a canopy conductance half that of deciduous forests and consequently lower QE and higher sensible heat flux (QH). There is a broad overlap in energy partitioning between 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 temperature and therefore heat flux to the atmosphere. Loss of permafrost would therefore amplify climatic warming. If warming caused an increase in productivity and leaf area, or fire caused a shift from evergreen to deciduous forest, this would increase QE and reduce QH. Potential future shifts in vegetation would have varying climate feedbacks, with largest effects caused by shifts from boreal conifer to shrubland or deciduous forest (or vice versa) and from Arctic coastal to wet tundra. An increase of logging activity in the boreal forests appears to reduce QE by roughly 50% with little change in QH, while the ground heat flux is strongly enhanced.

The surface energy balance of a polygonal tundra site in northern Siberia – Part 1: Spring to fall

“…During the entire observation period, the ground heat flux is a large component of the surface energy balance. About 20% of the net radiation is stored as latent and sensible heat in the ground, which is in the upper range of the typical values reported for other arctic permafrost regions (Boike et al, 1998;Lynch et al, 1999;Eugster et al, 2000;Westermann et al, 2009). The high contribution of ground heat flux to surface energy balance is caused by the cold permafrost temperatures, the shallow active-layer depth and the large annual surface temperature amplitude, which is related to the continental climate conditions.…”

“…Many publications have demonstrated the value of such regional studies, which show that measurements on energy and water balance are still needed to improve the parameterizations of climate models (van den Hurk et al, 2000;Betts et al, 2001Betts et al, , 2003. Several energy balance studies are already available for the North American Arctic (e.g., Ohmura, 1982Ohmura, , 1984, and more are contained in the comprehensive reviews by Eugster et al (2000) and Lynch et al (1999).…”

Controls on evapotranspiration in a west Siberian bog

“…Mean a values were 1.29 (±0.33) and 1.07 (±0.18) in the early growing season, 1.03 (±0.10) and 0.96 (±0.08) in the peak growing season (late June to mid-August), and 0.99 (±0.11) and 0.97 (±0.16) in senescence (after mid-August) in 1999 and 2000, respectively. Boreal wetlands commonly show a values nearing 1.0 [Hartog et al, 1994;Eugster et al, 2000;Eaton et al, 2001].…”

Inverse estimation of Vcmax, leaf area index, and the Ball‐Berry parameter from carbon and energy fluxes

“…[2] Evaluations of biogeochemistry and thermodynamics at an ecosystem scale with micrometeorological techniques are invaluable tools in understanding feedbacks between vegetation and the atmosphere [Baldocchi et al, 2000;Chapin et al, 2000;Eugster et al, 2000;McFadden et al, 2003;McGuire et al, 2002]. However, the value of these measurements of high temporal resolution is limited to the extent that meaningful parameters of ecosystem function can be derived from these measurements for general interpretation [Nichol et al, 2002;Running et al, 1999].…”