Ecosystem-friendly Forestry at Streams in Low Mountain Ranges

Wherever streams and their riparian margins are sited in the forest they create their own closed and sensitive ecosystem of longitudinally-zoned, connected expansion. Along with their diverse and linked ecological function the riparian margins and neighbouring wet forests are also habitats for tree species which are interesting from a timber perspective. The utilisation of these mainly productive sites need not be contradictory to the sensitivity of these sites and their protective functions. However, to fulfil both the production and protective functions it is necessary to adjust forest management in way that sees streams in their longitudinal expansion as a management unit and responds with site-adapted riparian and wet forest communities.

By respecting management and thinning guidelines, stable riparian forests remnants can and must be preserved. If they are not in optimal condition, they need to be developed using ecological measures so that both functional requirements are met.

Due to the EU Water Framework Directive’s requirements to ‘recreate a good ecological conditions’ of streams in a basin district larger than 10 km², forest owners are required to take action if their riparian forests are not in a semi-natural condition and if actions are not defined in a management plan. Many forest owners know little or nothing about ecological communities or their legal obligations to maintain and develop riparian forests.

In low mountain ranges destined for large-scale plantations of fast-growing coniferous species streams are often afforested right up to the stream bank. Often they are afforested with pure spruce stands or a large proportion of spruce. In submontane ranges they are also planted with Douglas fir. Silver fir is excluded because it can be assessed as a positive development.

Below recommendations to transformation forests along streams in low mountain ranges are presented. These are based on the results of the ‘Maintenance and Development of semi-natural Streams in Forests within Forest Management’ project (Support by Deutsche Bundesstiftung Umwelt, Project no. 2388). These recommendations are addressed primarily to foresters/forest managers.

General conditions

The reference region for these recommendations is montane and oreal low mountain ranges. The results of the study can be generalised into recommendations/guidelines for foresters (forest ranger and forest officer) and into forestry management tools for forest enterprises and owners. Based on the field surveys and the measures carried out, possibilities are shown on how to proceed with identifying existing water body-specific deficits. Furthermore, we show how to implement stream development measures and construct attractive and sustainably utilisable stands without damaging the flora and fauna.

Riparian forests that are not adapted to the site are able to develop themselves to semi-natural stands over several centuries. So the transformation of non-site adapted stands into stable, semi-natural stream-accompanying forests over the medium-term can not be left to the forests themselves. Before necessary measures can be implemented however, one has to clarify what the current condition of the stream is, what the stand should look like in the future and hence what the silvicultural objective is. Therefore, some intermediate steps have to be taken into consideration (see. Fig. 2). Without site knowledge and therefore knowledge of site-adapted forest communities, no silvicultural measures should be carried out. Its helpful to compare with reference-forests if they are available near by.

Water-body specific characteristics

Every stream is unique. This has to be kept in mind during planning and management. However streams can be typified by their morphology and natural plant community into natural landscape units. They should be managed accordingly.

Type of water-body in low mountain ranges

In the bedrock of low mountain ranges, the upper reach of the streams are generally V-shaped incised valleys in which erosion occurs due to the steepness of the slopes. Downstream they change into U-shaped valleys, where erosion and sedimentation appear at the same time (cf. fig. 3). U-shaped valleys with wide alluvial plains are also possible at the higher altitudes of low mountain ranges. They are of glacial origin. On very steep slopes it’s possible for young streams to have a box-shaped cross-section without the characteristic valley shape.

Forest communities at streams in the low mountain ranges

The most important riparian forest communities in low mountain ranges and their characteristics are listed in the ‘Forest communities of small streams’ article. Basically they are separated by their water balance and flooding frequency. Real riparian forests are flooded occasionally to frequently. Wet forests are characterised by mainly near-surface groundwater. Ravine forests accompany streams in humid and moist valleys (mainly V-shaped).

Apart from some exceptions (e.g. Salix multinervis) willows are irrelevant to forested low mountain streams. This is because semi-natural stands are heavily shadowed (whilst willows are light demanding pioneer species) and that deeply incised V-shaped valleys are not colonised or only their few dry shallow parts are colonised during summer. It is also possible that floods following heavy rains in June/July wash away the sensitive seedlings (Schwabe 1987).

Tab. 1:Valley shapes and forest communities in low mountain ranges.
ValleyVegetation TypeCharacteristic of the Vegetation
V-shaped Mainly zonal forest communities No riparian margins exist:
  • Dependent on the altitudinal zone beech-; beech-fir.- and beech-oak-forest communities
  • Humid and talus-rich V-shaped valleys: maple-ash-forest

Wide river systems

Mainly azonal forest communities Riparian margins exist:

Dependent on the width of the riparian margin, the frequency of floods and the altitudinal different forest communities such as

  • Black alder-ash-forest
  • Wood stitchwort-black alder-forest
  • Black cherry-alder-ash-forest
  • Local grey alder-riparian forest
  • Oreal: spruce-bog forest

Survey of the actual condition

By way of introduction to the silvicultural recommendations for riparian forest management the actual conditions guiding decisions about the measures implemented should be clarified. The following list of questions in tab. 2 will help. The impacts, consequences and specific details are shown in tab. 3.

Tab. 2: Survey of the actual condition as decision guidance for silvicultural measures (1).
Natural site factors
  • Shape of the Valley (erosion or sedimentation area)?
  • Altitude and local climate characteristics?
Artificial site factors Interference through:
  • Road constructions?
  • Dams or other constructions with barrier/dam impacts?
  • Straightening of the stream?
  • Water extraction?
Silviculture Situation


Water body environment (25 m-strips on both sides)

  • Composition of tree species?
  • Age?
  • Form of mixture and structure (vertical classification) and texture (horizontal classification?
  • Regeneration (amount, age and composition of species)?
  • Browsing by game?
  • Pervious management?
Operational Situation

Division into compartments

Opening up/ timber harvest

  • Is it possible to treat the water body and its surrounds as one management unit?
  • Is a soil-protecting and water body-friendly utilisation of the accompanying stands possible?
  • Is it possible to include neighbouring stands in the thinning operations to decrease harvesting costs?

Actual-target adjustment

It is possible to draw some conclusions about the actual potential natural forest community at the stream from the natural and artificial site factor parameters (see tab. 2 and 3). The potential natural forest community must be determined separately for every stream and is the role model for developing currently inadequate forest stands into semi-natural stream accompanying riparian and wet forests.

Because the Water Framework Directive requires a good ecological condition (ecological status classification ‘good’) and not very good (ecological status classification ‘high’), its not necessary that the actual development objective completely equates to the actual potential vegetation (PNV). Depending on the forest community, up to 15% of non-site adapted tree species, mainly spruce, is tolerated (refer to the ecological status classification of Baden-Württemberg below). In certain areas determined by the EU Habitat Directive the allowance of foreign species is twice as high – up to 30%.

Silvicultural recommendations

In tab. 4 recommendations and instructions have been compiled for practitioners to manage streamside forests in a sustainable and site-adapted way and to fulfil nature and water conservation laws.

Tab. 4 : Silvicultural recommendations for the development of semi-natural forests along small stream in low mountain ranges.
Reference area
  • Generally the entire riparian margin of a small stream should be used as a base for silvicultural measures, minimum strip 25 m wide (average tree height) on both sides of the water body (water body environment). Even if no riparian margin exists (V-shaped valleys with zonal forests), this strip should be considered as a spatial treatment unit;
Planning horizon
  • Streamside silvicultural measure priorities should be determined depending on the initial stand and the urgency: short-term (10 years), medium-term (20 years) and long-term (>30 years); openings of pure coniferous stands have top priority;
  • Arrange premature utilisation of immature stands (mainly spruce younger than 50 years) with the forest administration (Germany);
Composition of tree species
  • Depending on the forest community (riparian forest, wet forest, ravine and talus forest) a broad tree species spectrum is possible: black alder and valuable deciduous species should be maintained in particular along with pioneer species.
Structure and texture
  • In the course of several felling operations an uneven vertical and horizontal structures arises from successive opening of the canopy;
  • Adapted diversity of tree species enriches the texture of forests;
  • Allow site dynamics were possible;
Management of cuttings
  • No clear cutting (forests streams are oxygen-rich cold-water streams, abrupt exposure to sun inverts the existing ecological conditions);
  • In preparation of natural regeneration maintain or release in the upper and intermediate layer as many deciduous trees and silver firs as possible, independent of its quality. (For reasons of deciduous litter deposition, improvement of the germination bed, maintenance of root fungi, seed tree function, canopy, structure richness);
  • Consider sunburn when releasing light-sensitive tree species;
  • Keep in mind the surrounding stands to decrease harvesting costs (operational reasonable felling operations);
  • Consider the felling order (spatial order) at water bodies; intensive thinning of pure spruce stands increases the risk of windthrow in the remaining stand;
  • Semi-natural riparian forest remnants are valuable and must be saved. The most that should be undertaken is regeneration felling and/or removal of spruce trees;
Timber harvest
  • No crossing of the water bodies by forest machines, if necessary install temporary crossings (e.g. PVC pipes);
  • No driving along the water body or the water body surroundings;
  • Implement site-adapted timber harvesting techniques (cable crane, motor-manual processing);
Herbaceous and shrub layer
  • As soon as the canopy is opened up the felling flora appears. At wet sites and sites close to water bodies the natural seeding and growth is faster than at dry sites;
  • Generally allow succession and natural regeneration where possible (easier at nutritious sites than at nutrient-poor); usually only possible if enough generation capacity exists in the pre-stand (mainly at vigorous sites);
  • Assuming deciduous seed trees exists, it is possible to allow succession, where necessary supplemented by plantations; consider seed disposal from downstream drifting of seeds;
  • In deciduous-poor pure coniferous stands start early with advance planting (shade tolerant tree species in closed stands, e.g. beech; light-demanding tree species such as ash and sycamore maple in femel gaps. Open these gaps by peripheral fellings to control the light); planting is necessary where no seed trees exist;
  • To stimulate natural or artificial regeneration deciduous-(spruce)-rich stands or pure stands are opened with intensive thinning, semi-natural parts remain untouched;
Plant material and plantation
  • Use autochthonal plant material with a large proportion of fine roots;
  • Favour large plants and choose the planting site carefully in difficult terrain, use the protection effects of the old stand, but bear in mind the concurrency of the root and the reflection of the radiation; otherwise plantations with wide spacing;
  • Plant willows as cuttings from autochthonal material;
  • Use plantation methods with low impacts on soil structure: the water conductivity of the soil will be remained in this way (not possible in difficult terrain);
  • Stimulate main and secondary tree species evenly and preserve shrubs in young stands/natural regeneration;
  • Where necessary prevent natural regeneration of spruce
  • Regular annual check-up during the first 5 years;
Browsing by game
  • Planting soft-wooded deciduous trees such as willow cuttings or birch and mountain ash can help to decrease browsing intensity;
  • Felling flora (raspberry, blackberry, pioneer species such as willow and poplar) appear after opening the canopy and helps to draw browsing away from the regenerating stands;
  • Check if deer repellents are necessary (fence or single protection); keep in mind the entire situation;
  • Adjust hunting, raise awareness among the hunters, search for consensus;
Old timber and coarse woody debris
  • Retain coarse woody debris in the stand and in the water body as well as non-usable remaining timber, e.g. root collars or large branches)
  • Determine periods and amount in the forest management planning;
  • Plan buffers (no coarse woody debris in streams close to forest edges due to the risk to downstream floodplains during floods);
Open areas
  • Not all opened areas should be afforested immediately, partial exposure to light promotes diversity, especially near springs;
  • Basically remove neophytes during regeneration (rip them out!), e.g. Japanese knotweed;
Side effects
  • Consider implementing or initiating (soft-) deciduous species as measures to improve grouse habitat (hazel-grouse, capercaillie);
  • Opening of forest stands in areas frequented by tourists may be interesting (creation of view points);
  • Check accounting and refinancing opportunities from streamside habitat improvement measures via ‘eco-accounting’ or sponsorship programs/subsidies;


  • This article is part of the "Handbook Forest and Water"