Artificially established stands originating from afforestations tend to become single‑storeyed, even‑aged, uniform, and short‑crowned. Such dense stands are dark, poor in species, virtually without ground vegetation, and vulnerable to pests, snow pressure and strong wind. Natural mountain forests, in contrast, ideally display a group‑like structure and have extensive internal margins with green crowns reaching to the ground. In afforestation it is advantageous to establish clusters (German: Rotten) already at the time of planting and not only through tending operations later on.
Fig. 1 - At the upper timber-line stands of mountain forests display a group‑like structure and have extensive internal margins with green crowns reaching to the ground. Photo: Walter Schönenberger (WSL)
Why cluster afforestation?
There are many reasons to prefer cluster afforestation to the conventionally afforestation in mountain regions:
- In mountain regions, favourable locations, especially rises, should be given preference, i.e., more densely planted. Unfavourable locations such as gullies, depressions, or patches with well established tall forbs should not be planted at all. Observance of these principles automatically leads to a kind of cluster structure in the afforestation.
- Cluster structure in a mountain forest enhances the natural variety. More light can penetrate the stand, and this results in a more varied ground vegetation and better habitats for birds, game (possibility of browsing), and insects.
- In a cluster, the trees at the margin protect those in the centre from damage by game.
- The canopy does not have to bear the full weight of trapped snowloads, since the snow either falls or is blown into the spaces.
- Winds attenuate within a grouped stand. The risk of gaps being torn in the stand is reduced, since the stand is already open.
- Stands, also in a cluster pattern, can regenerate at a later stage without risk, since the old trees are storm hardy.
- Stands originating from cluster afforestations become open, multi layered, and non uniform, and consequently more resistant. The crown space is larger and extends to the ground.
- Soil and humus develop more rapidly within the clusters.
- Young growth tending is easier because of the reduced competition of the vegetation, the smaller tendency towards uniformity and the smaller area to be tended.
- Despite the smaller planting distance, the number of plants required per unit area is not greater than in continuous planting arrangements.
- The risk of fungal infection is probably reduced by the intervening spaces.
How big should a cluster be?
The size of the cluster is related to the expected tree height, i.e., dependent on altitude. The diameter of the cluster should equal half to the whole length of a tree. The cluster should be round to oval in shape, with the long axis parallel to the slope or the direction of the prevailing wind. At high altitudes, therefore, a width of 8 to 15 m and a length of 10 to 15 m is suitable; in mountain forests at lower altitudes, 5 to 10 m more. The main criterion is always the final objective: what kind of structure should the stand have in 100 years time?
Fig. 2 - Cluster structure achieved by tending a Norway spruce pole stand. Photo: Ulrich Wasem (WSL)
How is a cluster established, with what distances?
It is desirable that the advantages of the cluster structure become effective soon after planting, and that the number of plants required be kept within reasonable limits despite the close planting arrangement. To achieve this, a cluster can be established by planting 3 to 6 "smaller collectives" within which the saplings are planted very close together, i.e., 40 to 80 cm apart. Near timberline, a very close arrangement is necessary; at lower altitudes, the plants may be more widely spaced, since growth and survival are better. The small collectives are 3 to 4 m in diameter and comprise 20 to 40 saplings (see Figure 3). As a result, the canopy within each small cluster closes rapidly, within a few years, and the advantages of the collectives soon become effective (Figure 4). To facilitate the monitoring of success, it is advantageous to use the same number of plants in each small collective.
The distance between the small collectives may be 2 to 3 m, so that they unite in a second phase after 20 to 30 years to form the final cluster (Figure 5). At the time of planting the distance between the complete clusters should equal at least twice the branch extension of a mature tree, i.e., 7 to 10 m, so that the clusters never fully merge. For this reason, the distances should correspond to the final objective from the beginning (Figure 6).
Fig. 3 - At the time of planting: "small collectives'' of 20-40 seedlings, 3-4 m diameter and 2-3 m distant, seedling spacing 50-80 cm.
Fig. 4 - Five to ten years later, when seedling crowns close within the still separate small collectives.
Fig. 5 - Two to three decades later, when the small collectives will merge to form the final clusters.
Fig. 6 - Mature stand: the clusters remain distinct and touch each other only in places. The small collectives' are no longer visible.
Which species?
Only one species should be used within each cluster, but it is desirable to vary the species from cluster to cluster. Particularly upright conifers (cembran pine and spruce) are suitable. On less problematic sites without risk of erosion, weed infestation, etc., spaces the size of a cluster may be left unplanted. These spaces may serve for later afforestation or natural regeneration. On critical sites, on the other hand, for instance where protection against erosion is necessary, the spaces should be planted with shrubby pioneer species (green alder, mountain ash = rowan, birch, prostrate mountain pine, willows) for the time being. This procedure allows the development of uneven aged, multi layered stands.
Fig. 7 - Slope with very well structured clusters of trees. Photo: Ulrich Wasem (WSL)
How should the clusters be arranged on the terrain?
The size, shape, and composition of the clusters should not be fixed but flexible and adapted to the terrain. The small collectives should mainly be planted on rises, around tree stumps, on spurs, etc. On no account should gullies, depressions, or wet spots be planted.
