by Veronica Westendorff, PLA, ASLA, SITES AP
It doesn’t take a scientist to know. In the middle of summer, walking down the streets of almost any city, there is a notable wave of heat rising off the sidewalks where old trees have deteriorated or been removed and either no replacements or new, young trees which barely cast a shadow across the surface of the walkway are in their place. In contrast, sidewalks and streets lined with mature trees offer respite for pedestrians and cyclists. We cross the street to stand in the shade of a building or under the cooling canopy of the trees around us.
While this change in temperature, referred to as Urban Heat Island (UHI), is noticeable during the day, the real impact of UHI is felt at night, when the sun has set and the impervious surfaces around us hold and slowly release the heat of the day (Norton et al., 2015). This heat begins to compound, and the following day begins at a higher temperature, increasing the overall heat in these areas. Differences in temperature may vary by as much as 22 degrees Fahrenheit (Urban ReLeaf, 2016) and are markedly higher in urban areas with more impervious surfaces and less green space. Land cover type plays a large role in moderating these effects. Impervious areas and sealed soil act almost the same in the creation of UHI, while greened areas that include shrub cover and areas with trees and urban forests lessen the effects of UHI (Norton et al., 2015).
Symptoms of Climate Change
UHI is one of the negative impacts of climate change and certain types of urbanization. Urbanization—the movement of people from rural or suburban areas to more built up cities in search of work, education, housing, and more services and conveniences—can provide some very positive opportunities to reduce our global impact by clustering goods and services, as well as built infrastructure and economic hubs. But the negative impacts of UHI can be exacerbated in these areas, as seen in reduced ecosystem services, human discomfort, health issues like higher rates of heat stroke and asthma (Marando et al., 2013), and in higher energy consumption, habitat destruction, and strained resources (Miner et al., 2016).
As the size of our cities grows, more land area is covered with impervious surfaces that trap and retain heat, and less surface area is available for green spaces which reduce UHI and improve ecosystem services. Trees are one form of green space that are required by many cities in development plans and comprehensive plans that really do make an impact.
Trees Provide Relief from the Heat
Trees provide many benefits including cooling shade, evapotranspiration, creation of microclimates, wind barriers, and water adsorption (Gomez-Baggethun et al., 2012). Their leaves keep impervious areas from receiving as much direct radiation and reduce the heat adsorbed below their cover; their roots prevent erosion of soils and provide stability; and the entire organism acts as a carbon sink. These benefits are clearly desirable, but there is a lag in the time benefits are experienced from when a tree is first planted. It takes time to grow, and this is one of the greatest challenges in using trees to combat UHI. Tree height, spread, and density of leaf canopy influence the benefits provided by trees (Elmqvist et al., 2015). In general, it may take close to 10 years before their benefits are really felt. Healthy trees have a greater impact due to the denser foliage, wider spread, and age of the tree itself. Interestingly, as little as a 10% increase in tree cover can create a reduction of 3-4 degrees Celsius in ambient temperatures and decrease energy usage, which in turn reduces fossil fuel consumption and in turn, decreases the factors which cause climate change (Elmqvist et al., 2015). This type of impact is extremely beneficial to urban areas.
Tree selection matters and it is important to know your medium and make that part of your design. Evergreens are better for wind deflection, year-round shade, and are especially helpful in hotter climates. In cooler climates, deciduous trees can reduce UHI in the summer but when their leaves fall, this allows sunlight through, which can be a benefit in the cooler climates and seasons.
There are disadvantages linked to trees (Escabedo et al., 2010). Tree vegetation can actually trap heat below the canopy; however, the cooling effect of transpiration helps to balance this (Gomez-Baggethun et al., 2013). Male trees are pollen laden, and pollen means allergies as well as breathing issues. Some trees release volatile organic compounds (VOCs), which reduces air quality, while safety issues such as roots lifting sidewalks, fruit and seed drop as trip hazards, broken and falling limbs, low hanging branches, and security issues need to be considered in placement of trees. And, maintenance must be provided for in a long-term plan.
The Key Role of Landscape Architects
Landscape architects play a very important role in reducing the impacts of climate change through our mix of training in design, science, and big picture thinking. We are able to influence the decisions from site selection through final planting and construction. This gives us the ability to keep the project in line to meeting the overarching goal of improving climate issues, specifically urban heat island.
Site decisions to address climate issues like UHI begin with early involvement in the development process. Locating tree save areas, especially around large, mature trees with a healthy canopy, clustering development, and placing development in the most appropriate places on the site create the backbone to the process and the basis for better management of climate impacts. Determining which ecosystem services are being supported or are lacking then influences the types of green spaces that can provide the most benefit. Street trees and required plantings provide a means to ensure that tree coverage is planned for, but landscape architects can use these requirements to create more connected green spaces to support ecosystem services. Connections between sites on a local level are important, and then looking at how these spaces connect across the region in that larger context can make a significant impact on UHI.
Beyond the plant materials themselves, landscape architects specify other materials on the site, from paved surfaces and built structures to fixtures and amenities. Light colored materials reflect heat, reducing absorption and the accumulative effects that creates. Long term maintenance plans to stack the cards in favor of survival and can also be addressed through specifications, client education, and contractor relationships to clarify expectations.
Design Guidelines
Although urban trees are a great tool for reducing UHI and the impacts of climate change, they need to live to be effective! To this end, the diversity of tree species in an urban area is important, with native trees often hardier to these areas. As we so often remind ourselves, the right tree for the site is important. Considerations such as root space to avoid common hazards like sidewalk heave and roots circling trees, water and drainage requirements, sunlight versus shade (of surrounding buildings in particular), room for canopy spread, the type of wildlife it attracts and supports, and resistance to pests and diseases all need to be considered in the selection of urban trees. Availability in the trade is a large concern, so developing relationships with plant professionals can be vital in successful tree selection. Proper planting technique and long-term maintenance plans are needed to ensure plant health and proper pruning for safety and plant health. General awareness of plant health, diseases, and pests contributes to ensuring a long life for the trees. When it is time to remove and replace an existing tree, considering the success of the previous species, and changes to the current conditions is important.
Managing UHI can be achieved through “natural” solutions like urban trees and other green infrastructure, and ensuring the health of urban trees has a large impact (Langemeyer et al., 2014).
Do’s:
1. Save mature trees and vegetation.
- Larger trees with greater canopy spread cool more and provide more benefits.
- Protect the area under existing trees from cars.
2. Make the right plant selections for the space.
- Know the habit and culture of the trees you select. How much room is available for the tree in 10 years?
- Pay attention to cultivars and ensure the right cultivar is installed or the right substitute found; otherwise you design goal is lost.
- Reach beyond the trees that everyone go to! Diversity of species is important and can be achieved without sacrificing good design.
- Add native trees to your plans.
- Build relationships with local growers and nurseries for greater selection.
3. Trees need time to grow.
- Protect trees during and after planting with stakes and construction fencing.
- Plan for water the first one to five years with irrigation, water trucks, or water bags at the base or trunk of the tree. These need to be monitored and filled regularly.
4. Improve how trees are planted.
- Tree should be installed in spaces twice as wide as the root ball, with the top of the root ball above existing grade.
- Planting pits for street trees need drainage, which is essential to prevent drowning the roots.
- Amend the soil to increase both macro and micro pores in the soil structure to promote root growth. Several options include organic matter and structural soils.
- Water—via either an irrigation system that is installed with wind and rain monitors and maintained and observed regularly, or a planned schedule to water and monitor the water needs of the trees. Here again the importance of understanding the tree type is essential.
- Mulch around the roots of the trees to protect from mowers, cars (yikes), pedestrians, and cyclists, but NOT in a volcano mounded around the trunk form, which causes rot and poor root development.
Conclusion
As the impacts of climate change continue to increase, landscape architects can influence the shape and form of how cities develop. Understanding the role of green spaces in the context of both site specific and regional scales allows us to make decisions about the locations and connections of green spaces, particularly trees within the site that can improve UHI effects. Trees provide some of the best “bang for your buck” to improve ecosystem services and especially cooling within urban areas. They should be preserved when they are healthy and can be protected and saved, as density and canopy spread are very impactful. Careful selection, planting, and maintenance are needed for long term survival. Trees are the backbone of good design and the ecosystem services they provide in the urban setting are essential.
Sources:
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Norton, Briony A., Andrew M. Coutts, Stephen J. Livesley, Richard J. Harris, Annie M. Hunter, and Nicholas S. G. Williams. “Planning for Cooler Cities: A Framework to Prioritise Green Infrastructure to Mitigate High Temperatures in Urban Landscapes.” Landscape and Urban Planning 134 (February 1, 2015): 127–38.
Urban ReLeaf. “Benefits of Trees,” April 10, 2012.
U.S. Environmental Protection Agency, OAR. “Heat Island Effect.” Collections and Lists. US EPA, February 28, 2014.
Veronica Westendorff is a registered landscape architect of over 20 years, a member of ASLA, and is SITES AP certified. Currently working on her dissertation at UNCC in the William Lee States College of Engineering program Infrastructure and Environmental Systems, she is researching urban green spaces and ecosystem services.