Skip to main content

Is ash dieback under control?

Image by FERA
European ash tree is an important component of British woodlands. It has been stayed popular and recommended for planting due to its economic and aesthetic value, also the fact that its resistance towards grey squirrels. In UK, it has been estimated that among all the 141000ha big woodlands (>0.5ha), 5.4% of their composition is ash trees. However, since its first discovery in Poland in 1992, the ash dieback disease, caused by fungus Chalara fraxinea, has spread over the European continent and devastated ash populations in certain areas. On 19.Sep, Rob Spence for Forestry Commission came to Bristol to talk about thecurrent stage of ash dieback control in England.

Chalara fraxinea is the asexual stage of Hymenoscyphus pseudoalbidus, and also the infectious stage. Ascospores are produced from fruiting bodies on the dead branches in the litter, and can be transmitted by wind to more than 10km. Ascospores are not durable, thus its infection window is limited to summer months. The spores tend to attack the young trees due to their lower resistance to the disease, cause crown necrosis and eventually death. In mature plants, the effect of the disease is less severe. However, the disease can seriously compromise the condition of mature trees, and make them succumb to other diseases.

Source: BBC website
Current distribution of the disease in England is largely constrained in tree nurseries, except for East Anglia, where a number of cases have been reported in the wild. The prevalence of the disease in the nurseries all over the country is thought to be due to the fact that seeds are germinated outside of UK, and then saplings and young trees are imported back into UK from the continent, which may already be infected. However, the large outbreak in East Anglia is more likely attributed to extreme weather conditions which bring spores from the continent.

The control effort in southwest is focusing on confining the disease. Unlike East Anglia, the cases of ash dieback in wild are still rare. The Forestry Commission has been conducting aerial surveys to spot early infections, also, two smartphone apps, Tree Alert and OPAL can be used to take photos of suspected infected trees and send to the experts for identification. As the staff of the Forestry Commission is very limited, it becomes very unrealistic for them to come to field for most cases.

It is also worth noting that around 1-2% of the natural population is resistant against the disease. Researches are going on in The Sainsbury’s Lab and John Innes Centre in Norwich, as well as some European institutes trying to identify the resistant genes and possible approaches to deter the spread of the fungus through biological approaches. On country level, a ban has been placed on ash import from outside of the country and transfer of living ash tissues within the country, though the timber transport are still allowed as they are regarded as low risk.

In my point of view, ash dieback is well controlled at this stage. Despite the eventual widespread is inevitable, but this kind of selection bottlenecks has happened widely in nature since the evolution starts. Although there is no reason to reduce our effort in protecting ash trees, as long as we keep the genetic diversity with the susceptible populations while introducing and expanding the resistant traits within the population, the disease will be controlled in macro-scale. 

This blog is written by Dan Lan, Biological Sciences, University of Bristol

Popular posts from this blog

Converting probabilities between time-intervals

This is the first in an irregular sequence of snippets about some of the slightly more technical aspects of uncertainty and risk assessment.  If you have a slightly more technical question, then please email me and I will try to answer it with a snippet. Suppose that an event has a probability of 0.015 (or 1.5%) of happening at least once in the next five years. Then the probability of the event happening at least once in the next year is 0.015 / 5 = 0.003 (or 0.3%), and the probability of it happening at least once in the next 20 years is 0.015 * 4 = 0.06 (or 6%). Here is the rule for scaling probabilities to different time intervals: if both probabilities (the original one and the new one) are no larger than 0.1 (or 10%), then simply multiply the original probability by the ratio of the new time-interval to the original time-interval, to find the new probability. This rule is an approximation which breaks down if either of the probabilities is greater than 0.1. For example

1-in-200 year events

You often read or hear references to the ‘1-in-200 year event’, or ‘200-year event’, or ‘event with a return period of 200 years’. Other popular horizons are 1-in-30 years and 1-in-10,000 years. This term applies to hazards which can occur over a range of magnitudes, like volcanic eruptions, earthquakes, tsunamis, space weather, and various hydro-meteorological hazards like floods, storms, hot or cold spells, and droughts. ‘1-in-200 years’ refers to a particular magnitude. In floods this might be represented as a contour on a map, showing an area that is inundated. If this contour is labelled as ‘1-in-200 years’ this means that the current rate of floods at least as large as this is 1/200 /yr, or 0.005 /yr. So if your house is inside the contour, there is currently a 0.005 (0.5%) chance of being flooded in the next year, and a 0.025 (2.5%) chance of being flooded in the next five years. The general definition is this: ‘1-in-200 year magnitude is x’ = ‘the current rate for eve

Coconuts and climate change

Before pursuing an MSc in Climate Change Science and Policy at the University of Bristol, I completed my undergraduate studies in Environmental Science at the University of Colombo, Sri Lanka. During my final year I carried out a research project that explored the impact of extreme weather events on coconut productivity across the three climatic zones of Sri Lanka. A few months ago, I managed to get a paper published and I thought it would be a good idea to share my findings on this platform. Climate change and crop productivity  There has been a growing concern about the impact of extreme weather events on crop production across the globe, Sri Lanka being no exception. Coconut is becoming a rare commodity in the country, due to several reasons including the changing climate. The price hike in coconuts over the last few years is a good indication of how climate change is affecting coconut productivity across the country. Most coconut trees are no longer bearing fruits and thos