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News Article

Fragmentation of European ash gene flow networks


Isolated ash populations risk losing their genetic adaptive potential

The volume of planted forests has been increasing rapidly in recent decades, leaving natural woodland greatly fragmented. Separated populations may begin to differentiate due to genetic drift however inbreeding within isolated populations may also lead to loss of adaptive potential. These wild trees are also at risk of gene flow from nearby plantations, reducing their genetic diversity and possibly long-term adaptation to the environment. Alternatively the adaptive response of isolated populations may actually be bolstered by genetic variation introduced from plantations.

Either way, as introduced pests and diseases become ever more common in global forestry it is important to retain natural genetic variation in the hope that resistance genes for both current and future threats are preserved. These resistant individuals are, however, predicted to be few and far between, meaning that in the event of widespread mortality they will need to foster the next generation quickly across as large area. Understanding pollen and seed dispersal mechanisms will allow scientists to better predict how this recovery may occur and resistance genes are able to spread.

The aim of this paper was to increase understanding of the gene flow dynamics present in European ash, both within a fragmented population and with a neighbouring plantation. Fraxinus excelsior is a keystone species in European forests and has suffered multiple losses as a result of disease and human intervention making it a perfect study species. Ash dieback has ravaged many European ash populations, however, resistant traits have already been identified, highlighting the importance of retaining genetic diversity for this species and others.

Researchers at the universities of Copenhagen and Göttingen used a small natural forest and an adjacent planted avenue to test how gene flow functions both within wild populations and between wild and cultivated trees in Rosenbeck Germany. This was done using parentage analysis of seedlings and analysis of pollen and seed dispersal distances. The fragmented landscape studied covered agricultural land, forest patches and a highway lined with cultivated trees. A total of 268 adult trees were sampled, 58 of which were cultivated.

Results showed a slim majority of the studied seeds (55-64%) were the product of trees within the same patch. Marginally more (75-98%) seedlings shared this localised parentage. Whilst this suggests outcrossing is relatively low, 26-45% of pollen flow was found to originate from outside of the study patch. Neighbouring cultivated trees also contributed 2% to the pollen flow. The impact of wind was shown to determine the direction of distribution for both seeds and pollen however only seeds were shown to have their distance of distribution influenced by wind speed. Strong winds therefore strongly contribute to long distance ash seed dispersal

As the threat of exotic pests, new diseases and climatic changes are not likely to subside any time soon, understanding mechanisms of gene flow both within and between ash populations will be key in safeguarding the genetic future of this species in Europe as it continues to suffer from population limiting effects.

Semizer-Cuming, D., Kjær, E.D. and Finkeldey, R., 2017. Gene flow of common ash (Fraxinus excelsior L.) in a fragmented landscape. PloS one, 12(10): e0186757. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0186757

To find over 150 similar papers use the search string below in the Forest Science Database:

("population") AND ("gene flow" OR "genetic diversity") AND ("resistance") AND (tree*)