The Art and The Science
There's no getting around it—it takes a lot of knowledge to manage a forest. Since every forest is different and every landowner’s goals unique, there are no cookbook answers. To create the forest you want requires that you learn some basic forestry principles and understand how they apply to your specific forest.
Your forest exists within certain physical parameters. It has an elevation, slope, aspect, soil characteristics, climate, etc. These create the canvas upon which your forest grows. You can't change these factors very easily so your forest management choices are limited by its site characteristics. Your site determines what is possible for your forest.
How did we get here?
In addition to the physical canvas, your site has a history. This includes all of the past management practices, disturbances large and small, and other occurrences that made the forest what it is. This is another factor you can't change. The starting point for your management activities is the forest you see today.
A forest is composed of many individual species: trees, shrubs, herbs, lichen, fungi, birds, mammals, etc. These are the building blocks that make up your forest.
However, the forest is more than just a collection of species. Over many eons the individual species in a forest have adapted to living together as a community. Each has a role to play—a niche—and the various species interact with one another in numerous ways.
When we talk about silviculture we are primarily concerned with the trees, but remember that the trees grow in the context of a greater forest community.
Silvics—how does your forest grow?
Silvics is the science of how trees grow. Silvicultural systems use an understanding of silvics to manipulate vegetation and shape a forest stand (an area of forest that is similar and managed as a unit) through its entire lifetime.
Each tree species has its own needs and tolerances—for light, water, temperature, nutrients, and other requirements. These tolerances determine where a species can live, and how well it can thrive at that site.
Some trees are shade tolerant. These species can grow in the shadow of other trees. Others are shade intolerant and require more light to grow or thrive. They are successful when a disturbance opens up the forest canopy to allow more light in to the forest floor.
Competition for light, space, and water plays an important role in forest dynamics and some trees are more successful than others in this competition (see illustration to the right). Silviculture controls and manipulates these dynamics by removing trees and plants to release favored trees from competition.
Silviculture mimics the chaos of living systems
The classic model of forest succession begins with a clean slate of bare ground. Pioneer sun-loving herbaceous plants are the first to establish there. These are eventually replaced by shrubs, then fast growing shade-intolerant trees, and finally the climax forest of shade tolerant trees triumphs.
However, in the real world, this orderly sequence encounters frequent setbacks and variations. Disturbances of all kinds and sizes—insects, fire, disease, windthrow—change the succession pattern. Disturbance is vital to ecosystem function: it uncovers bare ground so seedlings can become established, contributes to biodiversity, and alters the forest structure. The cumulative history of disturbances creates the chaotic mosaic of habitats that we see in a typical natural forest.
Disturbance is a natural and inevitable process in any forest. It is the model for much of silviculture, which uses disturbance in the form of harvesting and other practices to manipulate forest structure, species composition, and age classes to meet specific goals and objectives.
Silvicultural systems are generally divided into two major categories: even-aged and uneven-aged.
Even-aged stands are created using clearcuts that remove everything to allow the forest to start anew. Generally, clearcuts are followed immediately by replanting, resulting in a new forest of trees that are all the same age. Clearcuts are the most efficient way to harvest timber and at the same time open up the forest floor to light for shade-intolerant species like pines. Clearcuts are also used to renew degraded forests, to change species composition of a stand, and for other purposes. In California, clearcuts are limited to a maximum of 20 acres.
Seed tree or shelterwood systems are a variation of even-age silviculture, where a few trees per acre are left in an otherwise clearcut area to regenerate the stand. These trees are often removed after the new forest is established.
Uneven-aged stands are those with two or more age classes. The uneven-aged structure is maintained with group or single-tree selection harvests.
Group selection is a system where trees are harvested in small groups that are 0.25 to 2.5 acres in size.
Single tree selection removes trees one at a time. This disrupts the forest structure the least, but is much more difficult to do without damaging nearby trees. Single tree selection usually does not provide openings large enough to benefit shade-intolerant species.
All timber selection types are considered more or less analogous to natural phenomena. Clearcuts mimic large disturbances like stand-replacing fires, group selection mimics smaller fires or windthrow, and single-tree selection mimics loss of individual trees from disease or other mortality factors. However, it is important to realize that these silvicultural activities are not completely equivalent to, and don’t provide all the ecosystem benefits of, their natural counterparts.
Intermediate treatments involve removing competing vegetation, such as shrubs and grass, and thinning trees to accelerate growth in the remaining trees. These treatments are undertaken to help improve the growth, quality, composition, and vigor of the stand.
Mix it up
You are not limited to one silvicultural system on your property. Often, a blend of treatments is prescribed to meet landowner goals. In addition, it may take a series of silvicultural treatments to achieve your goals. The forest reached its present condition over a long period of time with multiple disturbances. You shouldn’t expect to fix it all in one step.
The art of silviculture
How do you take the same silvicultural systems and treatments and use them to accomplish many diverse goals on a wide range of forests and sites? That is where the art of silviculture comes in.
Foresters and land managers develop this art over time. It includes an ability to “see” into the future, to visualize what the forest of today will become in the future and what changes are needed to move it toward the desired condition. It is an intuition about the forest, based on knowing the site and the trees intimately, and observing how they respond in different circumstances. It is the creativity to see new possibilities and the flexibility to change direction when necessary. None of these abilities can be taught, but they can be developed through experience and observation, trial and error.
Management decisions will shape the future
The silvicultural systems you choose and the management decisions you make are determined by your goals and objectives. For example, even-aged stands have a simplified structure that may be efficient for timber production but for many other objectives, such as wildlife and aesthetic values, a more complex forest structure is desirable. The choices you make will have profound consequences on the future forest.
Can’t predict the future from the past
Forests are dynamic systems that don’t always respond as expected. Now, in addition to the normal uncertainty we expect from living systems, we also live in a time of changing climate. Studies suggest that California overall will become warmer and drier with less precipitation, earlier snowmelt, less available water, longer fire seasons, more intense wildfires, and changes in plant and animal communities.
This will challenge all of our assumptions as we may not be able to predict future results based on past experiences. Trees in your forest, with their longevity beyond a human lifetime, will exist under these new conditions. How do you factor this uncertainty into your forest management?
Adaptive management is one approach for dealing with uncertainty. It requires clear goals, an understanding of alternatives, observation and monitoring, and the ability to adapt management decisions to new information. It is a critically important tool when decisions have to be made in uncertain circumstances.
Much of forest management is experimental. You need to build in the ability to revisit your actions and change course—to adapt—when circumstances warrant.
More About Silviculture
An Ecosystem Management Strategy for Sierran Mixed-conifer Forests. By North et al. 2009. Gen. Tech. Rep. PSW-GTR-220. Albany, CA: USDA Forest Service, Pacific Southwest Research Station. 49 pp.
This technical report has created a lot of interest and also some controversy. It was written specifically for the mixed-conifer forest and provides a conceptual framework for heterogeneity in the forest.