In many climatic regions the interface between the earth and the atmosphere consists of a mosaic of ecosystems, each possessing specific properties of absorbing and storing energy and matter. Large contrasts in these properties create differences in interface temperature and moisture, which, in combination with contrasts in roughness, produce differences in sensible and latent heat fluxes injected into the atmosphere, so that the boundary layer experiences rapid alterations to its structure and transport of energy and matter. In consequence, good data on fluxes from individual ecosystems and their contrasts are essential both to understanding the functioning of the boundary layer and energy/mass accounting at source and sink ecosystems.