Invasive Species and Oakville’s Urban Forest: Cause for Concern

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Recently, Oakville celebrated the results from its 2015 i-Tree forestry survey indicating that our city’s tree canopy had increased 1.3% over the past decade (Oakville’s Canopy Reaches New Heights, Nov. 2016).

Tree canopy is assessed by the Town to determine how we are doing at reaching our ‘Urban Forest Management Plan’ goal of having 40% urban canopy cover in Oakville by 2057. Oakville’s current tree canopy cover is estimated to be 27.8%. The original canopy cover estimate in 2005 was 29.2%. This 2005 data was reassessed in 2015 using a ‘more accurate’ methodology and the updated 2005 canopy cover was 26.5%. Therefore, while the report identifies a canopy cover increase, our current tree canopy cover is actually 1.4% lower than what we thought it was in 2005!

However, within the new report entitled Growing Livability – A Comprehensive Study of Oakville’s Urban Forest, are some troubling statistics about invasive species and the health of our urban forest that make a celebration somewhat premature.

According to the study, Oakville has added about 167,200 trees to its tree population over the past ten years, (page 8). Alarmingly, the report also noted a population increase of an invasive Eurasian shrub called European buckthorn (Rhamnus cathartica). It was estimated that the European buckthorn population has increased by approximately 192,000 trees (page 9).

Therefore, the population increase (number of trees) of this one invasive species exceeds the total tree population increase in the last ten years in Oakville. It is possible to conclude that the increase in canopy cover in Oakville can be largely attributed to the expansion of invasive alien species (IAS). It’s difficult to laud a canopy cover increase of 1.3%, when the composition of that increased canopy cover is likely invasive species.

Invasive Alien Species (IAS) are among the most significant threats to biodiversity in urban areas. IAS are species which have been introduced into an ecosystem that they didn’t evolve in and by monopolizing food and space, and altering habitats, IAS can have a range of negative impacts on native species and ecosystem functioning. In addition to their impact on biodiversity, IAS ultimately damage human health and negatively impact the economy through removal costs, damage to infrastructure and the loss of environmental services. For more info on IAS affecting Oakville’s environment click here

In Oakville there are numerous IAS expanding their populations. These species can work in tandem to decimate native species. Most of us are now familiar with the immediate impact of the Emerald Ash Borer, an invasive alien beetle, on our ash trees and woodlands. However, in the wake of the loss of mature ash trees in woodlands, comes the expansion of invasive alien plants. As buckthorn (Rhamnus cathartica and R. frangula) and groundcovers like garlic mustard (Alliaria petiolata) and dog strangling vine (Cynanchum rossicum) take hold, natural forest renewal processes are almost impossible, and costly IAS removal and herbicide applications are required. Our woodlands can no longer be expected to effectively regenerate naturally, and native species must be coaxed along with tree planting, watering and maintenance. Exacerbating the problem are invasive alien earthworms (Yes, earthworms are not native to Ontario!), which alter the soil, making it more difficult for native plant species to thrive. The result of all this is a rapid expansion of invasive trees, shrubs and ground covers and the loss of environmental services that we depend on our tree canopy and native ecosystems to provide.

Ash forest in decline due to EAB (Shell Park, Oakville)
Garlic mustard growth in the wake of Ash tree removal (Parkway 2, Oakville)
European Buckthorn (Rhamnus cathartica) increased from 2% of Oakville’s urban forest tree population in 2005 to 10.6% in 2015 (Buckthorn along trail at Langtry Park, Oakville)

Oakville’s forestry report acknowledges that European buckthorn will impact the viability of woodlots and natural areas and their attendant ecosystem benefits, however the report’s one recommendation about invasive species and Town policy action to date fall well short of what is needed.

If European buckthorn continues to expand its population and distribution at the expense of native trees, Oakville’s future urban woodlands could be characterized by understories dominated by European buckthorn and a gradually declining canopy of native trees. Over time, this will likely result in decreased overall species diversity, poor wildlife habitat, and unattractive aesthetics … (and) reduce the environmental services (page 33).”

A concrete IAS management strategy and budget for implementation should be a top priority to address this growing problem. The management of buckthorn (as well as other invasive species) should be approached with the understanding that we need a long-term approach and we need to identify areas and management tools that will be the most effective and efficient. The cost of IAS management and remediation will only increase with time.

Unfortunately, the ‘next steps’ identified in the staff report to the Community Services Committee about the i-Tree report on October 11, 2016, are very broad and it is unclear where, when or by whom, any specific policies or strategic action plans will be put in place.

The staff report identified implementing the 20 i-Tree report recommendations within a forthcoming update of the ‘Urban Forest Management Plan’ and in conjunction with the biodiversity strategy in 2017. A recommendation that the Town develop a strategy for the monitoring and control of invasive species was previously made in the 2008 UFMP (page 70)!

Oakvillegreen Conservation Association and Oakville Horticultural Society (OHS) have had success in organizing local action against IAS. As organizations on the front line of IAS community education and action, we are eager to continue to work with the Town to protect, plant and restore a healthy and diverse urban forest and native ecosystems. Together we can make a difference.

One good news story is the work initiated by a resident along Glen Oak Creek Trail. Representatives from Oakvillegreen and OHS met with field experts from Conservation Halton, Protect Our Water and Environmental Resources, (POWER), the Early Detection Rapid Response (EDRR) network and Oakville Parks and Open Space Department to assess the site and scope the problem. Neighbours were informed of the impending activity and invited to participate. While Town staff cut down large specimens and treated invasive tree stumps with herbicides, community volunteers worked on removing seedlings and saplings and planting native shrubs and trees. Having members of the community take local ownership of the problem and the solution was key to success. Multiple visits to the site by volunteers have resulted in a greatly reduced buckthorn population and native species are now able to regain their proper place. (You can read more about this project here: ).

Buckthorn seedlings in forest understory.
Ian Martin Group corporate volunteers with Oakvillegreen TreeKeepers, OHS volunteers and POWER at Glen Oak Trail for buckthorn removal

We invite more people to get involved in restoring Oakville’s forest health. Our urban forest faces many threats and challenges but with a little TLC local residents can help them thrive and expand!




Removing invasive species and adding native trees in woodlands requires some training and coordination with the Town of Oakville, but it can and must be done. Residents can join the Oakville TreeKeepers program. With financial support from the Oakville Community Foundation and Ontario Trillium Foundation, the program provides free trees for forest restoration, invasive species removal help, stewardship training, and fun neighbourhood events. Residents can also join Oakville Horticultural Society to learn about the care and culture of plants and work to protect the native plant diversity in our community.

Educating the public about the issue and encouraging the public to stem the tide of IAS is also critical. Oakvillegreen has produced a lawn sign to encourage residents to stop the spread invasive plants by not throwing garden clippings and yard waste into neighbouring woodlands and trails. Also, each spring Oakvillegreen hosts a Native Tree Sale to help Oakville residents purchase low cost, Halton-adapted native trees, shrubs and pollinator garden kits to support native biodiversity!


For more information:

Giuliana Casimirri

Executive Director, Oakvillegreen Conservation Association


Catherine Kavassalis

Co-Chair, Oakville Horticultural Society