Factors controlling evaporation and energy partitioning beneath a deciduous forest over an annual cycle

The energy balance components were measured above the ground surface of a t emperate deciduous forest over an annual cycle using the eddy covariance te chnique. Over a year, the net radiation at the forest floor was 21.5% of th at above the canopy, but this proportion was not constant, primarily becaus e of the distinct phenological stages separated by the emergence and senesc ence of leaves. The dominant response to seasonal changes in net radiation was through corresponding changes in the sensible heat flux, and both net r adiation and sensible heat flux peaked just before leaf emergence. Evaporat ion at the forest floor was typically less than 0.5 mm per day, and unlike sensible heat flux, was not closely coupled to seasonal changes in net radi ation. Instead, evaporation at the forest floor responded primarily to rapi d changes in litter water content. Forest floor evaporation was limited by the water-holding capacity of litter, and when the atmospheric demand was l arge, the litter layer dried on the time scale of several hours. After this rapid period of drying, net radiation and sensible heat flux dominated the energy budget. When leaves were present during the growing season, the sensible and latent energy fluxes at the forest floor were less than 10% of the total canopy f luxes, and the mean Bowen ratio was similar to that above the canopy. Howev er, during the dormant season, the controls of the energy budget at the for est floor largely determine the whole canopy fluxes. On an annual basis, th e fluxes from the forest floor are roughly 15-22% of those above the canopy and the evaporation was 86 mm. (C) 2000 Elsevier Science B.V. All rights r eserved.