In the natural forest reserve Aletschwald, the first full cruise under the lead of the forest reserve project was conducted in 1962. Thereby, all trees with a dbh ≥ 4 cm were measured within the entire perimeter of ca. 245 ha. The cruise was repeated in 1982 and again 2012. However, the last inventory encompassed only a smaller part of the reserve – 118.5 ha – but additionally habitat structures (hs) on large trees (dbh ≥ 36 cm) were recorded. In the analysis the two characteristic forest types 'Mature European larch-Swiss stone pine stand' and 'European larch pioneer forest' were treated separately. The forest structure observed in 2012 was then compared with the forest state described in earlier inventories, as well as with the first full cruise from 1942 (as far as a fair comparison was possible). This enabled us to describe the temporal development of the Aletschwald over up to 70 years.
The mature stand has clearly developed towards a natural forest, although traces of the former management are still visible. The observed increase in naturalness is evident from the general densification of the forest: stem numbers and stand volume have quadrupled since 1942, and reached 303.6 stems/ha and 116.1 m³/ha, respectively, in 2012. Tree species composition hardly changed over time, but the share of Swiss stone pine slightly declined, while larch slightly increased and deciduous trees such as birch, green alder, rowan and willow appeared in the lower diameter classes. In 2012, the share of deciduous species in stem numbers reached 12%. The mature stand showed already many valuable hs, i.e. bark injuries, holes and cavities in the stem or cracks, with a total density of 21.5 hs/ha. We recorded 7.3 giant trees (dbh ≥ 80 cm) per ha, of which 75% were Swiss stone pine, 23% larch and 2% Norway spruce. Thereby, giant trees had significantly more hs per tree than smaller trees, and live trees showed significantly more hs per tree than snags.
On the top moraine, which in 1942 was still almost treeless, a dense larch-pioneer forest has developed. Stem numbers of 445/ha and very low basal areas (8.3 m²/ha) point towards a stand composed of many small trees. Large trees (dbh ≥ 36 cm) were rare (1.5/ha), and thus, also trees with hs (0.15/ha). Giant trees were absent.
The strikingly increased ungulate population (chamois, red deer) seems to exert a strong negative influence on tree regeneration, as indicated by the decline of small diameter trees since 1982, and the low stem numbers in the smallest dbh classes (dbh < 15 cm). Viable tree regeneration is the prerequisite for maintaining a continuous forest cover. If there is no change towards an increased density of tree regeneration in the future, the original aims of the forest reserve 'forest conservation and natural forest development' will be at risk.