Forestry & Forest Sciences · Silviculture · also Tending Operations, Intermediate Treatments, Thinning and Stand Improvement
The intermediate treatments between regeneration and harvest that shape a stand's growth, quality and health.
In the exams: ICAR AICE-JRF 59 Unit 2 (tending: weeding, cleaning, climber cutting, mechanical, ordinary, crown and selection thinning, improvement felling, pruning, girdling); IFoS Forestry Paper I Section A (establishment and tending)
Tending is everything a forester does to a crop between its regeneration and its final harvest. The operations are weeding, cleaning, climber cutting, thinning, pruning, improvement and salvage fellings, release treatments, girdling of unwanted trees and fertilisation. Together they are called intermediate treatments in North America. They decide what the stand becomes: how fast the best trees grow, how straight and clean their stems are, which species dominate, and how well the stand resists wind, snow, fire, insects and disease. Two plantations raised from the same nursery stock on the same site can end up very different in value because of how they were tended.
This subject builds on two others. Silviculture principles covers crown classes, shade tolerance and stand dynamics, including self-thinning. Silvicultural systems covers the regeneration fellings that begin and end each crop. This manual does not repeat them. It takes the young crop as established and follows it to the final felling, explaining the purpose, method, timing and intensity of each tending operation, with the reasoning a forester needs to write or follow a tending schedule.
Tending is examined heavily. ICAR's AICE-JRF subject 59 lists weeding, cleaning, climber cutting, mechanical, ordinary, crown and selection thinning, improvement felling, pruning and girdling by name. The UPSC Indian Forest Service Forestry optional asks for descriptive answers on the same operations and on thinning grades and schedules for Indian species. Professional examinations such as the Society of American Foresters' Certified Forester ask about thinning methods, stand density measures, crop-tree release, pruning for clear wood and fertilisation response. Where definitions differ between Indian, British and American usage, this manual says so.
Weeding is the removal or suppression of herbaceous plants, grasses, shrubs and climbers that compete with young seedlings for light, water and nutrients during the first years after planting or natural regeneration. It is the most important tending operation in most tropical and subtropical plantations, where grasses and fast-growing shrubs can smother or starve young trees within a single growing season. Weeds compete above ground for light and below ground for water and nutrients; grasses are especially strong competitors for soil moisture, so weeding often matters most in dry seasons.
Weeding can cover the whole area (complete weeding), strips along the planting lines (strip weeding), or a circle around each plant (ring or spot weeding, often about a metre in diameter). Complete weeding gives the strongest response but is costly and exposes the soil to erosion on slopes; strip and ring weeding are cheaper and protect the soil. Methods include hand pulling, hoeing or slashing, soil working around the plant, mulching, mechanical cultivation between rows on flat ground, and herbicides where they are permitted and labelled for forestry. Slashing cuts weeds above ground but leaves their roots competing for water; uprooting or soil working removes root competition as well.
The number and timing of weedings depend on the species, the weed flora, rainfall and site. Fast-growing light demanders such as teak and eucalypts usually need several weedings in the first year and fewer in the second and third, until the crop overtops the weeds and begins to close canopy. Weeding is best done before the weeds set seed and before the dry season begins, so that the crop has the soil moisture to itself when water is scarce. The schedule for a particular species and region should be taken from the local working plan or plantation manual.
Cleaning is a tending operation in a young crop, not past the sapling stage, that frees the best individuals from undesirable trees of the same age that overtop them or are likely to do so. The undesirable stems may be coppice shoots of cut trees, which grow faster than seedlings in their first years; individuals of an unwanted species; or 'wolf trees', badly formed, heavily branched individuals of the crop species that take more than their share of space. Cleaning differs from weeding because it deals with woody growth of the same age as the crop, not with herbs and shrubs, and it differs from thinning because it is done before the crop has reached the pole stage and usually yields no saleable produce.
In naturally regenerated stands, cleaning may also reduce the density of very dense patches of seedlings, an operation sometimes called respacing or precommercial thinning in North America. Cleaning is done with a cleaning knife, slasher, brush cutter or by stem injection of herbicide where allowed. It must be done carefully: removing too much exposes the crop to sun scorch, frost, wind and weed invasion, and removing the wrong stems wastes the regeneration. In mixed stands cleaning is the main tool for controlling the species mixture at an early stage.
Early tending is labour-intensive and often underfunded, yet failures at this stage cannot be corrected later. A plantation that is not weeded in time may lose half its plants, and survivors that were suppressed for years grow slowly and form poor stems. Good practice is to schedule weeding and cleaning in the annual plan of operations, to inspect plantations before and after each round, and to record the operations in the plantation journal. In community and farm forestry, the timing of weeding must fit the agricultural calendar, because the same people do both. Taungya, described in silvicultural systems, solved the problem historically by linking tree weeding to food cropping between the rows.
Where herbicides are permitted and approved for forestry use, they can control weeds over large areas more cheaply than manual work and kill roots as well as shoots. Pre-planting herbicide application, spot spraying around plants and directed sprays between rows are common in temperate and some tropical plantations. Their use is governed by national pesticide laws, product labels and, for certified forests, certification standards that restrict or prohibit some active ingredients and require training, protective equipment and buffer zones along water.
Mulching with cut weeds, crop residues, plastic sheets or paper mats around each plant suppresses weeds and conserves soil moisture, and is widely used in arid planting and in farm and urban tree planting. Cover crops of low-growing legumes between rows can suppress aggressive grasses and add nitrogen, but must be managed so that they do not climb over or compete strongly with the trees. The choice among these methods rests on cost, labour availability, slope, rainfall and the weed species present.
Climbers, also called lianas when woody, are plants that use trees for support to reach the light. They are abundant in tropical and subtropical forests and on disturbed sites. Climbers harm the crop in several ways: they compete for light by spreading their foliage over the tree crown; they deform stems by twining, producing spiral grooves and constrictions; their weight bends or breaks young trees and leaders; they tie crowns together so that when one tree is felled, its neighbours are pulled down or damaged; and some, by girdling, kill their hosts. In plantations of teak, sal and other species in South Asia, climbers such as Bauhinia vahlii, Millettia species and Acacia pennata have long been treated as major tending problems. Invasive climbers and scramblers such as Mikania micrantha and Lantana camara, a scrambling shrub, can overwhelm young plantations and natural regeneration.
Not all climbers are harmful in every setting. In natural forest, lianas are part of the diversity, food supply and structure for wildlife, and complete eradication is neither possible nor desirable outside production areas. Climber cutting is therefore targeted at crop trees and future crop trees, and at trees to be felled or retained near harvesting operations.
The usual method is to cut each climber near the ground and again at a height of about a metre or more, removing the cut section so that the hanging part cannot root again or be grasped by new shoots. The upper part is left to die and fall from the crown rather than being pulled down, which would damage the tree. Climbers that coppice vigorously are cut repeatedly, or their stumps are treated with herbicide where allowed, or they are uprooted. Climber cutting is often combined with weeding and cleaning in young plantations, and with improvement fellings in older stands.
Climber cutting is done on a cycle, because cut climbers sprout and new ones establish. In tropical forest harvesting, cutting the climbers on trees to be felled, ideally six months or more before felling, is a standard part of reduced-impact logging: the dead climbers no longer bind crowns together, so felling damages fewer neighbouring trees and is safer for workers. Climber cutting on future crop trees after logging also speeds their growth.
Working plans in India and elsewhere often prescribe a climber-cutting working circle or an overlapping climber-cutting prescription, under which the whole forest is covered on a fixed cycle independent of the main fellings. A climber-cutting register records areas treated. Effective control also depends on the cause: heavy opening of the canopy by logging, fire or grazing encourages climbers, so tending that keeps the canopy reasonably closed reduces the problem in the long run.
A thinning is a felling made in an immature crop to improve the growth and form of the trees that remain, without permanently breaking the canopy. As a stand grows, trees compete for growing space; without intervention, the smaller and weaker ones are suppressed and eventually die (self-thinning, covered in silviculture principles). Thinning anticipates this natural process and directs it.
The objectives of thinning are: to concentrate the stand's growth on fewer, better trees so that they reach usable or valuable sizes sooner; to remove poorly formed, diseased or damaged trees and so improve quality and health; to use the wood of trees that would otherwise die and decay, providing intermediate yields and income; to control species composition in mixed stands; to improve stand stability against wind and snow by developing stronger crowns and roots; to reduce fire hazard and moisture stress; and, in some forests, to improve habitat, landscape or understorey development. The relative importance of these objectives varies: in a pulpwood plantation on a short rotation, thinning may be omitted altogether, while in a sawlog or veneer stand it is essential.
Thinning affects individual trees and the stand differently. Individual trees left after thinning receive more light, water and nutrients; their crowns expand and their diameter growth increases. Height growth of dominant trees is little affected by thinning over a wide range of densities, which is why top height is used to assess site quality. Stem form changes too: trees grown widely spaced develop more taper and larger branches.
At stand level, the classic generalisation is Langsaeter's curve, from Norwegian work of the 1940s. Over a wide middle range of stand density, gross volume increment per hectare is roughly constant: removing some trees does not much reduce the total volume the stand produces, because the remaining trees use the released space. At very high densities, growth may fall slightly because of intense competition; below a certain density the trees cannot fully occupy the site and stand increment falls. Thinning within the middle range therefore redistributes growth onto fewer, larger trees without much loss of total production, while very heavy thinning sacrifices volume for individual tree size. Because large trees are worth more per cubic metre, thinning usually raises the value of production even when it does not raise volume. Later research has shown the curve to be a simplification that varies by species and site, but it remains the standard framework.
Thinning is controlled by measures of stand density. The simplest are number of trees per hectare and basal area per hectare. Relative spacing, or the Hart-Becking index, is the average distance between trees as a percentage of top height; for square spacing the average distance in metres is the square root of 10,000 divided by the number of trees per hectare. A stand of 1,111 trees per hectare (3 m by 3 m) with a top height of 15 m has a relative spacing of 3 ÷ 15 = 20 per cent.
Reineke's stand density index (SDI) expresses density as the equivalent number of trees per hectare at a reference quadratic mean diameter of 25.4 cm (10 inches): SDI = N × (Dq ÷ 25.4) raised to the power 1.605, based on Reineke's observation that in fully stocked stands the log of tree number declines with the log of mean diameter with a slope of about −1.605. A stand of 900 trees per hectare with a quadratic mean diameter of 30 cm has an SDI of about 1,176. Each species has a maximum SDI, and management guides express target densities as percentages of it. Density management diagrams combine these measures with height and volume so that a manager can plan thinnings that keep the stand within a chosen zone.
North American usage separates precommercial thinning, which removes trees that have no market value and so costs money, from commercial thinning, which yields saleable produce and may pay for itself. Precommercial thinning, also called respacing, is done in dense natural regeneration or close-planted stands while trees are small, typically when they are a few metres tall; it reduces the stand to a target spacing so that the remaining trees grow quickly to merchantable size and the first commercial thinning comes sooner and yields larger stems. Because the cost is incurred long before any return, its economics depend heavily on interest rates and on the value of larger trees later. Commercial thinnings follow at intervals set by the schedule.
Some plantation regimes omit thinning altogether. In no-thin regimes, trees are planted at the final crop spacing or at a spacing that allows them to reach the desired size by rotation without intervention; this is common where thinnings cannot be sold, where extraction would damage soil, where wind risk makes thinning dangerous, or for short-rotation pulpwood. The trade-off is that the forester loses the chance to select the best trees and to take intermediate yields.
Mechanical thinning removes trees by a fixed rule of position or spacing, without regard to their crown class or quality. In row thinning, whole rows are removed, such as every third or every fourth row; removing every third row takes out one third of the stems. In spacing thinning, trees are retained at a fixed approximate spacing and the rest removed. Mechanical thinning is used for first thinnings in plantations, especially where the trees are still fairly uniform, where the produce has little value, or where machines need extraction racks. It is quick, cheap and needs little skill, but it removes good trees along with poor ones, and the trees left beside the removed rows develop one-sided crowns. A common compromise is to combine an outrow (a removed row serving as an extraction rack) with selective thinning in the rows between.
Low thinning, called ordinary thinning in Indian usage and thinning from below in America, removes trees mainly from the lower crown classes: dead, dying and diseased trees, then suppressed, then dominated, and in heavier grades some codominants. It imitates and anticipates natural self-thinning. Its advantages are simplicity of marking and the removal of trees that would soon die anyway. Its drawback is that light grades remove mainly small trees of little value and do little to relieve competition among the dominant trees, which are the ones that matter; heavier grades are needed to have a real effect on the main crop. In Indian forestry, the thinning grades (described in the next chapter) were defined for ordinary thinning. The ratio of the mean diameter of removed trees to the mean diameter of the stand before thinning, the thinning ratio d/D, is less than 1 in low thinning.
Crown thinning, also called high thinning or thinning from above, and associated with French practice, removes trees from the upper crown classes, chiefly codominants and some dominants, that compete with the crowns of selected best trees. Trees in the lower classes are mostly left, because they shade the boles of the main crop, suppress epicormic branches and protect the soil; they can be removed later if useful. Crown thinning concentrates growth on the chosen crop trees and gives a saleable yield at an earlier stage. It requires skilled marking, because each removal must be justified by the release of a better neighbour. Free thinning and crop-tree thinning are modern forms: a fixed number of well-spaced crop trees per hectare are selected and their direct competitors removed, regardless of crown class, while the rest of the stand is left largely alone. The thinning ratio in crown thinning is usually close to or above 1.
Selection thinning removes the dominant trees, usually the largest, to stimulate the growth of the trees of lower crown classes. It was proposed in nineteenth-century Germany (Borggreve's thinning) and has its uses where the dominant trees are poorly formed wolf trees, or where large trees are wanted for sale and the smaller trees can respond. Its danger is high-grading: repeatedly removing the best trees leaves a stand of poorer, slower-growing individuals, and in shade-intolerant species the suppressed trees may not respond after release. Selection thinning should not be confused with the selection system of regeneration, which manages uneven-aged forest and is described in silvicultural systems. Its thinning ratio is clearly greater than 1.
Choosing a method depends on the stand's structure, species, the value of thinnings, extraction methods and objectives. Many first thinnings are mechanical or a mix of outrows and selective thinning; later thinnings are usually crown or crop-tree thinnings in stands grown for quality timber, and low thinnings where simplicity or volume production matters.
Whatever the method, the quality of a thinning depends on marking. The marker walks the stand in strips, judges each tree against its neighbours, and marks with paint or a blaze the trees to be removed (or, in crop-tree thinning, the trees to be kept). Rules of thumb help: remove dead, dying and diseased trees first; favour the best-formed, healthy dominant and codominant stems; release crop trees on at least one side, preferably two or more; avoid opening large gaps; and keep good spacing rather than removing every poor tree, because an evenly spaced stand of good-enough trees grows better than a gappy stand of perfect ones. Markers check their work against the prescription by measuring basal area or counting stems on sample plots before and after marking. In Indian practice the marking officer records the trees marked in a marking list by species and girth class, which becomes the basis for the sale and for the plantation journal.
Harvesting damage must be considered too. Felling and extraction can scar the remaining stems, compact the soil and break roots; planned extraction racks, directional felling and careful machine operation keep damage low. Bark wounds on retained trees let in decay fungi and reduce the value of the very trees the thinning was meant to favour.
The intensity of a thinning is called its grade. Indian forestry, following the Forest Research Institute's classification, defined grades of ordinary (low) thinning in terms of the crown classes removed. The crown classes used are dominant, codominant, dominated and suppressed, together with dead, dying, diseased and whippy trees (whips are thin, flexible stems whose crowns lash neighbours). In the A grade, only dead, dying, diseased and whippy trees are removed; it is effectively a sanitation measure and leaves the stand almost unchanged. The B grade or light thinning also removes suppressed trees. The C grade or moderate thinning also removes dominated trees. The D grade or heavy thinning also removes some codominants, leaving mainly dominants and the best codominants. Some texts add a very heavy grade. Crown thinnings were graded separately as light and heavy crown thinning. Because these grades are defined by crown class, two marking officers can apply them consistently, but they are relative: a C grade in a dense young stand removes far more than in an older one.
Elsewhere intensity is usually expressed numerically: the percentage of basal area or volume removed, a residual basal area or SDI, a target number of trees per hectare after thinning, or a relative spacing.
A thinning schedule states when the first thinning is made, the interval between thinnings (the thinning cycle), the intensity of each thinning, and the number of trees or basal area to remain after each. The first thinning is due when crowns have closed and competition has begun to slow diameter growth, but before the live crowns of the best trees have receded so far that they cannot respond; in practice it is often triggered by a top height, a basal area or a relative spacing threshold set for the species. Thinning cycles are short in fast-growing young stands and lengthen with age. Schedules are drawn from yield tables, growth models and experience, and they differ widely between species, sites and countries; the figures for a given species should be taken from the local yield table or working plan.
Thinning intensity is often related to growth. In British management tables, the marginal thinning intensity is the largest intensity that does not reduce total volume production appreciably; it is set at 70 per cent of the yield class per year. For a stand of yield class 14 (14 m³/ha/yr maximum mean annual increment), the marginal thinning intensity is 0.7 × 14 = 9.8 m³/ha/yr, so on a five-year cycle each thinning removes about 49 m³/ha. Thinning harder than this reduces total volume production but grows larger trees faster.
The thinning yield is the volume removed; the ratio between thinning yield and the volume of the main crop before thinning measures intensity at stand level. The thinning ratio d/D describes the type of thinning: if the removed trees average 18 cm in diameter in a stand averaging 24 cm, d/D = 0.75, a low thinning; if the removed trees average 30 cm, d/D = 1.25, a crown or selection thinning. A mechanical row thinning gives d/D close to 1 because removal is unrelated to size.
Late or missed thinnings are a common problem. Stands left unthinned too long develop small crowns, slender stems and shallow roots; a heavy late thinning then exposes them to windthrow and snow break and gives little growth response. Where thinning has been delayed, it is usually safer to thin lightly and often, or to accept a no-thin regime, than to make one heavy intervention.
Improvement felling is a felling in a mixed or irregular stand, past the sapling stage, made mainly to improve its composition and quality by removing trees of undesirable species, poor form or condition, in favour of better trees. It is used where the forest is too irregular for regular thinnings, typically in natural tropical and subtropical forest and in degraded or previously high-graded stands. Trees removed include dead, dying, diseased, hollow and badly formed stems, overmature trees past usefulness, species of no value that suppress valuable ones, and climbers. Improvement fellings may be combined with selection fellings as the selection-cum-improvement system (see silvicultural systems), and they are often prescribed as a transitional regime until the forest can be brought under a regular system.
Not every 'defective' tree should go. Hollow and dead trees are habitat for birds, bats and many other organisms, and some 'weed' species are food trees for wildlife or valuable to local people. Modern prescriptions retain a share of such trees deliberately.
Salvage cutting removes trees that have been killed or badly damaged by fire, wind, snow, insects, disease or drought, so that their wood is used before it deteriorates. Timing is critical, because timber loses value quickly to stain, decay and insect attack, especially in warm climates; large salvage operations after storms or bark-beetle outbreaks can flood markets and lower prices. Salvage must also take account of soil and water protection on burned or storm-damaged sites, the value of dead wood for biodiversity, and the risk to workers from damaged and unstable trees.
Sanitation cutting removes trees that are infested, infected or highly susceptible, to prevent pests or pathogens spreading to the rest of the stand. Examples include removing bark-beetle brood trees before the new generation emerges and removing trees infected by root or stem rots in plantations. Sanitation differs from salvage in its purpose (protection of the stand rather than recovery of value) though the two often overlap.
Improvement, salvage and sanitation fellings are usually not part of the regular sequence of a yield table, so they need clear rules in the working plan: which trees may be removed, how removals count against the allowable cut, how fire- or storm-damaged areas will be regenerated, and how biodiversity elements will be retained. Salvage after large disturbances is also an opportunity to reconsider the species and system for the next crop.
Branches on the lower stem die when shaded by the crowns above; natural pruning is the death, decay and shedding of these branches and the healing over of the stubs. Its speed varies greatly among species: many eucalypts and some tropical hardwoods prune themselves well, while many conifers, such as most pines and spruces, keep dead branches for years, leaving loose, dead knots in the wood. Dense spacing hastens natural pruning by shading the lower branches early, which is one reason close initial spacing has been used for quality timber. Natural pruning is free but slow and uncertain, and dead branch stubs can let decay into the stem.
Artificial pruning is the removal of branches by hand or machine to produce clear, knot-free wood, to improve access, to reduce fire risk by removing ladder fuels, or to improve appearance. Dry pruning removes dead branches; green pruning removes live ones. Green pruning reduces leaf area and therefore growth if taken too far; a common guide is to keep at least about half of the tree's height in live crown, though recommendations differ by species. In some species, such as oaks, green pruning or sudden exposure stimulates epicormic shoots on the stem, which create new knots.
The cut is made just outside the branch collar, the swollen zone at the base of the branch, without leaving a stub and without cutting flush into the stem. A cut through the collar enlarges the wound and slows healing, while a long stub delays occlusion and lets decay enter. Tools include pruning saws, pole saws, shears and, in some plantations, mechanical pruners. Pruning is usually done in the dormant or dry season, when bark is less likely to tear and fungal infection is less active; local guidance should be followed because the best season varies.
Pruning is worthwhile only if it adds value to logs that will be sold for clear wood, so it is done only on final crop trees, chosen before the first pruning lift and protected through later thinnings. The aim is to keep the knotty core, the zone of the stem containing branch stubs, small. Pruning is therefore done early, while the stem and branches are still small. The defect core is the central cylinder within which knots occur; the rest of the log diameter is clear wood. If a log is 50 cm in diameter and the defect core 20 cm, the clear wood is 1 − (20 ÷ 50)² = 84 per cent of the cross-section area.
The lift is the height to which the stem is pruned in one operation. Pruning is usually done in two or more lifts as the trees grow, each lift keeping enough live crown. Radiata pine regimes in New Zealand, for example, have pruned final crop trees in successive lifts to a height of around six metres to produce clear butt logs, though regimes vary. Teak and other high-value hardwoods are also pruned in many plantations. The economics depend on the price premium for clear wood, the cost of pruning, the interest rate and the long wait until harvest; unpruned regimes are common where markets do not reward clear wood.
Pruning is planned as a sequence. Before the first lift, the forester chooses the final crop trees, usually a few hundred per hectare depending on the species and target log size, selecting healthy, straight, vigorous dominants evenly spread through the stand. Each lift is scheduled when the trees have grown enough that the target height can be pruned while keeping the required live crown. Records list the trees pruned, the lift height and the date, because the value of pruning can be realised only if buyers can be shown that the logs contain clear wood; some plantation companies record the diameter over stubs at each lift for this purpose.
Pruning at height is hazardous work. Ladders, pole saws and climbing equipment must be used correctly, and workers need training, protective equipment and safe working distances from each other. Pruned branches add to surface fuel for a time, so pruning in fire-prone plantations is often combined with removal or chipping of slash along firebreaks. Pruning wounds should not be painted or dressed as a routine; modern guidance, following research on tree wound response, is that dressings do not prevent decay and may trap moisture.
Girdling, or ring-barking, kills a standing tree by removing a continuous ring of bark, phloem and cambium around the stem. The roots, no longer supplied with sugars from the crown, eventually starve, and the tree dies over months or years, depending on species. The tree is left standing, so it does not damage neighbours as felling would, and it becomes a snag that provides habitat until it falls. Girdling is used to remove unwanted trees in stand improvement, to release crop trees from overtopping competitors, to kill trees that cannot be extracted economically, and to create dead wood for biodiversity.
Simple girdling is unreliable for species that sprout from the stump or roots, or that bridge narrow girdles with callus. A wider girdle cut into the sapwood, a double girdle, or frill girdling combined with herbicide (poison girdling) is more effective. Poison girdling was a standard part of the Malayan Uniform System and other tropical systems in the mid-twentieth century; the arboricides used then, such as arsenic compounds, are now banned in most countries because of their toxicity.
Where herbicides are permitted and labelled for forestry use, individual unwanted trees can be killed by stem injection (hack-and-squirt), in which herbicide is applied into cuts made around the stem with a hatchet or injector; by cut-stump treatment, applying herbicide to the freshly cut stump to prevent sprouting; and by basal bark spraying of small stems. Commonly used active ingredients include glyphosate and triclopyr, but approvals, application rates and protective measures are set by national law and product labels, and certification standards restrict or ban some chemicals. Herbicide use must consider effects on non-target plants, water and workers.
Timber stand improvement (TSI) is the North American term for the combination of operations that improve the composition, quality and growth of a stand without harvesting merchantable trees: cleaning, release, girdling, cull-tree removal and sometimes pruning. Liberation cutting releases young trees from older, overtopping trees, often left from a previous stand. Controlled or prescribed burning is used in fire-adapted forests, such as many pine forests, to reduce fuel, suppress competing hardwoods and prepare seedbeds. Grazing has been used to control grass and weeds in some plantations, with the risk of damage to trees if it is not controlled. Each method is chosen by cost, effectiveness, safety and environmental effects.
A release treatment frees a young crop from competition that threatens its survival or growth. North American terminology distinguishes three: weeding, which removes competing herbaceous and shrub vegetation; cleaning, which removes same-age competitors; and liberation, which removes older, overtopping trees. Release is most effective when done early, before the crop has been suppressed for long; seedlings that have lost their leaders or live crown under heavy shade may not recover. In many young conifer plantations in North America and Europe, the main competitors are fast-growing broadleaved shrubs and trees such as alder, birch, aspen, cherry and bramble; in tropical plantations they are grasses, shrubs and climbers.
Competition can be measured by the cover, height and distance of competitors around each crop tree, and some jurisdictions set free-to-grow standards: a crop tree is considered free to grow when no competing vegetation within a set radius is taller than a set proportion of its height. Such standards make release a measurable obligation after harvesting and planting.
Manual methods, such as cutting with brush saws and slashing, are labour-intensive but selective and widely acceptable; cut competitors often resprout, so repeated treatments are needed. Mechanical methods, such as rolling, crushing or cultivating between rows, suit flat, accessible ground. Chemical methods, where permitted, are usually the cheapest and most effective in killing roots as well as shoots, but face increasing restrictions and public concern and are limited under some certification schemes. Biological and cultural methods include mulching, cover crops, grazing by sheep or cattle under supervision, and planting larger or faster-starting stock so that the crop escapes competition sooner.
Integrated vegetation management combines these methods with site preparation, choice of planting stock and planting density so that the crop dominates the site as quickly as possible with the least intervention.
Competing vegetation is not wholly harmful. It protects the soil, shelters some seedlings from frost and sun, supports wildlife and pollinators, and in mixed stands may be a desired part of the future forest. Release prescriptions should therefore target the competition that actually limits the crop and retain vegetation where it does not. Over-release can leave crop trees exposed, encourage weed species, or reduce the value of the stand for other uses.
Fertilising an established stand adds nutrients to increase growth where a nutrient is limiting. Forest fertilisation is most common in intensive plantations and in some boreal and temperate forests. Nitrogen is the commonest limiting nutrient in boreal and many temperate forests; phosphorus is commonly limiting in old, weathered soils, such as many in Australia, the coastal plain of the south-eastern United States and parts of the tropics; boron deficiency causes leader dieback and deformity in pines on some soils. Fertiliser at planting, covered in forest nursery and plantation establishment, is different from mid-rotation fertilisation of established stands, which is covered here.
Diagnosis uses visual symptoms (such as yellowing, needle or leaf discoloration, dieback), foliar analysis compared with critical concentrations established for the species, soil analysis, and above all field trials that measure the growth response. A nutrient applied where it is not limiting gives no response and may be wasted or harmful.
In boreal forests of Sweden and Finland, nitrogen fertilisation of mature conifer stands, often around 150 kg of nitrogen per hectare applied once some years before final felling, has been used to increase volume growth; recommended doses and restrictions differ between countries and over time. In the south-eastern United States, mid-rotation fertilisation of loblolly and slash pine plantations with nitrogen and phosphorus, often after thinning, is widely practised. In Australia and New Zealand, phosphorus has been applied to radiata pine plantations on deficient soils. Eucalypt plantations in Brazil and elsewhere receive fertiliser at planting and sometimes later.
Fertiliser product needed is computed from the nutrient content: to apply 150 kg of nitrogen per hectare as urea, which contains 46 per cent nitrogen, requires 150 ÷ 0.46 = 326 kg of urea per hectare. Fertiliser is applied by ground spreading in young or accessible stands and from aircraft in large mature forests. Timing matters: response is usually greatest when canopy and crown development can use the extra nutrients, such as after thinning or at mid-rotation, and when the crop is free of severe competition.
Growth responses are measured as extra volume per hectare over a period after application, often several years. Responses vary with site, species, stand age and density, weather and the nutrient applied; a response measured in one region cannot be assumed elsewhere. Fertilisation pays when the value of the extra wood, discounted to the time of application, exceeds the cost of fertiliser and application; because the response comes some years after the cost, fertilisation close to harvest often gives the best return.
Environmental effects must also be considered: nitrogen not taken up can leach as nitrate into groundwater and streams, phosphorus can run off into water bodies and contribute to eutrophication, and fertiliser production and use emit greenhouse gases. Buffer zones along watercourses, correct doses and timing, and certification rules limit these risks.
Fertilisation interacts with other tending operations. Thinning releases growing space that fertilised trees can use, so the two are often combined; fertilising an overstocked stand mainly increases mortality among suppressed trees. Fertilising a stand still competing with weeds may benefit the weeds more than the trees, so release usually comes first. Nutrient removal by repeated harvesting of whole trees, including branches and foliage for bioenergy, can create deficiencies that did not exist under stem-only harvesting; on poor sites this is managed by leaving branches and needles on site, by returning wood ash, or by fertilising. Long-term site productivity trials in several countries are designed to detect such effects, and their results so far differ by site and harvesting intensity.
A student should be able to define and distinguish each tending operation and place it in sequence from weeding to final felling. Learn the crown classes from silviculture principles, then the thinning methods and Indian thinning grades by the crown classes they remove. Practise calculating trees per hectare from spacing, relative spacing, stand density index, thinning ratio, marginal thinning intensity, clear-wood proportion and fertiliser product. In the field, mark a thinning in a plantation and explain each tree marked, and practise correct pruning cuts at the branch collar.
A forester five years into practice plans and supervises tending: scheduling weeding and climber cutting before the dry season, marking cleanings and thinnings, choosing crop trees for pruning, enforcing correct pruning cuts, deciding whether delayed stands should be thinned at all, organising salvage after a storm or fire, and keeping the plantation journal. The forester also judges whether herbicide or fertiliser use is justified, legal and acceptable to certification auditors and local communities.
A senior forester with twenty-five years' experience sets tending policy and schedules for a forest estate or division: choosing thinning regimes and pruning regimes from yield tables and markets, deciding when to adopt no-thin regimes, allocating limited budgets between early tending and later thinnings, approving herbicide and fertiliser programmes, and responding to large disturbances with salvage and regeneration plans. The senior forester explains the long-term effects of tending decisions to owners, managers and communities.
A researcher with long field experience studies how tending changes growth, wood quality, stability, biodiversity and carbon: long-term thinning and spacing trials, density management models, pruning and wood-quality studies, fertiliser response trials and nutrient diagnosis, and the ecological effects of herbicide use and dead-wood removal. Such work updates yield tables and tending guidelines and tests whether long-held rules, such as Langsaeter's curve, hold for new species and climates.
This manual describes tending principles and methods; it does not give species-by-species weeding, thinning or pruning schedules, which belong to silviculture-of-species and to local yield tables and working plans. Herbicide active ingredients are named only as examples; doses, approvals and safety rules are national and label matters and are not given. Fertiliser doses are given only as examples from well-documented regions. Growth-model-based density management is introduced briefly and treated fully in growth-yield-modelling. The economics of tending (net present value of thinning and pruning regimes) are left to forest-economics-valuation.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
These notes are free to read here, with no account.
What the Laisha app adds:
Free for ten days, then a subscription. We would rather write that here than have you find it later.