River Raisin Watershed Canopy Assessment Project
As the largest freshwater system in the world, the Great Lakes account for approximately 94% of North America’s freshwater (Manivanan 2008). The quality of this resource affects the daily lives of residents who live, work and play within the Great Lakes region. The Great Lakes Restoration Initiative (GLRI) focuses on improving, protecting and restoring the integrity of this vital natural resource, in part, through grant-funded projects.
In 2024, ReLeaf Michigan, the Michigan Department of Natural Resources (MDNR), the River Raisin Watershed Council and Davey Resource Group, Inc. received a USDA Forest Service GLRI grant. This GLRI grant provided funding to assist four communities in the River Raisin Watershed to assess and understand their community tree canopy, and identify strategies to increase canopy in order to intercept stormwater, reduce runoff, and improve water quality within the River Raisin watershed and, in turn, the broader Great Lakes basin. The River Raisin Watershed is located within the Great Lakes Basin and spans the southeastern portion of Michigan, near the Detroit metropolitan area and Lake Erie.
In 2024, ReLeaf Michigan, the Michigan Department of Natural Resources (MDNR), the River Raisin Watershed Council and Davey Resource Group, Inc. received a USDA Forest Service GLRI grant. This GLRI grant provided funding to assist four communities in the River Raisin Watershed to assess and understand their community tree canopy, and identify strategies to increase canopy in order to intercept stormwater, reduce runoff, and improve water quality within the River Raisin watershed and, in turn, the broader Great Lakes basin. The River Raisin Watershed is located within the Great Lakes Basin and spans the southeastern portion of Michigan, near the Detroit metropolitan area and Lake Erie.
Map 1. The Michigan communities of Adrian, Milan, Monroe, and Tecumseh, located in the River Raisin Watershed.
The cities of Adrian, Milan, Monroe, and Tecumseh, Michigan participated in the project. The project partners worked closely with each community to:
- Complete a community tree canopy assessment using 2024 aerial imagery from the National Agriculture Imagery Program (NAIP).
- Assess urban tree canopy within each community’s full city boundary, roadway areas, HUC-12 watersheds, and U.S. Census Block Groups, as well as additional boundaries such as zoning classes or parks, where data was available.
- Evaluate equitable distribution of tree canopy at the Census Block Group level using the Tree Equity Score tool.
- Develop a prioritized planting assessment that identifies and ranks possible planting locations based on expected contribution to stormwater management and water quality improvements.
- Implement local outreach to strengthen understanding of the benefits of trees, the results of the community tree canopy assessment, and how each community can use their tree canopy to meet local goals.
- Using the data generated, support the planting of up to 50 trees in each community, at locations selected to maximize stormwater benefits and improve water quality within the River Raisin Watershed.
report and findings
General findings of the community tree canopy assessments are provided in the following sections. Each city’s Community Assessment Results page presents specific data, analysis and maps, including:
The report also includes a Story Map that provides an interactive format for viewing the maps and information for each community.
The information presented in this report and in the Story Map can be used to better understand the distribution of tree canopy within each community, the benefits provided by the tree canopy, and where expansion of the tree canopy through new tree plantings can provide the greatest benefits, particularly for stormwater mitigation. While this report provides general findings, trends and recommendations related to the community tree canopy assessment for each community, the accompanying data provide additional information that can be further examined and analyzed. Each community has been provided the full geographic datasets created during this project and is encouraged to continue evaluating the data for community-specific applications as new ideas, interests, or priorities arise.
- Land cover classification
- Tree canopy by various geographic boundaries
- Tree Equity Scores and discussion
- Ecosystem benefits analysis
- Potential tree canopy
- Identification of prioritized planting locations
The report also includes a Story Map that provides an interactive format for viewing the maps and information for each community.
The information presented in this report and in the Story Map can be used to better understand the distribution of tree canopy within each community, the benefits provided by the tree canopy, and where expansion of the tree canopy through new tree plantings can provide the greatest benefits, particularly for stormwater mitigation. While this report provides general findings, trends and recommendations related to the community tree canopy assessment for each community, the accompanying data provide additional information that can be further examined and analyzed. Each community has been provided the full geographic datasets created during this project and is encouraged to continue evaluating the data for community-specific applications as new ideas, interests, or priorities arise.
Community Tree Canopy Assessment Results
Based on an assessment of 2024 aerial imagery, the level of tree canopy cover in the River Raisin Watershed communities of Adrian, Milan, Monroe, and Tecumseh, MI ranges from 22% to 36% (Table 1, Figure 1). The communities participating in the 2024 ReLeaf grant program have similar overall canopy coverage when compared to previous year grant program participants (Table 2, Figures 2 and 3).
Tree canopy is one of five land cover classifications analyzed for this project. The other four classes include grass and other low-lying vegetation, impervious surfaces (e.g. concrete, buildings), bare soils, and water. Of these land cover classifications, impervious surfaces and tree canopy most directly impact the quantity and quality of stormwater runoff in a community.
Tree canopy is one of five land cover classifications analyzed for this project. The other four classes include grass and other low-lying vegetation, impervious surfaces (e.g. concrete, buildings), bare soils, and water. Of these land cover classifications, impervious surfaces and tree canopy most directly impact the quantity and quality of stormwater runoff in a community.
Table 1. Land cover classification of all four River Raisin Watershed GLRI communities.
Figure 1. Land cover classification of all four River Raisin Watershed GLRI communities.
Table 2. Tree canopy comparison of all GLRI communities. (Sources: 2016 Grand Traverse Bay Watershed Project; 2018 Lower Grand River Watershed Project; 2018 St. Joseph River Watershed Project.)
Figure 2. Tree canopy comparison of all GLRI communities, with River Raisin communities highlighted.
Figure 3. Number of GLRI communities within tree canopy cover ranges.
Canopy Cover Along Roadways
Tree canopy along roadways is beneficial to communities in many ways, ranging from reducing air pollution from vehicles to calming traffic to promoting the use of alternative forms of transportation. Unfortunately, tree canopy cover along roadways also tends to be lower than tree canopy cover in other parts of a city, due to the presence of the road itself allowing less space for trees and the accompanying challenges that roads pose to urban trees. Canopy cover along roadways was assessed for the four River Raisin communities, and all four had a canopy cover that was less than their citywide average. Despite having the greatest canopy cover along roadways of the four cities, Tecumseh also had the greatest difference between its roadway and citywide canopy cover. Tecumseh’s citywide canopy cover was 36%, while roadway canopy cover was 25%, an 11% difference. Monroe’s canopy cover along roadways was the most similar to its citywide average (20% compared to 22% citywide). Milan and Adrian each had 5% and 7% less canopy cover along roadways than their citywide averages, respectively. Prioritizing future tree plantings along roadways will help the River Raisin communities to enhance the benefits that trees offer to air quality and stormwater mitigation, discussed in the following section, in some of the areas of town that need those benefits the most.
Table 3. Distribution of tree canopy over streets in River Raisin communities.
Equitable Distribution of Tree Canopy
Census Block Groups (CBGs) are a useful metric by which to assess urban tree canopy because they are associated with U.S. Census demographic data, meaning they can be used to correlate tree canopy with information about a community’s socioeconomic characteristics. One industry-standard resource that pairs tree canopy data with socioeconomic Census Block Group data is American Forests’ Tree Equity Score. This web map-based tool utilizes information about a city’s tree canopy, building density, and land surface temperature, as well as their population’s income & employment, human health, languages spoken, race and age demographics, to calculate a score from 1-100 and automatically generate recommendations to help the community achieve greater equitable distribution of tree canopy by bringing more block groups up to a higher score through strategic tree planting.
The four River Raisin communities have composite Tree Equity Scores ranging from 81 in Milan to 57 in Adrian, and the percentage of block groups within each community that currently have a Tree Equity Score below 80 range from 91% in Adrian to 33% in Milan. In order to bring every block group (and therefore every citywide composite score) to a score of 80, the four River Raisin Watershed communities would need to plant a combined 2.9 square miles (1,879 acres) or approximately 137,055 trees in strategic locations.
The four River Raisin communities have composite Tree Equity Scores ranging from 81 in Milan to 57 in Adrian, and the percentage of block groups within each community that currently have a Tree Equity Score below 80 range from 91% in Adrian to 33% in Milan. In order to bring every block group (and therefore every citywide composite score) to a score of 80, the four River Raisin Watershed communities would need to plant a combined 2.9 square miles (1,879 acres) or approximately 137,055 trees in strategic locations.
Figure 4. Composite Tree Equity Scores in the four River Raisin Watershed communities.
Figure 5. Amount of Census Block Groups within each River Raisin Watershed community that have a Tree Equity Score less than 80.
Table 4. Amount of strategically-located tree plantings needed to bring all River Raisin Watershed communities Census Block Groups (CBGs) Tree Equity Scores to a level of 80 or 100, in terms of square miles, acres, and number of trees.
Ecosystem Benefits
Trees and urban forests provide quantifiable benefits to the community by improving air quality, storing and sequestering carbon, and reducing stormwater runoff. Trees protect and improve air quality by intercepting particulate matter (PM2.5 and PM10), including dust, ash, pollen, and smoke, which are filtered and held in the tree canopy. Trees and forests also absorb harmful gaseous pollutants like ozone (O₃), nitrogen dioxide (NO₂), and sulfur dioxide (SO₂). The tree canopy in Adrian, Milan, Monroe, and Tecumseh absorbs 335,640 lbs of pollutants annually, valued at $247,008. Trees and their canopy also directly reduce CO₂ in the atmosphere through growth and sequestration of carbon as woody and foliar biomass. The four River Raisin communities’ trees are storing a combined 146,430 tons of carbon, valued at $70,803,080, and sequestering an additional 5,565 tons, valued at $2,407,244, each year. Additionally, trees improve and protect the quality of surface waters, such as creeks, rivers, and lakes, by reducing the impacts of stormwater runoff through several mechanisms, including the interception of rainfall, increased soil moisture capacity and rates of infiltration, and reduced soil erosion. The urban forest in the River Raisin watershed reduces stormwater runoff by 11,850,000 gallons per year, valued at $105,846. The total ecosystem benefits provided annually by these four communities’ trees are worth $2,760,098 per year, or $73,563,178 including stored carbon (which is not an annual amount).
Table 5. Annual and total ecosystem benefits provided by tree canopy in four River Raisin Watershed GLRI communities.
Identifying and Prioritizing Future Planting Sites
While land cover analysis is helpful to understand existing tree canopy distribution and value, communities are often interested in expanding tree canopy to optimize the suite of ecosystem benefits provided by its trees, particularly stormwater reduction. To support community tree planting and canopy expansion efforts, this assessment identified and prioritized potential planting locations. This analysis calculated and prioritized realistic potential planting areas based on the total land cover that is open ground, such as those covered by bare soil, shrubs, grass and other low-lying vegetation. Vacant planting sites present opportunities to plant trees, however, not all open spaces are candidates for tree plantings - like roads or sports/agricultural fields. Since some locations are clearly better suited to meet community tree planting goals than others, this study attempted to eliminate areas unsuitable for planting and prioritize planting locations based on optimizing the ecosystem benefits that trees can provide to the community. In addition to the 4,706 acres of existing tree canopy, this assessment identified a total of 4,671 acres that could become potential tree canopy in the future. If all existing canopy is maintained and all vacant sites were planted, the River Raisin Watershed communities could collectively attain a maximum canopy cover of 53%.
Table 6. Potential future tree canopy and maximum potential tree canopy in the four River Raisin Watershed communities combined.
The priority models used for this analysis are based on the impacts of trees specifically to stormwater interception. Environmental data was assessed, including proximity to hardscape, soil permeability, tree canopy proximity, slope, and soil erosion factors. Overlapping these data produced a priority rating ranging from Very Low to Very High based on a calculated average. The resulting community maps identify areas that, if planted, have the highest potential impact to improving water quality. Final planting decisions should be made by each municipality. This report does not claim that all open locations should be planted with trees, but rather encourages that the prioritized planting areas provided in this study can be used to serve as a starting point and guideline for determining where future plantings may have the most impact on water quality and other tree benefits.
Table 7. Model inputs and weightings used for the stormwater priority planting analysis.
The maps of priority planting rankings can be used to identify potential locations where new plantings may deliver the most impact in terms of ecosystem and community benefits. The River Raisin communities can use their respective maps as a guide when deciding where future trees should be planted to have the greatest effect on lessening stormwater runoff. Of the four cities’ combined 4,671 plantable acres, 561 acres (12%) are assigned a planting priority of High or Very High. If only these areas are planted, the River Raisin communities’ combined canopy cover would grow from 4,706 acres to 5,673 acres, increasing total canopy coverage to 32%.
Table 8. Stormwater priority planting analysis results for the four River Raisin watershed communities combined, by number of planting locations and planting area in acres, as well as the resulting cumulative canopy acres and canopy cover percent if all sites of that priority level and higher were to be planted.
|
|
Figure 6. Stormwater priority planting analysis results for the four combined River Raisin communities by number of planting locations and planting area in acres.
References
R. Manivanan (January 2008). Water Quality Modeling: Rivers, Streams, and Estuaries. New India Publishing. p. 114. ISBN 978-81-89422-93-6.
