Controls on latent heat flux and energy partitioning at a peat bog in eastern Canada

Surface and atmospheric controls on latent heat flux (QE) and energy partitioning were assessed during three growing seasons at the Mer Bleue peat bog. The surface consisted of a sparse canopy (maximum leaf area index ∼1.3) of low, mostly evergreen shrubs over moss-covered hummocks and hollows. Available energy was partitioned mostly to QE (Bowen ratio often less than 0.5) throughout the growing seasons over an extensive range of water table fluctuation (as much as 50 cm). QE was often at or below the equilibrium rate due to surface (low moss water content, strong vascular stomatal control) and/or atmospheric (low vapour pressure deficit (Da)) factors. Turbulent energy fluxes varied with net radiation and the magnitude of the fluctuations were affected by Da and moss water content. It is suggested that a change in source partitioning for QE led to a change in QE − Da dynamics. Early in the growing season the moss was wet and the vascular canopy was replacing leaves, thus QE increased as Da increased because moss, which reacts passively to Da, contributed strongly to QE. Later in the growing season as water table declined and the evaporation load reduced moss and fibric peat water contents, moss contributed less strongly to QE and vascular contribution became more important. Also, stomatal control became more influential in reducing bulk surface conductance for water vapour and QE in response to increasing Da.