Silvics

Robert Teskey

University of Georgia

Definition

Silvics is the term used for the characteristics that define the life history, growth, behavior and ecology of a tree species.  It is often linked with silviculture, which is the application of silvics to the management of trees in order to enhance the reproduction, survival or growth of a specific tree species.  Silvics describes all aspects of a tree species life cycle, from germination to death, including the environmental and soil conditions that are suitable for its growth, and the insects and pathogens that can harm it.

Forest biologists have spent many years developing an understanding of the characteristics of tree species for the simple reason that each tree species has a unique set of requirements for growth and development, as well as some unique growth attributes.  Knowledge of these is essential for effective management of the species.

Management should be considered broadly in this context.  Management may include intensive practices such as growing a species in short rotation monoculture plantations for fiber, but it can also mean any activity that enhances the growth and development of a species, for example, the establishment of a tree species in a new rural or urban setting, the reintroduction of a species where it once grew, improving the growth and vigor of specific tree species for the use of wildlife, the introduction of exotic species that will be of benefit, or the eradication of introduced species that have become noxious weeds.  

An understanding of silvics also allows us to see the similarity and differences among tree species around the world.  Trees can be classified into three general categories based on whether they grow in boreal, temperature or tropical regions.  These classifications tell us that there is some similarity among species growing in the same climatic region.  Some species are more able to grow and survive where it is cold (boreal), or where it remains warm all year long (tropical), or in that in-between zone where cold and warm alternate (temperate).  Within each of these three climatic zones there are many, many tree species.  Importantly, the silvics of each and very tree species is unique. Why?

The answer is that no matter how much we humans like to classify and lump things together into groups, tree species are distributed individually across the landscape, and are not distributed in packets of like species.  That means that one species may grow best where it is wet, another were it is dry, another may have very specific soil nutrient or soil density requirements, while others grow well on many different types of soil. But each species has unique attributes and conditions allowing it survive and grow.  These attributes are often called a species niche, which is an ecological term meaning the species’ unique habitat or functional position in an ecosystem.  While species can share some characteristics in common, and others may share many similar characteristics, no two species will have identical life histories, growth, behavior or ecological characteristics, i.e., no two tree species have the same silvics.  Every tree species has some unique characteristics that make it successful in its environment.

The silvical characteristics of a tree species include life cycle, the environment and soil conditions in which it grows, and its response to biotic interactions such as competition or damaging insects.  In the Silvics of North America (1990) edited by R. M. Burns and B. H. Honkala, these factors are divided into the categories of Habitat, Life History, Special Uses, and Genetics.  Within these categories there are many factors that can differ among species and need to be considered.  The following is a brief discussion of the factors used to develop a silvical description of a species:
 

Habitat
 

The habitat of a species is where it is found in nature.  Habitat includes the species native range, or the historical geographical distribution of a species. It also includes the climatic range in which the species exists.  Climate is determined by the pattern and total amount of precipitation, including whether it occurs as rain, snow or fog, or a combination of those forms of precipitation; and by temperature, characterized by average annual and growing season temperature and the maximum and minimum temperatures the species can tolerate; length of the growing season, or frost-free period; the type or types of soil the species is found on. 

The total amount of solar radiation that a flat portion of the earth receives varies little on an annual basis, but it varies to a large extent seasonally and monthly.  The response to patterns of solar radiation can help define a species, because the timing of dormancy, or the winter resting period, is linked to changes in day length. Whether a species can grow in the low light conditions of a forest understory or the high light conditions of an open disturbed site is a particularly important habitat characteristic.     

Soils often exhibit a great variety of differences over small geographic areas, such as the depth of the soil, clay, silt and sand content, porosity, water holding capacity, and nutrient supply capacity.  These differences can affect the suitability of the site for a tree species. Topography needs to be considered as well.  Species often change from the bottom to the top of hills and mountains, and between sides of mountains.  The causes of this change can be attributed to changes in the environment (precipitation, temperature, solar radiation) and soils across gradients in elevation or aspect.  However, because topography integrates these factors together, it is often an excellent factor to use to help characterize a species.  Species may occur singly or in association with a large number of other tree species. In colder, dryer or extremely wet sites fewer tree species are found. On moist or fertile sites, or sites with long growing seasons, often a large number of tree species are found growing together in the same forest.  Tree species may also commonly be found with associated understory shrubs and herbs or grasses.   The present, absence and number of tree and understory associates further define the unique characteristics of a tree species.
 

Life History
 

Species differences in life history begin with their seeds.  Forest trees have a wide variety of seed characteristics.  Seeds can be large or small, heavy or light, covered with a thick or thin outside protective coating or shell, dispersed by wind, water or animals, and range in dormancy release requirements from almost none to very specific heat, cold, moisture, aeration or light exposure before germination is achieved.  Some conifers even require the heat of a fire to free the seeds from cones before dormancy release can be initiated.
Many of these dormancy factors are integrated into the concept of seed bed requirements of a species. 

Favorable seed beds range from open disturbed areas, for example, after a fire, to the moist humus of the soil in an undisturbed forest.  Seed beds across this range have different moisture, light, nutrient and temperature conditions, with each set of conditions favorable to different species. After germination, seedling development depends on the species ability to grow in the particular environment of the germination site.  

Although thousands of seeds often germinate on a site, only a handful survive through the first or second years.  This is often a time in which the seedling is exposed to intense abiotic and biotic stresses, and few seedlings are able to successfully establish in that situation.  Understanding the characteristics that allow a particular species to flourish after germination is often the key to successful establishment.  In intensive management situations, foresters avoid this critical period by germinating seeds in nurseries and greenhouses that offer a protected and resource rich environment to insure good survival.  The seedling may be out-planted when it is one, two years old or older.  This, coupled with proper site preparation to make the site favorable to the growth of the particular species insures a much higher rate of survival and faster initial growth rates.  

Species are also characterized by their growth form.  Aboveground, trees can be separated by whether they have evergreen or deciduous foliage.  Evergreen species can retain foliage from less than two years to more than 45 years.   In contrast, deciduous trees grow and shed their leaves every year.  Both evergreen and deciduous species can have multiple flushes of foliage and branch growth during a single growing season, although some have only a single flush each year and others have continuous growth during the growing season.  As trees age they can have fewer flushes than when they are young.  

The height and age of tree species also is an important attribute.  Maximum tree heights and longevities are species dependent and also vary with climatic region.  For example, evergreen conifers in the Pacific Northwest of North America achieve much greater heights and longevities than evergreen conifers in eastern North America, Europe or Asia.  Typically, trees that grow in more stressful environments are shorter, but they can have either deciduous or evergreen foliage displays.  Tree species that have fast growth rates tend to have short lives and smaller stature, but as with all silvical characteristics, there are notable exceptions to these generalizations among tree species.  The branching patterns of species also tend to be distinct, ranging from thick or thin, long or short, horizontal or upright, extending along the entire stem or occurring mostly at the top of the tree. 

Bark color, thickness and pattern are also distinctive among species.  Bark features help to protect the tree from insects and pathogens.  Thick bark is a notable feature in trees that are resistant to fire. The growth rate of trees tends to indicate its successional status.  Fast growth rates are advantageous in recently disturbed, early successional sites, whereas slow grow rates are commonly found in later successional species.  For this reason most of the trees planted for wood or fiber production are in the early successional category.

Within the stem, wood exhibits additional important differences among species.  Ring-porous wood consists of vessels that are evenly ranked from large to small each year.  Diffuse-porous wood has vessels of different sizes evenly distributed throughout the annual ring.  Tracheids, which are small diameter xylem cells, are yet another form of wood, mostly found in gymnosperms. Tree species with trachieds are called softwoods.  Tree species with ring- or diffuse-porous wood are called hardwoods.  However, the distinction is really not so clear.  Some softwood species have more dense wood than some hardwood species.  Species also have resin ducts in the wood for protection from insect attack.  Wood can vary greatly in its density and flexibility, affecting its suitability for use as building materials, furniture or paper manufacturing.   This makes wood traits particularly important in production forestry species.

Species can also be characterized by their patterns of root growth.  Some species have large tap roots, others none at all.  Some species are shallowly rooted, other deeply rooted.  Some species develop large and extensive root systems; others have much smaller root systems.  Even the size and color of the roots of different species can differ.  In some instances tree species have very distinct and easily observed roots, such as the large buttress roots in mangroves or the “knee-roots” of bald cypress trees. 

Reaction to biotic factors is also an important difference among species.  Competition occurs between plants when the resources needed for growth are in insufficient supply for all plants to achieve their maximum growth rate.  Tree species can be ranked by their ability to survive in various levels and types of competition.  The competitive ability of a species is difficult to characterize, but extremely important.  In the absence of competition, many tree species can survive and grow in a much wider range of conditions than those in which they are actually found in nature.  Species have evolved in association with different insects and diseases.  In some instances these damaging agents, such as root and stem rots, boring insects, or leaf herbivores prevent the species from achieving its maximum growth rate or potential longevity.   
             

Special Uses
 

It is often the ability to use a tree species for a specific purpose that makes its silvical characteristics of interest.  It might be for an ecologically oriented purpose such as habitat restoration, creation of forested wetlands, seed production for wildlife, or plantings for soil stabilization to prevent erosion.  Some species can agricultural crops from wind damage, and some are useful for surface mine site reclamation.  Others make excellent firewood for heating, cooking or charcoal production.  Yet other species are valued for their wood properties for making different types of paper, furniture, musical instruments, stencils, chemicals, or building materials for housing, boat and airplane construction.  Yet others are valued for their beauty in residential plantings (or for their resistance to air pollutants).  The list of applications of trees species for special and sometimes unique purposes is almost endless.
 

Genetics
 

Although we had discussed species’ attributes in this section as if each individual plant shares the same properties, in reality, there is genetic variation in many of the characteristics that make each species unique.  Species with wide geographical and environmental ranges, and those whose ranges are not continuous, tend to have more genetic differences.  In some instances they are so distinct as to be considered separate varieties. Genetic differences can be expressed in any traits, from time of budburst to timing of dormancy, wood properties, leaf, stem and root physiology, leaf shape, tolerance to environmental stress, susceptibility to insect predation or disease, growth rate and longevity.   However, overall genotypes within a species share many more similarities than differences, so that they are still recognizable as members of the same species.
 

Further Reading
 

Burns, R. M. and B. H. Honkala.  1990.  Silvics of North America: Volume 1. Conifers.  Agricultural Handbook 654.  U.S. Department of Agriculture, Forest Service, Washington, DC.  675 p.

Burns, R. M. and B. H. Honkala.  1990.  Silvics of North America: Volume 2. Hardwoods.  Agricultural Handbook 654.  U.S. Department of Agriculture, Forest Service, Washington, DC.  877 p.

Fowells, H. A.  1965.  Silvics of Forest Trees of the United States.  USDA Agriculture Handbook 271. 762 p.

Johnson. P. S., S. R. Shifley and R. Rogers.  2002.  The Ecology and Silviculture of Oaks.  CABI Publishing International, Oxford.  528 p.  

Kimmins, J. P. 2004. Forest Ecology, 3^rd ed.  Pearson Prentice Hall, Publ., Upper Saddle River, New Jersey. 611 p.

Smith, D. M., B. C. Larson, M. J. Kelty and P. M. S. Ashton.  1997.  The Practice of Silviculture, 9^th ed.  John Wiley and Sons, New York. 537 p.

Submitted: June 2006

Posted:  April  2007

Updated:  21 April  2007