Four decades of the coexistence of beech and spruce in a Central European old-growth forest. Which succeeds on what soils and why?

Abstract

The dynamics of forests dominated by European beech (Fagus sylvatica) and Norway spruce (Picea abies) have been studied intensively. However, mainly due to a lack of long-term data, little is known about how these dynamics interact with soil conditions. In an old-growth spruce-beech forest with high soil diversity we studied how the development of tree populations differs among different soils. Data from tree censuses carried out in 1972, 1996 and 2010 in the Boubin Primeval Forest in the Czech Republic were combined with detailed soil sampling to assess the relative abundance of beech and spruce and the role of the main drivers of population dynamics (tree growth, mortality and recruitment) in changes with respect to soils. The spatial distribution of populations of the two species primarily reflected a gradient of soil hydromorphism, with beech dominating drier soils and spruce dominating wetter soils. Over the 38 years, beech expanded on all major soils, yet the most important drivers differed. The only driver acting in favour of spruce on certain terrestrial soils was its faster radial growth. However, the effect was weaker than the effect of drivers that prioritized beech, mainly tree mortality. Fine-scale mortality (deaths of individual trees) was more significant on terrestrial soils, while the effect of coarse-scale mortality (deaths from a single severe windstorm event) increased towards hydromorphic soils. Certain soils (Histosols and Albic Podzols) diverged from the general trends because of their different disturbance regimes and specific tree–soil interactions. Soils play an important role in the dynamics of an old-growth spruce-beech forest. Their physical and chemical properties together with specific disturbance regimes determine fine-scale differences in tree species composition. At the same time, soils themselves are affected by trees, e.g. through acidification. The current expansion of beech is expected to continue on terrestrial soils but will probably slow down with increasing soil wetness.