Mimicking small scale disturbance regimes to enhance biodiversity in middle-aged Scots Pine forests – a forest restoration experiment

Abstract

In 2005 the Federal Government of Germany has started to transfer valuable areas for nature conservation purposes to the German Federal Environmental Foundation (DBU) and others as National Heritage Sites (NHS). These areas are characterized by being large, relatively undisturbed landscapes with a mixture of forests and open habitats. They are known to host a high share of endangered biotopes and species. The DBU National Heritage aims at sustaining the biological diversity on the sites with the long-term goal, to leave all of the forested area to natural development. Although most of the sites are embedded in relatively natural forest, coastal and water landscapes, a large share of forests still consists of mono-layered scots pine plantations. In order to accelerate forest development towards a more structured and species-rich habitat, it might be advisable to implement forest restoration measures.

The DBU NHS "Rüthnicker Heide" close to Berlin, was chosen to conduct a large scale experiment simulating small-scale disturbance events in approximately 70-year old scots pine stands to investigate short- and long-term effects on structural, functional and species diversity. The experiment encompasses a design of four sample alternatives in four replications, each on a five hectare sample plot. The alternatives are meant to depict an increased structural complexity from 1. small scale to middle scale gaps with timber removal to 2. active deadwood accumulation where timber remains as "fallen deadwood" (downed by harvester) and "standing deadwood" (trees ringbarked by harvester) and 3. active deadwood accumulation with additional planting of deciduous tree saplings. Plots with no active intervention have been left untouched as an example for "passive restoration" (4.). All of the plots were fenced on half of their size to account for deer browsing. Inventories of structural, functional and species diversity were carried out before and after the forest restoration measures.

The study aims to reveal if the increase in structural diversity, i.e. generation of dead wood, gaps and planting of saplings, results in an increased species diversity or a shift in community composition of beetles, fungi and herbaceous plants, as well as an increased density of microhabitats. Preliminary results show no significant effects on plant and fungal diversity, however, a trend towards a higher mean number of plant species can be seen. The most promising short term effects are to be expected for beetles, but results are not available, yet. The restoration measures were able to increase the number of microhabitats (mainly root plates and bark fissures) in alternative 2 and 3 by 730 and 500%, respectively, whereas in passive restoration the increase was about 25%. Natural regeneration showed a significant increase for alternatives 1 to 3 as response towards canopy opening. The outcomes can serve as a blueprint for the treatment of similar forests in protected areas.