Ecosystem-friendly Forestry at Streams in Low Mountain Ranges

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.

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.

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.

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).

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.

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.

• Appendix: Example: Recommendations to proceed with stand transformation by montane streams
• Full article

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