Chapter 8 - Variation
Contents - Class Homepage
Basic Concepts Tree Breeding Exotics


You should be able to do four things when you are done with this section:

1. Understand the causes and nature of variation within tree species.

2. Know terminology associated with forest genetics and tree improvement.

3. Understand the advantages and disadvantages of forest tree improvement programs.

4. Know methods used by forest geneticists to understand patterns of variation and apply them to tree improvement programs.


As with people, no two individual trees are exactly identical.  In the most basic terms this variation can be attributed to three sources: genetics, environment and the interaction between genetics and environment.

The genetic component of an individual consists of the DNA inherited from parents or transferred from the parent in the case of clones.  DNA acts as a blueprint for the construction of proteins and enzymes which control the development and function of the individual.

The environmental influences on a tree's form and function are extremely varied and can include both biotic and abiotic influences.
     abiotic factors- soils, nutrition, temperature, water
          availability, wind
     biotic factors- disease, insects, animals, competition

Two terms that are commonly used in forest genetics are genotype and phenotype.

Genotype refers to the genetic composition of an individual, with no regard for environmental influences.  Phenotype refers to the physical manifestation of genotype, environment and the interaction between genotype and environment.
For example two individuals may be identical genetically (same genotype) but have very different appearances (phenotype) due to differences in their environment.

Now that you are familiar with these terms, we can state  Kleb's concept- a phenotype is the result of genetic influence, environmental influence and the interaction of these factors.
     phenotype= genotype + environment + (genotype*environment)
     (or P = G+E+GE)

Understanding the nature of genetic variation between
individuals and groups of individuals (population) and how this
occurs is valuable for a number of reasons.
1. Conservation or restoration of species or populations.
2. Selecting appropriate genotypes for a particular site or use.
3. Breeding new varieties of a species.

The differences we see between individuals in a population (a group of interbreeding individuals) are affected by five processes, usually acting in combination with one another.

1.  Mutation- these are changes in the genetic structure occurring through breaks in DNA, errors in copying or changes in sequence.  This is the ultimate source of all variation.

2.  Selection is a force which decreases an individuals ability to reproduce.  For example, genetic variation in the ability of a tree species to survive severe drought may cause some individuals to die while others survive and reproduce.  An event like this may decrease the overall diversity of genes (alleles) in a population but a change in the overall genetic makeup of the population is produced.

3.  Migration is the movement of genes into or out of a population.  This includes pollen or seeds blowing into the population from a source that is otherwise considered to be a separate population.

4.  Random drift is a chance event unconnected with the genetics of an individual that affects the ability to reproduce.  Random drift can be thought of as "dumb luck."  A lightening strike or animal damage that kills a tree which is not directly related to the tree's genetic makeup can be thought of as random drift.

5.  Nonrandom mating reflects the fact that some individuals in a population are more likely to mate with one another than others in the population.  In a forest, a tree's flowers are more likely to be fertilized by nearby individuals.

Tree breeders work with the natural variability within a species to produce individuals which have desirable characteristics from several genotypes.  This requires a knowledge of where variation lies within that species.  Some species have discontinuous ranges meaning they consist of several populations.  In some species, individuals differ based on the population they came from.  Loblolly pines from the "lost pines" population in Texas are more drought resistant than pines from elsewhere in the range.  Spatial isolation can sometimes lead to the formation of a new species.  For example, eastern and western white pine may have been the same species at one point in time.  This may also be true for jack pine and Virginia pine.

Variation in a trait may be observed along a gradient.  An ecocline is a variation within a population caused by adaptation to an environmental condition that varies along a gradient.  In sycamore, trees from Missouri are more susceptible to a stem canker than those from Louisiana.  Trees originating between these north-south extremes are intermediate in tolerance.  These black spruce seedlings are a good demonstration of clinal variation.  Although they were grown in the same greenhouse, they exhibit very different growth rates.  Which seedling had southern Canadian parents?

Some species show family (a family consists of the offspring of a single tree or a pair of trees) differences for a trait that are unrelated to a gradient.  When these differences are due to local environmental conditions families are said to represent different ecotypes.  In some cases, ecotypic variation can lead to the formation of species even though they are close enough to one another to reproduce.  Good examples of this include black maple and sugar maple, and water tupelo and swamp tupelo.

Still other species, show a large amount of variation for a trait within families themselves unrelated to geographic origin, or local environment.

Speciation may occur in two other ways.  Habitat separation is the result of two populations delineated by environmental differences between their habitats.  Time may also lead to speciation.  Temporal separation results in populations delineated by differences in the timing of reproductive viability.  may result in increased variation between populations and, ultimately speciation.

Just as one species can split into two resulting in increased variation, two species can sometimes interbreed and form hybrids.  Hybrids commonly occur between some oak species, black and red spruce, and longleaf and loblolly pine.

Tree improvement is the application of the science of genetics to the selection and breeding of forest trees to produce trees that are better in one or several ways.  For thousands of years, people have bred animals and agricultural crops for a wide range of purposes.  For example, corn has been bred for greater yields, apples for sweeter taste and dairy cows for butter fat production.   Plant and animal geneticists have been able to produce these desirable characteristics by creating new varieties and hybrids from genotypes present within a species or among closely related species.  A large part of this work involves identifying individuals or populations with desirable traits and planting their offspring (mass selection) or mating individuals of with different genotypes (Tree breeding).  A simpler approach to tree improvement involves taking individuals and planting them in regions or continents where they are not native (introduction of Exotics).

Before discussing the various forms of tree improvement, it is important to keep in mind the reasons for doing this work.

1. Increased yield of timber, fruit and secondary compounds - As the human population grows, more and more resources are demanded from less and less land.  Regarding timber, increased interest in forest preservation are reducing the size of the land base from which timber can be cut.  For this reason, it is advantageous to timber growers to produce as much usable wood on as little land as quickly as possible.  To meet this objective, geneticists have identified and bred trees with fast growth rates and high yields such as Radiata pine, loblolly pine, and cottonwood.

2. Disease resistance - Fast growing trees are not always the most resistant to disease.  Therefore another rationale for tree breeding is identifying species, populations or individuals that are resistant to diseases     For example, rust resistant loblolly pine are commercially available, and work continues to produce a blight resistant American chestnut.

Fusiform rust can severely disfigure loblolly pine.

3. Land restoration or remediation - Trees are used to restore soil properties or increase values of land that has been damaged through various forms of human disturbance.  Reclamation of strip mines with black alder, waste disposal sites and reconstructed wetlands are all examples of situations where trees are planted because of their favorable impact on the soil and site.

4. Conservation biology - Populations or entire species may become rare due to changes in land use patterns or climatic conditions.  Tree species are very rarely threatened with extinction.  However, this issue has particular relevance to those of us in southwest Virginia and Virginia Tech.  The Virginia roundleaf birch is the only tree in the United States ever to be classified as an endangered species. Researchers in the forestry department at Virginia Tech conducted a program that has successfully preserved this species.

5. Aesthetic or landscape uses - Trees have been selected for planting in cities and yards because of their aesthetic beauty and ability to moderate the urban environment.  Some trees have been chosen because their appearance is considered beautiful or interesting (Colorado blue spruce, Bradford pear), others because of their resilience to urban stresses (ailanthus, ginkgo, London plane tree)
6. Combinations of traits - Since everyone wants to "have it all", geneticists select or breed trees that have more than one desirable trait.  Loblolly pines have been bred that contain the desirable growth of some genotypes with the disease resistance of others.  Hybrids of pitch and loblolly pine combine the resistance to ice damage of the former with the rapid growth of the latter to extend the range of productive southern pines into colder climates.

Before embarking on a tree improvement program, forest geneticists need to understand the variability within and between candidate species in order to make the best possible choices of individuals or populations.  Genetic variation can exist between individuals growing in the same stand, or between stands as ecotypes and ecoclines.  To determine where this variation lies, geneticists collect seeds from several families from across the range of a species and plant them at the same location.  This is called a provenance test.  By comparing growth attributes and survival between families and locations, several important questions can be addressed...

1. Does the greatest variation lie between individuals within a stand or between geographic areas?

2. Do trees from certain families or certain geographic areas perform better than others?

Based on this information, specific genotypes can be selected for the area around the test site and a better understanding of the genetic structure of the species has been developed

Once several favorable genotypes have been identified, they can be cross-bred to gain further improvements.  Three strategies involving breeding are the use of seed orchards, breeding orchards and hybrids.

Breeding orchards and seed orchards are used to mass-produce seeds of trees with favorable genotypes for large scale tree planting programs.  Breeding orchards involve controlling the source of pollen used to fertilize female flowers.  This process is expensive and difficult because it involves collecting pollen from male flowers and fertilizing female flowers by hand.  Before and after pollination, female flowers must be covered to prevent fertilization by pollen from other male flowers.  Trees growing from seeds produced in breeding orchards are called full sibs since they are the product of the same male and female flower.  In this way, if the offspring of two particular parents are  desirable in some way, these traits will be expressed by controlled crosses.

Seed orchards are used to produce seed where the source of the male pollen is not known.  Since the mother is known to be the same for all seed from a tree, but the pollen has come from any number of male flowers, these offspring are known as half-sibs.  Producing seeds in this way is much cheaper and easier than in a breeding orchard, but there is less known about the characteristics of the offspring.

Todays seed orchards produce progeny with superior growth characteristics and resistance to some diseases.

Seed orchard management can be very intensive, and expensive, maximizing seed production per tree.  Notice the wide spacing and weed control in this seed orchard.

To determine the growth characteristics of seedlings resulting from seed orchards and breeding orchards, geneticists conduct progeny tests.  Progeny tests are common outplantings that are used to evaluate seed producing parents.  On the basis of progeny tests, genotypes that produce undesirable offspring are rogued (removed) from the seed or breeding orchard to increase the overall quality of stock.  This has short term benefits but possibly may result in problems in the future by reducing the diversity of the overall orchard gene pool, thereby limiting the number of possible genotypes that can be produced.

Another method of testing the genetic potential of individuals is through clones.  Clones are genetically identical to their parents, and can be produced with clippings or tissue culture.

In some cases, forest geneticists use hybrids for tree improvement.  Hybrids sometimes combine the desirable characteristics of both parents, as is the case with pitch x loblolly pine hybrids.  Another phenomenon is hybrid vigor, where hybrids perform better in some way than either parent.  London planetree is a hybrid that formed in an English botanical garden between two species of sycamore, Platanus orientalis from western Asia and Platanus occidentalis from the United States.  The hybrid is very tolerant of compacted soils and is widely used as a street tree in cities around the world.

Forest geneticists, like many other researchers, need to justify their work from time to time.  By establishing demonstrations, they are able to show their bosses or others the value of this work.  These differ from other plantings in that they are designed to demonstrate differences between various breeding strategies or introduction of exotics relative to unimproved or native trees.  The point here is to clearly show that their work is resulting in trees that are better in some ways.

Exotic species are simply non-native species.  As a rule, it is probably safer to use native vegetation whenever possible since you know it is adapted to live in a particular environment.  However, native species are adapted to survive, but not necessarily produce the greatest economic gain or they may have a unique appearance that some people find attractive.  In that case, exotic species may be a good option.

When introducing exotic species, test plantings are often made to determine whether new species are compatible with a different  environment.  For example, knowing whether a species is susceptible to diseases in the new environment is crucial before expensive wide-scale efforts are made to plant this species.  In order to be successful, test plantings need to be of sufficient length to detect problems that may develop in trees as they become older.  Austrian pine planted in the eastern United States grows very well as a seedling and sapling but often falls prey to disease around age 15.

It also helps to know beforehand whether an introduced species has the potential to become a pest, in which case it should not be introduced.  With trees, poor survival of introduced trees is more often a problem than overly successful survival and spread, although there are important exceptions such as black cherry in Europe and tree-of-heaven in the United States  Over the years, geneticists have made the following observations, regarding success of introduced species.

1.  Species tend to do better if the climate of the new region where it will be planted is similar to where it came from.  For example, species originating in the Eastern United States are successful in East Asia, but not West Africa.

2.  Areas where introduced species tend to be successful tend to be those with an impoverished flora.

3. Areas where the climate or man-made changes have occurred more quickly than local species could adapt.

4.  In some cases, local species may not have the genetic variability needed to adapt to a new environment.  Such is the case when a disease destroys a species, such as with the American chestnut.  An exotic species may be less vulnerable to the pest while providing the same benefits as the attacked species.

5. Sometimes native species are difficult to replant, making exotics more desirable.  In the midwest, native hardwoods are difficult to plant.  Reforestation has concentrated on exotic coniferous species.  These introduced species may not be as good as the native species but sometimes the thinking is that poor trees are better than no trees.

6.  Monotypic genera.  Species which have no close relatives have often been successfully introduced into new areas.  One of the reasons for their success is that no close relatives are available which might host pests compatible with the newly introduced species.

At this point, we have learned that tree improvement has a number of advantages but also several real or potential disadvantages.

1. Planting improved trees often reduces the genetic diversity of a stand relative to the native vegetation.  This may increase the vulnerability of the stand to disease or insect outbreak.

2. Exotic pests are sometimes accidentally introduced on exotic trees.  Balsam wooly adelgid, a serious pest of Fraser fir, was introduced into the southern Appalachian mountains on planted tree seedlings.

3. Exotic trees sometimes become pests themselves.  Tree-of-Heaven is an unwanted species in many woodlots and even some relatively remote forests in the eastern United States.

4. Exotic or improved trees may not live up to their potential once they are mature.  Most genetic tests make recommendations on the basis of trees that are only a few years old.  Desirable characteristics like rapid juvenile growth may not be carried on in later years.  Further, desirable characteristics change over time as the nature of products made from these changes.

5.  Genetic improvement may not be the most cost effective way to invest in forests.  For example, weed control may result in much greater early growth gains than tree improvement.