Sensible and latent heat fluxes from the Boreal Ecosystem and Atmosphere Study (BOREAS) tower flux sites in 1994 are analyzed over both diurnal and seasonal cycles. We compare and contrast the southern and northern study areas and the behavior of five different land covers. For each land cover the evaporative fractions and surface conductances to water vapor are higher in the south than in the north, with the ranking from largest to smallest: aspen, fen, black spruce and jack pine. The conifer and, particularly, the jack pine sites show the greatest stomatal control of transpiration, as the vapor pressure deficit increases from morning to afternoon and as the soil dries during periods with low precipitation. The relation between surface conductance and the Priestley-Taylor coefficient α is consistent between southern and northern study areas but varies among land covers. The aspen and fen sites have higher α values than the landscape mean, and the mature conifer sites have lower α values than the landscape mean. We attribute the differences to the impact of spatial heterogeneity at the landscape scale. ¿ 2001 American Geophysical Unio