Chapter 9 – Forest Integrity

Release Date: February 2013

Forests provide numerous ecosystem functions including oxygen production, carbon sequestration, and climate moderation. Forests also reduce soil erosion, improve water quality, and create habitat for plants and animals (Larson et al. 1999). Despite these benefits, forests are sometimes viewed as obstacles to progress or solely as a resource for exploitation.

Conversion of forest to other land cover types leads to forest habitat loss. This reduces forest habitat to small, isolated, patches where plants and animals adapted to living in larger, well-connected landscapes are unable to acquire the necessary resources for survival and reproduction (Beier and Noss 1998; Fahrig 2003). Loss of forest habitat also reduces the benefits humans derive from forest ecosystems such as oxygen production, carbon sequestration and recreational opportunities. In addition to habitat loss, non-native species (including pests and pathogens) and climate change are also important stressors on forest ecosystems (Vitousek et al. 1996; Friedland et al. 2004).

Figure 1 - Monitoring forest integrity in the Credit River Watershed

Figure 1. Monitoring forest integrity in the Credit River Watershed.

CVC’s Forest Monitoring

To evaluate forest ecosystem integrity CVC’s Terrestrial Monitoring Program employs an Integrated Monitoring approach. This approach measures the composition, structure and function of forests at three levels of organization (e.g. species, community, landscape; Figure 1) at different spacial and temporal scales (Woodley 1993). Status and trends are then used to evaluate overall integrity of the monitoring plots and communities within which they are nested.

To evaluate forest integrity, plant and bird communities are surveyed annually at 29 forest sites throughout the Credit River Watershed (Figure 2). Plant communities are surveyed at permanent monitoring stations following plot-based methodologies developed by Environment Canada’s Environmental Monitoring and Assessment Network (EMAN). Forest bird communities are assessed according to Environment Canada’s Ontario Forest Bird Monitoring Program (Konze and McLaren 1997).

Figure 2 - Location and status (2011) of forest integrity monitoring stations in the Credit River Watershed.

Figure 2. Location and status (2011) of forest integrity monitoring stations in the Credit River Watershed.

CVC terrestrial monitoring selected five components to characterize forest integrity and created an Index of Biotic Integrity (IBI) to evaluate forest status and trends through time. The five components are: 1) Tree Health and Dead Wood, 2) Plant Communities, 3) Animal Communities, 4) Soils and 5) Landscape. This chapter focuses on the first three of these components.

Forest Integrity Status (2011)

In 2011, IBI rankings of forest integrity range from fair to good. Of the 29 monitoring stations, 19 are classified as fair and 10 are classified as good (Figure 2).

On average forests in the Middle and Upper Watershed have higher IBI scores than forests in the Lower Watershed (Figure 3). Forest plots with high IBI scores are generally located in large forest patches (i.e. >135 ha). These forest patches are able to support communities of specialized plant and bird species such as spring ephemerals as well as ground nesting and forest obligate/specialist bird species, resulting in higher IBI scores.

Figure 3. Forest monitoring station IBI scores for 2011 separated by physiographic zone. Bars with different letters denote a statistically significant difference in average IBI value within an indicator (p

Figure 3. Forest monitoring station IBI scores for 2011 separated by physiographic zone. Bars with different letters denote a statistically significant difference in average IBI value within an indicator (p<0.10).

Tree Health and Dead Wood

Tree health and dead wood IBI status is classified as good watershed-wide; however, densities of snags (i.e. standing dead trees) were higher in the Middle than the Lower Watershed. Snags are often removed from urban woodlands as a health and safety risk, which is a likely explanation for differences in snag densities between the zones. Nearly a third of sites (9 of 29) have insufficient dead wood to support the large number of watershed wildlife species that are dependent on snags, dead, and decaying wood in forests.

Plant Community

A total of 160 native plant species have been detected in forest communities throughout the watershed since 2005. Plant Community IBI parameters are good watershed-wide, although many differences exist among physiographic zones. Lower Watershed forest communities have lower species richness, fewer spring ephemeral species and a higher number of highly weedy species than communities in the Middle and Upper Watershed.

Non-native plant species richness continues to be a concern in the watershed. No monitoring plots ranked as good for this metric, and two sites, both in the Middle Watershed, ranked poor for non-native species richness. High non-native species richness at two Middle Watershed plots was unexpected, as non-native species are generally associated with disturbed habitat in close proximity to potential seed sources (White et al. 1993). Both plots are located in forest communities with high recreational use that are managed for hazard trees and firewood. Canopy gaps created by tree removal combined with recreational activities have the potential to increase non-native species by providing new habitat and increasing dispersal of weedy species.

Bird Community

A total of 108 forest breeding bird species have been detected at forest monitoring stations. Although bird community IBI ranking is fair watershed-wide, bird integrity scores are higher in the Middle and Upper Watershed than in the Lower Watershed. In fact, three forest sites in the Lower Watershed receive a poor IBI ranking for bird community integrity. The low scores result from the high proportion of birds known to be habitat generalists such as short-distance migrants (e.g. American Robin; Turdus migratorius) and shrub-nesting (e.g. Northern Cardinal; Cardinalis cardinalis) species. Ground nesting birds (e.g. Ovenbird; Seiurus aurocapillus) are also less abundant in the Lower Watershed relative to the Middle and Upper Watershed. A bird community consisting primarily of generalists is typical of highly urbanized parts of the watershed. Creating a natural heritage system that increases forest cover and connects large forest patches is an important step toward improving bird community integrity throughout the Credit River Watershed.

Forest Integrity Trends

Overall, forest integrity has been fairly stable between 2005 and 2011. A few indicators, however, are reflecting deteriorating conditions in monitored forest communities.

Tree Health and Dead Wood

Tree health in the Credit River Watershed has remained stable over the monitoring period. This is encouraging as Beech Bark Disease (Nectria coccinea var. faginata) and the arrival of the Emerald Ash Borer (Agrilus planipennis) are likely to stress forest ecosystems even more over the short-term.

Plant Community

Watershed-wide, most plant community parameters remained stable throughout the monitoring period. Of particular concern however is the increasing trend in non-native species (Figure 4). Along with habitat loss, non-native species are an important threat to biodiversity in Canada. In terrestrial ecosystems, non-native plant species may establish high densities that out-compete native species for light, water and space (LaPaix et al. 2009). Furthermore, some species are considered ecosystem modifiers that have the ability to change forest ecosystems (Francis and Austen 2000). Increasing populations of non-native species in the Credit River Watershed emphasize the importance of invasive species management programs undertaken by CVC.

Figure 4. Statistically significant increasing trends of non-native plant species richness across the watershed.

Figure 4. Statistically significant increasing trends of non-native plant species richness across the watershed.

Two individual plant species have shown significant trends over the monitoring period. Brown-seed Dandelion (Taraxacum officinale) increased in ground vegetation layer and White Ash (Fraxinus americana) decreased in the regeneration layer. Increasing occurrence of Dandelion reflects a trend of more weedy plants in forests. Decreasing occurrence of White Ash is also concerning as declines of a dominant tree species can potentially impact forest structure. Declining White Ash is particularly distressing as the spread of the Emerald Ash Borer will further stress a species that is already exhibiting signs of decline. Further monitoring, especially of White Ash regeneration and Tree Health is crucial to determine whether this observed decline reflects a long-term trend.

Bird Community

Overall, the forest bird community in the Credit River Watershed was fairly stable between 2002 and 2011 with no significant changes in species diversity and community turnover. Relative bird abundance and species richness, however, exhibited increasing trends over the monitoring period. Milder winters, increasing forest cover in regions across Ontario and increased popularity of bird feeders (Cadman 2008) are likely factors influencing these trends.

Some notable watershed-wide bird population trends were detected for a number of guilds and individual species. Similar to other bird monitoring programs, resident (e.g. Black-capped Chickadee; Poecile atricapillus) and short-distance migratory (e.g. American Robin) species increased. These increases were driven primarily by trends in the Lower Watershed, as were increasing populations of edge/early successional and shrub nesting species. In addition, long-distance migrants, habitat generalists, and canopy/sub-canopy nesting species showed increasing trends watershed-wide. Of concern, ground nesting species exhibited significant population declines throughout the Credit River Watershed.

Increasing populations of American Robin, Black-capped Chickadee and Brown-headed Cowbird (Molothrus ater) must be watched closely. These three species are urban associates that respond well to anthropogenic disturbance. Although it is not yet evident that these species are replacing more specialist, urban sensitive species, their increasing abundance may indicate greater disturbed habitat.


Overall, forest integrity in the Credit River Watershed is fair and stable for most components over the monitoring period (2005-2011). A number of indicators must be watched closely as they have the potential to influence long-term forest integrity. For example, although tree health in the watershed appears stable, American Beech tree health is concerning. Beech Bark disease is widespread throughout the watershed, with few beech trees free of scale infestation or disease symptoms. With the recent arrival of the Emerald Ash Borer to southern Ontario, the future health of ash trees is also cause for great concern. The increasing proportion of weedy plant species in forest communities is also alarming. The negative impacts of non-native plant species on forest integrity are well documented, and it is possible that forest integrity may be significantly compromised in the short-term. Densities of downed wood and snags must also be addressed watershed-wide to accommodate dead wood specialists (e.g. Hairy Woodpecker; Picoides villosus). It is important to note that although some concerns impact the entire watershed, most negative trends of forest integrity were either confined to, or were more severe, in the Lower Watershed. As urbanization intensifies, forest communities are predicted to continue to deteriorate unless steps are taken to protect sensitive features.

In the next Chapter we will examine the health of plants and animals in wetlands. Wetlands are biologically productive systems and play a vital role in carbon storage and water purification. How is the health of wetland communities changing in the Credit River Watershed? Learn more in Chapter 10.

Did you know?

In southern Ontario it is estimated that approximately 70% of the original wetland area has been drained for agricultural development.


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Integrity describes ecosystems that are resilient and resistant to change. These ecosystems are biodiverse and structurally and functionally diverse. They provide habitat for large species and predators and are self maintaining. Generally, high integrity ecosystems are subjected to low intensities of anthropogenic stressors and are comprised of components of a self-organizing system (King 1993). The loss of any key system component or change to their interactions results in a loss of integrity (Karr and Dudley 1981, Noss 1990, Aubin et al 2007, LaPaix et al. 2009).
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