Age Structure and Historical Development of Forests in “Bistrishko branishte” Biosphere Reserve in Vitosha Mountain (Bulgaria)

In 2001 the territory of the Bistrishko Branishte reserve of the Vitosha Mountain (Bulgaria) was affected by wind throw. Almost 100% of the Picea abies trees on an area of 60 ha were overthrown. After 2003 in the periphery of the wind throw an outbreak of Ips typographus has started, which developed at high speed and by 2008 affected 200 ha of the forests in the reserve. These natural disturbances raised questions about the past of the ecosystem and possible relation of these events to the previous history of the forest. To study the age structure and historical development we extracted 165 samples with increment borer from different parts of the forest of the water-catchment of Bistrishka River. Samples were taken from trees representing various diameter classes and perhaps different cohorts. The samples were prepared following standard dendrochronological methodology consisting of gluing to wooden holders, sanding with sandpaper No. 250 and 600, scanning at 1200 dpi and measuring the tree-ring widths with the CooRecorder software. The resulting series were cross-dated using visual characteristics of the tree rings and statistical similarity with the CDendro software. Our data showed that regardless of the location the age of trees is similar. The majority of dominant trees germinated after the 1870-s. Trees that are visually distinguishable with their larger sizes, had similar ages to neighboring dominants. There is no evidence for large-scale disturbances except for a known wind throw in 1956 in the tree line zone under Skoparnika peak. Over the past 100 years the forest has been affected primarily by small-scale disturbances. The similar age of dominant trees and forest structure are probably a consequence of fast forest recovery after reduced human activity by the end of the 19th century.