Influence of solar energy and tree-crown closure on the species richness of grasses at the south of forest steppe

The perpendicularity of sunlight incidence on the land surface, called slope insolation, is calculated as the nonlinear function of steepness and exposure. This variable better describes the light and thermal regimes of slopes. We demonstrate that grass-cover insolation can be estimated based on the slope insolation and tree-crown closure. It is found for a terrain at the southern boundary of the forest steppe that species richness and green mass of grasses are closely related to topography and grass-cover insolation (R 2 = 0.77 and R 2 = 0.83, respectively), and crown closure is closely related to topography and slope insolation from the south (R 2 = 0.85). A critical level of crown closure (15%) is determined, so that the limiting factor for grasses is soil moisture below and light above this level. It is shown that grass-cover insolation close to its average value (400 W/m2) differentiates phytocenotic and soil characteristics into areas of increased and diminished values.