Soil-moisture conditions indicated by field-layer plants help identify vulnerable forests in the forest-steppe of semi-arid Southern Siberia

High variability in soil-moisture conditions is typical for semi-arid forest-steppe ecosystems where precipitation varies greatly over time. Plant species that inhabit these environments integrate responses to broadly fluctuating wetness conditions. Indirect assessment of contrasting habitat wetness based on plant indicator values, species frequency, and species coverage was carried out in two sites representing the larch (Larix sibirica) and pine (Pinus sylvestris) forest-steppe communities. For the larch forest-steppe, we found that plant community composition and spatial structure depended strongly on wetness. In addition, we found that the vegetation was clearly differentiated into forest stands and steppe communities, depending on the slope aspect. There was also a strong correlation between dissimilarities of species composition and differences in habitat wetness revealed in the larch forest-steppe. In contrast, soil properties, such as gravel and stone content were found to be a key factor in the spatial distribution of plant species composition in the pine-forest-steppe communities. Indirect assessment of moisture conditions in the forest-steppe habitats, based on the field-layer plant species, was found to be preferable for indicating soil water deficits in the forest. Furthermore, as long-term observational data is often lacking, indirect assessment of the forest-steppe vegetation provides an opportunity to identify vulnerable forests at the marginal distribution. Based on indirect assessments of soil-moisture conditions, and taking into account differences in potential drought resistance between larch and pine forests, we concluded that increasing aridity will cause the replacement of Siberian larch by Scots pine in the South Siberian forest-steppe landscape. Consequently, in the future it is likely that forest-steppe typological diversity will decrease, and the semi-arid landscape may become more monotonous.