Processes Controlling Understorey Evapotranspiration

The understorey often accounts for a significant proportion of forest evapotranspiration. In this paper we discuss the role of the understorey radiation regime, and the aerodynamic and stomatal conductance characteristics of the understorey in understorey evapotranspiration. Values of the McNaughton-Jarvis parameter 12 for the understorey in two mid-rotation Douglas-fir stands indicate considerable coupling between the understorey and the atmosphere above the overstorey. However, the stronger coupling between the oveistorey and the atmosphere accounts for the observation that the fraction of stand evapotranspiration originating at the understorey increases as the water vapour pressure deficit increases and the soil dries. We also discuss the approaches to describing the process of evaporation from the forest floor and the results of understorey removal experiments. These show small decreases in stand evapotranspiration and root-zone soil water content, but significant increases in the transpiration and growth of the trees.