Vegetation change in the Astrakhanskiy Biosphere Reserve (Lower Volga Delta, Russia) in relation to Caspian Sea level fluctuation

During the twentieth century the level of the Caspian Sea dropped from -26 m (1930) to -29 m (1977) below global sea level and subsequently rose again to -26.66 m in 1996. We aimed to describe responses of the vegetation in t he lower Volga Delta to these substantial sea-level changes using an analys is of historic vegetation maps produced by aerial photography and satellite imagery. The sea level drop in the earlier part of the century was followed by rapid progression of the vegetation. The subsequent rapid sea-level rise in the 1980s did however not result in similarly rapid regression of the vegetatio n. This partial irreversibility of the vegetation response to sea-level cha nge is explained by the wide flooding tolerance of the major emergent speci es, namely Phragmites australis. Floating vegetation increased in extent, m ost likely due to the increased availability of more favourable conditions, particularly for Nelumbo nucifera, a tropical plant reaching its northernm ost distribution in the Volga Delta. This species increased in distribution from 3.5 ha in the 1930s throughout the entire Volga Delta to several thou sands of hectares in the Astrakhanskiy Biosphere Reserve alone in the 1980s . The reported sea-level changes swept the ecosystems in the Astrakhanskiy Biosphere Reserve back and forth within the Reserve boundaries. At longer t ime scales, ten-fold greater sealevel change has been reported. The ecosyst ems for which the Reserve is renowned might be pushed completely out of the Reserve under these conditions. We therefore question whether the current Reserve will be sufficiently large to guarantee conservation of the biota i n the lower Volga Delta at longer time scales.