Swiss Stone Pine – Portrait of a Mountain Forest Tree

The Swiss stone pine (Pinus cembra L.) stands out among the pine tree species of the Alpine region (fig. 1). It “crowns” the timberline, its altitudinal range explaining why it is sometimes also known as the “Queen of the Alps”. The stone pine also stands out for various biological characteristics (see profile). As well as its capacity for physiological adaptation, its interaction with the spotted nutcracker (Nucifraga caryocatactes) as a seed disperser is of particular note. And then the Swiss stone pine also plays an important ecological and economic role as the main tree species of the stone pine-larch forest community, especially in the Alps.

It is impossible not to recognise or hear the spotted nutcracker (Nucifraga caryocatactes), with its blackish brown, white-spotted plumage, its black tail with a white terminal band on the undertail, white undertail vents and characteristic, long and chisel-like beak, and its distinctive call (fig. 2). 

The spotted nutcracker feeds mainly on the seeds of the stone pine.  It collects them from late summer (fig. 3) onwards, and hides them in the ground as winter reserves. The bird has to find the stone pine seeds under the snow in winter and dig them out, often with a great deal of effort. The recovery rate of around 80 % is thus all the more astonishing. The seeds that remain buried are enormously important for the spread of the Swiss stone pine.  However, new studies show that, contrary to popular belief, the caches are not usually created on sites that are favourable for germination.

Ultimately, the decisive factor for the Swiss stone pine is how many seeds remain in sites that are favourable for the establishment of seedlings (fig. 4). Seed caches in clearings within existing forest areas and above the tree line, as they occasionally occur, may be of great importance for the Swiss stone pine in the current context of climate change.

The earliest evidence of the utilisation of Swiss stone pine comes from the Bronze Age (1400 BC). From the 16th century onwards, it is known to have been put to a variety of uses in addition to its use as fuelwood in the salt works, for example. This soft, easily workable wood was used to make utensils for mountain agriculture, such as bowls, vessels for milk, and spoons. Due to its antibacterial and insect-repelling properties, Swiss stone pine was popular for furniture used to store goods (cupboards, flour chests).

In addition to wood, forest litter was also used as fodder and animal bedding in certain regions until the middle of the 20th century. This had serious repercussions for the regeneration of the Swiss stone pine: because the raw humus layer was removed, not only the seed bed was raked off, but often also the seedlings developing in it. Out of concern for forest regeneration, litter and cone use was restricted or banned in many areas from 1900 onwards. As well as being limited by the use of litter, natural regeneration was further hampered by the hunting of spotted nutcrackers, which was financially rewarded in some regions until the 20th century.

Nowadays, more non-forestry products are being produced (fig. 6). In addition to the value chain of its products, for which no figures are available, the stone pine also has an intrinsic and above all an aesthetic value. Both these symbolic, centuries-old trees, marked by wind and weather, and the richly structured stone pine or mixed stone pine forests they form are much admired and appreciated. The quality of the stone pine landscape is an important argument used to underline the attractiveness of tourist destinations and thus for local value creation.

Rapid climate change does pose a major challenge, however. A case study in the Averstal valley (Canton Grisons) shows that the Swiss stone pines at lower altitudes have been growing less well since the 1980s. This is in contrast to the larch, which apparently benefits from climate warming (fig. 8). The long-lived Swiss stone pine can only move up the slopes slowly in response to the increasing competition as other tree species also migrate to higher altitudes.

The coming decades are expected to bring a wide range of environmental changes: warmer temperatures, rather less precipitation and longer dry periods. It can thus be assumed that the vegetation levels and thus also the habitat of the stone pines will shift upwards. It will not be easy for the Swiss stone pine: it does not compete well with other tree species advancing from below, and it grows slowly. In addition, the pine tree is almost only able to reach higher altitudes through the spotted nutcracker (fig. 9), and this species predominantly creates its seed caches in the already forested area. Likewise, the raw humus layer, which provides favourable germination and establishment conditions for the Swiss stone pine, is generally lacking above the timberline. 

In order to preserve stone pine forests as diverse habitats at the transition to the alpine zone under future climate conditions, an integrative approach is needed to protect biodiversity and the various functions and services provided by the forest, which include protection, utilisation and recreation. Local conditions and silvicultural history are important when assessing where and how to promote the Swiss stone pine. The two fungal pathogens snow blight (Gremmenia infestans, fig. 10) and Scleroderris dieback (Gremmeniella abietina) play a decisive role in the regeneration dynamics of Swiss stone pine and influence its spatial distribution. 

The Swiss stone pine prefers open forest structures (fig. 11). Although it is well adapted to mountain sites, a high stem density is a problem for the stone pine, especially in the shade of faster-growing tree species such as spruce. In order to preserve the Swiss stone pine even in the face of increasing competition, it should be promoted as early as possible through targeted tending of the young plants or later by removing competing species. In this way, the Swiss stone pine can also take on functions as a valuable mixed tree species even outside its predicted future distribution area, in the event that the spruce comes under increasing pressure there as a result of beetle damage.

When planting Swiss stone pines, attention should be paid to ensuring that:

• suitable small sites are selected; and that gullies and snowy depressions are avoided
• the trees are planted in small, well-spaced groups
• the trees are mixed with other high altitude tree species (e. g. Swiss mountain pine, larch)
• regional planting material in pots is used, taking into account a possible shift in altitude under future climate conditions
• where possible, the plants are bred at high altitudes (pre-adaptation)
• planting also above the current forest line is considered