Seed trees and natural regeneration potentials of pioneer tree species for calamity prevention in spruce forests
Pre-forest and secondary tree species: possibilities and limits of successful reforestation of damaged areas with regard to seed trees and natural regeneration potential of pioneer tree species for calamity prevention in spruce forests
In the past, single-layer conifer stands of Norway spruce and Scots pine that were not suitable for the site were established over large areas (ZERBE 2009). At the same time, naturally occurring pioneer tree species were consistently removed from the stand structure as so-called "forest weeds", regardless of their diverse ecological effects. This approach led to increasing species impoverishment and soil degradation in the forests (RÖHRIG et al. 2006). Furthermore, damage events (pest calamities, forest fires, snow breakage and windthrow) have repeatedly occurred and continue to occur in the unsuitable and consequently unstable pure conifer stands, resulting in extensive losses of entire forest stands (NOACK 2006). The sudden development of such clear-cut areas decouples water and nutrient cycles, with the result that more carbon dioxide is released and nutrients are washed out (BURSCHEL & HUSS 1997).
Due to their typical characteristics (large dispersal capacity, rapid juvenile growth, low site requirements), pioneer tree species offer the opportunity to mitigate the aforementioned environmental impacts of pure conifer-dominated stands on the one hand, and to contain the consequences of the effects of open spaces on the other (BURSCHEL & HUSS 1997). However, various studies on the reforestation of open areas have shown that successful colonization has not always been achieved (RICHTER & LEDER 1990, LÄSSIG et al. 1995, SCHMIDT-SCHÜTZ & HUSS 1998, WENT 2011).
The aim of this research project is therefore to gain new insights into the rapid and largely non-invasive reforestation of open spaces using pioneer tree species. The aim is to derive species-specific minimum densities of seed trees that enable recolonization. Furthermore, it should be possible to estimate the actual regeneration potential of the soft deciduous tree species currently present in the forests on the basis of area and stand parameters.
The dispersal potential of the various pioneer tree species will be investigated with the help of seed traps in open areas. For this purpose, seed traps for the anemochorously distributed species (birch, willow) and dung traps for the zoochorously distributed rowan will be set up on the clear plots. In addition, the seed trees in the neighboring stands will be measured in order to obtain precise information about the dispersal distance as well as the seed densities. At the same time, vegetation surveys are being carried out on Kyrill storm areas (hurricane 2007). These are intended to provide information on the actual colonization success or reasons for the absence of natural regeneration. The collected data will then be used to parameterize isotropic and anisotropic single tree models.
To conclude the project, the existing seed tree potential of soft deciduous trees will be investigated in an example region in the high altitudes of the Thuringian Forest. Taking into account the research results from the project, any deficits in the recolonization of new damaged areas in the model region will be identified and recommendations for action to compensate for these will be given.
The findings of the project should help to assess reforestation scenarios in the event of further storm disasters and, if necessary, enable precautionary measures to be taken to introduce pioneer tree species.