David Cracknell, George Peterken and John Healey discuss their recent article, ‘Neighbours matter and the weak succumb: ash dieback infection is more severe in ash trees with fewer conspecific neighbours and lower prior growth rate‘:
Lady Park is an ancient, mixed deciduous woodland on the flanks of the Wye Gorge between Symonds Yat and Monmouth that was designated as a research reserve in 1944 (now a National Nature Reserve) and has since been allowed to grow naturally. Permanent sample plots were established by the eminent Oxford forest ecologist, Eustace Jones, in order to study how natural woodland developed. He passed the study on to the Nature Conservancy in 1969, since when it has been continued by a relay of recorders co-ordinated mainly by George Peterken. In 2013, anticipating the arrival of ash dieback, George embarked on a new round of recording in the permanent plots, so we had an up-to-date record of several hundred ash throughout the reserve when ADB was first noticed there in 2017. This provided the foundation for the present study by David Cracknell, which David undertook for his MSc in Forestry at Bangor University with his supervisor, John Healey, and a group of collaborators they brought together. The recent paper is David’s first; but it is also a milestone for George, whose first paper was 58 years ago in the second volume of the Journal of Applied Ecology.
By 2019, the crowns of some ash were almost leafless, whereas others remained untouched by disease. Noting these substantial differences, David set out to investigate which of the many candidate environmental, tree community or individual tree factors might be contributing to this variation in susceptibility to the disease, capitalising on the detailed history of individual tree measurements and mapping in the permanent sample plots. This was an opportunity to see how ash dieback impacted an ash population that formed part of a mixture of beech, sessile oak, small-leaved lime, large-leaved lime, wych elm and others whose structure was as close as one can get in Britain to a mature stand whose dynamics have been dominated by natural processes.
What our results have shown is a combination of confirmation of the already known, the expected and the very surprising. We confirmed that the severity of crown dieback (the primary system of ash dieback disease) of individual ash trees was much greater in the moist lower slope parts of the wood and, independently, in trees with a smaller trunk diameter. A more novel, though expected, finding was that ash dieback was less in more “vigorous” trees that had previously been growing more quickly. Our surprising finding, only possible because of the accurate mapping of the position of every tree in the plots by our predecessors, was that individual ash trees with a higher proportion of other ash trees amongst their six nearest neighbours actually had significantly lower levels of ash dieback symptoms than did ash trees with more trees of other species in their neighbourhood. In other words, we found no benefit for ash trees from being surrounded by trees of different species. This is the opposite result to what we predicted based on ecological theory and previous studies of other tree pathogens or pests in other forests.
Thinking about why this might be and the potential implications, we recognised that the ash dieback fungus Hymenoscyphus fraxineus is an ascomycete that produces a huge number of tiny wind-dispersed spores that spread far and wide. This is a very different situation from pathogens or insect pests with a much shorter dispersal distance. Also, natural biological control of insect pests is more likely to be boosted in woodland habitats with high tree species and structural diversity if they have a greater diversity of predator species that eat the insect pests. In these cases, a woodland with higher tree species diversity would have a greater resilience.
There are other benefits of diversity that are not challenged by our results. Given that most pathogens and pests are restricted in the range of tree species that they infect, having a diversity of tree species in a woodland reduces the risk that the whole ecosystem will be devasted by a future pathogen or pest epidemic (as has happened to many Norway spruce dominated forests in central Europe due to the combination of droughts and the impact of spruce bark beetles). It is far from certain what pathogen or pest species will pose the greatest threats to British woodlands in the future, so managing them with a diversity of tree species still makes sense as a strategy to increase their general resilience against such potential threats. However, our research on ash dieback in Lady Park Wood shows that it provides no guarantee of protection against every pathogen.
Read the full article online: Neighbours matter and the weak succumb: ash dieback infection is more severe in ash trees with fewer conspecific neighbours and lower prior growth rate
