SUBGRID-SCALE VARIABILITY IN THE SURFACE-ENERGY BALANCE OF ARCTIC TUNDRA

Surface fluxes of energy, water vapor, and CO, over homogeneous areas of the major tundra vegetation types in arctic Alaska were measured us ing a mobile eddy covariance tower for 5-day periods in the middle of the 1994 growing season. In order to account for differences in weathe r and time of season, data were analyzed in comparison to a nearby, fi xed tower that operated throughout the summer. Among the different veg etation types, evaporation ranged from 1:3 to 2.7 mm d(-1). Net carbon uptake ranged from 0.5 to 2.4 g C m(-2) d(-1). Ground heat flux consu med 10-33% of midday net radiation. Typically, 38% of the net radiatio n was partitioned into latent heat flux, while the fraction of net rad iation removed from the surface in sensible heat flux varied from 16 t o 50% among vegetation types. The largest differences among vegetation types in surface energy partitioning were related to variations in so il moisture, with midday Bowen ratios ranging from 0.37 over wet sedge tundra to 2.25 over dry heath. Direct effects of vegetation on the dr iving gradients for energy and water vapor exchange were important in shrub tundra: shading of the moss layer by the canopy reduced ground h eat flux and increased sensible heat flux, while latent heat flux was similar to other mesic vegetation types because the moss layer account ed for a larger portion of total evaporation than did evapotranspirati on by shrubs. Scaling up from the vegetation types to the Alaskan arct ic using an area-weighted average of the observed energy partitioning gave results similar to regional energy budgets measured over larger, more heterogeneous areas of tundra. An extrapolation based on the hypo thesis that climate variability could cause a large fraction of the cu rrent tussock tundra vegetation to be converted to shrub tundra result ed in modest changes in the regional energy balance. However, nonlinea r variations of surface evaporation with leaf area and uncertainties r egarding changes in moss cover suggest that additional field experimen ts as well as modeling efforts will be required to predict the potenti al for changes in arctic tundra vegetation to feed back on regional cl imate.