Trees in Ottawa– and around the world generally– are facing many threats. Threats include climate change (drought, flooding, increasingly more violent storms, warmer temperatures in some locations and cooler temperatures in others), invasive species such as the Emerald Ash Borer, and tree loss to urban expansion. Pollution is also a concern, including air pollution and acid rain.


Climate Change

Climate change is transforming life on earth, and is a major threat to biodiversity, the environment, and humans. Climate change is a threat to trees for a number of reasons:

  • Climate change is causing normally rare, extreme weather events to occur more often, and with greater ferocity. Extreme weather events include hurricanes and storms, which can uproot and topple trees because of their high winds.
  • Due to climate change, droughts are becoming more intense, and are occurring for longer periods of time. Droughts cause trees to lose leaves through scorching and wilting. Roots begin to die, and when rain finally does return, the tree cannot take in water. Droughts also make trees more susceptible to disease and pests.
  • Climate change is altering the distribution of species, changing where they are able to live. This allows invasive species to invade new areas more quickly, and more often.

To learn more about Ecology Ottawa’s efforts to fight climate change, check out our Stop the Tar Sands Pipeline campaign and our Real Clean Energy campaign.

Invasive Species and Pests

Invasive species are one of the main threats to biological diversity (biodiversity) on this planet. Invasive species have changed, and are still changing, the urban forest in significant ways. Invasive species considered a threat in Ottawa today include the Dog-strangling Vine and the Emerald Ash Borer (EAB). Dog-strangling Vine is an invasive vine that grows in dense patches, suppressing the growth of native seedlings and saplings.

Dutch Elm Disease was an invasive fungus that, decades ago, decimated the elm tree population across North America and other parts of the world. In Ottawa, Ash trees were planted to replace the elm trees lost. Now, our urban forest is facing significant threat from the Emerald Ash Borer, a green beetle that is decimating ash trees. The EAB kills ash trees (but not mountain ash, which is not actually a type of ash) efficiently, and a tree is likely to die within 2-3 years of an infestation. It estimated that 25% of all trees in Ottawa are some species of ash, and all of these trees will be lost within the next 10 years.

We’re learning lessons from our past: clearly we need to plant a diversity of native trees–and many of them– in order to counteract this loss.

To learn more about the affect of Emerald ash borer on our ash trees: Emerald Ash Borer


Forests and trees are often referred to as the lungs of our planet. Trees take in carbon dioxide, a greenhouse gas with an increasing concentration that is causing climate change, and convert it to oxygen. Trees also filter air and remove fine particulate matter that is harmful to our health. For example, research and computer simulations by Nowak et al. about trees in the United States revealed that 850 deaths and 670,000 incidences of acute respiratory symptoms were avoided because of their tree canopy!

Despite the good work trees do cleaning and filtering polluted air, air pollution is also a threat to trees. Heavy pollution can cause particulate matter to accumulate on leaves, which can impair photosynthesis and negatively affect tree growth.

The major sources of air pollution include emissions from power generation and transportation sectors. These emissions also create acidifying particles, which cause acid rain. Acid rain weakens trees and makes them more prone to diseases and pests. Acid rain can also leach nutrients from the soil, and release toxic metals that are harmful to tree growth.

Pollution from urban runoff is also a problem for urban trees. Urban runoff is the rainwater that collects on and washes off of the surface of urban landscapes, such as pavement (roads, parking lots, sidewalks), urban green areas (lawns, golf courses, parks), and buildings. This runoff water often contains contaminants and pollutants that are harmful to trees, such as motor oil, gas, heavy metals, pesticides and fertilizer from green spaces.

Learn more about how Ecology Ottawa is doing their part to help keep our rivers clean: Ottawa River Acton Plan

Urban Expansion

Urbanization is a significant cause of tree loss. As cities expand, trees are often among the first casualties. Trees are cleared to build roads or to widen existing roads and highways, to create parking lots and new buildings, and for golf courses, stadiums, and other types of development.

As we clear trees for urban development, we also lose the diversity of birds, mammals, invertebrates (insects, worms, and other creepy-crawlies), plants, fungi, and lichens that are associated with these trees. This diversity of life is essential for maintaining ecosystem health. Ecosystems that are healthy and functioning provide us with many ecological goods and services, such as clean air, fresh water, pollination of crops in our gardens and of trees that bear fruit, etc. Healthy ecosystems also provide us with places, such as parks and walking trails, that give us joy and pleasure to spend time in, alleviating stresses that arise from day to day life.

Greening our cities, by planting native trees, is necessary for our mental and physical well-being, and for increasing biodiversity. So let’s get out there and plant some trees!


Doing Your Part

There are many steps we can take to help minimize how our actions contribute to these threats.

Climate change and air pollution: in many ways, these two go hand in hand. Power generation and transportation are the major sources of both air pollution and greenhouse gas emissions. By making simple changes, we can reduce the amount of green house gas and air polluting emissions that we produce.

  • Walk, take public transit, or cycle instead of driving your car
  • Turn off electronics and lights when not in use
  • Use energy-efficient appliances and devices
  • Don’t use pesticides or fertilizers on your lawn (as they contribute to water pollution)
  • Consider using an alternative to salt (eg: sand) to increase traction along your walkways and driveways in the winter, or use only the minimum amount of salt needed to get the job done (to limit run-off pollution)

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