Principles of Population Genetics and Willamette Valley Ponderosa Pine

Larry Miller, ODF Forest Geneticist

Consider what we know about the distribution of the Willamette Valley race of ponderosa pine:

While never existing in large continuous stands, the distribution of ponderosa pine in the Willamette Valley 150 years ago was much more widespread than today. Agriculture, human settlement, urban/suburban sprawl, and preference for Douglas-fir have significantly restricted the species distribution, not uncommonly down to just a few isolated trees here and there.

In the study of population genetics, we know that when populations become isolated and contain only a few individuals, crossing increasingly occurs between related individuals. This is called inbreeding, and the effects are almost always bad. Here’s why:

Most forest trees of the northern hemisphere, ponderosa pine included, rely upon wind pollination and an outcrossing mating system. There is a strong evolutionary basis for outcrossing, involving the same genetic principles that lead to incest taboos in human cultures. Genes that code for negative fitness traits, because they result in a loss of vigor and thus survival, are present at very low frequencies and are usually heterozygous recessive. This means that such traits are rarely expressed - a good thing for species survival. However, inbreeding often results in these sub-optimal recessive genes becoming homozygous. In trees, negative traits resulting from inbreeding include poor seed set, low seedling vigor, and reduced growth rate of the few inbred trees that do survive. Empirically, this is what we see today in the all-too-common poor performance of wild collected Willamette Valley Ponderosa pine seed.

Because of the long lifespan of trees, generations overlap in space and time. Thus, inbreeding includes many combinations of related matings, such as parent x offspring, half-sib x full-sib, grandparent x parent, etc. The most severe form of inbreeding is called selfing - when a tree pollinates itself. The results of selfing are so bad, that in most cases no viable seed is produced.

As one may well imagine, inbreeding within small groups that cling to existence on the edge of the species range has the potential to become an evolutionary dead end. Thus, the efforts of the Willamette Valley Ponderosa Pine Conservation Association are very important to the long term health and survival of ponderosa pine in the Valley. When trees were chosen for inclusion in the seed orchard at Schroeder, foresters were careful to select trees that were separate from one another. This means that the 160 + families in the orchard are, by-and-large, unrelated to each other. As such, the resulting seed should be predominantly out crossed, and thus will produce high vigor seedlings.

In 2009, the ODF is conducting a study to estimate the rate of inbreeding in Valley ponderosa seed produced at Schroeder. The National Forest Genetics Laboratory, located at the Institute of Forest Genetics in Placerville, CA will, via molecular genetics methods, sample seeds from 20 families and estimate the rate of inbreeding. Our expectation is that the inbreeding rate will be low, given the methods used to select the trees. This information will also help orchard managers make decisions about which trees to stimulate, and how to plan cone harvests.

Larry Miller is the current ODF Geneticist. He has wide experience in genetics and forest tree improvement. BS Forest Management - University of Maine, Orono 1977. MS Forest Genetics - North Carolina State University, Raleigh 1980. He has worked extensively in operational forest tree improvement in Ontario (Canada), Minnesota, eastern Washington, western Oregon, and northern Idaho.


Last Updated 03/01/09