Risk factors of ash dieback disease and consequences for carbon storage in natural ash populations

Ash ( Fraxinus excelsior ) is one of the most dominant species in UK broadleaved woodlands, contributing up to 10 million tonnes of national carbon stock. However, it is currently threatened by ash dieback disease ( Hymenoscyphus fraxineus ). Despite the pervasiveness of this pathogen, its impact on forests’ capacity to mitigate global change and the factors influencing symptom severity and mortality of infected trees are poorly understood. Analysing 40 years of data on ash dynamics, we assess impacts of ash dieback on the demography and carbon stocks of five natural populations. We further investigate risk factors for developing severe ash dieback symptoms or mortality. We observed an 8.7 % reduction in ash population numbers since previous surveys and a significant increase in mortality rates of small trees. There has been no significant increase in carbon stocks within ash populations since the conformation of ash dieback in the UK in 2012. Large trees have a decreased risk of mortality from ash dieback, while number of ash neighbours increased the risk. The severity of dieback symptoms decreased with increasing neighbourhood basal area and light availability. Trees with severe dieback symptoms hold 27.4 % of total carbon stored by ash trees, constituting a committed carbon loss or debt within these forests. Together, our results highlight the importance of stand density and light competition in modulating symptom severity and mortality of infected ash trees. We also suggest that the carbon debt within living, but fatally infected ash, requires consideration when calculating carbon mitigation capacities of effected forests. • Total ash population mortality rates haven't significantly increased since the first case of ash dieback in the UK in 2012. • Nearly a third of carbon stocks in ash constitute a committed debt of carbon, slowly lost as severely infected trees die. • Ash dieback symptom severity increased with decreasing light availability and neighbourhood basal area. • Likelihood of ash mortality increased for smaller trees and with number of ash neighbours.