The natural forest reserve Aletschwald is one of the oldest forest reserves of Switzerland and lies within the UNESCO world heritage «Schweizer Alpen Jungfrau-Aletsch». The aim of the reserve is to protect one of the most unique forests in Switzerland and to allow a natural development. During the summer of 2012, a sample plot inventory was carried out to complement the full cruise of parts of the Aletschwald with representative information on the forest structure of the larger perimeter Aletschwald and Teiffewald. Within this area of 422 ha, 107 permanent sample plots of 500 m² each were installed, and traits on stand structure, dead wood, habitat structures and tree regeneration were recorded. In this report, key figures are presented for the entire perimeter (called Aletschwald) as well as for the three main forest types separately (European larch-Swiss stone pine forest, Norway spruce forest and European larch pioneer-forest).
Stand density in the Aletschwald is generally low, most pronounced in the larch- stone pine forest (293 stems ha^–1 [±37]), followed by the larch pioneer forest (557 ha^–1 [±112]). The spruce forest shows the highest stem density with 895 ha^–1 [±201]), of which the amount of trees with a dbh of less than 16 cm is strikingly high (49%). Giant trees are only present in the larch-stone pine forest, with 6.9 haha^–1 [±2.0] they are already quite abundant. Almost negligible are giant trees in the spruce forest (0.7 ha^–1 [±1.1]), and they are lacking completely in the larch pioneer forest. Dead wood volumes only reach the lower limits of values found in virgin forests, with 23.8 m³ha^–1 in the larch-stone pine forest and 19.7 m³ha^–1 in the spruce forest. It is worth noting that snags in the Aletschwald consist mainly of small diameter trees with a dbh < 36 cm. Ecologically, it would be valuable to have more thick snags, because they provide more habitats for xylobiontic species and take longer to decay, providing their ecosystem service for a longer time span. The larch-stone pine forest is especially rich in habitat structures: 10.7 trees per ha have at least one hole or cavity in the trunk, but also dead branches, broken tops and bark injuries are frequent. The spruce forest features 5.9 trees per ha with holes or cavities, the larch pioneer forest only 1.4. The density of tree regeneration lies clearly below thresholds needed for a sustainable forest regeneration, this especially pronounced in the larch-stone pine forest. Additionally, the diameter distribution differs strongly from the negative exponential diameter curves typically found in undisturbed virgin forests – this divergence is especially distinct in the lower diameter ranges.
With respect to stand structure, tree species mixture, dead wood and the presence of habitat structures, the Aletschwald, and in particular the larch-stone pine forest, has clearly developed towards a natural forest. Tree regeneration, however, is noticeably influenced by the high ungulate population. If there is no change towards an increased density of tree regeneration until the next inventory in about 20 years, the original aims of the forest reserve 'forest conservation and natural forest development' will be at risk.