Deutsche Bundesstiftung Umwelt

DBU-Project No. 22388

„Maintenance and Development of semi-natural Streams in Forests within the Forest Management“

„Erhaltung und Entwicklung naturnaher Fließgewässer im Wald im Rahmen der Waldbewirtschaftung“

Ecosystem-friendly Forestry at Streams in Low Mountain Ranges

Dr. Regina Ostermann

1 Introduction

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.

2 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.

Figure 2-1: Procedure to develop semi-natural forest stands by streams

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. Figure 2-1). 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.

2.1 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.

2.1.1 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. figure 2-2). 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.

Figure 2-2: Characteristic shape of mountain streams. Deeply incised V-shaped valley without marked bank (left), sunken U-shaped valley with moderately wide river bottom (middle) enabling the formation of riparian margins (right). In wide river systems the bottom is less defined by the valley slope than by the floodplain water bodies (LAWA 2000).

2.1.2 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).

Table 2-1: Valley shapes and forest communities in low mountain ranges

Valley Vegetation Type Characteristic 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

U-shaped

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

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).

3 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 table 3-1 will help. The impacts, consequences and specific

details are shown in table 3-2.

Table 3-1: Survey of the actual condition as decision guidance for silvicultural measures (1)

Category Question

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?

• Composition of tree species?

• Age?

• Form of mixture and structure (vertical classification) and texture (horizon-tal classification?

• Regeneration (amount, age and composition of species)?

• Browsing by game?

Silviculture Situation

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

• Pervious management?

Operational Situation

Division into compartments

• Is it possible to treat the water body and its surrounds as one management unit?

Opening up/ timber harvest • 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?

Table 3-2: Survey of the actual condition as decision guidance for silvicultural measures (2)

4 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 tables 3-1 and 3-2). 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 ac-

companying 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%.

5 Silvicultural recommendations

In table 5-1 recommendations and instructions have been compiled for practitioners to man-

age streamside forests in a sustainable and site-adapted way and to fulfil nature and water

conservation laws.

Table 5-1: 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;

Age • 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 cut-tings

• 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 in-termediate layer as many deciduous trees and silver firs as possible, independ-ent of its quality. (For reasons of deciduous litter deposition, improvement of the germination bed, maintenance of root fungi, seed tree function, canopy, struc-ture 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 tempo-rary 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;

Regeneration • 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 down-stream 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 un-touched;

Plant material and plan-tation

• 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);

Care • 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 re-generating 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;

Neophytes • 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;

Appendix

Example: Recommendations to proceed with stand transformation by montane streams

Table 1 presents an example from the Aitern Stream in the Southern Black Forest. Forest

stands on immediately neighbouring stream sections are described as insufficient or

culturally influenced and very natural according to the naturalness criterion. Two segments

have been chosen.

Table 1: Comparison of actual and target conditions for a transformation of the stand: actual condition, development objective and measures to increase the naturalness of two stream segments of the Aitern Stream in the Southern Black Forest.

Actual condition Objective Measures

Culturally influenced segment

Stand Old forest, spruce dominant, dense to closed canopy with a dense understorey of deciduous poles, uphill progression of spruce poles to timber stands, depleted herbaceous layer, acidophilic mosses.

Semi-natural maple-ash-forest with silver fir

Removal of spruce with intensive thinning

Regeneration Barely existent due to darkness, where it does it existent its dominated by spruce

Establish and secure upcoming regeneration

Enough seed trees in the environment for deciduous regeneration, no planting necessary

Mixture of tree species

Spruce 70 %, silver fir 20 %, beech, ash, sycamore maple 10 %

sycamore maple 30 %, ash 30 %, mountain elm, large-leaved linden/lime 10 %, 20 % silver fir (spruce)

Retain all deciduous trees and silver fir

Structure and texture

Single-storied and homogenous Uneven-aged stand with gaps

Results from the spruce removal

Coarse woody debris

No coarse woody debris exists Adequate coarse woody debris

Retain dying trees and large amounts of residual wood

Very natural segment

Stand Poles to timber trees, numerous cop-pices, uneven ages, predominately closed canopy but several gaps exist (amongst boulder rubble), single and multi-layered.

--- Individual fallen spruces as applicable

Regeneration Little regeneration --- ---

Mixture of tree species

Maple 65 %, Ash 20 %, Beech 5 %, Linden und Mountain Elm 5 %, Silver Fir 5 %

As no activity is required in semi-natural forests

Structure and texture

Uneven-aged stand, therefore horizon-tally and vertically structured

--- ---

Coarse woody debris

No woody debris in watercourse or surroundings

Enrichment of dead-wood

Leave felled Spruces in the forest stand

Aitern Stream, Southern Black Forest: left side semi-natural maple-ash-stand with mountain elm, right heavily modified pure spruce stand (timber stand)

Naturalness-levels

Table 2: Classification of the naturalness according to the state-wide forest development types (reference is the potential natural vegetation based on the composition of tree species of the upper and intermediate storey of the regional and zonal forest without natural regeneration)

Naturalness Share of tree species of the PNV

Very natural All tree species of the PNV are represent, area percentage of tree species total >85 %;

Semi-natural All tree species of the PNV are represent, area percentage of tree species total >75 %;

Somewhat semi-natural

More than 50 % of the tree species of the PNV are represent, area percentage of tree species total >50 %;

Culturally influenced More than 30 % of the tree species of the PNV are represent, area percentage of tree species total > 25 %;

Culturally determined All other stands.