LAUSR

This study deals with the relationship between the species richness of vascular plants in 10 × 10-km plots and climate indices in the Middle Volga Region. The most significant indices have been revealed: precipitation and temperature in March, temperature in October, and winter precipitation, with their combined effect accounting for 74% of variation in species richness. In view of relatively low climate gradients, a special function of total precipitation in March has been used in analysis, which describes the nonlinear dependence of the richness of vascular plants on the amount of March precipitation in the form of a peak. An ecological interpretation of these relationships is given and a species richness map is constructed. It is hypothesized that minor variations of intra-annual temperature and precipitation indices in the area with low climate gradients may become critical factors for the spatial change in the number of vascular plant species.