Tree–herb co-existence and community assembly in natural forest-steppe transitions

Background: The effects of trees on understorey communities is a major driver of vegetation composition. However, we have little understanding on how isolated forest patches of the forest-steppe transition affect their herb layer as compared to adjacent grasslands. Aims: Our aim was to test whether trees had a protective effect on understorey herbaceous communities in the most arid regions of the forest-steppe transition, where the stress gradient hypothesis predicts positive net effects. Methods: We surveyed herbaceous cover and species composition in 135 forest-steppe sites and recorded soil moisture, microclimate and canopy cover in northern Kazakhstan. Results: Total cover and species richness were lower in the herb layer of groves than in the steppe stands and the soil of the groves was not moister than that of the steppes. Groves were dominated by grassland specialist plants, while forest specialist species remained scarce. However, these grassland species were different from the ones inhabiting the steppes, leading to little nestedness between groves and steppes. Conclusions: Although the presence of groves greatly increase the landscape level diversity of Middle-Asian forest-steppes, trees in the southern edge of the zone seem to have net negative effect on the understorey vegetation, seemingly contradicting the stress gradient hypothesis