Factors Controlling Forest Atmosphere Exchange of Water, Energy, and Carbon

Abstract

Forests play an important role in the water and energy balance of the land surface. It has been known since the early studies of Horton (1919) and Rutter (1975) that the water use of forests can be considerably higher than that of vegetation of different structure and height. Subsequent work has elucidated the main factors influencing this behavior (e.g., Shuttleworth and Calder 1979; Shuttleworth 1989). They showed that the combination of a high aerodynamic roughness, with a relative low and strongly controlled surface resistance, was the main cause for high evaporation rates from wet canopies and somewhat low transpiration rates from dry canopies. They also suggested that care must be given to the separate modeling of dry and wet canopy evaporation, and that total evaporation could not simply be derived from equations relating total evaporation to a net radiation estimate. Building on that work, numerous modeling studies at regional and global scale have provided evidence that the interaction of the forests with the atmosphere is a major component in shaping regional to global climate and weather (Nobre et al. 1991; Blyth et al. 1994). Forests not only use more water by evaporating more, they also influence the rainfall patterns and magnitude at regional and global scales by increasing the low level moisture convergence (supply of moisture through horizontal advection in the lower layers of the atmosphere).