ENERGY PARTITIONING AT TREELINE FOREST AND TUNDRA SITES AND ITS SENSITIVITY TO CLIMATE-CHANGE

Summertime energy budgets of contiguous wetland tundra and forest near Churchill, Manitoba along the coast of Hudson Bay were measured over a five year period, 1989-1993. An examination of differences in energy budgets between the two sites showed that net radiation was similar i n all years. Soil heat flux was greater at the tundra site in most, bu t not all, years. However sensible heat flux was always larger at the forest site and latent heat flux was always greater at the tundra site . Mean daily Bowen ratios at both sites were less than unity in all ye ars. Average Bowen ratios for the five rears were 0.45 for tundra and 0.66 for forest. Wind direction is used as an analogue for changing cl imatic conditions where onshore winds are cooler and moister than offs hore winds. Sensible and latent heat fluxes at both sites varied signi ficantly between onshore and offshore wind regimes. However, differenc es between onshore and offshore fluxes at the tundra site were larger than for the forest. Thus, Bowen ratios also varied more at the tundra site. We have plotted the ratio of tundra-to-forest Bowen ratios as a measure of the relative sensitivity of energy partitioning to climati c change. The ratio decreases with increasing vapour pressure deficit (and increasing air temperature). We interpret these results as sugges ting that energy partitioning over the wetland tundra is more sensitiv e to changes in climate than the treeline forest environment Thus, as the climate warms and becomes drier, more additional energy goes into evaporation of water from the wetland tundra than from the forest.