Variability of plant species diversity during the natural restoration of the subalpine birch forest in the Central Great Caucasus

摘要

The northern slopes of the Central Great Caucasus of Georgia were covered by birch (Betula litwinowii) forests in the past, but forest degradation resulted in subalpine secondary meadows. Over the past 20 years birch forest natural reforestation processes have taken place. The aim of this study was to determine the changes in plant species diversity during reforestation. Different habitat types were selected to trace the development of birch forests from subalpine meadows. Species richness was determined in 100 research plots (25 m2 each) of different habitat types. Pioneer succession was distinguished by the highest level of species richness due to the mixture of two habitat types. Degraded forest showed the lowest species richness. At the initial stages of forest restoration we observed several species of subalpine dwarf shrubs. They grow only in open canopy areas and are absent in closed forests. The alpine treeline habitat type and pioneer succession of subalpine meadows revealed close relations in species composition and diversity. These similarities show forest restoration in lower elevations as it is restricted in the treeline ecotone by local climatic conditions. The natural regeneration is apparently in close relation with the global climate change, but the most important factor is the reduction of uncontrolled sheep grazing.