Silviculture is the applied science of forest ecology and management. The foundation is based on silvics, which is concerned with the development and growth of trees and forests. The practice of silviculture is rooted in a broad understanding of forested ecosystems, which includes biometeorology, hydrology, geology and soils, and ecology...
The practice of silviculture has been around for centuries... The treatments that are applied at any stage must match the natural environmental factors of the particular region, as well as reflect the social value or purpose of the particular stand.
Soil Quality, as a general concept, can be thought of as the ability of a soil to function, in either natural or managed ecosystems... without impairing the resource base or harming the environment.
Soil Quality has the potential for many different interpretations... it should relate to the capacity of the soil to function effectively with regard to productivity; environmental quality; and plant, animal, and human health now and in the future.
| Physical | Chemical | Biological |
|---|---|---|
| Topsoil depth | Organic matter content | Soil Respiration (CO2) |
| Texture and aggregation | Salinity (electrical conductivity) | Microbial activity/biomass |
| Aeration and infiltration | Acidity/alkalinity (pH) | Earthworm counts |
| Surface cover | Nitrate nitrogen | Plant vigor |
| Compaction | - | - |
Figure 7: Inadequate soil cover on sloping surfaces can result in topsoil removal by erosive forces of water and wind. Here, rills and gullies are visible on a hillside where little fertile topsoil remains. Image courtesy of Tim Kettler
| Soil Quality Enhancing | Soil Quality Degrading |
|---|---|
| organic material additions | overharvesting |
| plant growth | bare fallow |
| fibrous root systems of plants | fire |
| cool, humid climate | hot, arid climate |
| vegetative cover | exposed soil |
| minimal tillage operations | intense tillage |
| wildlife | wildlife |
| erosion |
For plant growth, the topsoil is the richest and most valuable part of the soil. Topsoil formation is a very slow process, which makes it a non-renewable (but re-usable) resource in terms of human lifespans. Keeping the soil in place while it is used for construction or crops is one of the greatest challenges faced by engineers and land managers.
The seed-tree regeneration method is similar to some variations of the shelterwood method, in that some trees are left after harvest to provide seed for establishing the new stand. Unlike the shelterwood method, however, the residual trees are not intended to provide shelter during seedling establishment, so fewer trees are left. After seedlings are established, the seed trees are usually removed. They may be left, however, if it is not economically feasible to harvest, or if harvesting would cause severe damage to the new regeneration.
For adequate regeneration, the quality and spacing of seed trees are critical factors. The number of seed trees to be left depends on tree height, seed dispersal characteristics, prevailing wind direction, quantity and frequency of seed production, and seedbed characteristics. The number normally varies from 3 to 15 trees per acre.
The seed-tree method is best suited to windfirm species with wind-dispersed seed that produce fairly abundant and regular seed crops and that are best managed in even-aged stands. This method has been used extensively for regenerating southern pines such as loblolly, and has also been used with yellow-poplar.
The method resembles a clearcut in that it does not afford the new reproduction significant overhead protection. The seed-tree method has several disadvantages, though, when compared to the clearcutting method. These include uneven reproduction density as a result of seed dispersal patterns; risks of low viable seed production; less control of species composition, especially if the mature stand contained a large component of less desirable species; less accessibility for site preparation; risk of windthrow and snow and ice damage to the seed trees; and the risk of damaging the regeneration when the seed trees are harvested. In addition, many landowners do not like leaving significant volumes of lumber in seed trees. As a result of these disadvantages, the seed-tree method has largely been replaced by clearcutting as the intensity of management has increased.
Figure 12: Diagram of a forest stand regenerating under the seed tree method. (a) Prior to harvest, (b) immediately following the seed tree cut, (c) young regeneration following the removal of the seed trees, and (d) established seedlings and saplings growing under open conditions.
In southern pine management, clearcutting followed by site preparation and artificial regeneration with genetically improved seedlings is usually recommended because of the more uniform stand conditions and significant yield increases obtained. Several states have seed-tree laws, however, including Virginia. Title 10, Article 6, of the Virginia Code as amended in 1996 requires that landowners harvest by the seed-tree method to insure regeneration, unless an alternate reforestation plan is approved by a state forester. The law applies to stands of 10 acres or more where loblolly pine or white pine make up 25 percent or more of the live trees 6 inches or more in diameter at the stump height, and requires that:
When leaving seed trees, it is often advisable to leave more than the required minimum to assure better seed distribution and to provide enough timber volume to justify harvesting the seed trees at a later date. Because some trees produce more and better seed than others, seed trees should be carefully selected using the following criteria:
The seed-tree method has been used in upland mixed hardwood stands. These stands generally have an abundance of highly competitive seedlings from advanced regeneration and sprout origin. Stands of this type resulting from seed-tree cuts are similar to stands that were clearcut and naturally regenerated.
Wrapping and waxing, collecting scion wood, types of grafting or budding--bench grafting, cleft graft, bark and inlay grafts, budding, shield or T-budding, and chip budding--are discussed. Also included are photographs and a list of grafting supply resources.
Many people mistakenly believe that fruit trees grow true to name from seeds. In reality, if you collect seed from a fruit grown on a plant, the seeds will produce plants that will be a hybrid of two plants. The new plant will be the same kind of plant, but its fruit and vegetative portions may not look the same as the parent because the plant is "heterozygous." Therefore, all fruit trees must be vegetatively propagated by either grafting or budding methods.
Fruit growers frequently use grafting techniques to topwork new varieties or strains of fruit onto established trees bearing misnamed or obsolete varieties and to repair injury or damage caused by mice, rabbits, deer, or mechanical means. Commercial nursery workers propagate new fruit trees, and producing a tree ready for planting takes several years.
All of the temperate-zone deciduous fruit plants may be propagated by budding. Cleft, whip, and bridge grafting of apple and pear is possible, but such grafts are not often successful on stone fruits. Sweet cherry and, occasionally, peach may be successfully grafted using the side graft onto a 2- to 3 year-old limb.
Materials needed for any type of grafting are scions, rootstocks, a sharp knife, a cleft grafting tool, and a lightweight hammer. Depending on the type of graft to be prepared, you will need grafting "wax," rubber bud strips, wax-coated cotton twine, grafting tape, and wire nails (number 16 or 18). Having a sharp knife is of paramount importance--using a dull knife can lead to serious injury.
In this publication we will describe some successful propagation methods that can be used on tree fruit. The techniques described may be used in all instances where grafting is required. The choice of methods depends on the time of year, the type of material available, and the type of propagation desired.
As a general rule, all grafts made by budding or whipping should be wrapped. Grafts made by clefting, side limb insertion (often called "hip graft"), inarching, and bridging need to be waxed, and in some instances nailed tight.
Many cloth-backed and plastic tapes are available for use in wrapping the graft, although plastic tapes appear to be easier to use. Rubber composition bud strips are used to wrap the dormant buds. The plastic materials come in rolls of 3/8- and 1/2-inch widths and are well suited for grafting purposes. When carefully wrapped, they will form a very tight seal. The plastic tape is secured by pulling the free end under the last turn around the stem. Whether the tape is wrapped from the bottom up or vice versa makes little difference. However, making the wrapping as airtight as possible is important.
When rubber bud strips are used to wrap the bud or graft, no further attention is usually needed. When nursery workers' adhesive tape is used, care must be taken to slit the tape vertically about 4 to 6 weeks after growth starts to prevent girdling.
Cotton twine or cloth strips impregnated with wax may be used to wrap the graft. The waxed string is useful, but great care is necessary to ensure an airtight wrapping. The strips of impregnated cloth (1/2 inch wide) are useful in wrapping bark grafts or T-buds.
Several wax-based and water-soluble waxing materials are available. When using the more liquid water-soluble materials, be sure to recoat the graft several times to make sure that any cracks (which often occur with these materials) are closed. Water-soluble asphaltic compounds, commonly sold as wound dressings, are simpler to use than the wax-based materials. Containers of water-soluble materials must be protected from freezing to prevent breakdown of the product.
Grafting wax, also called "hand wax," does not require a heating device to liquify it. It is soft, pliable, and simply pressed around the graft. Hand wax is suitable when only a few grafts are to be prepared since it is very sticky and unpleasant to use. Commercial formulations of hand wax can be purchased from the companies listed in the "Grafting Supply Resources" section of this publication.
You can also make your own hand wax using the following recipe:
Melt the tallow or heat the linseed oil (to about 125°F), and then add the beeswax. When these two parts have melted together, then add the powdered rosin. Stir until thoroughly mixed, pour into a bucket of water, and allow to cool. Form the wax into a ball and pull (or knead) it until it assumes a yellowish or tan color. Divide into 1/4- or 1/2-pound balls and store in plastic bags until ready for use.
The fluid hot waxes and water-soluble materials flow freely around all cut surfaces of the graft and are less troublesome to apply than the hand wax; however, a special wax melter is needed for the hot waxes.
Sealing all cut surfaces is important for the graft to succeed. This includes all exposed surfaces where the stock and scion are joined and the free end of the scion.
In this paper, “productivity” will usually mean the cubic meters of harvestable wood that can be grown per year on a forested site. But it is more complicated than that. Productivity not only covers harvestable wood, but the quality of that harvested wood for various purposes. Even more broadly, it includes the productivity of other goods and services an indigenous or plantation forest can, or could, provide. In some cases, all of these elements of productivity are improved or harmed by a particular practice; in other cases, tradeoffs among them must be considered.
Many of the “factors” in this section have been studied in various settings, and the results
are often situation-dependent. For example, fertilizing on an
already fertile site often has little effect, while the same amounts of fertilizer on a
nutrient-deficient site can have dramatic positive effects, while that
level of fertilization on yet a third site may trigger an epidemic of some previously-mild
endemic pest attracted to the better nutrition then provided in
the trees’ foliage. Most of these many factors interact with each other. Many have been
studied separately, or in groups of combinations (for example,
Bower 1999), but rarely are all interacting factors accounted for. Getting it wrong in one
factor can negate the good that excellent inputs in several other
factors might have been expected to achieve (nicely illustrated in Davidson 1996; see also
Section 2.12 below).
In the topics that follow in Section 2, each factor will be briefly discussed, but precise quantification of changes in productivity with particular inputs will not be presented. A comprehensive review is beyond the scope of this paper; furthermore, many of the available quantifications are based on young trees, and they would probably be misleading. Some remarkable changes in wood and non-wood productivity will be presented in the case examples in Section 3. It is important to acknowledge here that establishing and managing a productive forest plantation requires high levels of professional knowledge, experience and judgement.
Indigenous forests are often remote or even inaccessible. Since people need to get to a forest plantation site in order to plant it, access to most forest plantations is usually available for their later management and harvest. The relative availability of access does not directly affect productivity. It can indirectly affect it by its influence on the ability of management to exert leverage through such things as post-planting release, thinning and pruning, and other management activities. Furthermore, accessibility affects the energy it requires to deliver forest products to their users, and thus the economic feasibility of using them.
As agriculture increasingly mechanizes, it has been noted that forest-management activities are becoming relatively more labor-intensive compared to agriculture. Thus, it makes increasing sense to have forests near villages and agricultural operations more distant from them. This change carries an added benefit for villagers who use wood as fuel for cooking and/or heating their dwellings, particularly because food can usually be more easily transported long distances than can the wood to cook it (Smith 1981).
A common spatial relationship has been one of villages surrounded by forest clearings growing crops, with forests at ever-greater distances from the villages as the villages increase in population and more land is cleared. As this relationship inverts, forests will probably be re-established near the villages as forest plantations, although natural regeneration could be used to maintain them once established
Of all the factors in this section, it appears that site quality has the greatest and often longest-lasting effect on differences in the productivity of various indigenous and plantation forests. The important components of site quality are: soil depth and drainage; soil physical and chemical composition (including pH); amount and pattern of yearly soil moisture availability; frequency and nature of common and occasional winds, storms and fires; and the general climate of the area. The presence and importance of such things as competing vegetation, and of populations of animals, insects and microorganisms that are either damaging or beneficial to trees, can and should also influence selection and later management of forest plantation sites. The size and health of trees already present on the sites are good but imperfect indicators of site quality. Different species of trees on candidate sites will index them differently, and genetic differences within the same species can also result in substantial differences in derived site-indexes between similar sites, or vice versa . In most forested regions of Earth, people have recently and historically cleared forests for pasture and crops. In retrospect, many such cleared sites have been too erosive for continued pasturage or cropping, or have proved to be (or been degraded to become) marginal or sub-marginal for their intended agricultural uses. The good news is that such erosive or marginal and sub-marginal agricultural sites often prove to be good or even excellent sites for forest plantations. Furthermore, some non-forested sites very unpromising for agriculture, such as the coastal dunes of southwestern France, northwestern New Zealand, eastern China and western Senegal, have been planted with forest trees and are now productive forest plantations. Finally, many forest plantations are established for a variety of important primary purposes other than supplying wood (Palmberg 1989; Laarman and Sedjo 1992), for example, to rehabilitate sites degraded by uses such as strip-mining. Where such sites are not evaluated and chosen for their potential wood productivity, any wood eventually produced by them may be viewed as a side-benefit
The first decision to be made about each forest plantation site is whether it will be a mono-species plantation, or whether it will be planted to two or more species in mixture. European tradition has favored mono-species forest plantations, largely based on European experience in trying to domesticate their indigenous species. However, if the forest plantation is to provide aesthetic, wildlife-habitat, biological diversity, and other services in addition to wood-production, multi-species forest plantations will often be favored. If the forest plantation species of choice are (is) indigenous to the region, then assigning a species or mixture of species to available forest plantation sites can usually be done with reasonable accuracy. In many regions, introduced species outperform the local species in wood production by multiples of 2, 5, 10 or even more. Extensive species introduction trials should precede widespread commitment to even the most promising introduced species. But even with such species-introduction trials indicating outstanding performance, small-plot performance in such trials is not always repeated in widespread forest plantations. This may happen, for example, when some pests or pathogens miss the early small trials but later find the large areas of host trees. This can be particularly damaging if the pests or pathogens are also exotic, and arrive without their natural biological controls. During the past several millennia, a remarkably short list of plants and animals has been successfully domesticated from a remarkably long list of plants and animals that were hunted and gathered for food or befriended as pets and houseplants. The same may or may not be true for forest trees (Hansen and Kjaer 1999).
If indeed many species of trees are to be domesticated, rate of progress on many of them is likely to be even slower than occasioned by the constraints of their large size and long generation times. A list of the species that currently seem likely to be domesticated would include many members of the Pinaceae, surely several pines (Pinus radiata, P. taeda, P. pinaster, P. caribaea, P. patula and others), perhaps a few spruces (Picea abies, P. sitchensis and others), maybe Douglas-fir (Pseudotsuga menziesii), likely several members of the Cupressaceae (Cryptomeria japonica, Cunninghamia lanceolata, Sequoia sempervirens and one or more Cupressus) and perhaps others (one or more Taxodium, the hybrid Cupressus macrocarpa by Chamaecyparis nootkatensis, and Sequoiadendron giganteum). Few or no Southern Hemisphere conifers are likely to make that list. Among temperate and boreal angiosperms, several poplars (Populus) and the many promising hybrids among them, several willows (Salix) and their hybrids, walnut (Juglans nigra and perhaps others), and maybe some cultivars from genera such as Quercus, Betula and Acer seem possible or likely domesticates. Among subtropical and tropical species, Eucalyptus grandis and several other members of the promising Eucalyptus genus, several Acacia species, Gmelina arborea, Tectona grandis, Swietenia macrophylla, Paulownia tomentosa, one or more members of Leucaena, Prosopis, Casuarina, Grevillea and Dalbergia, and a few others are likely candidates for that short list. Some of these will no doubt fall out of favor as attacks by pests and pathogens result in unacceptable levels of loss, or as difficulties such as recalcitrant reproductive biology or weedy invasiveness, become apparent.
“Domestication” implies genetic changes in populations to suit human needs or purposes. But it is more than that. Domesticated plants and animals rarely reach anywhere near their full potential productivity or performance without husbandry that is adapted to the domesticates. Forest trees will probably be no different. To some people, natural selection as a force for adaptation and change in nature is good, or at least acceptable, while human-directed selection is suspect. But (at least until modern biotechnology came along) the rules were fundamentally the same. Humans, as a component of the environments of the plants and animals being domesticated, changed how some traits influenced reproductive success. But most of the rules of nature were and are still enforced, and that will be particularly true with respect to domesticated forest trees. Since most forest plantations are left pretty much on their own most of the time, plantation trees with human-directed trait-changes must still function well in what is mostly still the domain of nature
For any breeding program using parent trees not indigenous to the local region, a first step is to learn or determine the species’ pattern of provenance variation. Efforts are then concentrated on selecting parent trees from populations that are already well-adapted to the region. Detailed examples for several tropical species are given in Vichnevetskaia (1997). In the first generation of most tree-domestication (sometimes referred to as “tree improvement”) programs, much attention is given to find