Reconstructing spatiotemporal dynamics of Central European natural beech forests: the rule-based forest model BEFORE

Abstract

Without humans, large areas of central Europe would be covered by forests dominated by beech ( Fagus silvatica). The spatiotemporal dynamics of natural beech forests are hence a subject of interest for both forest management and conservation. However, since in most regions there are no longer any natural beech forests, their structure and dynamics cannot routinely be analysed and compared to managed forests. The forest model BEech FOREst (BEFORE) is therefore, designed to reconstruct the spatiotemporal dynamics of natural beech forests. BEFORE is a grid-based and partly individual-based model which divides beeches into four different height classes. Changes to the forest structure due to growth, mortality and storm disturbances are entirely described by empirical ‘if-then’ rules. BEFORE is capable of reproducing two patterns which have been observed in remnants of natural beech forests: beech forests consist of a mosaic pattern of small areas (on average 0.3 ha) which are at different developmental stages; at the scale of these small areas, a cyclic succession of three developmental stages occurs, which are characterised by different vertical structures. One typical feature of natural beech forests is hence their very high structural diversity. Gaps in the canopy induce a local pulse of vitality and growth for younger beech trees. These pulses are spread into the vicinity by two mechanisms. Firstly, windfalls affect not only the site of the tree knocked over itself but also neighbouring sites due to the damage caused by the tree falling over. Moreover, since the light is diffuse and oblique, canopy gaps affect also vitality and growth in the neighbourhood of a gap. The results obtained with BEFORE show that natural beech forests achieve quasi-stationary dynamics, demonstrating considerable fluctuations in the forest structure. For example, the percentage of forest area at the optimal stage, which is characterised by a closed canopy and almost no understorey, varies between 10 and 40%, and after extreme storm events even between 0 and 60%. Beech forests with an inner area larger than 40 ha (corresponding to a total area of 70 ha) develop spatiotemporal dynamics which do not differ qualitatively or quantitatively from larger forests, but even very small natural beech forests would exhibit very high temporal and structural diversity. Thus, even small ‘islands’ of unmanaged stands within larger, managed forests would contribute significantly to providing structures typical of natural beech forests.