Waiting for Ash Dieback

16_8 Taf Fechan SO0409 ash dieback Morlais bridge 2Across Britain ash trees are declining, if not dying, as a consequence of the ash dieback caused by Hymenoscyphus fraxineus(https://en.wikipedia.org/wiki/Hymenoscyphus_fraxineus). Even in sheltered gorges in south Wales infected trees can be seen.

At Wytham Woods we await the disease with some trepidation – the disease has been found on trees within a few kilometres of us, as the spores blow.  Should we just leave the Woods to cope, or prepare to clear up the infected trees?

Ash makes up about a third of the canopy overall and in places is over 50% of the cover.  It is the tree that has been growing fastest and showing the best regeneration over recent years (Kirby et al. 2014).

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A major reduction in ash will have substantial effects on the ecology of the woods, as well as on that of the surrounding landscape (Mitchell et al. 2014). We have not seen as serious a threat to a native woodland tree since Dutch Elm Disease. However, if we look more widely there are comparisons we might make and perhaps lessons we could learn from other pest and disease outbreaks.

17_4 Bayerischer National Park regen after dieback 4Ten years or so ago the Bayerischer National Park (BNP) in eastern Germany (http://www.nationalpark-bayerischer-wald.de/english/) was hit by a bark beetle attack that killed large numbers of mature native spruce. The pressures to salvage log the stands were strong but resisted. It was left largely to natural regeneration. Even now the appearance for a British visitor is dramatic. Would we tolerate scenes like this in our forests?

In part, it is a matter of scale.  The BNP has also large areas of young to middle aged stands. So, the dieback areas are effectively creating the new open stage stands that we expect to be created by management, as in traditional coppice cycles. Our ancient woods are tiny by comparison and, rather than creating more diverse conditions, a severe ash dieback attack could lead to more homogenisation of woodland structure if the whole site is affected over a few years.

17_4 Bayerischer National Park regen after dieback 3In BNP there is also dense natural regeneration occurring, the bark beetles kill old trees but not the young ones.  Ash dieback is, if anything, more deadly on seedlings and saplings and often any that escape are likely to get browsed off by high deer numbers.  So, we cannot assume that we do not need to manage the recovery of small ancient woods, just because minimum intervention works on the BNP scale.

Wytham Woods represent an interesting intermediate situation.  They are large (c400 ha) by British standards and the ash tends to be most abundant in the more recent woodland areas.  We know that the Woods were much more open 70 years ago from post-war aerial photographs and the deer are reasonably under control.

So when the (almost) inevitable happens and the ash start dying, this might be one site where we could follow the consequences of leaving the woodland alone and, using the past research on the site, see to what extent the ecological consequences match the current projections. Of course all researchers might need to issued with hard hats to cope with the increase in falling dead wood, and leather trousers for the probable increase in bramble cover.

Kirby K.J., Bazely D.R., Goldberg E.A., Hall J.E., Isted R., Perry S.C. & Thomas R.C. (2014). Changes in the tree and shrub layer of Wytham Woods (Southern England) 1974–2012: local and national trends compared. Forestry, 87, 663-673.

Mitchell R.J., Beaton J.K., Bellamy P.E., Broome A., Chetcuti J., Eaton S., Ellis C.J., Gimona A., Harmer R., Hester A.J., Hewison R.L., Hodgetts N.G., Iason G.R., Kerr G., Littlewood N.A., Newey S., Potts J.M., Pozsgai G., Ray D., Sim D.A., Stockan J.A., Taylor A.F.S. & Woodward S. (2014). Ash dieback in the UK: A review of the ecological and conservation implications and potential management options. Biological Conservation, 175, 95-109.

 

 

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