The Urban Forest

Urban trees

In many cases, outbreaks of invasive species begin in urban settings. The spread of invasive species is heavily influenced by human activity, following common shipping and trade routes among urban ports of entry (See ‘Pathways’). In urban settings, individual trees are valued for their many environmental and economic services and for the social and health benefits that they provide. These include, increased property values, reduced energy costs for homes with trees near them, improved air and water quality, and aesthetic values, among others. In fact, TD Bank recently released a report that places a dollar value on urban trees in Toronto, which can be seen by clicking here.

Invasive forest pests can have huge economic costs to Ontario and Canada for prevention efforts, control efforts, and other mitigation and regulatory efforts. The costs associated with such prevention and regulatory controls, as well as with scientific monitoring and research of introduced pests, reforestation of impacted areas, and potential processing and treatment of wood products intended for export can be extremely high and hard to predict (FIAS, NRCan, 2013). The City of Toronto, for example, estimates that it will cost $37 million over five years to cut and replace the city-owned ash trees that are killed by the emerald ash borer (EAB). Further, as of 2012, the Canadian Food Inspection Agency had already spent over $30 million to manage the invasion of EAB and had cut over 130,000 trees to slow the spread of the beetle (Ontario Ministry of Natural Resources, 2012). Over the next decade, some estimates from the United States suggest that 17 million trees will need to be removed and replaced within communities alone. This would cost approximately $10.7 billion, but could double if both urban and rural land is taken into account (Kovacs et al, 2010). These estimates only take into account one invasive pest, the emerald ash borer. Costs continue to rise as more invasive pests and pathogens spread throughout North America.

Invasive species also have the potential to cause extensive ecological impacts on individual urban trees, as well as to the overall urban tree canopy. As invasive species can cause damage to various parts of the tree, can reduce its health, or even kill a tree, they subsequently have a negative impact on the health of the overall urban forest. By damaging or killing trees, forest pests can reduce habitat or food sources for native animals within urban centres. Further, the quality of the environmental services that trees provide – such as shade, water and air filtration, and oxygen production – can be greatly impacted by invasive species in cities. 

The social impacts related to forest pest infestations are the most difficult to value, but they can also be extensive. Many communities, especially in rural northern Ontario rely on forestry for their livelihoods and reduced harvest levels due to forest pest infestations can greatly impact the stability and well-being of these communities. Damaged forest ecosystems could also mean a negative impact on tourism and recreation, especially in areas where a healthy forest or green space is highly valued for outdoor activities, such as campgrounds, or hiking trails. Many invasive species infestations begin in urban areas, attacking trees on streets and in parks. This can negatively impact the aesthetic value of neighbourhoods, communities, and cities.

The Working Forest

Lodgepole pine are impacted by mountain pine beetles

Socio-economic impacts from invasive species are also experienced in the forest industry. Social impacts within the forest industry itself can arise from a reduction in income for forestry workers who rely on stable and high wood supplies. Ontario’s forest industry is responsible for over 200,000 jobs across the province (OMNDMF 2011) (Ontario’s Biodiversity Strategy (MNR, 2011)), and the future of these jobs is uncertain as timber resources are threatened by invasive species.

The greatest direct economic impact of forest pest outbreaks is felt by the forestry sector, through reduced wood supply or through reduced wood quality. This can greatly lower revenues for forest companies due to reduced harvest levels, impacting the economic strength of the sector (NRCan, 2014). Pimental et al. (2005) estimate that $2.1 billion worth of forest products is lost each year to invasive species in the United States. Canada’s annual timber losses due to invasives are estimated at 61 million m3 which is equivalent to $720 million in losses (Canadian Action Plan for Invasive Alien Terrestrial Plant and Plant Pests (CFIA, September, 2004). 

There is special concern over the potential Asian longhorn beetle outbreak. This beetle attacks a wide range of hardwood trees and poses a threat to the hardwood products and maple syrup industries, whose products were valued in 1997 at $480 million and $130 million respectively (An Invasive Alien Species Strategy for Canada, 2004). 

The Natural Forest

2167010-SMPT (1)

Widespread ecological damage can occur when an insect or disease becomes invasive. The introduction of an invasive forest pest can cause a decline in the biodiversity and health of a forest ecosystem and a large reduction in available wood fibre within a forest (NRCan, 2014). To make matters worse, since invasive pests often do not have natural predators in natural forests outside of their native range, they can spread rapidly and uninhibited to cause extensive amounts of ecological damage.

The ecology of an insect or disease determines the direct ecological impact it will have on a tree. Some forest pests feed on a tree’s leaves, reducing the health of its canopy. Other species feed on the tissues beneath a tree’s bark, reducing the tree’s ability to transport water and nutrients. Invasive insects and diseases can cause trees to develop one or more of a number of unhealthy conditions, such as leaf blight, root rot, volume loss, dieback, deformity, growth loss, cankers, or death (NRCan, 2014). While some trees can be healthy enough to resist or even recover from the attack of an invasive insect or disease, others, usually older or weakened trees, cannot survive the attack and will decline in health and eventually die (NRCan, 2014).

The negative impacts incurred by invasive species are not limited to the tree they infest; they have the potential to cause widespread impacts on the health of the whole ecosystem. The loss of trees in Canada’s forests can reduce habitat for native animals and insects, create canopy gaps altering the microclimate of the forest, make forests vulnerable to subsequent invasive species, and can reduce the overall biodiversity within forest ecosystems. Specifically, populations of native species that have specialized interactions with the threatened host, such as terrestrial arthropod species with a high level of association with ash, might be at increased risk (Gandhi and Herms 2010). Ecological impacts caused by introduced species are hard to predict, and even harder to assign a dollar value to. Healthy, functioning ecosystems provide a wide range of ‘ecological services’ such as air and water purification, erosion control, carbon sequestration, nutrient cycling, wildlife habitat, and climate regulation. Troy and Bagstad (2009) estimate that these ecosystem services in southern Ontario provide billions of dollars of economic benefits (Ontario’s Biodiversity Strategy (MNR, 2011)).

Forest pest infestations can also have social impacts on traditional Aboriginal activities associated with a healthy forest. Many First Nations people rely on natural forests for their livelihoods and for maintaining their culture. Therefore, a less resilient forest ecosystem could negatively impact the traditions and activities of these communities. Further, the aesthetic and spiritual values that various people associate with forests and natural ecosystems could be impacted as a result of invasive forest pests damaging natural areas (FIAS, NRCan, 2013).