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Forest genetic diversity and clonal propagation


Genetic impacts of introducing clonally propagated trees into forest stands

As sessile organisms, plants are unable to escape changes in their environment or the onset of new pests. Instead they must rely on their ability to adapt and endure in the face of changing conditions. A highly heterogeneous population has a much greater chance of containing the right mix of genes capable of resisting a new disease or rise in temperature, therefore so it is in the interests of those seeking to preserve or benefit from forests to encourage this genetic diversity.

Mutation, gene flow, genetic drift, and selection all contribute to the maintenance of genetic diversity within a population, but historically, selective breeding has led to a homogenisation of plant genetic resources as vegetative propagation largely bypasses these natural processes, creating genetically vulnerable monocultures. Humans have been breeding crop species for centuries and in comparison, domestication of forest trees is in its infancy. However the use of selective breeding in forest trees is on the rise and the use of clonal propagation becoming more popular. In order to assess the possible impacts this may have on forest stand populations and their genetic diversity as a whole, researchers at Umeå University and the Swedish University of Agricultural Sciences reviewed work comparing genetic diversity in managed and natural forests. They also developed their own quantitative model for understanding the genetic and genomic impacts of clonal forestry in the context of Swedish forestry.

The authors consistently refer back to population size as a contributory factor to changes in genetic diversity; a large reduction in population size can lead to a bottle neck effect, increased vulnerability to genetic drift and a greater threat of inbreeding and inbreeding depression. Conversely, larger populations are able to harbour more genetic diversity and are less susceptible to genetic drift. Tree populations are naturally pre-disposed to maintaining a high genetic diversity as they are often large, long-living and capable of pollen and seed dispersal between distant populations. Data reported so far supports this, especially for boreal-temperate species, finding high heterozygosity and allelic richness. Of course this wide dispersal can also be a disadvantage; seed orchards may suffer from unintended cross-pollination with wild populations, diluting the intended product. Similarly, gene flow from orchards to wild populations can introduce poorly-adapted genes and remove natural variation.

In their quantitative model, the authors show that the number of clones used to replant forest stands greatly influences the level of genetic diversity a population retains over time; even an increase from two to ten clones prevented the loss of much diversity. They also found that genotypic diversity was affected more strongly than genetic diversity, with even a high number of clones still leading to a very substantial loss of genotypic diversity.

The development of vegetative propagation had allowed for faster attainment of new traits compared to orchards however the usage of largely vegetative propagated material in in forest stands still has unknown effects on the genetics of a forest as a whole. As a conclusion the paper suggests that there should be ‘baseline’ levels of genetic diversity established for forests in order to monitor changes induced by the introduction of clonal material. Both genetic and genomic diversity should be monitored and across a wide sampling range, both geographically and temporally, to better understand the effects breeding programmes may have already had. In summary, whilst any use of clonal propagation will likely lead to a lower genetic diversity than natural forests, the paper suggests that the effects can be mediated if properly monitored and controlled for.

 

Read the full paper below: Ingvarsson, P.K. and Dahlberg, H., 2018. The effects of clonal forestry on genetic diversity in wild and domesticated stands of forest trees. Scandinavian Journal of Forest Research, pp.1-10. https://doi.org/10.1080/02827581.2018.1469665

 

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