1 Department of Silviculture, Forest Research Institute Sofia, 132 St. Kliment Ohridski Blvd., BG-1756 Sofia, Bulgaria
2 Swiss Federal Institute for Forest, Snow and Landscape Research, Zürcherstr. 111, CH-8903 Birmensdorf, Switzerland
3 Icelandic Forest Research, Mogilsa, IS-116 Reykjavik, Iceland
4 Institute of Silviculture, Department of Forest and Soil Sciences, University of Natural Resources and Life Sciences (BOKU) Vienna, Peter-Jordan-Straße 82, AT-1190 Wien, Austria
Components of structural diversity of abandoned chestnut (Castanea sativa Mill.)-dominated and co-dominated forests along an altitudinal gradient in the Belasitsa mountain region of Southwest Bulgaria were evaluated, including: (i) tree species composition; (ii) differentiation in diameter, height and age; (iii) tree crown defoliation and light transmission; and (iv) regeneration composition and abundance. Competition between tree species and its influence on current stand structure were analysed. Lack of management had triggered rapid structural and successional development in formerly chestnut mono-dominated forests which have now been invaded by midseral and later seral vegetation dominated by European beech (Fagus sylvatica L.) and sessile oak (Quercus petraea Liebl.). Distribution of sample plots according to diameter differentiation for chestnut showed positive differentiation values (dominance of chestnut with respect to other species) in 43 of a total of 46 plots sampled. Structure is much more balanced in terms of height differentiation. In the absence of management, chestnut blight has been a major stress factor and is likely an important driver of chestnut decline. The proportion of chestnut trees infected by chestnut blight disease exceeded 80% in 28 plots. Nearly one third (31%) of all sampled trees were characterised by a degree of defoliation of more than 60%. Despite the low levels of light at the forest floor, the density of the regeneration stratum was relatively high (averaging 19300 ha–1). An important finding is the retained regeneration potential of chestnut (31% of all seedlings). Chestnut seedlings, however, appeared to be poorly adapted to shading and rapidly declined in density and growth while seedlings of most competitors survived longer and dominated the regeneration stratum.
Keywords: Castanea sativa, structural diversity, tree crown defoliation, Cryphonectria parasitica, regeneration potential, forest succession, forest management