Observations on the role of treeline in controlling atmospheric circulations

Previously it has been considered that the position of the northern limit of boreal tree line was controlled by the summertime position of the Arctic front. Recently, however, Pielke and Vidale (1995) have suggested that instead the position of the northern tree line may influence the position of the Arctic front through contrasts in surface heating over the tree line. This study expands on their work by providing observational measurements on the energy balance contrasts between Arctic tundra and Boreal forest at Council on the Seward Peninsula, Alaska. Here the boreal forest had a lower albedo and hence absorbed more incoming radiation. The forest also had less ground heat flux due to greater canopy closure, resulting in generally larger sensible and latent heating over the forest. Midday values of sensible heat flux averaged 250 Wm ^-2, which was around 40 Wm ^-2 greater than the adjacent tundra. Measurements of boundary layer profiles suggest that this increased heating causes deeper and drier boundary layers over the forest. These ecologically induced contrasts may impose a significant influence on climate through the manipulation of synoptic scale weather features such as the Arctic front. These biotic influences need to be considered in assessing possible future vegetation and climate change in the arctic.