Guest Contributor: George Profous, Senior Forester, Division of Lands & Forests, New York State Department of Environmental Conservation.

What was once dubbed the Urban Heat Island (UHI), is now ominously termed Extreme Heat.  But whatever the terminology, trees can help to reduce this warming.

The increase in urban temperatures can be addressed by air conditioners, cooling centers, painted rooftops and individually planted shade trees. While these approaches make sense, dealing with effects only puts a temporary Band-Aid on the problem. The world is becoming increasingly urban, and this development  influences heat distribution.

Trees are most valuable in our cities, where they shade streets, sidewalks, buildings and people. They increase the reflection of sunlight. In the few cases where they absorb a bit more sunlight than a lighter surface, they compensate by evaporating and transpiring water, providing even more cooling. Simply put, trees don’t get hot. No umbrella or awning behaves in the same way!

By shading building materials that absorb and re-radiate a lot of heat, trees cool the streets and the people on the sidewalk who are absorbing the re-radiated heat – and keep the nights cooler. Overall heat reduction from trees in cities from around 2 to 7 F during hot summer months are common. In the 2022 European summer heat wave, fatal heat stress occurred when 104 F daytime temperature were compounded by nighttime temperatures above 86 F, providing no break from the heat. Steps to decrease surface temperatures, which can be up to 45 F warmer throughout the day, prove critical to reducing nighttime temperatures.

In one study, temperatures in a 1.2-acre forested park (about the size of a football field) were 90 F in the treetops, 80 F at pedestrian height, and 104 F elsewhere. Estimated heat savings were 10,500 kWh per day or about 500 large air conditioning units operating daily.

Guidelines for Trees in the Urban Landscape

Guideline 1: Location. Trees should be placed on the higher-priority, sun-exposed sides of east-west oriented streets, targeting streets with lower building heights and wider street widths, also termed shallow street canyons. In deeper street canyons, buildings tend to overwhelm the influence of trees and dominate air temperatures and distribution.

Main Street, Poughkeepsie, facing west. A deeper canyon with an east-west orientation. Tall structures provide more shade on the south side of the street (left side in photo). Very tall skyscrapers on narrow streets change the canyon’s heat dynamic completely. The benefits tilt to other than direct heat reduction, and prevailing winds and pollution become more of a factor.
Shallow street canyon with a low building-to-street width ratio and a north-south orientation. A very wide two-way street can provide some opportunities for design change. Start with trees on the east side.
In a narrower north-south oriented street with taller buildings, there is much more shading by buildings. Greenspace might include smaller stature trees with shorter duration street and wall shading. These areas are lower-priority locations for designs that mitigate extreme heat, though the trees still provide cooling, psychological and other benefits. Planners should strive to increase the canopy cover above thirty percent. Street narrowing might be a consideration to augment growing space and reduce impervious surfaces.
Broadway, City of Newburgh, Orange County. Wide street/shallow canyon (lower building height to width ratio), east-west orientation. Prevailing east -to-west winds provide additional air movement. The north (left) side of the street is first priority, as it receives the most sunlight.

Guideline 2: Spacing. Trees should be placed to maximize their shade area yet planted far enough apart to allow night-time cooling and ventilation. Air flows between the tree canopies. However, the daytime improvements in Human Thermal Comfort (HTC) from street trees far outweigh the minor heating impacts at night, which remains comfortable. In order to maintain maximum cooling and air flow between trees, locate large trees at least 40’ apart, 30’ for medium, and a closer spacing of 20’ for small trees.

Guideline 3: Growing space. Small green infrastructure projects planned for underplanting with shrubs and grasses should be limited to pocket parks so as not to water stress the new trees. Alternatively, understory plantings should be added in the third growing season. Trees provide more than seventy percent of the cooling value of green infrastructure. Long-term management responsibility to maintain the underplantings must be in place for this more complex arrangement. Otherwise, understory plantings should be limited due to cost and staffing considerations.

Trees in this green infrastructure design almost died due to drought. Proper tree root planting depth and grading are critical. In this case, the problem was competition with understory vegetation and a grade level initially too high for roots to establish in an area fed by stormwater.

Guideline 4: Canopy cover. Prioritize areas for tree planting that have a street Sky View Factor (SVF) of over fifty percent in order to improve overall heat stress. However, greening neighborhoods may not result in cooler neighborhood temperatures until a sufficient expanse of vegetated area is achieved (32% of a 200-meter buffer) through interventions that increase vegetative cover and decrease impervious surfaces, such as converting paved parking lots and derelict  building lots into vegetated space.  The minimum goal should be a thirty percent canopy cover.

Parking on one side of a very wide street with an east-west orientation. The north side here is a tree-planting priority. Parkland on the south side could provide additional cooling benefits.

Guideline 5: Height. Urban areas in the northeastern U.S. are aerodynamically smoother than surrounding rural areas, so urban heat dissipation is less efficient and urban warming occurs. Varying tree heights in relation to structures can improve roughness and cooling by influencing air circulation. Don’t hesitate to provide a row of tall trees alternating with a row of medium or small trees.  But don’t forget to adjust the spacing and diversify the plantings.

Guideline 6: Parks. Parks form mini low-pressure areas which create breezes from their interiors to city neighborhoods. The average park cooling intensity (PCI) and park cooling distance (PCD) vary widely depending on local conditions such as prevailing winds, aerodynamic roughness and adjacent canopy cover. In any case, an effective way to offset the urban heat island is determined by how vegetation is used in parks.

Trees need to be spaced apart to allow some night-time long-wave radiational cooling.  Grouped trees are more effective than individual trees, which are better than large turf areas. A higher canopy cover  is better than a low canopy, but some variation can support surface roughness (improving air circulation).  More space between the tree canopy and the surface allows for increased air movement and improves human thermal comfort. A forest with a dense understory is a barrier to downslope winds, which are then confined to clearings, stream channels and roadways. Openings of up to sixty feet in diameter have daytime relative humidity levels that equal those beneath tree canopy.

May 12, 2023, 2:30 PM, temperatures in the 80’s. A high leaf area index (LAI) is important for dense shade. Pedestrians are more likely to congregate and use a shaded sidewalk.
Mansion Square Park (left) and One City Center (right), City of Poughkeepsie. Clustering trees interspersed with open spaces and unobstructed park perimeters are recommended for air movement to improve park cooling.

The park cooling edge effect applies within distances of about 2.2 to 3.5 times the height of a border obstruction. Simply put, a square park should feature at least twice the dimensions of the height of a border obstruction. For example, a park’s width should be twice the height of a line of buildings or trees near the perimeter, to keep the edges permeable.  One study indicates that plantings near the park or radiating from the park may extend the park’s cooling influence. Another study found reductions of 3 to 14 F  (2 C to 8 C)  between 0 and 200  feet of a small 3.7-acre park.

Busy streets appear to inhibit cool air movement. These streets should be positioned away from the edges of parks and parallel to prevailing summer winds to allow cool air extension from parks. Also, it’s best to create a network of smaller, interconnected parks spread over the entire urban area.

Pocket parks should be included in any neighborhood design and can add considerably to the success of a commercial zone. They can add to a sense of community as they  encourage pedestrians to linger. Enlarging the root growing space in a pocket park increases its ability to withstand drought and sequester stormwater, lowering heat, improving air flows, and improving carbon sequestration, tree health and survival. Planting in groups improves tree health by reducing temperatures.

Leaf Area Index (LAI), high to low. Linden (Tilia), Oak (Quercus) and Robinia (Gleditsia).

Guideline 7: Shading and resilience. Identify drought-resistant trees and trees with a higher Leaf Area Index (LAI). They will provide greater shading and transpiration. For example, Human Thermal Comfort (HTC) has been shown to improve  by 11 C under linden (Tilia) trees compared to black locusts (Robinia) at 4 C  (Rahman 2022). It is important to know the origin of a cultivar to make sure it is compatible with the local climate. It’s necessary to know whether the tree grows naturally in the understory or overstory, as this also influences photosynthesis and survival.

Guideline 8: Growing space. For  a tree to have enough room to grow, a high-quality root environment is key. Increase planting space to at least 5 feet by 8 feet and provide water for the first two years. Mulch the tree by maintaining three inches at its base to provide cooler soils in summer and warmer soils in winter, extending the growing season. Above around 77 F (25 C) photosynthesis begins to diminish.  All these approaches are critical in high-heat areas, as they will help trees establish deeper roots and provide access to soil water in droughts. They are sure to  keep your natural air-conditioner working at optimum efficiency!