Regeneration dynamics in mixed mountain forests at their natural geographical distribution range in the Western Rhodopes.

• Conversion of formerly spruce-dominated stands at their distribution margin. • Analysis of management and microsite effects on regeneration success. • High regeneration potential in mixed forests at their south-eastern limits. • Single-tree selection cuttings favour fir recruitment at the expense of spruce. • Ungulates played a negligible role in tree recruitment processes. Mixed mountain forests consisting of Norway spruce (Picea abies (L.) Karst.), European beech (Fagus sylvatica L.), and silver fir (Abies alba Mill.) are among the most productive and stable forest ecosystems in Europe. Their southeasternmost geographical distribution range is located in the Western Rhodopes, where they have high economic, recreational, and ecological value. In the past, shelterwood cuttings dominated forest management practices in these forests and were mainly aimed at maintaining and reproducing conifers. During the past two decades, single-tree and group-tree selection systems have been promoted as alternative management approaches to support the conversion of spruce-dominated stands to close-to-nature mixed forests of fir, beech, and spruce. However, the natural regeneration dynamics in these stands are barely known, and their dependence on microsite and management effects needs to be better understood. The objective of this study was to investigate ecological factors under management regimes of different intensity ("single-tree"-selection and "group-tree"-selection) that influence the regeneration processes in mixed mountain forests in the Bulgarian Rhodopes. Data on regeneration and microsite conditions were collected on 105 systematically distributed plots (25 m²/100 m²) in four 100–120 years old stands located in the regional forest district of Smoljan, Bulgaria (1580–1650 m a.s.l.). We relied on generalizeds linear mixed models to analyse for each species the (1) size-dependent regeneration density and (2) height increment in dependence on management practices, competing vegetation, as well as soil and light conditions. Our study revealed an overall high potential for recruitment in the Western Rhodopes. Regeneration density was highest in fir (median 12800 N ha-1), followed by spruce (median 1600 N ha-1) and beech (median 1200 N ha-1). Fir benefited most from "single-tree" selection cuttings, while "group-tree" selection cutting tended to promote beech and fir but also spruce. Competing ground vegetation was detrimental for seedling density of all species. Annual height increment increased with plant size, was lowest in spruce, and similar in fir and beech. Sapling increment was driven by light, whereas seedlings did not react to increased radiation. Browsing was species-specific and was highest in beech (15–30 %), followed by fir (5–10 %) and spruce (<1 %). It was not a crucial factor in impeding tree recruitment. We conclude that frequent harvest activities of low intensity which consider advanced regeneration are a promising approach to successfully convert the formerly spruce-fir-dominated forests to climate-adapted fir-beech-(spruce)-mixed stands. [ABSTRACT FROM AUTHOR]