Boreal and arctic siliceous scree and block field

This habitat type includes all kinds of boreal and arctic unvegetated accumulations of siliceous boulders, stones or gravel, except for the littoral habitats. They form a base-poor substrate that harbours acidophilous plant communities. The habitat type is heterogeneous in regard to its biota, as it extends from the small blockfields of the southern boreal taiga to the highlands of Iceland. Screes and blockfields are produced by various geological processes. Scree usually refers to a collection of broken rock fragments on slopes or under cliffs produced by slope processes (also called talus formations). Screes often show a sorting of rock fragments. The largest blocks falling off from cliff faces roll down the furthest, whereas the finest material accumulates in the uppermost part of the slope. Other boulder and gravel fields originate from glacial deposition, frost action breaking rock outcrops in situ, or e.g. frost heaving from moraine. Rock glaciers and ice-dominated moraines are distinguished as a separate habitat H4.3, however. Yet another type of boulder fields is related to ancient beach deposits constituted by former coastal constructional processes. Sparsely vegetated screes and block and gravel fields are distributed across the whole boreal and arctic region but with varying abundance. The largest screes are found in Iceland (incl. gravel fields) and along the Scandinavian Mountain range, where they reach mid- or high-alpine levels. Large stable blockfields cover mountaintops in northern Fennoscandia and some quite large ancient beach deposits encircle high hills along the ancient coasts of the Baltic Sea. The vegetation of scree slopes represents a complex that covers many vegetation types from forests at the foot of the slopes to scrubs and sparsely vegetated unstable screes in the upper parts of formations. The forest and scrub types are not included in the habitat described here, but included under G- and F-types, respectively. More or less flat boreal blockfields may be also covered by trees or sometimes by scrubs, but also in this case, the habitat type in question only refers to open areas that do not have a tree or scrub layer. In such screes, the most characteristic assemblages of vascular plants are found in unstable patches, where also weak competitors can persist. In other blockfields, the role of vascular plants is small. The vegetation typically consists of lichens and bryophytes with different growth forms dominating different microhabitats, e.g. crustose and foliose lichens and small cushion-forming bryophytes on the sides of boulders, and fruticose lichens and mat-forming bryophytes in the hollows between blocks. Where vascular plants find enough soil between blocks, they form sparse vegetation. Transitions may occur towards grasslands or heathlands while near mountain summits transitions towards fjell-fields (H5.1) may be found. Probably the most important steering factor in screes and blockfields is a natural or semi-natural disturbance regime, which maintains characteristic species assemblages. In the arctic, gravel and blockfields are kept open just by the harsh climate, but in the boreal zone slow overgrowth has been observed in some regions. In screes, the disturbance regime is characterized by the continuum of periodic rockfall, instability of the substrate, and in some regions also by the long tradition of grazing. In some cases, grazing or, e.g., hiking or mountaineering activities may cause additional erosion in the habitat to such an extent that it disturbs the formation of typical vegetation. In forested areas, especially small blockfields tend to become more and more vegetated, starting gradually from the margins. In these habitats, the characteristic scarce vegetation may be in the long run dependent on regular forest fires.

Indicators of good quality:

• natural or seminatural disturbance regime, with a continuum of periodic rockfall and instability of the substrate or (in some cases) forest fires
• no or little succession towards scrub and forest
• continuation of traditional grazing (where relevant)
• no disturbance (for example by hiking, grazing, etc.)
• diversity of lichen, moss, and vascular plant species

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats