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17/3/2006

Vegetation

Vegetation history

From the point of view of vegetation, Sweden is a young country, recently formed after the last glaciation. Immigration of plants has taken place in steps and proceeded from different directions. Norway spruce, today our most common tree species, came as late as about 3000 years ago and reached its present southern boundary just some hundred years ago.

The south-western parts of the province of Skåne (southernmost Sweden) appeared out of the ice about 14000 years ago and by that time had land connection to Denmark and the British Isles. The climate was that of tundra. Herbs and grasses initially established were in part what today are considered alpine plants, and in part species originating from the large steppes, which at that time were found in Europe. The virgin soil was at the beginning completely sterile and the pioneer plants were probably tied to nitrogen fixing fungi and bacteria.

During the period from ca. 14000 until 12000 years ago climate improved and the ice melted at a high rate. Scots pine, birch, aspen and mountain ash trees moved in from the south. Around 11000 years ago the climate became more severe and the recently migrated tree species were forced back. After a cold period lasting ca. 500 years climate improved rapidly and Scots pine, birch and other tree species returned, this time accompanied by several species of vascular plants. The migration north was temporarily stopped in southern Svealand (southern Sweden), the reason being the Yoldia sea, which at that time connected the early Baltic with the The North Sea, one connection being the narrow Närke sound.

In the period 10000 to 8000 years ago the land bridge to Denmark stepwise broke up and a simultaneous land rise slowly drained central Sweden (Svealand). During this period plus the following 3000 years the climate was very favourable, many vascular plants and deciduous trees moved in from south. In the same period Scots pine, birch, different willow species and grey alder moved in from the north. The forests in southern Sweden were dominated by hazel, elm, and oak and somewhat later also lime and ash, whereas the northern parts were dominated by birch and pine.

The climate slowly became cooler. At about 3000 years ago beech and hornbeam moved in from the south and spruce from the north and east. In the south Man to an increasing extent started to force the forest back in order to develop cultivation and pasturage. This was a process taking place at the time of the younger stone age. From the early days of the Bronze Age the influence of Man has strongly affected vegetation. Cultivation and pasturage have supported plants that prefer open, disturbed and sometimes also nitrogen-rich environments. In addition Man has both intentionally and unintentionally moved plants.

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Distribution of species

Distribution of species strongly depends on climate, and many species therefore have a northern and a southern limit in Sweden. Plants adapted to a maritime climate are found in the southern and south-western parts, while plants preferring a continental climate may be found in the eastern parts of Sweden. The migration itself also affects plant distribution. Most of our vascular plants moved in from south-west via the land connection with Denmark and the British Isles, whereas many of the species found in the north, like spruce, moved in from north-east and east. These migration patterns, starting after the last glaciation, may still influence today's distribution of plant species.

Because of the relatively late colonisation of Sweden only few endemic species have developed, compared to older plant geographical areas. Plant distribution strategies also are of importance. Some species form a huge number of seeds easily spreading a long way to new habitats while other species spread a lesser number of seeds in their vicinity where growth conditions obviously are sufficiently good. Species distribution depends also on their ability to compete other species and to endure strain from weather, pasturage, parasites, and diseases. Extreme weather or damages from for example insects can in short time cause changes in vegetation, particularly for species living at their limit of distribution. One example is the mountain birch, which distribution strongly is affected by epidemic attacks by the autumnal moth (Epirri'ta autumna'ta). Further, long-term climate or environmental changes, like the global warming caused by increasing emissions of carbon dioxide, or acidification caused by sulphur emissions, severely can disturb species distributions.

Man has always had a strong direct influence on species distribution and plant community composition. In course of time this influence has increased, and today there are very few, if any, unaffected areas. Much of the influence is unintentional and is the result of changes in agriculture, forestry, social structure and nature conservation. Sometimes the changes are intentional, for example afforestation and extermination. Transfer of plants may involve risks because they can bring pests or diseases attacking local species or vice versa. Introduced species may spread rapidly, like for example Reed Sweet-grass (Glyceria maxima (Hartm.) Holmb.), Hogweed (Heracleum sphondylium L. ssp. sibiricum L.), Nootka Lupin (Lupinus nootkatensis Donn ex Sims) and Canadian goldenred (Solidago canadensis L.), and thereby drive more original species out from the plant communities.

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Plant communities/vegetation types

A coarse description of vegetation in a given area may be obtained by dividing the vegetation into types according to the characteristics of species composition. One kind of division is based on dominating species. Another kind of division is based on so called indicator species, species that without being dominant ones, simply through their presence indicate special environmental characteristics. Normally such indicator specie is accompanied by a well-defined group of other species. The Nordic Council of Ministers (NCM) has supported a division of vegetation into types common for the Nordic countries based on dominant species in tree, and field layers. This division is based mainly on two gradients; soil moisture and nutrient availability in soil, respectively. The types, of course, have a limited geographical distribution that is described for each such type. Forest vegetation is divided into main groups according to tree composition. These are:

Coniferous forest

Pine forest

Lichen, shrub, Sphagnum and herb types

 

Spruce forest

Shrub, fern, low herb, tall herb and Sphagnum types

 

Other coniferous forests

 

Deciduous forest

Birch and aspen forests

Mountain birch forests: lichen, shrub, grass, low herb and tall herb types

 

 

Birch forests: shrub, grass, herb and Sphagnum types

 

 

Aspen forests

 

Beech and hornbeam forests

Beech: shrub and Wavy Hair-grass (Deschampsia flexuosa (L.) Trin.), low herb and tall herb types

 

 

Hornbeam forest

 

Oak, elm, ash, lime, and a mixture of these species (nemoral forest)

 

 

Alder forest

 

 

Other deciduous forests

 

Mixed deciduous and coniferous forests

Lichen, shrub, grass, herb and Sphagnum types

 

 

Young mixed coniferous-deciduous forests (open ground spontaneously forested)

 

The above main types may be further subdivided according to species composition.

The characterisation (vegetation types) recommended by the NCM aims primarily at producing a background material for planning of land use, for example in protected areas. It has also proven to be useful at vegetation inventories for environmental monitoring and biodiversity. In forestry there is a need to use easily distinguishable criteria to identify soil fertility and the production capacity of a given site. This is of special interest when the existing stand does not give a proper picture of the site capacity. Such cases appear when the existing stand is uneven, in part damaged or mismanaged. A system for site quality classification based on site properties was developed by Björn Hägglund and Jan-Erik Lundmark at SLU. Together with information on climate, soil mineral material type, soil type and soil moisture, vegetation is used to indicate site productivity. Starting with the bottom layer, vegetation is primarily classified as belonging to either moss or lichen type. Moss-type sites are thereafter classified into either:

Tall herb type

Low herb type

Soil without field layer

Broad-leaved grass type

Thin-leaved grass type

Sedge-horsetail type

Bilberry type

Cowberry type

Crowberry-heather type

Poor dwarf-shrub type

 

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Vegetation data of the Swedish Survey of Forest Soils and Vegetation (SK)

The Swedish Survey of Forest Soils and Vegetation (SK) is a taxation of forest site properties encompassing 23500 permanent plots. Plot frequency is highest in the south and decreases with increasing latitude. The inventory covers a small part of Sweden's surface only and, thus, describes in very general terms where and at what frequency different plants appear. Common species with a wide distribution are easily registered in the survey, whereas less common and rare species with special environmental requests are very likely not to be found at the permanent plots. All results about the distribution of such species are therefore very uncertain. In MarkInfo we therefore have focussed on the more common species. The few data registered about less common species may still have a value indicating possible temporal changes taking place at a given plot. During the years 1983-87 the inventory encompassed ca. 70 species and groups of species of vascular plants, mosses and lichens. For the 1993-2002 inventory this number was increased to ca. 230. In this inventory are also included a certain number of stem-living lichens.

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Evaluation of the vegetation data of the Swedish Survey of Forest Soils and Vegetation

At present an evaluation is taking place of the data on understorey vegetation collected in the Swedish Survey of Forest Soils and Vegetation. The primary intention is to describe the current status of vegetation with emphasis on biodiversity in relation to environmental factors. In connection with this evaluation the usefulness of the data are investigated, as well as the response of indicator species to different pH values and nitrogen status in forest soils.

According to the evaluation so far it is not very meaningful to estimate biodiversity using ca. 230 species and groups of species, and even less meaningful using only 70 species/groups of species. In addition plot data show that almost half of the plots housed only one species. In spite of this limitation a map of the diversity has been made which mainly indicates regional differences. It is likely that differences in diversity also indicate a real difference.

An important result from the investigation is that several common species have been related to environmental factors, such as humus pH and nitrogen, although an uncertainty still exists due to the fact that data for vegetation and soil chemistry are not collected at the same plots.

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