To understand silviculture, one must first understand silvics. Silvics involves understanding how trees grow, reproduce, and respond to environmental changes.
Some tree species thrive in shade – sugar maple, red maple, hemlock and basswood are good examples. These species can live, grow, and reproduce in shade and semishade conditions. Many tree species prefer or require full sunlight – yellow poplar, walnut, some oaks, black cherry, yellow pine, and hickory are good examples. These species require full sunlight to reproduce, after which they grow best in full sunlight or as part of the overstory canopy of the forest.
They also tend to be the fastest-growing species and, to a great extent, the most valuable species. Still other species such as white pine, white ash, and some oaks, elm, and birch are intermediate in their sunlight requirements. These patterns are easily noticable in the woods. Normally, large overstory trees are oaks and poplar while seedling and sapling composition is generally maple, beech, and other oaks.
Silvics also is concerned with seeding requirements, elevation, and location. Different species will show up in different areas, on different soils, and at different elevations. If this sounds like ecology, then it can be stated accurately that silvics is the ecology of the forest.
Silviculture involves managing and handling the forest in view of its silvics.
Silviculture imitates a natural change – such as a windthrow, beetle infestation, or fire. However, silvicultural methods harvest forest products for human use rather than wait on nature to burn them, eat them, or blow them down. Silviculture can be practiced at any time in the life of a timber stand.
It is worthwhile to mention what silviculture is not. “High-grading” a timber stand is not silviculture. High-grading is “taking the best and leaving the rest.” Sometimes after high-grading, the remaining timber is of decent quality and the soil is productive, resulting in a future stand that is not noticeably degraded, except after a long-term look at growth.
Sometimes the remaining timber is poor, and the poor-quality, poor-growth stand that results is apparent to everyone. High-grading is not a plot by loggers against landowners; simply, it is a purely economic practice. Loggers want to cut only what they can sell easily.
Silviculture looks not only at the timber crop currently available, but also at the effects of present day harvesting on the next timber crop. Foresters are in a unique business in that they commonly make management decisions that will affect crop quality and growth 40 to 80 years into the future.
Common Silvicultural Harvesting Methods
Selection systems are partial removals of trees based on the silvicultural objectives of the landowner. This method is used when species of shade tolerance or intermediate tolerance are considered desirable. Each tree is assessed, determined to be cut or to be left, measured, tallied, and marked. Although economics always plays some part in determinations, it is not the only factor. Rate of growth, potential for further growth, health, quality, spacing, and species are some of the factors that also must enter each determination.
It is important that a landowner get a qualified forester to do this assessment; a so-called “thinning” or “select cut” can be offered by anyone with a saw in hand. A thinning based only on economic factors can easily become a high-grade. Furthermore, a small tree is not always a younger tree. Although many small trees in the woods are that size because of their youth, just as many are small because of poor genetics, stunted growth, or being a poor or inappropriate species for the site. It takes many years of experience to tell the difference.
It is important to combine some spacing and some weeding or cleaning while conducting a selection system harvest. Part of this method is manipulating the amount of sunlight on the ground to successfully regenerate desired species. Selection system harvesting has the advantage of allowing a timber stand to retain its forested appearance in the years immediately following harvest. It has particular advantages in higher elevations and farther north where shade-tolerant species are considered very desirable.
It has disadvantages of providing for slower long-term growth, for allowing undesirable species to predominate, for allowing undesirable epicormic branching on future crop trees, for holding back valuable sun-loving species, and for being an easily and frequently abused method. Furthermore, it is very difficult to use successfully on steep ground due to high potential for heavy logging damage on residual trees.
Selection system harvests can be designed to suit each individual tract, each site, each timber type, and each individual landowner. Each type of selection system has a separate set of objectives for the manipulation of sunlight, reproduction, present crop, and future crop.
Seed tree cuts allow the harvest of all trees except 2 to 10 trees per acre. These remaining trees are chosen for their good form, genetics, species, and ability to produce seed crops. The job for these remaining trees is to rain down genetically good-quality seed on the freshly disturbed areas. Although this method leaves fewer trees per acre, these tend to have a pleasing appearance because their spacing is very consistent. This method is rarely used in hardwood management, but often used in pine management. It is used with hardwoods only in rare cases where the desirable species are wing-seeded (i.e., ash and maple). However, this is not a good method for oaks or other heavy-seeded species.
Clearcutting is also a silvicultural method. This much-embattled method is truly a viable silvicultural practice. It is most often prescribed where sun-loving species are desired for the future timber stand. It is also prescribed in poor-quality or problem stands which have been abused by fires or repeated high-grading. In pine management, a clearcut normally is followed immediately by planting seedlings.
However, where quality hardwoods grow, natural regeneration supplies more than enough seedlings from existing seed, existing seedlings (also called advance regeneration), root sprouts, and stump sprouts. Thankfully, natural regeneration of hardwoods is one of nature’s strongest and most inevitable forces in our region. Where desirable hardwoods can be grown, natural regeneration is the proper plan; attempting to plant or artificially regenerate in such an area is neither necessary, wanted, nor advisable.
The desired effect of a clearcut is to start all regeneration at ground level so that the resulting timber crop is made up of desirable sun-loving species, which are the fastest growing, straightest, healthiest, and most superior trees possible. A 20- to 60-year-old clearcut is a textbook case of survival of the fittest. Because full sunlight is provided for future crop trees, rate of growth is at its greatest. Clearcut areas show 1.5 to 2.0 times the growth rates per acre than in selectively cut areas.
Clearcutting is not “cutting everything we want.” A clearcut should truly be a clear cut. Clearcutting is cutting everything. The objective is to provide full sunlight – not partial sunlight, with a heavy dose of shade from runt, cull, and unwanted trees. Additionally, a truly clearcut area looks uniform – immediately after the cut and after regeneration has begun. This uniformity is much more pleasing to the eye, not the “hairy” look of a pseudo-clearcut or a heavily high-graded stand.
Clearcuts also require professional management. Streamside buffer zones should not be clearcut to protect the temperature characteristics of that stream, or should buffer areas bordering towns or roads.
Obviously, clearcutting makes a lot of news. The method will arouse a tremendous amount of emotion and opinion.
Clearcutting is not a universally accepted practice. Opponents of the practice of cutting every tree within a specific area contend it degrades the environment. Forestry professionals and resource managers argue that the practice is sound – a complete clearcut “usually creates the best conditions for regenerating stands” under certain conditions and should be used when those conditions occur.
This is opposed to a “commercial” clearcut where only trees of marketable species, size and quality are cut. “Commercial clearcutting, sometimes called high-grading, is appropriate only where nearly all trees in the stand are marketable or where the harvesting process will knock down all the unmarketable trees.”
When are conditions right to consider clearcutting a forest?
Here are seven conditions:
When regenerating tree species that need full sunlight to stimulate seed sprouting and seedling growth.
When dealing with sparse or exposed or shallow-rooted trees that are in danger of being damaged by wind.
When trying to produce an even-aged stand.
When regenerating stands of tree species that are dependent on wind blown seed, root suckers or cones that need fire to drop seed.
When faced with salvaging over-mature stands and/or stands killed by insects, disease or fire.
When converting to another tree species by planting or seeding.
To provide habitat for wildlife species that require “high-density, even-aged stands”.
The system employed in timber harvest may affect the site preparation necessary to establish a new stand in several ways. For example, partial cuts prior to final harvest may salvage dying timber and may increase the growth of final crop trees or facilitate advanced natural regeneration by stimulating seed crops and providing favorable seed beds, but such practices also favor establishment of understory species which will proliferate rapidly after final harvest.
The choice of logging method will affect greatly the proportion of the cutover area which has exposed mineral soil. Mineral soil favors either seeding or planting of conifers, but if there is a seed source of a competitor nearby, exposure off mineral soil can lead to almost instant thickets of shrubs which are expensive to remove.
Both rubber tire or track laying vehicles have been shown to cause significantly more soil compaction than do moving cable logging systems. Losses of seedling survival and growth in areas of soil compaction may be severe. Many land management organizations compensate for such compaction damage during logging by scarifying (“ripping”) all cat trails and landings after harvest.
Chemical site preparation differs from mechanical manipulation primarily in that it generally has a much less severe impact upon the environment and in that chemical site preparation is generally much more selective than mechanical site preparation techniques which employ power machinery. It differs from mechanical site preparation, too, in that the active agents, the chemicals, may affect crop as well as target species and may be dispersed beyond the boundaries of the project area. For these reasons, only persons thoroughly conversant with the mode of action of chemicals and with the technology of their distribution should employ herbicides in forest management.
The use of chemicals in vegetation management has been greatly restricted because: a) relatively few compounds are registered for use in the forests; and b) strong resistance to the use of chemicals by environmentally oriented groups. As an example, in the state of Oregon, 40,000 ha. have been treated with aerial applications of herbicide each year. In contrast, chemicals have been used less than 10% as frequently in British Columbia, and virtually all the use of chemicals in British Columbia has utilized ground systems for controlled application.
Prescribed burning is defined as fire applied in a knowledgeable manner to forest fuels on a specific land area under selected weather conditions to accomplish predetermined, well-defined management objectives.
Few alternative treatments can compete with fire from the standpoint of effectiveness and cost. Chemicals are expensive and have associated environmental risks. Mechanical treatments have the same problems. Prescribed fire is much more affordable with much less risk to the habitat and destruction of site and soil quality.
A properly conducted burn will release mineral elements such as ash on the soil surface and within topsoil, and higher concentrations of nitrogen, phosphorus, potassium calcium and magnesium in the upper few inches of mineral soil. The net result may be an increase in the quantities of available nutrients for a short period after the fire. However, in areas of high rainfall, such elements may be rapidly leached through the soil horizon, rather than released slowly from natural decomposition of organic matter.
Whether such leaching loss of nutrients results in a reduction in site productivity is dependent upon the original distribution of the nutrient capital. Sites with relatively deep, fertile soils may not be seriously affected by even intense fires, while the shallow, rocky soils on steep slopes in higher elevations may have a significant proportion of the nutrient elements in the litter layer and upper mineral soil. Such sites may be severely degraded by injudicious use of fire.
Prescribed fire is a complex tool to be left in the hands of a certified fire prescriptionist. Proper diagnosis and detailed planning is mandatory before every burn, as well as having the right tools. An incomplete assessment of any factor in a plan can lead to serious loss of property and life with serious liability questions to both the landowner and the one responsible for the burn.
Natural regeneration has a definite place in some situations and may become more important again with increasing reliance on harvesting methods other than clearcutting. In any event, more consideration than at present should be given to utilization of natural regeneration in combination with artificial methods of reforestation. Desirability of, and problems with, natural regeneration vary, however, with forest types, topography, and management objectives.
Regardless of tree species and geographic location, the basic requirements for successful natural regeneration are a supply of viable seed, a suitable seedbed, and an environment compatible with germination and seedling establishment. If these requirements are not met, regeneration fails.