The following definitions of silviculture were proposed by Stephen Spurr, a famous American forester (Spurr 1979):
1. The art and science of producing and tending a forest.
2. The application of silvics in treatment of a forest.
3. The theory and practice of controlling forest establishment, composition, and growth.
Silviculture is at the heart of forestry. It is about decision-making, and it enables people to get what they desire from forests in an efficient, sustainable way. To practice good silviculture, a forester needs sound training in basic biology, soil science, ecology, dendrology, physiology, mathematics, economics and management science.
A single stand is the focus of most silvicultural planning. Other important aspects of forestry cover decision-making when many groups of stands are contained within an estate, and decisions made at a stand level may be changed during estate planning.
The need for silviculture
In earlier times, when population densities were low and forests seemed inexhaustibly vast, there may have been less need for silviculture. Forests were often mined with little regard for regeneration and composition of future vegetation on a site. It is now realised that highly desirable products obtained from natural forests arose through a combination of fortuitous circumstances and long periods of growth, and that the cost of producing such products by simply replicating natural processes is often prohibitive.
Silviculturists are more urgently required now than ever before. Many commentators predict huge shortfalls in global wood supplies, and an emerging environmental awareness is leading the public to demand that forests are wisely husbanded, a task which demands silvicultural skills.
If all forests and sites were identical, silviculture would be simple and boring. We are confronted, however, with enormous variation in sites, from rainforests to tundra and everything in between. Understanding and effectively managing the diverse vegetation which occurs on these sites is a difficult but exciting task.
The importance of defining objectives
People demand different things from their forests, and these differing objectives also add to the complexity silviculture. The first step in making silvicultural decisions is to identify how a forest is to be used. Possible uses include maintenance of oxygen in the atmosphere; erosion control; watershed regulation; provision of habitat for wildlife; maintenance of natural beauty; recreation; wood production; and/or fibre production.
The nature of the task
Generally speaking, a silviculturalist helps people meet objectives for their forests by controlling the establishment, growth, composition and quality of forest stands.
Wood production may often be achieved by maintaining a simple, efficient stand structure, whilst mixed objectives often require complex structures. There are exceptions to this generalisation, however.
Intervention in natural processes costs money, and it is incumbent on forest managers to demonstrate that proposed actions will be worthwhile financially. Predictions of the effects of actions must be combined with calculations of costs and benefits. Discounted cash flow analyses are sometimes used to compare alternative treatments of single stands, especially when financial benefits are paramount among the objectives of a forest owner.
It is usually necessary to predict the effects of for instance, nursery practices, site preparation, thinning or pruning on the outturn of forest products (be they wood or less tangible products such as natural beauty). In some circumstances these predictions may be made roughly from general principles. In many cases, however, more precise predictions are required, and managers rely on numerical models of natural systems.
Constraints must also be considered. These can be divided into ecological, managerial, and social constraints. Ecological constraints include factors like site quality. Highly productive sites usually offer more opportunities than infertile sites. Existing vegetation, the micro-environment, and pests are also important.
From a managerial point of view, some treatments may be infeasible, too expensive, or inadequately researched to be used. Company policies may also rule out certain actions. Sites located close to mills or markets offer more opportunities.
Lastly, social concerns place very real constraints on managers. These may be enshrined in laws (our Resource Management Act is a good example), or might be considered out of respect for wider social costs of otherwise useful activities.
Managers usually plan silviculture by analysing regimes, or sequences of actions. It makes little sense, for instance, to decide to fertilise a forest without considering what effects it might have on the timing of other treatments such as pruning and thinning, on the growth of non-crop vegetation and on the financial viability of the entire crop production cycle. Fertilisation will cost money, and this investment must be recovered by higher yields, a shorter rotation, or perhaps some change in the nature of the wood or form of the trees.
Silviculture is challenging because although you plant or establish trees for a reason, those reasons change over time. Every silvicultural action or inaction comes at a cost. With every decision a bit of flexibility is lost. How much flexibility should be maintained? How can flexibility be maintained at an acceptable cost? These are some of the challenges facing silviculturalists.
This European beech regeneration in Denmark was assured by appropriate logging and site preparation practices.
Students of silviculture should expect to become very familiar with classical systems of silviculture for natural or near-natural stands. These systems were invented in Germany during the last three centuries, and are meant to provide wood products while ensuring natural regeneration and maintaining the ecological integrity of a forest. They can be adapted to many different situations by a sufficiently informed and creative forester. An understanding of ecological principles is perhaps the most important prerequisite for using them. For example, a manager can control which species regenerate on a site by carefully choosing the right intensity and timing of canopy openings made by harvesting and maintaining the right sort of ground surface conditions after harvest.
Plantation silvicultural systems are often, but not always more intensive than classical systems. Decisions about what species to plant, what genotype to plant, how to propagate the plants, how to prepare the site, how many trees to establish, how to treat non-crop vegetation, whether or not and, if so, when to thin and/or prune, what spacings to grow the trees at, when and how to harvest, and how to put these decisions together as one package need skills which can be acquired after much study. Costs of crop establishment and tending should be carefully balanced against returns, leading to an optimum harvest age just before interest charges overwhelm any returns from further growth. Consider a typical rotation of radiata pine grown for clearwood in New Zealand which would involve:
Note from this time line that objectives must be identified before step 1. In practice it is best to begin at the end of the rotation and work backwards, so that decisions about crop establishment are made only after the rest of the regime has been designed.
Forest protection is also an aspect of silviculture, but is so complex it is often offered as a separate course in its own right.
A silviculuralist's task, therefore, is usually directed towards creation or maintenance of desired forest structure, composition, stand density & stocking, stem quality, and rotation length, while protecting both the crop and the site. He or she will predict gains and losses of proposed actions from ecological, social and economic points of view in order to design management regimes for stands which satisfy human desires.
Daniel, T.W., Helms, J.A., & Baker, F.S., 1979, Principles of silviculture, McGraw-Hill
Shepard, K., 1987, Plantation silviculture, Martinus Nijhoff
Smith, D.M., 1986, The practice of silviculture, 8th Edition, Wiley
Spurr, S.H., 1979, Silviculture, Scientific American 240: 76-82, 87-91