Open Access

Dry calcareous grasslands in Europe are renowned for their high plant diversity. However, declining habitat areas and highly fragmented distribution threaten the long-term persistence of this valuable habitat type. In Estonia the decline of traditional grassland management and subsequent encroachment of shrubs has resulted in a substantial loss of alvar grasslands – a particularly rare and species-rich type of calcareous grassland. It is known that a shrub cover of more than 70% decreases the alvar grassland species richness. At the same time, a shrub cover of 30% is considered optimal for alvar grasslands and thus a target state for habitat restoration. However, very little is known about the effect of low shrub cover on environmental conditions and species composition of alvar grasslands. Our aim was to detect to what extent the small-scale plant diversity and species composition is influenced by low shrub cover (less than 30%). We hypothesized that even a low shrub cover can have an effect on the environmental conditions of alvar grasslands. We sampled small-scale plant species richness, shrub cover and abiotic environmental conditions such as light, soil moisture, soil pH and soil depth in 10 metre long transects (n = 33) subdivided into 10 cm × 10 cm plots in Estonian alvar grasslands. Structural equation modelling was used to quantify the direct and indirect effects of shrub cover on the richness of characteristic alvar species and on the richness of generalist species. We found that low shrub cover of up to 30% increased total and generalist species richness directly and indirectly by increasing the light heterogeneity. Alvar characteristic species richness was not relat-ed to low shrub cover values. This suggests that when estimating the effects of shrub cover on species richness and on conservation needs of grasslands, habitat specificity of species needs to be taken into account.