The Sky Canopy Gap Irradiance (SCANGIR) Model: a technique for the estimation of ‘effective gap’ solar radiation regimes in closed-canopy forests

There is now widespread consensus among researchers that treefall gaps play a key role in the structure and function of closed-canopy forests, such as rain forests, with virtually all canopy tree species requiring growth in a gap to reach maturity. Until recently many believed that latitude and the size of treefall gaps were the main factors determining the availability of solar radiation in gaps and associated microclimates and vegetation dynamics. However, if solar radiation availability is the main limiting resource in such forests, then studies of gap dynamics need to take into account the other factors affecting the penetration and distribution of direct and diffuse irradiance within gaps. As a consequence, this is the first attempt to develop a universal model for the estimation of ‘effective gap’ solar radiation regimes that, along with latitude and gap size, incorporates factors such as slope angle, slope aspect, height of surrounding vegetation, sky conditions and daily and seasonal variation in the position of the sun. Results of three computer simulations are presented to demonstrate how latitude, topography and sky conditions affect solar radiation availability beneath circular canopy gaps of various configurations. Finally, the implications of these simulations for forest regeneration, reforestation and forest management are discussed.