Information and Advsiory Note Number 76, January 1997
1.1 Choosing appropriate methods of ground preparation for forestry has become more complicated in recent years for a number of reasons. Trees are being grown to meet a wider range of objectives, and deliberate management for nature conservation and visual amenity has become more important. Commercial forestry is increasingly involved in restocking areas of felled plantations and natural regeneration is being used as a means of establishing both native woodlands and conifer plantations. These changes, together with technical advances in machinery, have resulted in the development of techniques capable of various degrees of soil disturbance.
1.2 Over the last 50 years, much of the expansion of forestry has been on relatively infertile unwooded sites and it quickly became apparent that cultivation could result in improved early survival and growth. Machinery was adapted from agricultural equipment and by the 1960s and 1970s forest ploughs were making intensive cultivation possible. Despite the benefits to establishment, there proved to be problems with deep forestry ploughing. Root development was impeded, leading to poor tree stability, and early improvement in growth rates was not necessarily carried forward into the rotation. There were also significant environmental impacts both on and off site connected with soil disturbance, effects on water run-off and the visual impact on the landscape. These problems led to a review of guidelines and the development of a range of alternative techniques.
1.3 The woodland manager today is faced with an array of machinery, opportunities and choices. This Note summarises a more detailed report (Worrell, 1996). It aims to provide an introduction to the options, and an overview of the criteria for selection of ground preparation techniques. To begin with, some explanation of terminology may be useful.
2.1 Ground preparation refers to the clearing, drainage and cultivation of forest sites. This advisory note concentrates on cultivation which covers any operation designed to provide a favourable planting site for young trees. Drainage involves engineering to control runoff from sites, and guidelines are given in the Forestry Commission’s Forest and Water Guidelines (FC, 1993).
2.2 The standard forest plough with a furrow depth of ~45cm is very deep by agricultural standards. Forestry Commission terminology puts a shallow forest plough at up to 30 cm, a deep plough at between 45 and 60 cm, and very deep ploughing at anything up to one metre. Ploughs have one or two mouldboards (the plate which displaces and turns the soil). Where two are fitted there will be two ridges (turves) made on either side of the furrow. Ploughs may also have a tine attached which extends below the level of the furrow and rips a thin line in the ground beneath. This helps to break up impermeable layers which restrict rooting or prevent drainage. A turf plough has no tine attached. Ploughs can be mounted on, or trailed behind, a tractor and ploughing can be spaced at regular intervals or complete where the entire ground surface is disturbed.
2.3 Mounding and dolloping operations create a 10-50cm pile of soil into which trees are planted. When the mound is made at regular intervals by a continuous acting machine at the back of a tractor, it is termed mounding. When a backactor digger is used to scoop out individual piles of soil creating a mound beside the pit, it is termed dolloping. This is sometimes carried out together with drainage, and the spoil from ditches is used to make regularly spaced mounds - a technique known as ditch dolloping.
2.4 Scarifying is where the surface of the soil is scraped into heaps or turned in patches. A disc trench scarifier is like a rotating harrow or disc of tines which clears a continuous strip and leaves a line of loose soil for planting. A patch scarifier has mattocks mounted on a wheel which scrape a clear planting patch at regular intervals. A turfer scarifier is essentially an intermittent shallow plough.
2.5 In indurated soils or those with an iron pan, simply dragging a tine (30-105cm deep) through the ground can help to loosen the soil and increase the effective rooting depth. This is known as ripping. Attachments can be added to the tine which increase the amount of soil disruption either below the soil surface - termed subsoiling - or at surface level as a shallow plough in a technique known as ripper screefing. In moling, a bullet-shaped tine is dragged through the soil beneath the surface producing a small subterranean drainage channel.
2.6 The traditional method of manual cultivation is pit planting where trees are planted in a hand-dug hole refilled with loosened soil. Hand turfing involves cutting squares of turf up to 20cm deep and planting on the inverted turf which is either returned to the pit or placed alongside. Simple hand screefing with a spade or mattock clears the vegetation and humus layer, creating a planting site. Often trees are planted without clearing the vegetation in a notch made with a planting spade.
3.1 The primary aim of any cultivation is to provide a well drained, aerated planting site free from competing vegetation. It can also help to remove run-off, improve drainage, increase the rooting depth of soils and disrupt soil horizons - so releasing nutrients to young trees. Cultivation makes planting easier and faster and usually allows trees to be found more easily when weeding. It should increase early growth, so reducing the period when young trees are vulnerable to browsing and frost, and make growth throughout the plantation more even.
3.2 There has been little research comparing the benefits of different cultivation techniques. While it is apparent that cultivation improves both survival and early growth rates compared with direct planting into unprepared ground, there is little evidence of significant differences between the cultivation techniques used. There is insufficient information to say whether alternatives to ploughing will affect rotation length or total productivity and therefore have an impact on financial returns.
3.3 Of course, techniques are designed to meet the requirements of particular site conditions. In restock sites the tree stumps make scarifying the surface layers a more feasible option than ploughing and, in freely draining soils, scarifying may be all that is required. Scarification alone, however, will do nothing to break up an iron pan or induration where some form of sub-soil disruption is necessary. Ripping with a tine may be sufficient although ploughing is likely to disrupt a greater area. Ripping is less effective in gley soils where moling provides the added advantage of creating a channel for drainage. Mounding and dolloping are increasingly being prescribed as alternatives to ploughing, particularly on restock sites. As less ground is disturbed and no continuous channels formed disruption of the site and sediment run-off are much reduced. These methods are less visually intrusive and avoid the problem of restricting rooting in a particular direction which has led to windthrow in many ploughed sites.
3.4 Survival rates appear to be increased where there has been some soil cultivation -direct planting and hand screefing giving lower survival rates. This may be less important when creating more natural type woodland where higher levels of losses may be acceptable. The quality of planting stock can be a more important factor in initial survival than the type of cultivation.
4.1 Ground preparation impacts on the environment in a number of ways. It obviously affects the structure of the soil but will also impact on amenity and recreational values. It is not always obvious whether changes are detrimental or beneficial. Changes could be compared with the status of the soil under the preceding land use or with comparable changes under natural conditions. For example, the effect of windthrown trees on soil profiles is not dissimilar to some forms of cultivation, and ‘improved’ soil conditions may be expected under new native woodland. The aim of conservation, however, should be to conserve relatively unaltered soils and encourage practices which promote natural processes of improvement.
4.2 In conservation terms there are readily identifiable negative impacts of cultivation. Direct effects on soils include changes to soil profiles and pedogenic processes; erosion from exposed soil and drainage; soil compaction by heavy machinery and changes in organic matter status, water status, aeration and nutrients. There are also effects on vegetation both indirectly through soil changes and directly from removal and deposition of soil.
4.3 The impact on watercourses is likely to take the form of changes in yields and flow patterns, sedimentation and changes in chemical content. Cultivation will also significantly affect natural landscapes and, in some cases, may physically impede access. The scale of any of these impacts is likely to be a function of two main factors; the amount of soil disturbed and the proportion of the ground surface directly affected. Average figures for these factors are indicated in Graph 1 although individual measurements can vary substantially depending on site conditions and machine settings.
4.4 Ploughing does cause gross disturbance of the soil which can be easily detectable decades or even centuries later. However, the amount of soil displaced can be of the same magnitude as that displaced in windthrown areas of semi-natural forest. Dolloping and mounding would normally produce less disturbance since the cultivation is intermittent at each planting site. Disc scarification inverts less soil than mounding techniques.
4.5 The rate of erosion resulting from cultivation depends on both the amount of soil disturbed and the way that water is removed from the site. Almost any cultivation will increase levels of erosion in the short term, but the impact will be considerably greater if there is excessive transport of soil off-site into watercourses. Sedimentation resulting from erosion off-site can impact on river flora and fauna and the careful design of ploughing patterns and drainage schemes is vital to keep this to a minimum. Ripping and moling which provide obvious drainage pathways may contribute to higher levels of erosion than intermittent cultivation such as mounding. The impact of ploughing and drainage on water yields and flow patterns is considerable with an acceleration of run-off leading to a shorter time to peak flows, an increase in magnitude of flows and lower watertable levels. Flows will return to more normal levels as the forest matures.
4.6 Cultivation and drainage will change the water status and aeration of the soil since that is the aim of the process. This in turn will impact on the vegetation although it may have less effect than that of a new woodland canopy. Cultivation will also make a range of new habitats available.
4.7 Changes in organic matter status are likely where peat sites are drained and cultivated. In deep peat areas the impact will be significant and cultivation is not normally recommended.
4.8 The visual impact of ploughing is considerable and has led to a review of guidelines. Ditch dolloping at a high density can also be very noticeable. Generally, mounding and scarification have less impact and are noticeable for a shorter time.
5.1 The choice of technique will depend on a number of factors but the objectives of management and sensitivities of the site are probably the most important. Cultivation will speed up the process of afforestation or regeneration and the intensity of cultivation should balance the imperative of establishment with the cost and extent of the impact. In areas where the natural heritage is considered a priority, it would be most appropriate to opt for techniques which promote natural processes and maintain site variability. Inevitably this may mean a longer timescale for establishment. Normally there should be a presumption against drainage and more effort made to match species to the existing site potential.
5.2 There are, however, practical reasons for speeding up establishment. Grant schemes have a limited time window for establishment; protection measures (fences and rangers) may need to be maintained until establishment is complete; and, there are likely to be financial considerations relating to returns over time.
5.3 The costs of cultivation are normally between £50-£100/ha although dolloping with a digger and manual methods of cultivation can cost as much as £200-£300/ha. Costs will increase where ground conditions are poor and on steep slopes. Mechanical cultivation of very steep sites can sometimes only be carried out safely with a plough or ripper tine which help to control the tractor.
6.1 The preferred options for native woodland establishment where natural heritage considerations are a priority would be natural regeneration or where planting is necessary, zero cultivation or manual cultivation - particularly hand turfing in wetter gley or peat sites. Where some form of intervention is deemed necessary to create a seed bed or planting site, scarification and mounding are the preferred mechanical option since these minimise the amount of soil disturbed and the impact on watertables.
6.2 The preferred options for planting and restocking of plantations to minimise the impact on the natural heritage interest would again be manual methods of cultivation or direct planting near existing stumps on felled areas. Mounding and disc scarification are the most appropriate mechanical options in most sites, although in peaty gleys scarification is inappropriate. On lowland brown earth sites, an agricultural plough or subsoiler with screefer could be used. Ripping and medium depth ploughing is a final option in podsol sites or where iron pans must be disrupted. In wetter sites a combination of subsoiling and moling or dolloping would be acceptable where less intrusive methods are impractical.
Worrell, R. 1996. The environmental impacts and effectiveness of different forestry ground preparation practices. SNH Research, Survey and Monitoring Report No 52, SNH, Battleby. (An illustrated appendix is also available for reference from the library of Scottish Natural Heritage).
FC, 1993. Forest and Water Guidelines. The Forestry Commission, HMSO.
Roland Stiven (on behalf of SNH)