Environmental Change Affects Maple Trees

Acid rain and drought are among biggest threats

Laura Benakoun holds core samples from tree trunks.

Laura Benakoun holds core samples from tree trunks.

One of Canada’s most beloved and iconic symbols – the maple tree – is fighting a war on multiple fronts. Environmental threats such as climate change, acid rain and insects are taking their toll on maples, and researchers in the Department of Geography are trying to find out why.

Despite their prevalence in Canada, maple trees are sensitive to environmental changes. For her master’s research, Laura Benakoun spent last summer in the forests of southern and central Ontario sampling sugar maples to look for signs of distress and poor growth. Yellowing leaves and bare branches are some of the visual signs of decline in maples, while slowed growth of the tree itself can be seen only by sampling the trunk and examining the tree’s rings.

“We don’t actually know how the growth of the tree is related to those visual signs,” she says. Tree decline is a “chicken or egg” scenario: Is abnormal tree growth responsible for these signs of illness, or do external factors stunt growth? “It will let us predict a little better what’s happening to the forests, and how they may react in the future if we understand how they’ve been reacting in the past.”

The Ontario Forest Biomonitoring Network, where the maple samples were taken, was established in 1986 by the Ontario government to monitor the effects of acid rain and, more recently, climate change. Acid rain doesn’t get much media attention today, says Benakoun, but it still poses a threat to trees. She’s on a leave of absence from the Ontario Ministry of the Environment while she completes her master’s degree at U of G.

Atmospheric pollution such as nitrogen and sulphur can cause soil acidification when it comes down in the form of acid rain. “Maples are very sensitive to the acidity of the soil,” she says. “Acids tend to release more toxic minerals that are in the soil for uptake, and that can harm the tree.” Soil acidification can result in nutrient imbalance and leaf loss.

Benakoun measured tree health by taking trunk core samples using an instrument that resembles a corkscrew. Turning its handle carves a narrow hole through the tree trunk. She then extracts the core sample to examine the layers of wood grown each year. Each ring indicates how much water and nutrients the tree received. Thicker rings tend to indicate a better year for the tree.

Trees undergo two growth phases per year, she explains. In early spring and summer, trees grow more due to additional rainfall. “That’s when they put on a lot of their wood.” Tree growth slows down toward the end of summer when rainfall is less plentiful. This late growth shows up as a thinner, darker part of the ring.

Trees that grow in the same forest tend to respond similarly to their environment because they’re exposed to the same amount of rain and nutrients, she says. Topography can affect how much water trees receive. A tree at the bottom of a hill, for example, receives more runoff water than one on top of a hill.

Healthy trees are better able to withstand environmental stressors such as drought, excessive rainfall and ice storms, says Benakoun. Forests may become less resilient as they face more stressors, which may be exacerbated by climate change. “It’s the added stress of predacious insects combined with drought combined with the effects from acid rain and soil acidification.”

Maple trees are also susceptible to insects, such as the Asian long-horned beetle and forest tent caterpillars. Even earthworms can pose a threat. During her fieldwork, Benakoun noticed some forests had very few maple seedlings. She attributes this to deer feeding on the seedlings and to earthworms that eat the leaf litter needed by seedlings to grow.

As temperatures rise, tree species that were once native to southern parts of North America may begin to migrate north, causing competition with northern species. “You may end up with different competition for habitat,” she says. “Maples may be able to expand more successfully in the north or do better in the more northern parts of their regions.”

What can be done to help maple trees? Benakoun highlights Ontario’s elimination of coal burning, which used to release sulphur into the atmosphere. Cleaning your footwear before you go hiking or camping can reduce the transmission of diseases and invasive species. If you go fishing, don’t dump earthworms.

Aside from providing food and habitat for animals, maple trees also fuel the tourism industry, from maple syrup festivals to fall colour tours. “It’s on the flag,” she says. “It’s this iconic species that we need to value.”