On measuring and modeling energy fluxes above the floor of a homogeneous and heterogeneous conifer forest

Information on mass and energy exchange at the soil surface under vegetatio n is a critical component of micrometeorological, climate, biogeochemical a nd hydrological models. Under sparse boreal and western conifer forests as much as 50% of incident solar energy reaches the soil surface. How this ene rgy is partitioned into evaporating soil moisture, heating the air and soil remains a topic of scientific inquiry, as it is complicated by such factor s as soil texture, litter, soil moisture, available energy, humidity defici ts and turbulent mixing. Fluxes of mass and energy near the forest floor of a temperate ponderosa pi ne and a boreal jack pine stand were evaluated with eddy covariance measure ments and a micrometeorological soil/plant/atmosphere exchange model. Field tests showed that the eddy covariance method is valid for studying the mea n behavior of mass and energy exchange below forest canopies. On the other hand, large shade patches and sunflecks, along with the intermittent nature of atmospheric turbulence, cause run-to-run variability of mass and energy exchange measurements to be large. In general, latent heat flux densities are a non-linear function of availab le energy when the forest floor is dry. Latent heat Bur densities (lambda E ) are about one-quarter of available energy, when this energy is below 100 W m(-2). Latent heat flux density (lambda E) peaks at about 35 W m(-2) when available energy exceeds this threshold. A diagnosis of measurements with a canopy micrometeorological model indicates that the partitioning of solar energy into sensible, latent and soil heat flux is affected by atmospheric thermal stratification, surface wetness and the thickness of the litter la yer. (C) 2000 Elsevier Science B.V. All rights reserved.