74 FOREST REPORT 2005
4.1a T ree species diversity
Switzerland’ s surface would naturally be covered mostly by beech and spruce forests, which contain relatively few species.
Most Swiss forests today contain mainly commercially interesting species, such as spruce, fir , beech and ash.
The species diversity of trees in Swiss forests has increased during the past ten years by 4%.
This also promotes the diversity of other plants and animals.
Promoting competitively weaker, but ecologically more valuable tree species increases biodiversity.
Number of species
The twoSwissNationalForest In- ventories to date have revealed a pleasing trend: between 1985 and 1995 the species diversity of trees increased by 4%.The in- crease was largest south of the Alps (10%) and smallest in the Central Plateau (1%). The total area of monotonous forest also decreased slightly during the same period.This isanother pos- itivedevelopment becausemon- otonous forestscontain far few- eranimalandplant species than mixed stands. Programmes pro- moting tree diversity therefore make ecologicalsense.Thebeech andsubalpine spruce forests that today dominate the Swiss land- scape naturally contain rather few species. Forests rich in dif- ferent tree species, on the oth- er hand, which are often creat- ed through human intervention, provideamultifacetedmosaicof habitats. Silvicultural treatments can today promotecompetitive- ly weak tree species that require light in the sameway as the tra- ditionalforest management prac- tices,coppice forestand coppice with standards,used todo.Thus Zurich’s Sihl Forest, for exam- ple, which was once intensively used,is today richer in tree spe- cies thana beech virgin forest in theCarpathians.The situation is, however, different for those in- sects and fungi that depend on dead wood: they find betterliv- ingconditions invirginandnat- ural forests (> 4.5 Dead wood).
Nevertheless, around 1300 spe- cies of plants, including 12 in- digenousconifersand 43 indig- enousbroadleaf trees,still grow in Swiss forests. However, 70%
ofSwiss forest ismadeup ofjust three species of tree:spruce(40%
of the trees counted in 1995), beech(18%)andfir(11%).
The individual tree spe- cies have developed different- ly.The number of spruce,beech, fir, Scotch pine (Pinus sylves- FURTHERINFORMATION
Swiss Federal Research Institute WSL 8903 Birmensdorf
National Forest Inventory Research Dept. LANDSCAPE Section Landscape inventories (0) 44/ 739 23 43
Swiss Agency for the Environment, Forests and Landscape SAEFL 3003 Bern
Forest Agency
Section Forest utilization and timber industry
Section Forest conservation and biodiversity
(0) 31/ 324 77 78
75 4BIOLOGICAL DIVERSITY
tris), common oak and sessile oak went downconsiderably be- tween1985and1995,while those of sycamore,linden, rowan and whitebeam went up. The abun- dance of wych elm fell by 30%
when an introduced fungal dis- ease spread.In contrast, the oc- currence of English yew, a rare species,has increasedslightly.
SAEFL started a project in 1997 togetherwith the ETH Zu- rich to promote relatively rare tree species(SEBA),suchas yew, wildservice tree(Sorbus tormina- lis)or small-leaved linden (Tilia cordata).The aim of the project is to increase the populations of these species and to raise forest owners’ and managers’ aware-
Forest management
Aforest rich in tree species is of- ten the result ofcareful manage- ment. If no trees are felled, the forest will become increasingly older,andshade-tolerant species suchasbeech,firandsprucewill eventually dominate at the ex- pense of species that needlight.
Left to its own devices,a forest willgrowdenser overalongperi- od and the treelayerwillbecome more uniform. Felling trees, on the other hand,bringslight into the forest and thus promotesdi- versity. Although at most sites natural regeneration is prefera- ble,planting young treescan en- hancebiodiversity solongas the treesarelocalspecies.Nowadays, many forestsare less intensively managed,so that fewer regener- ationareasarebeingestablished and fewer stands thinned. This has, in the short and medium term,anegative effect on biodi- versity.
4.1a.1
Mean number of tree species
Trees12 cmin diameter andlarger on the500m2 sample sites of theNationalForest Inventory NFI2.
Switzerland: 25
■ up to 2.0
■ 2.1–2.3
■ 2.4–2.6
■2.7–2.9
■≥ 3.0 3.0
3.0 3.0
2.8
2.8
2.8 2.5
2.5
2.5 2.4 2.3
2.3 2.2
1.8
4.1a.2
Distribution of tree species
Trees12 cmin diameter andlarger on the500m2 sample sites of theNationalForest Inventory NFI2.
■≥ 3 species
■3 species
■2 species
■1 species
LFI1 LFI2 LFI1 LFI2 LFI1 LFI2 Broadleaf forest area Conifer forest area Total 100
75 50 25 0
[%]
ness. In addition to the SEBA species, the government is pro- moting the cultivation of oaks, especially on areas affected by the stormLothar.
The diversity of tree species in the Central Plateau is, with- out human intervention, great- er than it is in themountains,as biodiversity decreases with alti- tude.Speciesdistributedinareas with so-called Atlantic and sub- Mediterranean climates thrive betterat loweraltitudes because of the longer vegetation period and milder winter. Species be- longing to thiscategory are oaks, Norway,Italianandfieldmaples, elmsandwildservice trees.
