APPLYING SINGLE AND 2-LAYER CANOPY MODELS TO DERIVE CONDUCTANCES OF ASCOTS PINE PLANTATION FROM MICROMETEOROLOGICAL MEASUREMENTS

A single layer (Penman-Monteith) and a two layer (modified Shuttlewort h-Wallace) evapotranspiration (ET) model are used alternatively to der ive conductances related to the dominant fluxes of water vapor from a semi-closed Scots pine plantation. The derivations are based on microm eteorological measurements of above canopy energy flux densities and a simple resistance network. For a period of consecutive fine weather d ays, below canopy net radiation and below canopy ET were about 20 perc ent of the corresponding above canopy values. Resulting conductances f or latent heat flux agreed well with porometric measurements of pines and understory scaled to canopy level. The shift from single to two la yer modelling reduced the canopy conductance to pine conductance by th e fraction of understory ET. However, characteristics of porometer res ults and micrometeorologically derived conductances were quite differe nt: The porometer estimates of conductance were highly variable due to stomatal response to local environmental conditions or ''natural'' va riability within the tree canopy and vegetation patches which characte rized the forest understory. Micrometeorologically derived conductance s integrate spatially resulting in relatively smooth and repetitive da ily patterns that lack the information of small scale variability. Thi s is seen as a favorable feature of micrometeorological derived conduc tances when used for the parameterization of atmospheric models for cl imate research as long as small scale bio-diversity is irrelevant.