What we know about tree species
A Protective Forest generally should have a stable and a durable structure. A forest stand with trees of different ages and several tree species is less vulnerable to damaging events. If, for example, a tree species falls out due to a harmful organism, the important protective effect can still be ensured by the remaining ones. Due to their higher center of gravity and weight old, relatively large trees are more susceptible to windthrow or wind breakage than trees of medium diameter. In case of a storm event the younger trees are quickly in place and able to maintain the protection against natural hazards.
The optimal tree species composition does not exist as such, it depends on the imminent natural hazard, on the potential vegetation at the forest site as well as on the altitude, the exposition, the slope or the basic bedrock type.
In order to build and shape the best-performing Protective Forest with its optimal age structure and tree species composition, constant maintenance interventions and proper management are needed to preventively counteract possible damage. Wild game affects the forest stand; if there is a lack of food in winter, they damage the bark or bite the tops of young trees. In order to support natural regeneration in the protective forest in particular, a well functioning wildlife management with an integral and large-scale approach is needed.
Overview of important tree species in Protective Forests
Spruce: The natural range of the spruce covers a large area in Austria, its lower, natural limit in the past was about 500 m, in the Alps it often reaches up to the vegetation limit. Especially in subalpine and montane altitudes spruce naturally occurs in pure stands as well, making it one of the most important tree species in Protective Forests. The spruce is sensitive to oxygen deficiency in the root zone and with its shallow roots it can only access poorly penetrable and strongly water-saturated soil horizons with great difficulty. In severe drought and long periods of heat the spruce suffers, becoming easily susceptible to the most important organism of damage, the bark beetle. In future, due to climate change, it is expected that its range will be limited to higher altitude areas.
Silver fir: Silver fir is an important tree species of the montane mixed forest. In Austria its distribution area is found in the low mountain ranges as well as in the high mountains especially in the entire montane to low subalpine region. The distinctive shade tree species forms a tap root and is thus less susceptible to wind damage than the spruce. Due to the historical development of forests in Austria the fir has lost much of its original range. In Protective Forests it can still grow vigorously into the regrowth phase even after a long period of shade due to its pronounced shade tolerance. Game browsing is the most frequent cause of its absence in the forest stand.
Larch: Larch has a very wide distribution in Austria so it can be observed from the Wienerwald to the highest forests in the Central Alps. The pioneer tree species is important after large-scale calamities in Protective Forests such as after windthrow. Due to its high need for light it is sensitive to overshadowing or lateral constriction of other tree species. Due to its heart-root system, it is firmly anchored in the soil, is only slightly susceptible to storms and is hardly at risk to snow damage. In Protective Forests it is relatively insensitive to rockfall.
Swiss Stone Pine: The Swiss stone pine is an important tree species in Protective Forests especially at the highest inner-alpine altitudes. It colonizes sites with extreme environmental conditions close to the timberline as well as in rock and debris piles. The Swiss stone pine actually develops a taproot; in addition numerous lateral roots in rock crevices provide stability.
Common beech: Beech occurs naturally in submontane and montane locations and is therefore an important tree species of Protective Forests at this altitude level. It can absorb high impact loads (rockfall) and, due to its growth form, hardly tends to break crowns under dynamic impact loads. Except on wet or very shallow sites it is well anchored by its heart root. In a mixture with sycamore maple and conifers (fir, larch, pine), beech is therefore well suited for protection against rockfall at these altitudes. Similar to the taproots fir and oak, beech can use water from deeper soil layers and is therefore an important tree species for protecting against floods.
Mountain maple: This tree species does particularly well in cool, moist climate; it occurs mainly in medium to higher locations of the mountains in Austria. Older maple trees are rather light-demanding but young plants are extremely tolerant to shade. Mountain maple also roots in screes and thereby stabilises them. Even large injuries of the stem heal very well, so it is excellently suited for the protection against rockfall. However, it is more susceptible to frost than coniferous trees.
Trees versus natural hazards
Rockfall: In the case of rockfall trees in a Protective Forest act as natural barriers, slowing or stopping falling rocks when they graze a trunk or hit it with full force. In addition, the trees stabilize the ground with their roots. The best protection against rockfall is provided by a dense forest with trees of different thicknesses and several tree species. Ideally, with those tree species that can heal trunk injuries well such as sycamore maple. In spruces, large bark injuries can lead to rot fungi entering the tree causing decomposition of the wood structure. They are therefore not particularly suitable in a protective forest against rockfall.
Avalanches: The forest is able to prevent the start of an avalanche. Especially wintergreen trees, such as spruce or Swiss stone pine, a lot of snow is deposited on the tree tops. This makes the snow cover underneath less thick. Part of the intercepted snow on the treetops sublimates again, the remaining part drops down to the ground and can thus prevent an unfavorable snow deposition. Due to the far-reaching branches at the edge of the forest stand, lower wind speeds prevail, and in addition, the interior forest climate usually has a positive effect on the snow cover. In Protective Forests trees, branches or lying wood provide a high degree of ground roughness which stabilizes the snow cover and can help prevent it from sliding off. An ideal Protective Forest against avalanches is thus well structured, contains wintergreen tree species and the trees have a sufficiently large diameter to avoid being affected by snow sliding and creeping. The avalanche protection of the forest is of course limited only to those areas where tree growth is possible. If avalanches occur above the tree line and make their way down into the forest, the forest only acts as a decelerator and can reduce the range of the avalanches.
During floods, the forest, and especially the forest floor, acts as a natural reservoir and it is proven to reduce runoff peaks. Activities of terrestrial organisms loosen the soil and thus provide improved infiltration. In addition, trees consume a lot of water through transpiration. During rain or snowfall the tree canopies intercept some of the precipitation, with most of it being evaporated to the atmosphere again. The interception capacity depends on the tree species as well as the stand structure. The most interception - over the entire year - is provided by evergreen tree species.
Landslides: The roots of trees consolidate the soil particles and have a stabilizing effect. Especially in shallow landslides the forest stand helps to prevent erosion. Due to the water consumption of the trees as well as interception, there is generally less water available in a forest stand. This has a positive effect against the initiation of landslides. In the case of deep-seated landslides, the cause is usually below the range of influence of tree roots, meaning a forest stand has little or no effect here.
There is no such thing as the one protective forest that is effective for all elevations or against all natural hazards. Protective forests against avalanches require dense stands, especially of wintergreen tree species. Protective forests against rockfall should also not be too loose in order to effectively slow down or stop the stones. Here, a good spread of diameters is also needed, i.e., sufficiently thin trunks in addition to thick ones. Although these thin stems cannot completely slow down a stone, their deflecting effect ensures that they often influence the direction of the falling stones, thus increasing the probability of hitting a sufficiently thick tree. In alpine high altitudes, so-called "Rotten" trees serve as stabilization. Groups of dense trees provide a reduction in wind speed and support each other's growth by creating an improved microclimate.
Typification of forests is to help forest owners in selecting tree species
Protective forests are of high importance in Austria. Until 2023, a climate-related classification of sites and forest types will therefore be undertaken for all site- and object-protective forests in Austria. This classification is to provide concrete recommendations for a suitable tree species composition on specific forest sites and is to foster sustainable management.
Tree species in protective forests
Tree species in protective forests
ÖWI 2007/09 (Austrian Forest Inventory)
Natural or close-to-natural tree species composition
61 percent of the forest area
MONE 2015 Indicator Report