82 FOREST REPORT 2005
4.4 Naturalised tree species
Only 0.6% of the trees in the Swiss forest are naturalised foreign species. These pose no threat to the forest.
Planting exotic species plays merely a minor role in today’s forest economy.
The proportion of exotic species in Swiss tree stands rose by 0.1% between 1985 and 1995, but this increase has not worried the experts.
Distribution
Roughly 12 000 plant species fromallover theworldhavebeen introducedinEuropeduring the past 500 years, but until today only about 400species(3%)have managed tobecome permanent- ly established.Such newcomers, known as exotic plants orxeno- phytes,arealso part of forest flo- ra. These plants may be useful, asare,for example,afewforeign tree species in thewoodindustry, but they may alsocausedamage if they displace native plantsand eventually theanimals that feed on them.
The threat for Swiss forests is, however, small as only 0.6%
of the treesare exotic andinjust 0.4% of theSwiss forest area do they makeupmore than50% of thegrowingstock.Exoticspecies are said tobe dominant if they form 50% or more of the stock.
In recent years the situation has remained stable with the frac- tion of exotic species in Swit- zerland’s tree stands increasing by just 0.1% from 1985 to1995, which does not worry the ex- perts.Our native florais very re- sistant becausemigratingpeople are thought to havebeenbring- ingnon-native plantswith them since theIceAge.This hasled to
4.4.1
False acacia
False acacia comes fromNorth America. The treewas planted throughout Europe as anornamental and forest tree and has since gonewild.
83 4BIOLOGICAL DIVERSITY
thedevelopment ofamixedflo- ra, which can also hold its own against the newexoticspecies in- troducedfromoverseas since the discovery of theNewWorld.
Non-native plants still pose a certain risk.Those species that can spread without human as- sistance and outcompete the native plants in their habitats are especially dangerous (inva- sive species). False acacia (Rob- inia pseudoacacia),for example, which originatedinNorthAmer- ica,prefers poor soiland displac- es native pioneer plants that also specialize in nutrient-poor habi- tats.Falseacaciais therefore one of eleven invasive species on a
“blacklist”of plants that should be preventedfromspreading.
FURTHERINFORMATION Swiss Federal Research Institute WSL 8903 Birmensdorf
Research Dept. LANDSCAPE Section Landscape inventories National Forest Inventory (0) 44/ 739 23 43 Research Dept. Forest Section Silvicultural strategies (0) 44/ 739 24 86
Sottostazione Sud delle Alpi (0) 91/ 82152 30
Swiss Agency for the Environment, Forests and Landscape SAEFL 3003 Bern
Forest Agency
Nature and Landscape Division (0) 31/ 324 05 37
4.4.3
Number of stems and percentage of naturalised species
Trees12 cmin diameter at breast height or larger.
Species Scientificname Number Percent False acacia Robinia pseudoacacia 1103 000 0.21 Douglas fir Pseudotsuga menziesii 917 000 0.17 European
blackpine
Pinus nigra 317 000 0.06
White pine Pinus strobus 186 000 0.03
Cottonwood Populus spec. 157 000 0.03
Northern red Quercus rubra 151 000 0.03
Other natura- lised species
153 000 0.03
Total no. of naturalised species
2 984 000 0.56
Use
Cultivatingexotic tree species is an important source of income in the forest economies of some centralEuropeancountries.This is not the case in Switzerland, where the quantity of such tim- ber produced is so small that there is only aniche market for themost frequent specieslike the Douglas fir.
In just 0.4% of the Swiss forest area do exotic tree species make up more than 50% of the growing stock. This situation has remained stable in recent years.
4.4.2
Areas dominated by naturalised species
Percentage of forest areas where over 50%of the growing stockconsists of foreign species.
Switzerland: 0.4%
■up to 0.0%
■0.1-0.5%
■0.6-1.0%
■1.1-1.5%
■over 1.5%
0.0
0.0
0.0 0.0
0.0 0.0
0.2
0.5 0.4
1.9 0.6
2.2
0.8 0.5
