Energy partitioning in relation to leaf area development of short-rotation willow coppice

Recently, deciduous forests have been included in more sophisticated climate models as one of the various vegetation types. As the canopy of a deciduous forest changes considerably during the growing season, knowledge of how the energy balance components are affected by the developmental stage of the canopy is important for proper simulation of climate. From previous studies of the surface energy balance for a willow short-rotation forest it was concluded that partitioning of energy between the components varied clearly with time. In this study the leaf area index (L) was used as an index of canopy development and the effect of leaf area changes on the energy partition was analysed. It was found that at the beginning of the growing season, when the stand was leafless, the maximum hourly Bowen ratio had a value of about 5.0. A rapid decrease in Bowen ratio with increasing L was observed during the period when L changed from zero to 1.5. Later, the Bowen ratio was seemingly independent of L and the typical hourly values ranged from - 0.5 to 0.5. In similar manner, the evaporation ratio, defined as latent heat flux over net radiation, increased from about 0.1 to unity when L increased from zero to 1.5, and thereafter the dependence on leaf area was much smaller. It was also shown that the amplitude of the soil heat flux decreased with increasing L. For similar clear days, the daily maximum soil heat flux decreased from 15% to 6% of the maximum net radiation when L increased from 0.2 to 4.0. A good coupling between latent heat flux and net radiation was found when L exceeded 2.0. The ratio of actual evaporation or transpiration to potential (Penman) evaporation as a function of L was compared for various agricultural crops and for the willow forest. In general, the ratio increased up to an L value of 3-4, and thereafter the dependence was much smaller. The difference in threshold values found for direct energy partitioning estimates and relative evaporation, respectively, was tentatively proposed to be caused by the improper definition of potential evaporation for a developing canopy.