The spruce and climate change: An area for retreat in the alps?

The human influence on natural forests, which has lasted for almost 2.000 years, has changed the composition of tree species in favor of spruce at the expense of beech (Table 1).  The "industrialized" use of wood served large scale harvesting practices and was interested primarily on conifers rather than beech. On the other hand, forest usage from peasant communities utilized single stems which resulted in the preservation of various structured uneven ages forests in private woodlands.

The reduction of species diversity and the emergence of pure spruce stands have diverse anthropogenic causes such as a high demand for timber, clear cutting practices, excessive grazing or the exorbitantly high population of wild game. The largest contributor to the promotion of spruce in recent decades may have been made by hoofed game which significantly hindered the development of spruce competitors.

In table 1 the average tree species composition for the entire forested area of the Bavarian Alps was calculated but it doesn't show the actual mixing proportions. There are, to a greater degree, significant areas with practically pure spruce stands in the mountain forests. Results of the second federal forest inventory shows that on almost three quarters of the forest area in the Bavarian Alps practically pure stands of spruce are to be found and are predominately in the age classes 2 to 4. This corresponds to five times  the proportion of naturally occurring forest types composed primarily of spruce, especially at altitudes of 1.200 meters above sea level (table 2). In lowland areas the spruce can be found naturally, however it rarely takes a dominating stance.

The Alps are particularly susceptible to climate change and  the warming there in the recent past was about three times as strong as the world average. According to a regional climate model a temperature increase of around two degrees Celsius is to be expected. The optimal climate for the spruce lies between 5°C and 7.5 °C. This means that the spruce could hold its dominant position in the subalpine and alpine zones (Table 2). However, it will be subjected to increasing competition through classical mixed mountain forest species such as silver fir and beech.

Algorithms from the forest information system for the northern Alps calculated habitat models for the most frequent tree species using regionalized climate scenarios (temperature increases during the growing season from 1.5°C and 3,2°C) for the year 2100. From these models it can be deduced how climate change will affect the vitality of the spruce in various forest types (Table 3). In the entire montane area (this means half of the mountain forest area) the spruce will lose vitality and presence in forest stands. In subalpine areas the vitality of the spruce could increase.

This is supported by the analysis of tree-rings which examined the reactions in growth in dry years within  the various altitudinal zones in the mountain forest. Thus it can be found that the spruce has the relatively strongest losses in growth after dry periods in the lower elevations (<1.000 m above sea level). On the other hand, spruce shows a predominately positive effect on growth in elevations over 1.4000 meters above sea level. This means that the high mountain areas can become a place of retreat for the spruce. 

The proportion of spruce will not only decline in the future due to climate change but also because of silvicultural operations. This arises since the forests in the Alps fulfill an important protective function on many sites. Additionally, pure stands of spruce are very susceptible for calamities. The highest commandment of the hour is therefore the cultivation of mixed stands of spruce, silver fir and beech.

If spruces from high elevations are planted in lowlands, then they will usually break their buds and sprout in the spring earlier than the lowland spruce trees. With a rapid warming in  the higher elevations this effect could possibly also occur there. As a result, the spruce in the higher elevations could be exposed to an increased risk of late frost - at least until they have become adapted to the new climatic conditions.

Parallel to the decreasing vitality of the spruce in  the montane areas, the vitality of the eight-toothed spruce bark beetle (i.e. typographer beetle) is strengthened. It can create more than one generation a year in  the high mountain ranges and can become a primary pest. A visual witness to this are the bark beetle "nests" which can now occur in all elevations (Figure 2).

Shallow, humus poor sites are found in the montane, moderately dry, carbonate dominant mixed mountain forest. This affects a significant five percent of the Bavarian Alps. On these sites the spruce regeneration has difficulties to establish itself and clear cutting has led to a degradation of the soil. On these sites clear cutting caused a massive mineralization of the humus layer and thus the loss of the nutritionally favorable acidic organic layer. The availability of the elements phosphorous, potassium and manganese decreased. A deficient supply of these elements can lead to severe damage in spruce monocultures. Stable protection forests with the climax tree species spruce can only work out in the long run if  the humus layer is regenerated. Degraded sites can be reforested with the pioneer species larch and pine. The natural regeneration of spruce has to occur by itself. This can, however, take between thirty to forty years.

The nutrients nitrogen and phosphorus limit the growth of the spruce. However, particularly in the middle areas of the Bavarian carbonate Alps, these elements are commonly deficient. In order to avoid a further reduction in yield the humus layer should be regenerated so that nitrogen and phosphorus can be mineralized in the future. The protection of the humus layer on productively weak calcareous soils has to be prioritized over whole tree harvesting. The protection of the humus layer is of fundamental importance for the whole Alpine region since humus loss is probably also a result of climate change and occurs on sites which are not used for forestry.

Even on sites with better nutrient availability the spruce also has a problem to establish regeneration. Spruce regeneration can be out competed by that of beech, maple and silver fir where these mixed tree species are not disparaged by excessive browsing from wild game. The spruce can be helped on such sites through appropriate silvicultural measures.