Analysing Betula litwinowii encroachment and reforestation in the Kazbegi region, Greater Caucasus, Georgia

AimsThe encroachment of tree and shrub species in high mountains is an increasing worldwide phenomenon, which is expected to dramatically alter high-mountain ecosystems and their functioning. Moreover it indicates in some cases a reforestation process, which will result in important ecological and social benefits, including carbon sequestration and protection against landslides. We therefore examined the spatial extent of forest growth and shrub encroachment mainly of birch (Betula litwinowii) in the sub-alpine belt of the Central Greater Caucasus between 1987 and 2010 and its relation to topographic site conditions. LocationKazbegi district, Central Greater Caucasus, Georgia. MethodsWe analysed 155 vegetation releves sampled in 2009, 2011 and 2015, mainly derived from the Caucasus Vegetation Database, to obtain information about topographic site conditions and structure of B.litwinowii stands. B.litwinowii forest growth was assessed by digitizing the forest outlines from aerial and space-borne imagery (1987, 2005 and 2010). To identify areas of B.litwinowii encroachment as an indicator for different encroachment stages, we modelled the tree and shrub cover using the Random Forest algorithm. ResultsWe found four types of B.litwinowii stands, characterized by different tree and shrub coverage (initial Bromus variegatus-Betula litwinowii encroachment indicating the first stage of succession, Aconitum nasutum-Betula litwinowii forest, Rubus idaeus-Betula litwinowii forest and Rhododendron caucasicum-Betula litwinowii tree line scrubs). B.litwinowii forest increased 25% compared to 1987 mainly in an uphill direction. Furthermore the modelled tree and shrub cover (R-2=.69) could be related to the four vegetation types. ConclusionsOur results indicate a recent trend towards shrub encroachment and consequently reforestation in the Kazbegi region.