Abiotic disturbances in Bulgarian mountain coniferous forests – An overview

Abstract

Disturbances are among the most important processes that shape forest dynamics and landscapes. However the historic ranges of variability (HRV) of many coniferous forests in Europe and specifically in the mountains on the Balkan Peninsula are not well understood. We present the first overview of available data on disturbance events in coniferous forests in the high mountains in southwestern Bulgaria. Our study included data from historical publications, documents and archives, newer documentary data obtained through interviews with foresters and verified using satellite images, published studies and new tree-ring data. We documented at least 188 abiotic disturbance events in the Bulgarian mountains including fires (39%), windthrows (31%) and avalanches (20%) and fewer disturbances caused by snow and ice.Fires primarily affected Pine-dominated ecosystems, especially pure Pinus sylvestris and mixed Pinus sylvestris-Pinus nigra forests. Our tree-ring analysis also provided evidence for repeated fires in subalpine Pinus heldreichii and Pinus peuce forests during the last 500 years. Some of these fires affected different valleys within the same year, suggesting either large-scale events or the simultaneous occurrence of fires related to appropriate fuel and climate conditions. We also found data on several large fires in Picea abies-dominated ecosystems. Most of the fires were less than 100 ha, but some were larger, the most extensive of which was the Batalach fire (year 1890, approximately 9000 ha) in the Western Rhodopes. The highest frequency of fires was in 1880–1910, 1940–1950 and 2000–2010. We found evidence for at least 59 windthrows greater than 1ha, mainly in pure Picea abies and mixed Picea abies-Pinus sylvestris forests. There was high variability in the size of windthrows, ranging from small-scale, gap-sized blowdowns to large and infrequent disturbances affecting more than 300ha, the largest of which was the Beglika windthrow (1961, 3000ha). Although avalanches are considered important for subalpine forests on steep slopes they were generally poorly documented and data were limited to several avalanche cycles in 1955, 1963–1974 and in the last two decades. Snow-related damage was important mostly for young pine forests in which several events affected a high stock of wood in the 1930s, 1987, 1988 and 2015.Our data show that natural disturbances of various types and of a range of sizes are part of the natural dynamics of coniferous forests in Bulgaria. Further, our study contributes to the understanding of the historic range of variability of mountain forests in Europe.