Spatial Differentiation and Structure of Beta Diversity of Woody Plant Communities in Forest-Steppe Complexes of the Kalba Ridge (Eastern Kazakhstan)

The Kalba Ridge in eastern Kazakhstan is a transitional forest-steppe system in which woody vegetation forms a mosaic of forest, shrub, meadow, steppe, and riparian habitats. We analysed a presence/absence matrix of 76 woody taxa across 11 plant complexes grouped into eastern, central, and western sectors. We assessed species richness, distribution breadth across plant complexes, Jaccard similarity, Whittaker beta diversity, Sørensen dissimilarity decomposition, Hellinger ordination, PERMANOVA, and local and species contributions to beta diversity. Species richness decreased from 71 taxa in the eastern sector to 52 in the central sector and 29 in the western sector. The multi-site Sørensen dissimilarity across all complexes was 0.740, with species substitution (0.508) exceeding nestedness (0.232), whereas east–west sectoral differentiation was driven mainly by nestedness. The first two PCA axes after Hellinger transformation explained 52.9% of the variation in species composition. Sector compositions differed significantly (PERMANOVA: pseudo-F = 2.30, R2 = 0.365, exact p = 0.013), with no evidence of differences in multivariate dispersion. The eastern dark coniferous forest and western meadow-steppe had statistically significant LCBD values. Notably, the western meadow-steppe contained only 12 taxa but remained compositionally distinctive, showing that species-poor assemblages can contribute disproportionately to regional beta diversity. These results show that conservation planning should represent both species-rich ecotones and compositionally unique habitats.