Diversification of coniferous monocultures in the last 30 years and implications for forest restoration: a case study from temperate lower montane forests in Central Europe

Abstract

Abstract Planted monocultures of even-aged coniferous tree species are abundant worldwide but increasingly damaged by biotic and abiotic stressors and disturbances. In Central Europe, a fundamental goal of ecologically oriented forest management is thus the conversion of pure and often even-aged coniferous stands into structurally more diverse and mixed broad-leaved forests. This conversion is often achieved by single-tree selection resulting in small canopy openings that promote artificial or natural regeneration. Consequently, forest conversion aims at altering stand structure and tree communities. In order to describe the status quo of forest conversion and derive implications for forest management, we investigated changes of tree composition and forest structure in the Bavarian Spessart mountains in southwest Germany. We conducted a resurvey of 108 semi-permanent plots in four different coniferous stand types of Norway spruce, Scots pine, Douglas fir, and European larch about 30 years after the initial survey. We found significant differences in the stratification and cover of respective forest layers between the two sampling periods that indicated an increase in stand structural heterogeneity. While species richness of the overstorey remained constant, species richness and diversity of the shrub and lower canopy layer increased significantly. Regenerating “winner” species included late-successional broad-leaved (e.g., European beech, sessile oak), pioneer broad-leaved (e.g., silver birch, rowan) and shade-tolerant coniferous (e.g., silver fir, Douglas fir) species. Although Norway spruce was significantly reduced in the overstorey, it regenerated in parts extensively in the understorey. We conclude that the forest conversion in the Spessart mountains was overall successful in terms of diversifying forest structure and tree species. Its effects are, though, still emerging and the stands are in a transitional phase. Besides the preferred natural regeneration of target tree species, forest management may consider active measures to guide the facilitated diverse tree community of previously pure and even-aged coniferous stands towards stand maturity.