One prime objective of any silvicultural measures must be to promote natural regeneration in the stand, because natural regeneration offers considerable advantages in comparison with planting: undisturbed root development, improved tree vitality and stability, and lower costs for establishment of the regeneration. Different measures are necessary - depending on the type of stand - to achieve a particular natural regeneration.
Natural regeneration in coniferous forest stands
In coniferous stands and mixed stands dominated by coniferous tree species, a diversity of vertical and horizontal vegetation structures and tree age structures is important. By implementing advance planting measures as early as possible, it is possible to secure the establishment of high proportions of deciduous trees and fir. Depending on the light requirements of the trees, they can be planted in small clusters, groups or bigger patches.
Spruce: In pure spruce stands, the forest owner must introduce mixed tree species early by advance planting. The individual trees and the stand as a whole must be stable enough to allow for sufficiently long regeneration periods. Once the mixed tree species planted in advance have become established over a sufficient area, the natural regeneration of the spruce can be stimulated on suitable sites by carefully removing individual trees to allow in more light. By controlling the crown cover, forest owners and foresters can promote differentiation and structure in the natural regeneration.
Caution is advised in unstable stands. In stands with high h/d ratios, short crowns or considerable tree damage, intervention must be restricted to stabilising measures. Moderate but frequent intervention measures are implemented to remove primarily trees with short crowns and damaged trees. It is essential to leave mixed tree species as seed trees. In undifferentiated natural regeneration areas without a canopy, tending measures are essential to regulate the growth of mixed species and to stabilise the stand.
Pine: It is essential to preserve all deciduous trees and shrubs coming up as elements creating a mixture. If there are no mixed tree species suitable for the site already present, they should be introduced before the regeneration is stimulated. The genetic basis of the pine stands is important. Only high quality, source-identified pine stands suitable for the site are suitable for natural regeneration. The natural regeneration is stimulated by creating large gaps in the canopy over parts of non-regenerated areas to allow light to reach the forest floor.
Fir: Structural diversity (vertical, horizontal, tree-age) in the standing stock boosts the natural regeneration. Natural regeneration with fir will flourish even in relatively dark conditions and develops particularly well if it can grow for a long time under shelter of the old stock. Only when the fir regeneration is at least three to four metres higher than brambles and/or competitor tree species does it make sense to open up the canopy and allow more light in.
Douglas fir: Natural regeneration is not appropriate where the standing trees are of unknown origin. This applies particularly for the Rocky Mountain Douglas fir (Pseudotsuga menziesii var. glauca), as it is particularly susceptible to the fungal disease Rhabdocline needle cast (Rhabdocline pseudotsugae). With the Coast Douglas fir (Pseudotsuga menziesii var. viridis), genetics also play an important role. Forest stands of unsuitable provenance should not be regenerated naturally. To prevent the development of pure Douglas fir stands, a sufficient proportion of mixed tree species in small clusters or groups must be established. Longer periods of canopy cover help to produce a differentiated structure in the regeneration.
Natural regeneration in deciduous forest stands
For deciduous timber stands and mixed stands dominated by deciduous trees, the general objective is to create high quality mixed stands. Depending on the light requirements of the tree species to be regenerated, they are established in small clusters, groups or bigger patches. In close-to-nature forestry, individual old trees and islands of old timber should be left standing.
Common beech: Natural regeneration should be stimulated early to ensure a long regeneration period. This can quite happily be up to 50 years. The result is structurally diverse, high quality regeneration in a mixture of small clusters, groups and larger patches. The regeneration can be stimulated by harvesting unevenly distributed target diameters (group selection method). Shelterwood cutting over an extensive area and the extensive natural regeneration of beech that results from it is to be avoided. Patches of regeneration of mixed tree species (deciduous and evergreen species) must be safeguarded early by removing trees to give more light.
Pedunculate and sessile oak: The regeneration in pedunculate and sessile oak stands is induced only in full mast years in mature stands. These must remain dark beforehand. First of all, single trees of the target diameter are removed. Later, a patch of at least 0.5 hectares is felled. Advance regeneration of shade-tolerant deciduous trees should be avoided. Only once the oak has become established are areas without regeneration filled up with mixed tree species or shading tree species.
Valuable hardwoods: With cherry, maple, common ash, alder and the Sorbus species, natural regeneration is encouraged by felling target diameters using the group selection procedure. Having different levels of canopy cover boosts differentiation among the trees. In extensive areas of single species natural regeneration, it is essential to introduce shade-tolerant deciduous trees in the areas where regeneration has not yet developed.
The trees in the stand compete for light, water and nutrients. Natural regeneration can be promoted by reducing the competition between
- the relevant tree species,
- the standing stock and natural regeneration, and
- understorey vegetation and natural regeneration.
The water supply for the soil and for the natural regeneration is lower in coniferous stands than in deciduous stands. This is because more of the precipitation is intercepted or withheld from the soil (interception). The water supply for the natural regeneration can be improved by
- a more permeable crown canopy (group selection, opening up of canopy),
- less root competition from old trees (removal of trees)
- low sunlight and
- maintenance of a favourable soil humidity.
Shade at the beginning of the regeneration period facilitates the latter two requirements. Regional differences with regard to the level and temporal distribution of precipitation must of course also be taken into consideration. The change in the amount of sunlight (quantity and quality of radiation) has an influence on
- the germination percentage of the light-dependent germinators,
- the survival and vitality of the seedlings/young trees (especially of light-dependent germinators),
- the competing vegetation (blackberries, touch-me-not (Impatiens)),
- the proportion of light-dependent tree species and
- the survival and vitality of additionally planted trees.
Close-to-nature forestry means working in harmony with nature, but it also means encouraging natural regeneration and exploiting the potential of existing old forests that are suitable for the site and climatically tolerant.