Surface energy balance and surface temperature sensitivity in northern boreal ecosystems

Understanding the surface energy balance (SEB) is the key to assessing ecosystems’ ability to modify their local climate, and how sensitive they are to changes in their biophysical properties. Atmospheric interactions connect the local phenomena to the global climate system.

We assessed long-term measurement data from six northern boreal sites, three mires and three forests between 67 and 69°N. The results complement existing knowledge by presenting SEB data from areas previously under-represented in the literature. The importance of these high-latitude ecosystems is increasing as areas north of the Arctic Circle warm faster than the globe.

Ecosystem available energy was largely defined by the shortwave albedo, which was lower for the darker, coniferous tree-dominated forests than for the open mires. Daily radiative energy did not decrease towards the north during summer as the long polar day compensated for the lower intensity, but lower temperatures during spring and autumn made the seasonal cycles more pronounced than in the more southern areas. The wet, moss-dominated mires were characterised by a low Bowen ratio and high evaporative fraction, low bulk surface resistance to evapotranspiration and high surface layer decoupling. In the forests, evapotranspiration was more tightly controlled by the vegetation, which was sensitive to the soil water content at the pine-dominated sites. High decoupling was observed to occur in the forests during humid conditions.

Mires were more sensitive to changes in their surface properties due to their low vegetation, high aerodynamic resistance and latent heat-dominated energy partitioning, but the sensitivities depended on the season.