A Pain in the Ash Tree

A downed 101-foot red oak, thought to be around 200 years old

Hurricane Sandy left a wide path of destruction throughout New York and New Jersey, and one area that was uniquely damaged by the storm, was the New York Botanical Gardens. Over one hundred native trees, including some ancient oaks (like the one pictured left) were destroyed by the storm (see Why Do Trees Topple in a Storm). Trees falling in the woods are a natural part of the forest life cycle, a downed tree will provide habitat for animals, return nutrients to the ground, and the light that can now penetrate the canopy will allow for new growth to occur. While it is sad to see trees destroyed, (especially when there is malicious intent, or by accident – read this account of the accidental death of a 5,000 year old tree, it took root only a few hundred years after human history was first recorded!!), it is comforting to know that this is part of the natural process. But how about when a tree is under attack by a pest or disease (10 common tree diseases), sure it may be a natural process, but is that of any comfort?

Recently, the UK has been waging a war against the tree fungus Chalara fraxinea, which has decimated ash tree species throughout northern Europe – affecting more than 90% of ash trees in Denmark and Sweden. Once infected with Chalara, the ash tree exhibits symptoms that include leaf loss and crown dieback in affected trees, and it can lead to tree death.

The BBC reports that Chalara dieback of ash has been listed as a quarantine pathogen under national emergency measure. The types of emergency measures to be implemented are similar to those used to tackle the spread of animal diseases like foot and mouth or bird flu. Currently there is no chemical control for this fungus, the only control is for infected trees to be felled and burned; so far, more than 100,000 ash trees have already been destroyed in England and Scotland. By burning these trees, forestry officials are hoping to slow the spread of the fungus, however this management practice has been criticized. Conservationists note that 80 common insects, at least 60 of the rarest insect species in the UK have an association with ash trees (ironically, many of those insects depend on the dead and rotting form of the ash tree), and that the ash tree provides many other services, from habitat for birds, substrate for lichens and mosses which grow in its bark, to food in the form of seeds for small mammals. By burning these infected trees, these functions are removed, and the rotting timber is no longer able to provide habitat or nutrients. Others agree, noting that simply chopping and burning is not only a futile management strategy (Chalara spores can travel more than 30 km in the wind), but highly disruptive and unnecessary. They maintain that the most effective strategy for dealing with this epidemic is time.

If the spread of ash dieback can be slowed (through measures such as banning the import of ash trees and the movement of trees around the country), and its impact minimized (through management programs that do not included chop and burn), there is hope that scientists can find trees with a genetic resistance to the disease. In Denmark, where ash dieback severely decimated the population, between 1 and 2% of ash trees are showing signs of immunity from the disease. Once resistant trees are found, they can be cultivated and used as new stock for nurseries. One of the keys to finding a resistant ash, will be the engagement of the public and their help in surveying diseased and healthy ash trees, fortunately there is an app for that.

The underlying question is how best manage this outbreak. Is this just a natural process and cycle that should be allowed to continue, until nature balances itself out with resistant forms of ash slowly emerging, and other species dependent on the ash learning or failing to adapt along the way? Or has the introduction of this disease by man warranted active management practices, such as chop and burn? Like most things, the answer isn’t so clear cut (sorry of the pun), and requires a management practice that is a balance of both strategies.

For a unique perspective on dealing with pest infected trees, check out Core77’s  series on the Blue Pine Beetle disaster. Part 1: A Raw Materials Nightmare: The Blue Pine Disaster, part 2: What is BKP (Beetle Kill Pine) Good For?, part 3: What Will the Future Bring for BKP? Canadian Innovation FTW, and part 4: How Can You Help as an Individual?

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