There is no bush on the face of the globe exactly like another bush; there are no two trees in the forest whose boughs bend into the same network, nor two leaves on the same tree which could not be told one from the other.For the multitude of plants, animals and insects living in British Columbia and Pacific Northwest forests, there is no reason to believe that logging or the many forms of natural disturbance result in a net loss of genetic diversity.
— John Ruskin, Modern Painters
The numerous species of ferns, mosses, lichen, and fungi are so prolific and widespread that it is hard to imagine inbreeding or temporary local loss of habitat as a serious threat.
For the trees and other plants there are nearly always some seeds or seedlings ready to spring back to replace the old forest. These contain the genetic material of their parents.
Most birds, mammals, and other animals either escape to surrounding forest or adapt to the new environment. Some species, such as salamanders, will survive in reduced numbers initially but there is no evidence that their genetic diversity is lost as they eventually recover in the new forest. Other species, such as field mice, will increase in numbers after logging but there is no reason to suspect significant change in the genetic make-up of the population.
There is a notion among some environmentalists that when native forests are cut and replaced by tree seedlings grown in nurseries that there is a loss of genetic diversity in the newly planted forest.1
The uniform appearance of nursery seedlings suggests to the observer that they are genetically identical. In fact, in most cases in Pacific Northwest forests there is actually an increase in the genetic diversity of the planted tree species when harvested areas are replanted with seedlings grown in nurseries.2
When an area of forest is cleared of trees by fire, windstorm, or logging, and left to regenerate on its own, the new forest will be composed mainly of trees from seed that was on the site or blown in from trees growing close by. This means that a certain amount of inbreeding will occur over time and the trees in a given area will tend to be related to one another.
When seedlings from nurseries are used they are from seeds that were collected over a far wider area than the area that is replanted. The strategy in nursery production is to increase genetic diversity by outbreeding and to use selective breeding to enhance desirable traits such as growth rate and disease resistance. As a result, there is usually a higher genetic diversity in a given species that is planted than in the same species that is naturally regenerated.
The use of the word 'genetic' is often confusing because there are so many ways of influencing the genetic composition of an individual or population. The impression is sometimes given that plantation seedlings have been "genetically engineered" when this is not the case.
Most genetic work with trees simply involves controlled breeding programs using normal sexual reproduction by fertilizing seeds with pollen. The only difference between this and what occurs in the wild is the conscious effort to avoid inbreeding while at the same time selecting individuals with desirable traits.
A word about cloning
Another word that is often misinterpreted is 'clone'. The term tends to conjure up images of test-tube babies and armies of androids ruled by evil masters. As it is applied to the breeding of plants and trees, the use of clones is not quite so sensational.
Anyone who has taken a cutting from a house-plant and rooted it in a glass of water has created a clone. Much of agriculture and horticulture has always been based on the use of cloning in order to ensure uniformity of food and garden plants.
All the beautiful varieties of roses and rhododendrons are the result of first breeding and then cloning the desired offspring.
Staple foods such as potatoes are also grown using cloning techniques. With cloning there is a real potential to reduce genetic diversity as each clone is genetically identical to the other. This is easily avoided by appropriate selection of clones and planting programs.
The use of clones is not nearly as common in forestry as in food and garden plants but there are some notable examples where this technique is used.
The most advanced and widespread use of cloning in forestry is practiced with eucalyptus in tropical and sub-tropical climates. While all species of eucalyptus originally derive from Australia, some are now grown for timber production in many other parts of the world including Brazil, Chile, Portugal, and California.
The most successful programs involve highly controlled breeding and cloning through rooted cuttings. The result has been a very high quality of wood and phenomenal growth rates.
In the Pacific Northwest there are two tree species that are cloned for forest planting. The largest program involves hybrid poplar trees that are grown on river flood plains for pulpwood. At a more experimental stage there are field trials underway with yellow-cedar (Alaska-cedar) in south-coastal British Columbia.
In all tree cloning programs a great deal of attention is paid to ensuring genetic diversity both in breeding stock and in the production forest by using clones from a large number of parent plants.
References
1. Elizabeth May, At the Cutting Edge — The Crisis in Canada's Forests, Key Porter Books, Toronto, 1998, p. 19.
2. Hamish Kimmins, Balancing Act — Environmental Issues in Forestry, UBC Press, Vancouver, 1992, p. 160.