by
Adam Bond, Architectural Preservationist
Urban Forestry
The urban tree canopy is among the most undervalued forms of public infrastructure in American cities. It is also among the most inequitably distributed and the most poorly maintained. The research on the ecological functions, public health benefits, and equity dimensions of urban forest cover has advanced substantially in the past decade, producing a body of evidence that supports treating tree canopy as critical infrastructure — to be inventoried, funded, protected, and regulated with the same institutional seriousness as roads, water systems, and electrical networks.
The cooling function of urban trees has been established beyond reasonable scientific doubt, but its quantitative dimensions have been substantially refined by recent research. A 2024 systematic mapping review in Arboriculture & Urban Forestry, synthesizing findings from 115 studies published between 2018 and 2024, found that urban trees consistently reduce air temperature by approximately 0.5 to 5.8°C and surface temperature by 2 to 12°C, depending on canopy density, climate type, and measurement method. The range is wide because the cooling effect is highly sensitive to specific conditions: dense, tall canopies in humid climates with adequate soil volume produce the highest cooling effects; sparse, stressed trees in compacted urban soils with insufficient root space produce much lower effects.
A 2019 study in the Proceedings of the National Academy of Sciences measured intraurban air temperature variation along transects spanning a range of canopy cover in a mid-sized Midwestern city comparable to Allentown and found that daytime air temperature varied by 3.5°C on average across the urban landscape, with the greatest cooling occurring where canopy cover exceeded 40 percent. A 2022 field study in Tacoma, Washington found that the probability of daytime temperatures exceeding regulated high-temperature thresholds was up to five times greater in locations with no canopy cover within 10 meters compared to locations with 100 percent cover — a finding with direct implications for heat-related illness risk in summer heat events.
The health implications of this cooling function have been quantified at the population level by a 2023 European study that found 40 percent of urban heat island-related deaths to be preventable if cities guaranteed a 30 percent canopy cover. This figure, derived from a continent-scale analysis, represents a population health intervention of a magnitude comparable to major vaccination programs — but achievable through the much simpler mechanism of planting and maintaining trees. The Baltimore urban tree canopy and cooling demand research (2026) found that tree canopy cover was among the strongest predictors of neighborhood-level cooling electricity demand, with higher canopy cover associated with substantially lower cooling loads — a direct fiscal benefit to households and a direct grid stability benefit to the utility system.
The distribution of urban tree canopy in American cities is not equitable. It reflects, with striking fidelity, the pattern of historical investment and disinvestment that has shaped urban development more broadly: neighborhoods that were red-lined, targeted for highway construction, or subjected to urban renewal demolition in the mid-20th century have substantially lower tree canopy cover today than neighborhoods that were protected from these interventions. Research by American Forests’ Tree Equity Score project has documented tree canopy cover gaps of 20 to 40 percentage points between the highest-canopy and lowest-canopy neighborhoods in comparable American cities — gaps that are consistently correlated with race and income.
A 2021 analysis in Science of the Total Environment found that urban tree canopy has greater cooling effects in socially vulnerable communities — that is, the marginal cooling benefit of a tree planted in a low-canopy neighborhood is larger than the marginal benefit of a tree planted in a high-canopy neighborhood. This creates a strong equity-efficiency argument for targeting tree planting in low-canopy neighborhoods: the environmental benefit per tree is highest precisely where the social need is greatest and where current coverage is least. The inverse argument — that tree planting in high-income neighborhoods is more cost-effective because the trees are more likely to survive in better-maintained conditions — has been used to rationalize inequitable investment patterns, but the marginal benefit evidence undermines it.
For Allentown specifically, the American Forests Tree Equity Score data indicates substantial canopy cover variation across the city, with several lower-income neighborhoods falling significantly below the citywide average. The city’s established parks system and historically tree-lined residential streets in some neighborhoods provide a strong foundation, but the gaps documented in lower-income areas represent both an environmental justice failure and a missed opportunity for the most cost-effective urban heat mitigation available.
The cooling function, while the most immediately quantifiable urban tree benefit, is one of several ecosystem services that make canopy infrastructure valuable. The stormwater management function is particularly significant for Allentown, whose aging combined sewer and stormwater infrastructure faces increasing pressure from both deferred maintenance and the more intense precipitation events documented in the Northeast’s changing climate profile.
Urban trees intercept precipitation before it reaches the ground (through canopy interception), absorb water through root uptake and transpiration, and — where root systems extend into permeable soil — contribute to groundwater recharge that is otherwise eliminated by impervious surfaces. Studies by the US Forest Service have estimated that urban trees in the United States collectively intercept approximately 17.4 billion gallons of rainfall annually, reducing runoff loads on stormwater and combined sewer systems by a measurable margin. In cities with aging combined sewer systems that overflow during heavy precipitation events — a significant source of both water quality impairment and infrastructure cost — the stormwater interception function of urban trees has quantifiable fiscal value in terms of avoided overflow events and avoided capital expenditure on sewer capacity upgrades.
The root system of a mature street tree also contributes to soil stability and reduced surface erosion in ways that are difficult to replicate by any other means. The loss of a mature tree to disease, storm damage, or deliberate removal produces a hydrological deficit in its immediate vicinity that persists for 20 to 40 years, until a replacement tree achieves comparable root mass. This multi-decade replacement lag is the reason that tree removal should be treated as an infrastructure decommissioning decision with a quantified replacement cost — not simply a property management convenience that requires only a permit fee to authorize.
The gap between tree planting and urban forestry benefit is determined primarily by the survival rate of planted trees through their establishment period — the first three to five years after planting, when the tree has not yet developed the root mass to sustain itself through summer drought without supplemental watering. Research by the USDA Forest Service documents that urban tree survival rates in planted programs vary from approximately 40 to 90 percent over the first three years, with the primary determinants being planting site quality (soil volume, impervious surface coverage, drainage), post-planting care (watering, staking management, pest protection), and community stewardship (engagement of nearby residents in monitoring and reporting).
Programs that invest exclusively in planting without investing in post-planting stewardship achieve survival rates at the lower end of this range, effectively wasting the majority of their investment in establishment-period mortality. Programs that combine planting with community stewardship components — training and supporting local volunteers to water and monitor planted trees — achieve survival rates at the upper end. The difference in program outcomes is not primarily a function of investment level but of investment allocation: a program that spends 60 percent of its budget on planting and 40 percent on stewardship significantly outperforms a program that spends 100 percent on planting. Cities with active stewardship programs, including New York City’s TreesCount program and Chicago’s Openlands tree stewardship network, have documented higher three-year survival rates and lower per-surviving-tree costs than cities relying on municipal maintenance alone.
Gillerot, Lennart, Dries Landuyt, Pieter De Frenne, Bart Muys, and Kris Verheyen. ‘Urban tree canopies drive human heat stress mitigation.’ Urban Forestry & Urban Greening 90 (2023): 127876.
Arboriculture & Urban Forestry. ‘Urban Trees and Cooling: A Review of the Recent Literature (2018–2024).’ Early view (2025).
Iungman, T., M. Cirach, F. Marando, P. Barboza, J. Khomenko, M. Masselot, X. Quijal-Zamorano, et al. ‘Cooling cities through urban green infrastructure: a health impact assessment of European cities.’ The Lancet 401, no. 10376 (2023): 577–589.
Proceedings of the National Academy of Sciences. ‘Scale-dependent interactions between tree canopy cover and impervious surfaces reduce daytime urban heat during summer.’ PNAS 116, no. 15 (2019): 7575–7580.
PMC / Environmental Health. ‘Street trees provide an opportunity to mitigate urban heat and reduce risk of high heat exposure.’ Environmental Health 23 (2024): 15.
Science of the Total Environment. ‘Urban tree canopy has greater cooling effects in socially vulnerable communities in the US.’ Science of the Total Environment 864 (2023): 160929.
American Forests. Tree Equity Score Methodology. Washington: American Forests, 2021.
Urban Science. ‘Urban Heat and Cooling Demand: Tree Canopy Targets for Equitable Energy Planning in Baltimore.’ Urban Science 10, no. 1 (2026): 61.