Much of the emphasis for a planting project is placed at the beginning with seed selection, nursery development, and outplanting. Unfortunately, the project manager often reduces his efforts after the seedlings are in the ground. Given the high risks during this period of establishment, such reduced efforts could prove disastrous to the stand.
Once planted in the field, seedlings no longer have the safe nursery environment. Instead, they are subject to several natural and human-caused hazards. Establishment thus becomes a period of protecting and tending the tree crop. In addition, establishment also determines how the tree develops for the remainder of the stand's life. Stands that have a vigorous, hazard-free start are more likely to develop into vigorous, healthy plantations. Those stands that have to compete with vegetation and other factors will have a slower start and may even fail.
This bulletin, the fourth in the World Bank "Forests and Forestry" series, describes management techniques used to reach plantation establishment, or when trees begin to dominate the site. Developed specifically for anyone involved with forest tree planting in the field, this bulletin gives a broad overview of preferred methods to ensure a rapid start and survival of the plants during the first few years after planting. It also includes preliminary silvicultural techniques to ensure stand quality.
Bulletins one through three in this series deal with seed supplies, planting stock and site preparation, respectively.
Sustainability is the goal
Newly planted stands can be damaged by many factors including adverse weather conditions, insect pests, disease, fire, wild and domestic animals, and people. The amount of risk from each cause varies with environment and location.
One of the more difficult situations to assess occurs when exotic species are planted untested in new environments. There is the chance that native insects or diseases may readily adapt to and destroy the exotic host. In such instances, the planter should prepare a small trial of the species before committing to a large-scale operation.
Other potential causes of damage can be identified early on through a site assessment (See Bulletin No.3, Site Analysis and Outplanting.) With this knowledge the operations manager can prepare remedial measures to reduce or prevent these occurrences.
Aside form identifying the hazards, the manager must also assess the cost-benefit ratio in identifying solutions. Total protection against all risks will prove too costly. Any potential profits from the stand would be eliminated by excessively high investment in protection. Also, in some cases physical protection measures cause social antagonism, such as impeding migratory routes. In this instance, other approaches are needed.
Thus, plantation managers must weigh the compromises and accept some degree of risk. In some cases, it may prove infeasible to establish a plantation at a given site.
Protection Is Essential
Tending the trees after planting creates favorable conditions for both the plants' survival and to stimulate a healthy, vigorous growth. Tending operations keep the plants from being suppressed by natural vegetation and protect them from other potential sources of harm such as animals, insects and disease. Generally these operations include a wide range of treatments including access control, soil and nutrient management, pest management, and so forth.
Animal control takes various forms
Animal damage can be caused by smaller animals, such as rodents, and larger animals, like cows, deer and elephants. Left unprotected, seedlings may be subject to browsing and trampling by both wild and domestic herds. Domestic animals, such as goats, pose an especially severe problem.
However, several means are commonly used to protect the plants. Control may be done by physical barriers, or by removal of the offending animals. Physical barriers include fences, walls, thorn hedges and ditches or a combination of any two or three of these. Fencing is often the easier means to install, but is an expensive option.
When protection against smaller species is desired, a wire-mesh enclosure can be used. The lower part of this type of fencing should be buried to prevent animals from burrowing underneath. Experience in many countries, however, has shown that wire fencing is often stolen and is therefore useless for long-term protection.
Plantation managers should also note that when fences are used, they can restrict community access to other areas otherwise available for free-range grazing. Migratory patterns, too, may be interrupted.
There are many instances where land appears to be severely degraded and therefore ideal for forest plantation establishment; yet in certain seasons these degraded sites are critical grazing areas for domestic or sometimes wild animals. Such factors must be taken into account as alternative arrangements will need to be made.
In areas prone to animal browse, a chemical repellent can be applied to young plants to discourage grazing animals. In the U.S., such products as Deer Away™ or Plant Pro-Tec have been effectively used to discourage large and small animals from browsing small trees.
The treatments typically have a strong repellent odor or taste and provide a quick, effective deterrent. However, the compounds last for only a short period and must be reapplied frequently if the problem continues. The method is best suited to smaller plantations as it is usually applied by manual rather than mechanical means. But one drawback is that such products are rarely available in developing countries.
For individual stems, protective staking or cut thorn bushes can help discourage goats or other animals from eating the seedlings during the first few years. Some companies market "stem guards" of various materials. These devices are favored for urban planting, particularly to protect trees planted along new roads.
In extreme cases, poison baits can be effective in controlling some incursions of specific animals. Various poisons can be used next to the seedlings or directly in the animal burrows. These compounds are usually applied in the form of poison-soaked grains such as wheat or oats. In the U.S., ORCO brand Gopher Grain Baits and Field Mouse Plus are examples. In developing countries, many local products have been used by farmers to protect their agricultural crops.
Extreme care must be used when using poisons to prevent harm to humans and non-target animals. Poisoned grains especially must be kept distinctly separate from food storage areas.
Controlling unwanted vegetation
Unwanted vegetation or weeds interfere with the development or survival of young plants by competing with them for sunlight, water, space and nutrients. In addition, a thick ground cover such as grass not only hinders nutrient uptake, but may also harbor harmful animal pests, which can girdle and destroy the small plants. Grass swards lose water very quickly at the onset of dry seasons or dry spells, significantly affecting the soil moisture of the upper soil layer, which will affect tree growth.
The objective is to reduce or remove this vegetation to promote better growth and development of the planted trees. In general, the intensity of the weeding operation should be done at a level that effectively encourages establishment at a reasonable cost. Also, the types of treatment should not be so severe that it degrades the site through soil or nutrient loss.
The intensity and type of treatments will vary according to four main factors: labor costs, tree species, spacing and vegetative density. Typically, species less tolerant of competing vegetation will require a more intensive weeding. However, some shade-tolerant species can withstand the shading effects of weed growth. In such cases it may not be necessary to do an intensive weeding, thus saving time, costs, and labor. Also, a limited weeding may benefit some specific plants by allowing the vegetative cover to protect them from drought, sun scorch, or desiccating winds. Vegetative cover can also reduce erosion and improve moisture infiltration.
Furthermore, as a general rule the smaller the tree, the more frequently weeding is necessary. Treatments are typically done for the first two to three years after planting, depending on how long it takes for the trees to be competitive on the site.
This timeframe also highlights the importance of planting stock quality (see "Forests and Forestry" Bulletin No. 2. Often, small, good quality plants raised in root trainers will outgrow larger poly-bag plants, thereby reducing weeding costs.
Foresters use thinning practices, described later in this bulletin, to manipulate stocking levels for weed control. Increased stocking per hectare at the time of planting will promote early crown closure and reduce weeding frequency; but there is a caution as a greater stocking may lead to increased competition among trees.
If a narrow spacing (2 square meters or less) is used, early thinning will be needed to permit tree growth while maintaining site control.
Care must be taken because not all species respond in the same way. For example, most eucalypts will not recover from early competition. This characteristic had a serious effect in Haryan State in north India. Farmers planted at one-meter spacings, which resulted in massive production of small-sized trees flooding the market and significantly lowering the selling price.
Had better spacing been used, the farmers would have had a greater chance to market different sized products.
Narrowing spacing can also cause a permanent reduction in growth rates where sites are deficient in some nutrients such as phosphate.
In terms of the third factor, vegetation density, planters can expect heavier weed growth on the more fertile sites. On sites with a light vegetative cover the goal is to keep the vegetation down to allow a more homogenous growth permitting a quicker establishment of the site. Conversely, a dense vegetative cover can suppress and kill newly planted trees. In this case, the goal is to reduce or eliminate the cover to increase plant survival and maintain an adequate number of trees for establishment.
Weed suppression methods include trampling or beating down the vegetation, or cutting it back either manually or by using machinery. When warranted, weed removal can be done by cultivation or by the use of herbicides.
Manual cultivation, done with hand implements such as a hoe, is quite effective, but is very labor intensive. Mechanical cultivation, when possible, calls for equipment like that used in agriculture.
Aside from eliminating weeds, cultivation can increase rainfall infiltration in the short term and reduce soil moisture losses. Cultivation can, if properly timed, also make a considerable growth difference in areas with a significant dry season.
Depending on the need and required intensity, weeding may be total or partial. Partial methods include spot weeding around the plants or line weeding, which follows the planting rows. Whenever possible, partial weeding is preferable as it minimizes site disturbance.
When weeding in especially heavy vegetation, it is common practice to stake seedlings that are hard to see to prevent accidental cutting of the plants.
Herbicides are also used as a removal method. These compounds are typically applied as a granular mix or liquid application. But note, the planter must carefully follow instructions for any required mixing and for proper application. Strict adherence to recommended usage is absolutely necessary to prevent harm to nontarget plants and especially to humans and animals.
One preventative measure in weed control is the use of cover crops. The type of crop selected, however, should not interfere with the growth of or access to the tree crop. A more thorough discussion and example of cover cropping is provided in Forestry Technology #3 , Site Analysis and Outplanting, available from The World Bank.
Closely spaced Acacia mangium outpaces imperata grass
A fast-growing South Pacific acacia is showing promise of breaking the hold of unyielding imperata grass. The tough, spreading grass has claimed many hectares of former forested areas, turning them into wastelands.
The tree, acacia mangium, "seems to be shaping up as a potential new tool for tropical reforestation," said Noel Vietmeyer in a recent concept paper. "Seedlings planted directly into this pernicious weed (imperata) shade it out and create a forest within three to five years." This is because imperata has a weak link--it cannot tolerate shade.
Beyond this exciting discovery, Vietmeyer goes further, explaining that the tree can eventually transform an imperata site back into its original productive condition. For instance, on former mangium lands in Sabah, a mangium forest now has native vines, shrubs, and ground covers growing under the shade and protection of its canopy.
Furthermore, "The soil is improving with the leaf litter," says Vietmeyer. This improvement includes the nutrient exchange created by mycorrhizal fungi and nitrogen-fixing bacteria.
Insect and disease prevention and control
Trees in natural stands are generally in balance with native insects and diseases. Problems occur when the natural balance is upset. One example is the introduction of an exotic tree species that is not adapted to endemic pests and diseases.
In another case, monoculture plantations can be vulnerable to widespread damage if a virulent pathogen is introduced and remains unchecked. For example, American southern pines planted in some provinces of China are being severely infested by an accidentally imported mealy bug which has no significant parasites in China.
The pathogen in such cases can easily move among the host trees uninterrupted and, if the organism's life cycle is short, its population may quickly attain epidemic proportions.
Despite this natural balance, healthy trees are sometimes attacked, especially in years of heavy infestation. Like the trees, both insects and fungi have their own pathogens with which they are usually in balance. If this balance is upset, the tree pathogens can cause a great deal of damage. Furthermore, few practical controls are available for most fungal and viral diseases of forest plantations.
To counter potential hazards, preventative measures are the first line of defense. The best precaution is to plant species that are resistant to, or tolerant of, the prevailing insects and diseases and are suited to the site's climate and soils. Afterward, proper tending operations that promote a healthy, vigorous growth of the plants are a must. Otherwise, the risk of damage by pathogens increases if the trees are weakened.
Plantations of mixed species also will help prevent widespread infestations. Typically, insects and diseases are specific to their host species and are not likely to switch species. Mixed plantations may be either composite stands or alternating sections of trees, which act to block the spread of pests or pathogens.
Even with the best prevention, attacks will occur. In this case, plantation managers must promptly investigate the cause and identify its origin. The life cycle is studied to determine at what stage in development the pest is most vulnerable. Once the pest is known and located, mechanical, chemical, or biological control measures can be taken, targeting the weakest link in its development chain. Good managers set up systems to monitor levels of pests and are ready for immediate action should any imbalances appear.
In newly planted sites, prompt removal and destruction of affected trees may be enough to stem the pathogen's spread. Preferably, the infected plants should be piled and burned to ensure a thorough eradication. Burning may pose a problem, however, if it is a viral or fungal pathogen. In this case, burning can aerially disseminate the spores or resting bodies.
Mechanical control is also an option. This involves the physical removal of the pest, which is possible if the insect is large enough and is caught early in the infestation. Use of traps may also be a possibility. Or, if alternate hosts are present, their removal will assist in stopping the spread. And, where the invading insect resides in the topsoil or leaf litter, these areas can be raked or tilled to interrupt the development cycle.
A chemical insecticide or fungicide also can be applied to check the advance of damaging pathogens. These compounds are applied as liquids, dusting powders, or mists. They may be applied using portable sprayers, blowers, or even squirt bottles for limited applications. In Asia, products from the ubiquitous neem tree (Azadirachta indica) are extensively used against pathogens.
However, any chemical applied should be used in strict accordance to the directions to prevent misapplication that could cause further damage to the stand, upset the pathogens' own equilibrium or, worse, pose a threat to human health.
Biological methods include the use of insect predators, parasites, or diseases of insects. It also includes the option of releasing sterile populations, which hinders reproduction of the organism. The advantage of biological controls over chemicals is a reduced risk of toxicity to nontarget plants and animals. However, biological controls are highly species specific and may cost more than chemical controls depending on the location of the stand and type of pest that is to be controlled. Management should liaise with the nearest research laboratory dealing with forest or agriculture pests to check on biological control possibilities.
The threat of fire
Fire is a particular threat during the dry season, but should be of concern in all but the wettest months. Farmers often burn areas to promote new growth for their cattle, while in many Indian states forest is burned to encourage development of young "tendu" leaves, which are used locally for making cigarettes (beedis).
However, simple measures can be taken by plantation managers to assess, prevent and control fires when they occur and there will be no serious damage if burning is properly controlled.
Assessing the degree of fire hazard involves a simple check of four factors: air temperature, relative humidity, wind speed, and fuel buildup. Dry conditions and strong winds expose the site to its greatest fire risk. Fires started during these periods can quickly burn out of control.
Many villages and communities periodically burn vegetation as a common practice. Fire prevention for plantations should be a main consideration in such areas. Methods to reduce the chance of fire include reduction of the ground level vegetative fuel source, controlled burns, and firebreaks. Reducing the fuel source can be done through either cultivation of the soil or cutting and chopping the grass and weeds. Burnable materials can be removed between rows or around each plant.
Firebreaks provide a direct barrier to ground fires and allow access to the plantation for maintenance or in case fire does occur. Main firebreaks are kept entirely free of vegetation. They should be laid at right angles to the prevailing winds.
Plantation managers must also consider the fire threat from both within and outside of the plantation. A full perimeter break will reduce the risk of outside fires, while intermittent breaks will stem the spread of fire from within the stand.
In addition, firebreaks should be engineered to allow proper drainage, thus preventing undue soil erosion. Also, the design and layout of access roads is important for protection as mentioned in Site Analysis and Outplanting.
Greenbelts, too, can be planted as a fire retardant border to the plantation. The use of vetiver grass is one example. Historically, in some areas of the United States, large stands of old timber were protected for centuries by small patches of shrubs and small trees, which acted as barriers to fires frequently set by Native Americans.
When feasible, periodic controlled burning can be used to reduce the vegetative fuel source. Controlled burns require appropriate weather conditions including light winds, some ground moisture, and mild temperatures and most of all careful planning and supervision. Otherwise, there is potential the fire will spread beyond the designated area. In the case of dense vegetation, complete removal should be done by successive burning. Removal in a single operation might cause too intense a burn, creating a hazardous condition which may burn out of control.
In areas prone to frequent fires, plantation planners should consider planting fire-tolerant species.
Even with the best preventative efforts, fires will occur. Fire needs heat, fuel, and oxygen to continue. Take away any of these three ingredients and the fire will stop. This means that during hazardous periods, a team of fire fighters will be needed on some form of standby to take rapid action on any identified fire.
When fires ignite, fire fighting is often limited to whatever materials can be found on hand. It is therefore best to be prepared by having appropriate handtools stored nearby. For instance, small ground fires can be extinguished by beating the lead of the fire with branches, wet blankets, or by throwing dirt on it with shovels in a fan-like motion. These actions cut off the oxygen supply.
Also, firelines can be made rapidly by a team of workers who, working in a single line, use handtools to scrape the earth. The fireline, which is cleared to bare mineral soil in the approaching path of the fire, contains the fire's spread. For small ground fires, the line only needs to be about 50 cm wide. Wider lines should be dug for hotter fires or where wind is a consideration.
When the fire is contained within the fireline, it can then be extinguished. The area must be watched until the fire is absolutely out. Many fires thought to be dead have restarted after being left too early.
Human-caused hazards
Hazards created by humans on newly planted sites can take many forms. However, these risks are often due to careless rather than purposeful action. Increased fire hazard from traditional field burning, noted above, is one example. Other factors might include unlawful trespass or unknowingly herding animals across the site.
Plantation managers might avoid these potential problems through good communications and involvement of local communities; foresters sometimes face problems with this because of their earlier policing role. If villagers are made partners in the potential benefits of a stand, they will more likely take an interest in the stand's protection.
To encourage long-term care of the plantation, some type of incentive program can be established to encourage villagers to care for the trees. A food for work program is one example. Participants in the planting program are compensated with food for work given during each stage of the planting and tending operations.
Possibly the most effective method, however, is to ensure that those involved have direct benefits from the plantation they are protecting in terms of the intermediate and final yields. For instance, forest village programs in Thailand provide community services, product marketing, profit incentives and transportation as a means to create agroforestry programs.
In each of these approaches consistent support is required throughout the project, both to ensure the project's success and demonstrate the potential of reforestation or afforestation efforts.
A checklist for safe pesticide applications
Used properly, pesticides--including herbicides, insecticides and fungicides--can effectively control unwanted vegetation, insects and fungi. Misapplication or inappropriate use, however, can endanger the crop, or worse, human and animal health.
Specific recommendations for local treatments are beyond the scope of this bulletin. The types of chemicals to use and specific circumstances under which they should be used are too numerous to list. However, if the forester determines that chemical control is an appropriate response to unwanted vegetation, insects, or disease, then he should seek appropriate guidance from either a manufacturer's representative or other qualified applicator.
The compounds should not be selected and used based on advice from third parties, or from sources only vaguely familiar with their usage.
The forester should follow basic precautions when using these chemicals:
- Know the chemicals you are using and read the container labels thoroughly before application. Additional information should be available for most materials. Be certain the chemical is appropriate for the target species, type of site and desired result. Never use unlabeled or leftover compounds as this practice may prove disastrous or even fatal;
- Use safe application methods. Be sure that conditions such as wind and ground moisture are favorable to application. Wear appropriate protective clothing, such as coveralls and impermeable, unlined gloves and boots. Nitrile gloves are best; latex gloves are useless. Also avoid wearing leather gloves and boots as these will absorb the compounds. Use soap and water to wash any exposed skin areas thoroughly after application;
- Rinse and dispose of the chemical containers properly. Never wash equipment or containers near freshwater sources such as lakes, wells or streams. Also, dispose the rinse water properly, away from freshwater sources. Puncture or crush the containers to ensure they are not reused for food or fodder storage, or other purposes. Make sure they are emptied thoroughly. Also, triple rinse tanks and spray equipment;
- Keep good records of the application noting location, date, time of day, wind speed and direction, temperature, and type of chemical used;
- Prior to application, properly store the chemicals by keeping the containers upright in a cool, dry, protected shelter. Make sure they are well away from food sources and inaccessible to children and animals. Never store the chemicals in empty food or beverage containers.
Plantation Surveys
The post-planting period is an ideal time to establish sample test plots in the stand to monitor stand development. Monitoring preferably should continue through the entire rotation. Ongoing surveys help chart the stand's progress and assist in making early detections of potential problems. For example, remedies can be more effective in the early stages of an insect or disease attack, as opposed to battling the problem after the pests have become established. Of course, the main purpose is to be continually aware of the condition of the tree crop.
Through the survey, or stand exam, foresters have developed techniques to review tree and non-tree characteristics. Tree factors include growth, distribution, species, and survival rate. Other items might include signs of insect or disease damage, vegetative growth, and animal damage. These technologies should become part of any planting project, whether large or small.
In carrying out a survey, it is unnecessary to examine each tree in the entire stand as this would be cost prohibitive. Instead, a properly prepared sample survey of the site will accurately reflect the condition of the stand.
In designing his survey and choosing sample plots, the manager must keep potential bias to a minimum. For example, just sampling the edges of a stand, which are more accessible, will not give a true picture of the stand as a whole.
To get an accurate reading, the distribution of sample plots must be uniform throughout the stand. However, if the stand has variable conditions, such as flat ground, streams, hills, or other distinct site characteristics, then each section should be sampled separately, or stratified. Separate tally sheets should be kept for each stratified section so that separate estimates can be made if so desired.
Systematic sampling using circular sample plots is preferred in areas that are familiar to the forester and for which maps are available. Circular plots are easy to measure because a single measurement, the radius, is used to locate each plot's perimeter.
If a map is unavailable or the total size of the area is unknown, then a systematic strip inventory would be appropriate. This method divides the area into strips of equal widths. In most countries, the common width of a strip is 20 m (sometimes one chain). Strip lengths will usually vary because of irregularly shaped borders of the stand. Also, the strips should be layed at right angles to the drainage pattern.
Field tallies
For his field work, the manager can design a standard form that lists the desired survey traits. The form can have boxes to check off the various factors, making compilation easier once the survey is complete.
Typical factors include:
- number and species of trees, heights, diameters;
- notes on gaps in the stocking levels;
- tree mortality and apparent causes;
- amount and type of vegetation present including brush and grasses;
- signs of animal presence and animal damage;
- signs of natural site disturbance such as flooding, wind damage, or erosion.
In addition, if permanently marked sample plots are used, the plantation manager can gauge the quality of various seed sources. He can monitor such factors as growth, form, and vigor, given that other factors are constant among the seed lots.
Such records can help reduce the amount of experimentation from one rotation to the next.
Siting sample plots in a stand
Basic measures must be followed when preparing a survey to ensure the sample plots adequately reflect the true condition of the stand. Otherwise, the readings will be inaccurate, which could lead to inappropriate or wasteful management decisions.
The inventory design depends on several factors including topography, composition and variability of the stand, skill of the survey personnel, and the time and funds available for the work.
Simple systematic random sampling using fixed circular plots is a commonly employed method that is also easy to use. Circular plots have the advantage because a constant radius can be used to measure the plot area. Typical plot sizes used in the United States and Scandinavia range from .01 ha. to .1 ha. The smaller plots are suited to stands with small-sized trees.
A map of the area should be drawn and fixed spacing between plot centers is chosen, for instance 20 m. Using this interval, the forester can then select a starting point in one corner of the stand and lay out a grid using columns and rows. Preferably, the sample lines should run diagonally to the planting lines to provide a better cross section of the stand. In this manner, the plots are systematically distributed resulting in a homogenous sampling.
The number of plots selected will depend on the size of the area and the starting point location. Typically, a five percent inventory is used, though a lower intensity survey may be used if the stand appears homogenous in its makeup.
On this basis, the forester can make the requisite calculations:
Ap = PA
n = Ap/a
P = a/BL
where:
A = total stand area
Ap = area of the stand that will be measured
P = equals intensity of inventory as a decimal %
a = area per plot in square units of B and L
n = number of plots
B = spacing between plots on a line in a given unit
L = spacing between lines in same units as B
For example a 5 percent inventory on a 50 ha. site using .1 ha. plots would require 25 sample plots (n = Ap/a).
Young Plantation Management
With proper tending and protection, trees should dominate the site within at least the first five years after planting and even earlier in the tropics. At this point, the trees have essentially become established and are ready for the preliminary phases of silvicultural treatments. These include thinning and pruning. Fertilization is another option, but should be applied before crown closure (See box).
Precommercial thinning
Thinning essentially reduces the number of trees on site to allow more growing space for those remaining. The goal is to reach the optimum spacing for mature trees to maximize the desired products or end use.
For most species the sale value of wood increases significantly with tree size and a suitable thinning regime will guarantee the largest number of the biggest trees possible.
Thinning also results is a more uniform distribution of the crop trees throughout the stand. If stand is not thinned, trees will become crowded, restricting their development and growth.
Some species, notably eucalypts as mentioned earlier, will not respond to thinning if left in competition too long. For pulpwood production, such species are established at their final stocking, which eliminates the need for thinning.
The operation is also an opportunity to remove undesirable plants including weed trees and trees that are diseased, infested, misshapen, or in poor health. Removal reduces the risk of spreading disease or insect infestations. Climbing vines and other threatening vegetation might also be eliminated at this time.
There is no general set age during which thinning should take place. Timing the activity will vary according to species, spacing of the plants, and the quality of the site. As a general rule, thinning is timely when tree branches touch or nearly touch, almost creating a thicket.
Plantation managers should consider three factors for a thinning operation: thinning intensity, cycle, and type. Intensity is the amount and frequency of tree removal. In some cases, thinning may be done in more than one operation over a short period. This temporarily retains some trees to maintain the protection of the crop trees and helps avoid the risk of wind damage.
The thinning cycle is the number of years between each thinning operation. In addition, the type of thinning has two strategies. Low thinning removes the slower growing and suppressed trees and is often referred to a "hygienic thinning." Conversely, crown thinning keeps the more dominant trees, thus favoring the better individuals. In all cases thinning cycles must relate to the crop growth for its most valued end-product.
Initial pruning
Pruning is the removal of branches to a height as far as can be reached. It is generally done when the stand is a decade old. Again, correct timing depends on the species and growth rate.
Pruning benefits the stand by adding value to timber crops by creating more clear, knot-free wood. The operation also reduces fire hazard by eliminating a fuel source and improves access through the stand.
The forester must ensure that workers cut branches as close to the stem as possible. Cutting can be done either manually with hand saws or by using power saws. Special long-handled pruning saws are available for higher pruning without climbing. Moreover, the activity should take place during a dormant season or slow growth period.
Furthermore, not all trees need to be pruned, or need to be pruned to the same general height. Decisions on what to prune should be made based on the type of species or the desired products. Some species may rapidly self-prune after crown closure, thus saving the effort and associated costs.
Other species may not respond well to pruning as they may require a larger percent of live crown. In any case, trees that are pruned must have an adequate percentage of live crown remaining to support the tree. Trimming too much can weaken the tree, thus slowing growth and making it vulnerable to disease or insect attack.
Fertilization quickens growth, corrects deficiencies
Fertilization can be used to either enhance the stand's growth or to amend the soil to correct any detected nutrient deficiencies. Treatments should be done early in the establishment phase, soon after planting. This is the point at which young plants can most use a growth spurt. Also, crown closure in a developing stand could inhibit proper application if the treatment is put off for several years.
Trees require an adequate dose of essential elements for proper growth and development. Lack of these may cause stunted, slow and less than optimal development.
Suspected deficiencies will likely show in the foliage as discoloration, deformation, or dieback. Field tests and lab analysis should be used to support initial suspicions. Elements most often in short supply are nitrogen and phosphorous. In addition, micro-nutrients are often overlooked, but can be extremely important, especially on degraded sites.
Before deciding to fertilize, the forester should complete a cost-benefit review. Benefits of fertilization include: allowing normal tree growth on marginal sites; increasing growth rates, which give the plants a better chance at survival; and permitting a quicker establishment. Costs include labor, materials, and transportation of materials to the site. Also, the forester should consider the presence of nitrogen-fixing species, which could negate the need to fertilize or reduce the application levels.
Application can be done manually by hand, by machinery such as tractors, or, in sufficiently large projects, by aerial spreading. Also, timing of application is important. For example, application without adequate rainfall may result in leaf burn or damage to the plants because of high salt concentrations in the soil.
This bulletin was originally published by The World Bank/AGRNR.
Author Contact:
by Norman Jones
1 Bradfield Avenue, BRIDGEND, Mid-Glam CF31 4HL, United Kingdom
Tel [44 16560] 656726; Fax [44 1656] 768369;
e-mail: njones18@compuserve.com
Why work hard to collect good seed?