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A study on the impacts of climate change on the
urban forest in the Greater Toronto Area
– using a climate change risk assessment framework and the climate envelop analysis
Z. He
Toronto and Region Conservation
Abstract
Climate change is one of the greatest yet most uncertain stressors on urban trees, and
may present both opportunities and challenges in terms of tree selection and survival.
The climate conditions in the Greater Toronto Area are projected to be slightly warmer
and wetter in the future. Our study utilizes two methods to assess the potential impact of
projected climate change in the next 40 years on the intensively managed urban forest
(e.g., street and park trees). First, the climate change risk assessment framework
developed by the Toronto and Region Conservation and Beacon Environmental was
used to assess the impacts of anticipated changes in key climatic drivers, such as
increasing temperature, on important growth indicators for tree species. Second, the
climate envelope analysis by the Canadian Forest Service was used to evaluate the
changes of the optimal distribution ranges of tree species by comparing their optimal
climatic ranges with the projected future climate. The two methods are shown to be
valuable and complementary. Some preliminary results have been obtained and can be
used to make preliminary recommendations for urban tree species selection and to
direct future research.
Executive Summary
Urban forests are under stress from a range of environmental factors, including climate
change. Data generated from a number of climate models suggests that the Greater
Toronto Area (GTA) may be warmer and slightly wetter in the future. The implications
for the urban forest are not well known. Higher temperatures and increased
evapotranspiration may have negative impacts on tree growth, but an elevated
atmospheric CO2 level has potentially positive growth effects. To better understand
these impacts, the Toronto and Region Conservation Authority (TRCA) has initiated a
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study to assess the risks associated with climate change on the urban forest in the GTA
over the next 40 years.
This study would take a phased approach. This report summarizes the results of the
first phase of the study. The objectives during the first phase were to:
• Evaluate the efficacy and utility of TRCA’s climate change risk assessment
framework in assessing the risks associated with climate change to the intensively
managed urban forest in the GTA;
• Evaluate the efficacy and utility of the climate envelop approach in assessing the
risks associated with climate change to the intensively managed urban forest in
the GTA;
• Determine gaps in the existing literature and provide recommendations for future
research and monitoring; and
• Provide preliminary recommendations for urban forest management.
This study utilized two methods. The first is the climate change risk assessment
framework developed by the TRCA to assess the impacts of changes in key climatic
drivers on important natural heritage features. Increases in temperature, precipitation,
and atmospheric CO2 level were selected as the primary climatic drivers of change.
Three urban tree species were selected as pilot species – red oak (Quercus rubra), red
maple (Acer rubrum), and tulip tree (Liriodendron tulipifera). The risks posed by the
primary drivers to a set of four growth indicators for these species were assessed. The
second method used is the climate envelope analysis by the Canadian Forest Service
(CFS). The changes of the optimal distribution range for 20 study species were
analyzed by comparing the optimal range of four bioclimatic indicators for each species
to the predicted future conditions.
Results showed both positive and negative effects of climate change on the study
species. With the risk assessment framework, the primary drivers tend to have
counteracting effects on tree growth. Increased temperature and drought generally have
negative impacts on tree growth while elevated CO2 levels seem to benefit tree growth.
Overall, the risk associated with climate change is low on tulip tree and medium on red
oak and red maple. The climate envelope analysis also suggests that the tree species
will respond differently to changes in the different climatic parameters. Some species
such as white spruce may not be able to adapt to future climates, whereas other
species such as honey locust may prefer future climatic conditions.
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The two methods can be utilized together to evaluate each study species. The study
results can be used to identify research gaps and guide monitoring programs. The
results can also inform the selection and placement of more experimental species, such
as species currently found in forest regions south of the GTA. Planting the right species
at the right location will be very important for climate change adaptation. Other biotic
and abiotic stressors must also be managed, including poor soil conditions and physical
injury. A more diverse and resilient urban forest will be better able to adapt to future
environmental changes.
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