The rowan in Switzerland

The rowan or mountain ash (Sorbus aucuparia) is found throughout virtually all of Europe – a vast area, spanning from deep Siberia to the northern forest line in Scandinavia.

This wide distribution, including in terms of altitude, is due to its extraordinarily high tolerance to different sites. According to Leibundgut (1984), marshes, black alder stands and virgin soils are the only places this often shrub-like tree species does not appear.

90% of rowan trees grow in the upper montane and lower subalpine zones, with half of these lying more than 1,300 metres above the sea (Figure 4). The NFI rowan sample plot at the highest altitude is currently located 2,167 metres above the sea at Saas-Balen, Valais. When combined with the alpine alder (green alder), the rowan can be found at altitudes of up to 2,300 metres. It is the deciduous tree with the greatest altitudinal distribution, just ahead of the birch and grey alder (Figure 4). The occurrence of this photophilous pioneer species is largely conditioned by competition. It is more common than average in sparsely populated mountain forests, in selection-type, non-uniform high forests, in young growth, and in uniform high forests with clearings.

The rowan usually thrives on slopes (lateral light) at inclines of 10° or more. At lower altitudes, it prefers the cool, wet climate of the north-western to north-eastern slopes, in contrast to the whitebeam. Like the spruce, the rowan favours acid subsoils but can also be found in alkaline soils. It appears in combinations in numerous forest communities and dominates only in the very rare green alder/rowan forests. More than any other deciduous tree species, the rowan is mainly distributed in coniferous forests. Two thirds of rowans populate spruce stands; the rest are found in fir, larch, beech and maple forests.

With a population of 2.7 million trees (BHD ≥ 12 cm) and a proportion of 0.5%, the rowan appears rather insignificant. However, if plants 10 cm or taller are taken into account, the tree is found on 31% of NFI plots. This widespread tree species is short-lived, typically living 80 to 100 years, but no more than 150 years. Rowans taller than 15 to 20 metres (NFI1: max. 24 metres) or wider than 25 to 30 cm (NFI1: max. 37 cm) are rare.

Game browsing is often highlighted as a threat to a sustainable population. According to the NFI, previous year browsing affecting rowans 10 to 129 cm tall has fallen from 44% to 35% throughout Switzerland over the past two decades. At the same time, despite the considerable damage caused by browsing, the number of young rowan trees in this height class has increased significantly, as has the number of rowans in secured regeneration (classes BHD 0.1 to 3.9 cm and BHD 4.0 to 7.9 cm) throughout Switzerland. However, these predominantly positive average values should not hide the fact that browsing damage is often substantially higher at local level and the population may be at risk.

According to the fourth NFI 2009/2017, the rowan has a stock share of 0.08% and an average stem volume of 0.1 m³, making it a typical secondary tree species with little value for the timber industry, similarly to the whitebeam, field maple, bird cherry and grey alder, for instance. After rapid juvenile growth, the increment decreases after 20 years.

The timber is only of economic importance in northern Eurasia as wood for furniture, for example. Thin trunks make good walking sticks and tool handles, while thicker specimens are valued by wood turners and carvers. This soft wood was previously also used for gears, dowels and axles. Today it is sold as firewood at best.

In silviculture, the rowan plays an important role as a natural pioneer species in the regeneration of subalpine spruce forests or in the natural succession of tall forb/fir forests. It is resistant to frost and snow load and, as a pioneer species, provides the necessary protection for climax forest species with regard to windthrow or clearings. Being very elastic when young and able to reproduce through seed formation as well as basal sprouting, offshoots and stump sprouts (Figure 6), in the subalpine zone the rowan is essential to the afforestation of rock avalanches and as a protective forest species in biological barriers against torrent and avalanches.