Genetic Effects

Excerpt from: Buchert, G. P. 1994. Genetics of white pine and implications for management and conservation. Forestry Chronicle 70(4): 427-434.

Silvicultural Treatments Influence Mating Systems and Population Structure

Because of genetic consequences on the offspring population, the effects of silvicultural treatments on mating systems are of concern. Although specific information for white pine is lacking, available experimental information indicates that properly applied silvicultural treatments need not cause genetic degradation. For example, shelterwood harvesting in old-growth Douglas fir appears to have had no negative effects on mating systems or residual stand genetic variation. Neale and Adams (1985) reported no difference in selfing rates between natural stands and residual trees in shelterwood cuts as compared to nearby uncut natural stands. They determined that an actual decrease in measurable inbreeding occurred among shelterwood residual trees compared to natural stands, probably due to the elimination of related neighborhoods during harvesting. Differential residual tree density of the two shelterwood treatments did not affect selfing rates, although one stand had more than twice the residual density than the other (35 trees/ha versus 15 trees/ha). After measuring genetic variation in protein gene markers from these stands, Neale (1985) found no observable change in amount of' genetic variation between harvested stands and uncut stands. He attributed maintenance of genetic variability in Douglas fir shelterwood management to the initial high within-stand variability, to the large effective residual population size, and to low levels of selfing among residual trees.

Studies in Scots pine (Pinus sylvestris L.) seed tree stands in central and northern Sweden however, indicated that the 24% selfing rate in a low-density (18 trees/ha) stand was two times the 12% level of a high-density (122 trees/ha) stand (Yazdani and Lindgren 1992; Yazdani et al., 1985). There were additional differences in the genetic structure of natural regeneration under the two different seed tree spacings. When Yazdani and Lindgren (1992) analyzed established regeneration from the low density seed trees, they found that 25% of the seedlings within 5 m of a given seed tree had that tree as a seed parent, suggesting that genetically related neighborhoods of half or full-sibs were being established in the naturally regenerating stand. This number dropped to about 5% in regeneration under the high-density stand (Yazdani et al. 1985). The authors suggested that both wider spacing between seed-trees and poor climatic conditions at the more northerly low-density site (which would have limited pollen production in the stand) could account for development of related neighborhoods.

Types of Selection and their Effects on Populations

Natural selection against the genetic load carried by inbred individuals has the effect of balancing and even cancelling out the effects of inbreeding in regenerating populations (Shaw and Allard 1981). For example, Knowles et al. (1987) found that few inbred tamarack seedlings survive to adulthood, and Yazdani et al. (1985) found that evidence of inbreeding disappeared in Scots pine between the ages of 10-20 years. However, the possibility of inbreeding depression exists when a significant number of inbred individuals do survive to maturity. Tigerstedt et al. (1982) were able to detect inbreeding effects in 80-year-old Scots pine trees. Fowler and Heimburger (1969) suggested that inbreeding depression is the cause of poor growth performance in Iowa sources of eastern white pine, where, at the southwestern limit of its present range, it is found in small groups or as scattered individuals. This being the case, the genetic load is such that a portion of the trees are able to survive, but at much reduced growth rates and with such high incidences of disease and insect attack that form and merchantable volume are greatly affected (Abubaker and Zsuffa, 1991).

Silvicultural selection, in combination with the legacy of natural selection working over a rotation, can be a powerful force in effecting genetic change in the next generation. By choosing the trees that are to be left to regenerate the site, tree marking and harvesting crews will be making critical genetic decisions. Their choices will determine which genes will be expressed in the next forest generation, to what extent inbreeding will be a factor in the distribution and expression of the residual genes, and ultimately, whether the genetic potential of the population is improved or degraded. For example, Wilusz and Giertych (1974) reported that the classical nineteenth century Prussian silvicultural practice of decreasing stand density at increasing age through removal of the poorest individuals resulted in significant diameter improvement of Scots pine progenies at age 59 years in a replicated common environment test. Basal area of progeny from older, silviculturally managed stands (140 and 170 years) was about 30% greater than that from a 16 year-old stand, and 10% greater than that from a 47 year-old stand. They suggested that a regimen of silvicultural thinnings, over the rotation, appears to be very effective in improving progeny volume production. Such periodic thinnings would also reduce related neighborhoods, thus reducing inbreeding. Another benefit of this silvicultural regime would be the removal of trees with high genetic load from the production population, resulting in an increase in overall stand wood production.

In another example of positive genetic improvement through silvicultural selection, Ledig and Smith (1981) reported an increase in genetic resistance to white pine weevil (Pissodes strobi Peck) by progeny from thinned eastern white pine stands compared to progeny from unthinned stands. The silviculturally treated parent populations were thinned by removing weevil-damaged trees, and a replicated test was established from post-treatment seed. Weevil attacks in the 12-year-old progenies were 10% less frequent than in offspring from the unthinned stands. In the same study, Ledig and Smith reported a 5% difference in 12-year height between thinned and unthinned stand progenies. They attributed this to a reduction in inbreeding by removal of closely related trees during thinning.

Negative silvicultural selection, also known as high-grading creaming, diameter-limit cutting, or logger's choice, is an effective way to change the genetic structure of a population. Leaving the poorest trees to form the parent pool for the next generation will ensure that the genes conditioning slow growth, maladaptation to the environment, and poor tolerance to stress-related insect and disease attack will be in high frequency in the offspring population. It is expected that due to this negative selection, the offspring generation will produce a lower total amount of harvestable volume than did the parental generation. Experimental evidence of the consequences of negative selection practices on forest populations is not available. However, Kang (1988) demonstrated mathematically that negative selection for traits in forest trees has the potential to significantly change the genetic structure of populations. This suggests that applying management practices without taking into account their genetic impacts carries a high risk of genetic degradation.

A further consequence of repeated negative silvicultural selection may be the rapid decline of' growth and survival in subsequent generations due to the phenomenon known as asymmetry of response to selection (Howe 1991). Because of interactions among several genetic processes, including effects of inbreeding and natural selection, Falconer (1981) has suggested that characteristics which confer environmental fitness in populations are particularly susceptible to asymmetrical selection responses. The result is that selection against environmental fitness traits will decrease offspring population fitness (and subsequent productivity) more rapidly and effectively than selection for those same traits will increase population fitness.

Genetic Management "Rules of Thumb"

Very little information is available on mating patterns and genetic structure in white pine populations, and much needs to be learned before optimized genetic management strategies can be applied via silvicultural prescriptions (Namkoong et al. 1988). However, information gathered from other conifer species suggests several "rules of thumb" Which, when used, will contribute to genetically sound silvicultural practices.

First, practice positive silvicultural selection. By increasing the frequency of "good" genes in the population, the silviculturalist increases the probability of producing "good" trees in the next generation. Second, minimize the possibility of inbreeding. By doing so, inbreeding depression will not limit growth potential. Third, maintain sufficient genetic variation in local populations to enable them to adapt to changing environments.

It is likely that many mature second or third-growth stands may have regenerated from high-graded parental stands. The genetic consequence may be reduced productivity in the current and subsequent stands. In such cases, genetic improvement will result in the regenerated stand if care is taken to ensure that only the best trees are left as seed parents, and if their spacing encourages cross-pollination and disrupts related neighborhoods. In addition, residual trees with large, healthy crowns will produce large amounts of male and female strobili, thus reducing the potential for selfing.