Information and Advisory Note Number 19                                                Back to menu

Rivers and their catchments: potentially damaging physical impacts of commercial forestry

1 Introduction

1 1 This information and Advisory Note describes the potentially damaging physical impacts of commercial forestry on streams, and outlines practices which limit, mitigate or ameliorate these impacts. This Note should be read in association with Rivers and their Catchments an Overview (Information and Advisory Note No 18) Area officers should consider these hydrological impacts along with wider landscape and ecological issues when assessing commercial forestry planning applications. If good practice is followed, many of these impacts can be avoided or reduced.

1 2 In Britain, planted forests account for approximately 10% of the total land area. The majority of new planting occurs in Scotland, where around 13 5% of the land is occupied by commercial woodland. Forestry activities such as planting and ditching may alter the catchment hydrology, increase sediment erosion and transportation, and alter stream water chemistry. These impacts may affect the abundance and diversity of aquatic biota downstream of the afforested area

1 3 The aims of the Government's forestry policy are:


1 4 The Forestry Commission is the Government Department responsible for forestry in Britain. The Commission comprises two main departments - Forest Enterprise Agency and The Forestry Authority Forest Enterprise manages the national forest. The
Forest Authority is responsible for regulation, advice, and grant-aid

1 5 Advice should be aimed at good forest management rather than reactive management of impacts. The Forestry Commission produced Forests and Water Guidelines (Forestry Commission, 1993) which seek to ensure that forests and woodlands are managed and developed so that their social, environmental and economic benefits are realised for the community at large. These guidelines should be consulted for further information and practical details on good practice and mitigation measures.

1 6 In recent years, modification to forest establishment practice has reduced many of the potentially damaging impacts. Mounding has largely replaced ploughing, producing a smaller hydrological impact due to the discontinuous nature and smaller volume of soil affected. Increasingly, the matching of species to sites and greater use of broad- leaves has meant the need for aggressive drainage is reduced. The use of watercourses as compartment boundaries, open space, broadleaved areas and windfirm edges has reduced the direct impacts of shading and sedimentation. There is increasing evidence to suggest that native woodland in particular is beneficial to aquatic ecosystems (SNW1996)


2 Impacts and mitigation of forestry activities on rivers


2 1 Hydrology

211 Ploughing and restructuring of drainage patterns may occur as part of ground preparation work prior to commercial tree planting. Drainage ditches are often aligned at right angles to the slope, with interception ditches to reduce run-off within the plough furrows. Drainage causes peak flows to arrive more rapidly in the receiving watercourse, particularly during moderate rainfall events. This effect decreases with increasing storm size and no major increases in peak flows have been detected for large storm events, when the saturated catchment would already be supplying all the run-off directly to the streams. Poor forestry drainage can result in localised downstream flooding. Figure 1 shows a sample hydrograph for a moderate storm pre - and post - ground preparation



2.1.2 Catchment water yield may be increased initially by the drainage of water previously stored in the soil, and the reduction in the storage capacity of drained ground. As a plantation matures, water yields from the afforested catchment may decline due to greater interception by the canopy and increased evapotranspiration losses. Water yields from young forests or felled areas are unlikely to be significantly different to those from moorland catchments. However, as the forest matures it is estimated that for every 10% of the catchment planted with coniferous forest, there is a 1 5 to 2% reduction in the annual run-off. The reduction in water yield from mature forests may be critical during low flow conditions if base flow is affected by a reduction in the rate of aquifer recharge.

2 13 Correctly spaced cut-offs can be used to slow down run-off from cultivation channels and to minimise and delay discharge reaching the receiving watercourse. A 40m spacing is good practice on a 5% (3°) slope. Drain gradients should not exceed 3 5% (2°) and should be less on erodible soils. Drains should be installed immediately afforestation begins.

2 1 4 Drain design should incorporate sediment traps which must be accessible and maintained periodically. Good practice should ensure that the maintenance of traps and drains does not coincide with salmonid spawning, periods when young fish inhabit the gravels (approximately October to May in upland catchments) or other environmentally sensitive periods such as bird nesting. Drains must end before the riparian zone of the watercourse, and planting furrows should include a 3-5m wide strip of vegetation between the end of the furrow and the dram.

2 1 5 Careful planning and good practice will help to mitigate the impact of forest drainage systems on catchment drainage. Care should be taken to avoid damaging small streams and the natural drainage pattern. Natural watercourses should not be diverted into drainage channels and surface run-off should not be routed to adjacent catchments.

2 1 6 During commercial coniferous harvesting operations (usually 30 to 50 years after planting), clear felling exposes the bare ground surface and thus increases the risk of erosion. Tree felling increases run-off and stream flow, with less rainfall being intercepted and evapotranspired, particularly as the decay of the underlying mat of dead branches (brash) allows more rainfall to reach the ground surface. The size of stream flow increase is dependent upon the age and composition of the forest at harvest, the soil and the climate.

2 1 7 In-stream and riparian flora and fauna have adapted to the existing range of flows Changing the flow regime through forestry activities may alter the community structure.

2 2 Sediment inputs.

2 2 1 Forested catchments generally have greater in-stream sediment loads due to erosion, than undisturbed non-forested catchments. The amount of sediment in the stream is highly dependant on forest age. Particularly large soil losses have been found during ground preparation and harvesting.

2 2 2 Forestry practices may increase the input of sediment to the drainage system. Fine sediment increases turbidity in watercourses, while coarser material settles out on the river bed and alters the morphology and habitat availability within the channel (see Information and Advisory Notes 22, 23 and 79). Sediment can also block pipelines and damage hydroelectric schemes or water treatment plants Sedimentation may, in time, affect the lifespan of reservoir storage.



2 2 3 Increased levels of suspended sediment can affect stream biota by reducing light penetration, by affecting fish respiration and feeding, by smothering spawning gravels, and by smothering plant life and altering the community structure of invertebrate populations Information and Advisory Notes Nos. 22 and 23 discuss turbid water.

2 2 4 During ground preparation, bare soil and spoil is exposed to the erosive forces of water, wind and frost. Impacts are greatest in upland catchments where the elements are more extreme. The potential for erosion is further exacerbated by the increase in peak discharge associated with reduced interception and evapotranspiration following ground clearance.

2 2 5 Forest roads are a major source of sediment in catchments with commercial forestry, particularly from well-used roads and roads that have been regraded. Studies in the Balquihidder Catchments, Central Scotland (Johnson and Bronsdon, 1995), showed that for a given rainfall event, the total sediment load from a used forest road (before regrading) was in excess of 2 5 times that from an unused road. Total sediment yield from a used regraded road (40kg in 4000I) was up to 5 times that from an unused forest road (8kg in 4000I) Freeze/thaw events produced less erosion than rainfall.

2.2 6 Road erosion can be reduced using a gravel surface layer over 20cm thick. Silt traps within road drains will reduce sediment throughput. Where practicable, roads should avoid crossing streams, and any stream that is crossed should be culverted. Roadside drains that may contain sediment should discharge to a buffer zone rather than directly to a stream. Good practice should prevent drains becoming contaminated with oil and chemicals. Any embankments associated with roads should be vegetated to aid stabilisation and prevent excessive erosion.

2.2 7 Harvesting activities re-expose soil to erosion by wind, water and freeze/thaw action, particularly on steep valley sides The mat of brashings left in place during harvesting provides some protection, though the intensive use of forest roads during harvesting is a major sediment source.

2 2.8 Buffer zones aim to trap and filter out sediment, mitigate direct inflow of acidic surface water, and reduce the nutrient concentrations in run-off before it enters the stream. The buffer zone should be wide and gently sloping to allow water to spread across the zone. Buffer widths vary and the Forests and Water Guidelines should be consulted for recommended widths. Buffer zones prevent excess shading of the stream by mature trees and should allow the growth of diverse bankside vegetation.

2.3 Stream chemistry

2.3 1 The planting, growth and harvesting of trees can have significant impacts on stream water chemistry. The use of fertilisers (e g phosphate, potash, nitrogen and potassium) is common to encourage tree growth in nutrient-poor upland soils. Nitrate concentrations in the soil can also increase as a result of bacterial breakdown of brash. Approximately 10% of an aerial application of phosphate fertiliser can be lost in run-off during the first three years after application, but losses thereafter are small.

2.3.2 Nutrient inputs from fertilisers can be limited by applying fertiliser by hand or ground machine rather than aerial spraying, and consequently, aerial spraying is now being undertaken less often. Fertilisers and pesticides should not be applied during wet weather, high winds, snow cover or on frozen ground because of the increased risk of wash-off and off-site pollution, and they should not be applied close to watercourses or waterbodies, buffer zones or stream corridors. All chemicals should be stored away from watercourses in bunded areas with a 110% capacity In addition, a contingency plan must be formulated to deal with accidental spillage. Containers must be safely disposed of by a registered waste disposal handler.

2.3 3 Nutrient enrichment has the potential to cause algal blooms, resulting in dissolved oxygen depletion, fish and invertebrate mortality or migration. Few instances of algal blooms have been reported in the uplands due to the rapid turnover of water. In addition to chemical impacts, the shading of streams as the forest grows, and deposition of needles leaves on the stream bed and banks, can reduce the general productivity and diversity of stream-dwelling flora and fauna.

2 3 4 Acidification is a long-term process caused by industrial pollution. Acidification occurs where the inputs of acidic sulphur and nitrogen compounds exceed the buffering capacity of the soils and geology through which run-off passes, prior to entering a stream or loch. The most acidified areas are upland catchments where base-poor soils and geology coincides with polluted precipitation Central and South-West Scotland have been identified as particular problem areas, and fish populations are low there due to the low pHs, low calcium and high aluminium concentrations. Mineral soils are generally more erodible than peats, and they can be a significant source of toxic aluminium in acid-sensitive areas.

2 3 5 There are four major mechanisms by which forests contribute to surface water acidification


In dry weather, water from deeper soil horizons accounts for more of the input to a stream, and the acidity of the water can be neutralised by base cations in mineral layers. During wet weather the deeper soil layers are saturated and run-off is generated by shallow organic horizons that contain acidic cations and anions In upland catchments in particular, surface run-off accounts for a large proportion of run-off, and therefore the potential for acidification is greater in such catchments

2 3 6 Acidification of surface waters can be treated by the use of lime. However, this is not suitable for conservation purposes since it alters the water chemistry, which is then reflected in the range of aquatic species present. Liming is costly, particularly in remote upland areas, and targets the effects rather than the causes of acidification. Reduction in pollutants is the only way to solve acidification.

2 3 7 A strategic approach is slowly being adopted to reduce atmospheric emissions of acid pollutants at international, national and catchment scales


3 Further reading

The Forestry Authority 1993 Forests and Water Guidelines HMSO, London

Forestry Commission 1991 Forestry Practice Handbook 6 11th Edition Edited by B G Hibberd HMSO, London

Johnson, R C and Bronsdon, R K 1995 Erosion of forest roads Agricultural Engineer, 50, 4, 22-27

Scottish Native Woods (1996) Why manage riparian woodlands? SNW, Aberfeldy
 

4 For further details contact:

Earth Science/Forestry and Woodland Groups,
Advisory Services
Scottish Natural Heritage
2 Anderson Place
EDINBURGH
EH6 5NP


5 Author and Editors:

Jonathan Clark, RPS Cairns Ltd
Editors: Katherine Leys, SNH
John Kupiec, SNH
 

 

 

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