Ecological Impacts of Tree Diseases

Diagram of the cascading effects of tree diseases on biodiversity and ecosystem servicesThere are an increasing number of non-native pests and pathogens that are impact the UK’s trees.  The decline in a particular tree species will impact a wide range of other species that use that trees species as well as driving changes in ecosystem functions. 

Many species use trees directly for either feeding such as insects or as a habitat/living space such as bryophytes and lichens.  Not all trees are equal.  Some species will only use one or a very limited number of tree species.  Therefore a wide spread decline in the population of one tree species may drive changes in species that are heavily dependent on it.

Tree diseases will have cascading effects on biodiversity, ecosystem function and services.  Woodland management may change as a response of tree diseases which in turn will influence biodiversity and ecosystem services.  Ultimately the diseased species will be replaced by other tree species which in turn will influence the biodiversity and woodland functioning.


Impacts of Ash Dieback

Dead ash leaves due to ash dieback   Photo credit: Ruth MitchellAsh dieback is caused by a non-native fungus Hymenoscyphus fraxineus, which arrived into eastern Europe in the 1990’s on imported trees.  Since then the fungus has spread eastward killing large numbers of ash trees (Fraxinus excelsior). The fungus was first confirmed in the UK in 2012 and as of December 2017 was found in 44% of 10 km squares in the UK. 

The ecological importance of ash in the wider environment was previously unknown.  Staff at the James Hutton, in collaboration with staff from the RSPB, The Royal Botanic Garden Edinburgh, Hodgetts consultants and Forest Research have found that ash fulfils a unique role in the functioning of our woods.  Ash supports 955 species 45 of which only use ash (obligate) and 62 of which are highly associated with ash meaning they rarely  use tree species other than ash.  It is these 107 species that are most at risk from a decline in ash.

In comparison with other native deciduous tree species in the UK the functioning of ash is very different. Ash loses its leaves when they are green meaning the leaf litter contains high levels of nutrients and the leaf litter breaks down quickly.  Replacement tree species are unlikely to have leaf litter with similar qualities. Thus if ash declines the nutrient cycling and ecosystem functioning of ash woodlands will change.

The ground flora within an ash woodland is also unique as ash casts a very light shade meaning many species can grow underneath it.  If ash is replaced by other tree species with a darker shade than some ground flora species may decline.Lichen rich ash wood habitat at Rassal   Photo credit: David Genney

Table showing the number of ash associated species and their level of association with ash

Group Obligate High Partial Cosmopolitan Uses Total
Bird     7 5   12
Bryophyte   6 30 10 12 58
Fungi 11 19 38     68
Invertebrate 30 24 37 19 131 241
Lichen 4 13 231 294 6 548
Mammal     1 2 25 28
Total 45 62 344 330 174 955
















Tools to aid conservation of ash associated biodiversity

Dying ash tree   Photo credit: Ruth MitchellAshEcol is an excel file containing a list of all 955 ash associated species and information on if they do or not use 48 alternative tree species.  The database is user friendly and aimed for woodland managers.  AshEcol allows managers to identify ash associated species and which alternative trees species they will also use.  Management to encourage these alternative species either via planting or natural regeneration may help mitigate some of the impact of a decline in ash.  Obviously there is no suitable management that will help mitigate impacts for species that only use ash.

AshEcol may be downloaded here


AshEcol has been used in case studies at 15 sites across the UK to assess how woodland management could change if ash dieback established at these sites and the aim was to conserve as much ash associated biodiversity as possible.

Map of case study sites which can be downloaded here

For further information please contact Dr Ruth Mitchell



Publications on ash dieback

Mitchell RJ , Broome A, Beaton JK, Bellamy PE, Ellis CJ, Hester AJ, Hodgetts NG, Iason GR, Littlewood NA, Newey S, Pozsgai G, Ramsay S, Riach D, Stockan JA, Taylor AFS, Woodward S (2017) Challenges in assessing the ecological impacts of tree diseases and mitigation measures: the case of Hymenoscyphus fraxineus and Fraxinus excelsior. Baltic Forestry 23 116-140

Mitchell, R.J., Hewison, R.L., Hester, A.J., Broome, A. Kirby, K.J. (2016) Potential impacts of the loss of Fraxinus excelsior due to ash dieback on woodland vegetation in Great Britain. New Journal of Botany, 6, 2-15.

Mitchell, R.J., Pakeman, R.J., Broome, A, Beaton, J.K., Bellamy, P.E., Brooker, R.W., Ellis, C.J., Hester, A.J., Hodgetts N.G., Iason, G.R., Littlewood, N.A., Pozsgai, G., Ramsay, S., Riach, D., Stockan, J.A., Taylor, A.F.S. and Woodward, S. (2016).  How to replicate the functions and biodiversity of a threatened tree species? The case of Fraxinus excelsior in Britain. Ecosystems, 19, 573-596 (doi:10.​1007/​s10021-015-9953-y)

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. and 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

Mitchell, R.J., Broome, A., Harmer, R., Beaton, J.K., Bellamy, P.E., Brooker, R.W., Duncan, R., Ellis, C.J., Hester, A.J., Hodgetts, N.G., Iason, G.R., Littlewood, N.A., Mackinnon, M. Pakeman, R., Pozsgai, G., Ramsey, S., Riach, D., Stockan, J.A., Taylor, A.F.S. & Woodward, S. 2014. Assessing and addressing the impacts of ash dieback on UK woodlands and trees of conservation importance (Phase 2). Natural England Commissioned Reports, Number 151.

Mitchell, R.J., Bailey, S., Beaton, J.K., Bellamy, P.E., Brooker, R.W., Broome, A., Chetcuti, J., Eaton, S., Ellis, C.J., Farren, J., Gimona, A., Goldberg, E., Hall, J., Harmer, R., Hester, A.J., Hewison, R.L., Hodgetts, N.G., Hooper, R.J., Howe, L., Iason, G.R., Kerr, G., Littlewood, N.A., Morgan, V., Newey, S., Potts, J.M., Pozsgai, G., Ray, D., Sim, D.A., Stockan, J.A., Taylor, A.F.S. & Woodward, S. 2014. The potential ecological impact of ash dieback in the UK. JNCC Report No. 483