Wet woodland

Wet woodlands are an important habitat often overlooked in floodplain restoration. They are a rare and extremely fragmented habitat need of expansion.

They can be created on mineral sites, but will take many years to develop their full characteristics. As they develop they will provide opportunities for a range of wildlife.

Most are dominated by alder and various willow species. Black poplar once occupied areas of recently deposited mineral soil, but now only a few thousand trees survive along lowland streams.

The benefits of re-creating floodplain woodland are:

Biodiversity conservation – excellent habitat for birds and invertebrates.
Water quality improvement.
Flood control through wooded washlands.
Enhancement of landscape integrity.
Timber production – hardwoods and short rotation coppice.

Design

Hydrological conditions need to be suitable. There must be a permanently high water table.

Important criteria are:

Regular inundation. Floods scour channels between trees and re-fill former channels and pools. Accumulated woody debris should be allowed to hold water in shallow lakes and swampy areas.
Channel movement. Natural floodplains have a distinctive disturbance regime with the river constantly re-shaping, undercutting banks and moving sediment around.
Any floodplain woodland creation would ideally be integral to wider floodplain restoration.
Planting schemes should match the topography, with alder-sallow on the lowest land to oak-ash on the higher. Open areas of swamp and shallow water are valuable.

Today, the majority of floodplain woodlands are recent stands around flooded gravel pits. However, there are plans to incorporate wet woodland in some large wetland restoration schemes. Milton Keynes Parks Trust aim to create a floodplain woodland on the River Ouse through the process of gravel extraction. The existing river and channel features will be retained and create new multiple braided river channels, pools, islands, marshes, and sandbanks. An additional benefit will be the reduction of peak flood flows downstream, hence reducing flooding.

Many floodplain forest species are pioneers that readily colonise new disturbed ground, so natural regeneration is a good option. Species that are slower to colonise can be planted to create a mosaic of diverse woodland types mixed with wetland habitats. Grazing can be introduced after about five years, using small herds of suitable, hardy animals at around one animal per six hectares.

Establishment techniques

Natural colonisation

Natural colonisation will suit sites adjacent to existing semi-natural woodland.

Preparation of a rough seedbed will often enhance germination and take of new trees. Do not use fertile topsoil as this will encourage weed species.

Pioneer species, such as willows and birch develop a dense scrub cover relatively quickly (around 10 years). Thereafter natural succession will gradually change the balance as new species colonise. This can take a very long time, but it is a natural process so the value for wildlife will remain throughout.

Benefits of natural colonisation are:

Low cost – although rabbit fencing and deer control may be needed.
Natural composition – of species adapted to the conditions found on the site.
Natural distribution of trees – natural look to the landscape as opposed to planting in rows.
Promotes conservation of local biodiversity.

Disadvantages:

Slow to develop full assemblage of species (but valuable in the meantime)
Can look untidy during establishment

Tree and shrub planting

This would be appropriate where natural colonisation is unlikely eg where the site is far from existing woodland, or results are required especially quickly.

Whatever the planting method used it is important to ensure that:

Trees and shrubs are planted in loose friable soil to ensure adequate water storage.
Rootstock is always protected from desiccation.
Competing rank grass and other plants are managed for 4-5 years (see below).

When preparing the soils, redress any compaction where necessary by ripping.

Do not improve soil fertility - in areas with extremely low fertility consider other habitats such as mires. Tree establishment depends on root penetration and low competition. Poorer soils help to reduce competition from grasses and herbaceous vegetation, and develop a diverse ground flora.

Planting cuttings

Willows and poplar will readily establish from cuttings:

Take cuttings from one-year old material, these should be 200-250 mm long with the top cut about 10 mm above a leaf bud and the bottom cut just below a bud.
Heel cuttings into loose, friable soil (to half the height).
Protect from grazing and competitive plants as above.

Wet Scrub establishment

Establishing wet scrub will add conservation value to open wetlands and wet woodland edges. However, ongoing management is critical to maintain the desired balance.

Interfaces between scrub and surrounding habitats are important; certain species need this, and it provides warmth and shelter for species. Avoid regular straight edges.

A diverse structure increases the variety of species can shelter, feed and breed. Natural colonisation by scrub produces a varied age structure, spatial distribution and species diversity. Several species, such as blackthorn and willow, are ready colonists of suitable ground. Where possible, and where establishment time is not a constraint, this should be the favoured option.

Once established, the structure is likely to need diversifying through rotational cutting, thinning and natural regeneration. It will then require periodic rotational management to maintain it.

Planting patterns and spacing

Plant naturalistically, not in straight lines - this leads to wind tunnelling and a simplified structure of limited value for wildlife.
Sparse planting leads to the development of more natural woodlands than the dense planting traditional in commercial silviculture.Wider spacing involves fewer trees, lower planting and lower maintenance costs. It produces bushy trees and a slower closing canopy, as well as reducing weeding during establishment, and delaying thinning for several decades.
» at an average spacing of 3.6 m, 722 trees are required per ha.

Closer spacing, results in taller trees, earlier canopy closure and requires earlier thinning, otherwise a simple structure develops lacking many of the features needed by birds and other wildlife. Closer spacing than 2m should not be considered.
» at an average spacing of 2 m, 2,500 trees are required per ha.
Randomised spacings, where some areas are denser and some sparser, reflecting species and topography, creates woodland that is more naturalistic.
Plant slower-growing canopy trees, either singly or in small clusters, at varying spacing of 2 – 10 metres. Enhance structural diversity by planting in between with shrubs and rapid-growing, short-lived trees, or by allowing natural regeneration.
Around 40% trees to 60% shrubs throughout the planting scheme will create structural diversity.
Plant shrubs randomly, some in tight clusters forming thickets, others more widely spaced; some clusters of mixed species, others of single species, depending on soils and topography. Slow-growing species should not be planted in association with faster-growing, competitive species.
Larger planting schemes should include changes in composition to match variation in soils and slope.

The use of fast growing nurse species to help soil formation, and to add shelter and warmth is common, but some species can be competitive so follow-up management to progressively remove unwanted nurse species will be required after 5-7 years. Minimise problems using nurse species that suit the target woodland community, e.g. native willows. Native alder is good on wet sites but is poor at establishing from natural regeneration.

Where nursery-grown trees are used, whips (small, bare-rooted stock) generally out-perform standards. They are usually more cost effective as prices range from £0.50 to £1.00 per unit, depending on species.

Planting should be undertaken during the dormancy period – November to February – avoiding frosty weather. Roots must not dry out. Heel unplanted stock in until it can be planted out. One person can plant 800-1,500 whips a day by hand.

Where they occur, protection from deer and rabbits is essential. Tree shelters are costly but increase survival rates, aid early growth and reduce the need for weed control. Fencing costs are high: ca £5 m-1 for rabbit fencing, ca £10 m-1 for deer fencing, but reusable types are available. Remove tree guards and fencing once the crop is well established (5+ years).

New plantings will need weed control for the first 4-5 years to limit competition. This need not be rigorous, as occasional losses will create useful gaps. Options include: biodegradable mats fitted around base of tree; wood chip or bark mulch application; glyphosate application once or twice a year; mowing or tillage. Mulching is effective but large supplies would be needed for large schemes. This and matting help to retain soil moisture.

Long-term management

These are typically managed by grazing. Stocking rates depend on sward productivity and are best prescribed locally - aim to produce a mosaic of tussocks and shorter turf. Where grazing is not an option, cutting and collecting a hay crop may be necessary.

Wet grasslands require low intensity or no grazing until July, to ensure that breeding birds are not affected by nest trampling, followed by more increased grazing in order to reach optimum sward heights for the next season. Where grass growth is particularly rapid, it may be necessary to introduce a low level of grazing earlier in the year to maintain the community structure.

Relevant case study

Manor Farm Quarry case study

The information set out within this advisory sheet in no way constitutes legal or regulatory advice and is based on circumstances and facts as they existed at the time Nature After Minerals compiled this document. Should there be a change in circumstances or facts, then this may adversely affect any recommendations, opinions or findings contained within this document