Ottawa: A city under siege
In March, Nick Hill was one of four winners from the UK and Canada of the prestigious Prince of Wales Forest Leadership Award. Now on placement in Ottawa, Canada, Nick reports the current Emerald Ash Borer outbreak in the city, and invites you to take part in trans-Atlantic discussion on the subject. A follow-up interview addressing readers’ questions has been posted here.
For anyone with an interest in trees, one of the first things to catch the eye on arrival in the city of Ottawa is the abundance of dead ash trees. They stand on street corners and in park woodlands as skeletal monuments of a rapidly changing city tree population and landscape.
Dead ash trees along a busy city cycleway, a common site across Ottawa.
Like in the UK, ash is a common tree in the landscape of eastern Ontario, accounting for some 25% of the region’s woodland cover. Similarly, this is a result of human influence on the landscape. During early European colonisation eastern Ontario was cleared of species diverse, old growth forest for the expansion of agriculture. Land of marginal productivity has since been abandoned and reclaimed by successional vegetation. As a wind dispersed pioneer, ash has thrived under such conditions.
The three native ash species found commonly in the Ottawa area are Fraxinus pennsylvania, F. americana and F. nigra. With tolerance to pruning, pollution and to the seasonal extremes of Ottawa’s climate, ash has been widely used as street trees in the city. Beautiful ash avenues have provided shade, character and living climbing frames for generations of suburban communities. However, in recent years such idyllic, leafy living conditions have come under threat.
The first Emerald Ash Borer (EAB) infected trees in Ottawa were spotted in 2008 by the daughter of an Ottawa City forestry inspector, from the back window of the family car as they drove onto a city flyover. The combination of pamphlets brought home from an EAB training session and a keen youthful eye had proven surprisingly effective. The infested trees were situated in a small treed area, close to industrial units and shopping centres where trucks and shoppers travel in from the region and from across the continent. They were some 400 km from the beetles’ previous range. Early containment efforts proved to be futile as the beetle appeared to have already completed a number of life cycles. The beetle gradually spread across the city and to surrounding rural areas on the wing as adults and perhaps by hitching lifts to new sites on firewood movements and passing vehicles.
A handful of adult Emerald Ash Borer and some signs of their presence. Emerald Ash Borer eggs are laid on the bark from where larvae bore into the tree and feed on the cambium layer. After pupating the adult beetle leaves the tree through a “D” shaped exit whole.
Dead ash trees are now the most significant driver of tree works across the Ottawa. Tree dismantling and replacement planting is tackled by the city’s arboricultural and planting teams. For felling of higher volumes in city park woodlands, heavy forestry machinery is called in. Taking advantage of snow cover and frozen ground in the winter, feller-bunchers and skidders cause surprisingly little ground disturbance. The sight of such forestry equipment rolling through wooded areas of Hyde Park or Hampstead Heath is pretty unimaginable and its achievability here is a testament to a healthy woodland culture amongst the people of the city and nation.
A clearing created in a city woodland by removal of dead ash trees. In this instance ash felling has provided an opportunity for a diverse range of herbaceous vegetation to flourish and for planting of a mixture of tree species.
With movements of wood restricted in EAB infected areas, Ottawa City has had to act creatively in order to utilise an unprecedented local supply of ash timber. With the planned opening of a city wide light railway for 2018, demand of timber is thankfully high and the city have commissioned the milling of ash for use in furniture and paneling throughout the new stations.
Felling and restocking is not the only EAB treatment available. The spread of the beetle across North America has led to the development of chemical treatments. Ottawa city is now responsible for the annual injection of some 2,100 trees. TreeAzin is the treatment of choice, containing the natural pesticide azadirachtin which is extracted from neem tree seeds (Azadirachta indica). The expense and repetition of treatment means that trees of only the highest amenity value are injected. However with long term side-effects to tree health unknown and the complete eradication of EAB in the region extremely unlikely, inoculation cannot be seen as a long-term solution for EAB, but merely a delay of its effects on the landscape.
Annual injection of this mature ash (marked with green paint) with TreeAzin delays the impact of EAB on the park, while neighbouring mature trees are felled and replaced. Here the newly planted tree is a Basswood (Tilia americana).
The benefits of trees in urban areas prevail where streets have been planted with a mix of tree species. Here cherry, birch, maple and a beautiful butternut cool the street as temperatures rise above 30°C away from the shade. The butternut is another (Juglans cinerea) native species under threat, the plight of which is a story for another blog!
Get in Touch: Questions for Pest and Disease Strategy Specialists in Ottawa
With the prevalence of ash in Europe, the steady spread of Chalara dieback of ash (Hymenoscyphus fraxineus) and with EAB already in western Russia, there are clear parallels between Ottawa’s EAB story and our ash populations in the UK. Furthermore a historical taste for single species planting regimes in our cities, such as the plane trees of London and Bristol, challenges the resilience of such urban treescapes to species specific pest outbreaks. For this reason I would like to open the table to readers with any questions for the pest and disease strategy specialists of the City of Ottawa. If you would like to find out more or share an experience about any of the approaches discussed above or on the wider topic of urban forestry management for pest and disease outbreaks, please get in touch via one of the methods below. I can then relay these points of interest to the relevant specialists here in Ottawa in a follow-up interview.
Instagram: @the_wood_worm (Follow for my discoveries and musings from my placement here in Ottawa and in Algonquin Provincial Park)