Regional Tree Planting and Protection Plan

Tree canopy cover is defined as the layer of leaves, branches, and stems of trees that cover the ground when viewed from above. Canopy cover within the Charles River watershed is currently around 53% (2021), which is relatively high compared to other urban areas in the United States.

This percentage accounts for all trees within the watershed and represents trees that are growing on city-owned property, including streets and parks, on campuses, in parking lots on commercial and industrial properties, in private and publicly owned forested natural areas, in private front and back yards, and on boundaries of all of the above.

The Charles River watershed has over 100,000 acres of land shaded by trees, 41% of which is protected through a variety of federal, state, and municipal measures. The municipalities within the Charles River watershed range from highly urban and densely populated to largely suburban areas with large tracts of open space and conservation lands. These conditions drive significant differences in canopy type and quality across the watershed.

Urban tree canopy (UTC), in this context, will refer to both urban and suburban towns in the Charles River watershed. UTC cover is considered a highly effective “nature-based solution” to increase community resilience to the impacts of climate change and is associated with:

1. Mitigation of extreme weathers: UTC lowers ambient temperatures anywhere from 10-15 degrees compared to areas with no canopy cover
2. Reduced stormwater impacts: UTC can also reduce stormwater impacts, particularly combined sewer overflows, by intercepting, slowing, and diverting runoff. These ecosystem services can be amplified through green stormwater infrastructure (GSI), including tree wells incorporating engineered materials that can hold excess stormwater.
3. Other co-benefits: In addition, UTC brings significant ecological, human health, and social “co-benefits” to communities, including increased biodiversity, improved air quality, reduced pulmonary disease, improved mental health, lower crime rates, and increased social cohesion.

In 2021, CRWA completed a land use inventory of the watershed. It showed that the majority (62%) of tree canopy is on forest lands, with a significant portion on residential lands (44%) of various densities.

Due to the diverse urban conditions, UTC is not evenly distributed throughout the Charles River watershed. Canopy cover by town area within the watershed ranges from 11% in Somerville to 71% in Dover. The most concentrated UTC is found in the upper and middle subregions, each of which had approximately 59% canopy, while the lower subregion has only 32.9%.

🔍 Throughout this storymap you'll see this symbol indicating when a map is interactive. Move and zoom around the map on the right to explore tree canopy cover.



As a result of these differences in canopy cover, the social and ecological benefits of UTC are not equitably distributed. Areas of the watershed with lower than average canopy cover are generally correlated with communities considered to be at higher risk of climate impacts based on the state  Executive Office of Energy and Environmental Affairs (EEA)  definition of environmental justice. In Massachusetts, an environmental justice (EJ) population is a neighborhood where one or more of the following criteria are true:

1. The annual median household income is 65 percent or less of the statewide annual median household income
2. Minorities make up 40 percent or more of the population
3. 25 percent or more of households identify as speaking English less than "very well"
4. Minorities make up 25 percent or more of the population, AND the annual median household income of the municipality in which the neighborhood is located does not exceed 150 percent of the statewide annual median household income.

The map on the right shows block groups within the watershed meeting the Mass EEA criteria requirements for EJ and how many criteria they meet.

Tree canopy cover within the Charles River watershed has been declining. Between the years of 2008 and 2021, the average UTC loss by town was 6.9%, and ranged from a low of 2.7% to a high of 23.5%.

Tree canopy loss appears highest in towns with small portions of land in the watershed (Foxborough, Mendon, Ashland), but this is an artifact of analysis methods. Even so, it shows that significant tree loss has occurred in areas within the watershed boundary in these towns.

Among towns with a majority of their land in the watershed, Milford and Millis had the most significant losses by percent change. 

🔍 Swipe left on the map to compare tree canopy between 2008 (right) and 2021 (left).

Land use conversion from permeable to impervious surface was quite moderate over the period from 2008 - 2021. Since 2008, the amount of impervious surface in the watershed has increased from 25.3% to 26.4%, a change of 4%. New impervious surfaces pop up across the watershed, and larger spots appear in the less urbanized Middle and Upper subregions.

73% of the tree canopy in the watershed is privately held, 25% is publicly held, and 2% could not be easily classified. The average canopy coverage on public lands is slightly higher, 59.6%, compared with 54.3% on private lands and 40% coverage on lands designated here as ‘unknown.’

🔍 Explore Tree Canopy by Ownership Type:

 _{Source: MassGIS Parcel data (2024) overlaid with 2021 Tree Canopy Cover data from USFS} 

As an urban area, ongoing development pressures are one of the biggest challenges the watershed faces. Models from the Harvard Forest’s  New England Landscape Future  (NELF) have outlined different land use change scenarios that may occur based on various future conditions and policies. Specifically for forests (areas with over 40% canopy coverage), under a ‘business-as-usual’ scenario, the watershed may lose 53,910 acres (45%) of unprotected forests watershed-wide by 2060 if no action is taken to protect these areas from development.

Land use conversion from forest to development should be a specific concern because the ecosystem services delivered by a forest are both different and greater than those of individual trees in our communities.

Greater average winter temperatures, increased summertime heat events, and longer growing seasons will have diverse effects on trees:

1. Decreased plant survival: This will be due to increased stress, increased mortality of newly tranplanted trees, and the expansion of both native and introduced pests
2. Increased competition from invasive species: Longer growing seasons will give invasive species a competitive edge over native trees
3. Potential increase in volatile organic compounds (VOCs) produced by trees
4. Ecosystem disruption: Temperature is a critical signal that deciduous trees use for breaking dormancy in the spring, which provides seasonal food sources, habitat, and other essential benefits

Climate change will continue to increase the frequency, intensity, and duration of fall and winter rainfall in Massachusetts as well as the intensity of winds. This will likely increase catastrophic tree failures from high winds and saturated soils.

Increased frequency and duration of extreme drought is also expected and will threaten the establishment of newly planted trees. These trees will require increasingly significant human inputs (regular watering) to survive.

Milder and wetter winters will have large impact on pests and disease. Current concerns include (click on each to learn more):

•  Southern Pine Beetle 
•  Beech Leaf Disease 
•  Emerald Ash Borer 
•  Spotted Lanternfly 
•  Jumping Worm 
•  Dutch Elm Disease 
•  Spongy Moth 
•  Asian Longhorned Beetle