Human activity has contributed to increased forestry disease

European forest
...the influence of pathogens on vegetation is usually chronic, rather than fatal. However, when you have an additional stress factor such as climate change, a lot of trees begin to feel the strain. They become much more vulnerable and find it difficult to cope with the pathogens that they have.
Professor Martyn Waller
A study has explored the relationship between human activity, climate change, and increased forestry disease...

Human activity and abrupt climate change have contributed to a century of increased forestry disease, according to a Kingston University academic.

Professor Martyn Waller, whose findings have been published in the Journal of Quaternary Science, studied the impact of forestry disease over the last 11,000 years in order to determine whether or not pathogen outbreaks have significantly influenced vegetation change.

His review reveals that abrupt climate change has coincided with large-scale pathogen outbreaks in the past. Moreover, it is likely that human activities, such as the international trade in seedlings and nursery specimens, have increased the frequency of such events during the last 100 years.

Ash dieback disease


Ash dieback – a pathogen to which common ash trees are vulnerable – is the latest in a long line of forestry diseases to threaten European vegetation. The pathogen, which is thought to have originated through a process of hybridisation, was initially identified in Poland during the early 1990s. It has since spread rapidly across Europe, reaching the United Kingdom in 2012. The Kingston-led study was conducted, in part, to place this disease within its long-term context.

In an interview with ScienceOmega.com, Professor Waller explained more about the relationship between human activity, climate change, and forestry disease.

"It was the outbreak of ash dieback disease in the UK that initially piqued our interest into what could be gleaned from the long-term records," he began. "Also, nobody had undertaken a review of this kind since the 1980s. We decided, therefore, that it was time to have a look through the literature to see what it was telling us."

Professor Waller identified several instances where large-scale pathogen outbreaks appeared to coincide with abrupt changes in climate.

'Abrupt climate change'


"The earliest identified episode took place approximately 8,200 years ago," he said. "This was a very short-lived, but quite severe, period of climate change. The evidence suggests that pathogen outbreaks coincided with this change in climate.

"There has also been a lot of debate about events that took place 5,000 years ago; two tree species declined – one in Europe, the other in North America – at roughly the same time," Professor Wallerf continued. "This period has been associated, at least in the United States, with abrupt climate change. Again, records indicate that the pathogen outbreaks were probably triggered by this change in climate."

There are, it appears, historical examples where abrupt climate change resulted in large-scale pathogen outbreaks. I asked Professor Waller how much researchers understand about the mechanisms involved in this relationship.

Additional stress factors


"For the most part, trees have co-evolved along with many of these pathogens," he replied. "As a result, the influence of pathogens on vegetation is usually chronic, rather than fatal. However, when you have an additional stress factor such as climate change, a lot of trees begin to feel the strain. They become much more vulnerable and find it difficult to cope with the pathogens that they have. Pathogen numbers start to rise and pathogen loading increases; it’s a snowball effect."

Whilst the notion of anthropogenic climate change is disputed by some, it is widely accepted that we are currently undergoing a period of significant climatological variation. So, to what extent has the frequency of large-scale pathogen outbreaks increased over the course of the last 100 years?

"I should point out that I am primarily interested in longer-term, historical trends, so I haven’t spent a great deal of time examining the last century," Professor Waller explained. "Even so, just by taking a cursory glance, one can clearly see that there has been a massive increase in forestry disease over the last 100 to 150 years. It is possible to trace some outbreaks back to slightly earlier in the 19th Century. These were most likely caused by people transporting trees around."

Putting aside the issue of whether or not humanity is responsible for global warming, Professor Waller is confident that human activity has played a major role in increasing levels of forestry disease.

The role of human activity


"Humans have certainly made a significant contribution to this increase," he stated. "When trees are moved around, diseases are sometimes introduced into new areas. Moreover, hybridisation can occur when a pathogen comes into contact with a closely related species. Hybridisation often results in pathogens that are more vigorous than the ones that created them. This has proven to be a particularly aggravating factor in the case of ash dieback disease."

Evidence suggests that factors such as abrupt climate change and human activity have had a significant impact on the prevalence of forestry disease in the past. My final question, however, concerned the future. Should we expect the frequency of pathogen outbreaks to continue to increase over the coming years?

"Yes, I think that we most definitely should," concluded Professor Waller. "If predictions are correct and we are indeed entering a period of climate extremes, we are likely to encounter exactly the type of environment that pathogen outbreaks favour. When you add in factors such as human activity and hybridisation, you have the requisite conditions for a perfect storm."

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