How Climate Benefits from Trees Scale with Cities
Trees influence meteorological and climatic effects at all scales of a city, from pocket parks to entire neighborhoods. However, while research has been done on the effect of trees canopies on urban environments, in general, we are only now beginning to understand how urban canopies affect climate at all scales of the city – from an individual building to a street, park, overall pattern of land use and finally, the city as a whole.
Back in 1978, G.W. Grey and F.J. Denke stated in Urban Forestry that most cities were forests. This statement surprised many people. However, the definition of a forest is that "at least 10% of the land is stocked with trees" (Rowntree 1984). A typical North American city may have a commercial core with less than 10 percent tree cover, but immediately adjacent areas usually show greatly increased cover, with residential areas often in the 15 percent to 40 percent range, and parks with 20 percent to 60 percent tree cover. By viewing the city as a forest, designers and planners can gain a fresh perspective on what trees mean to an urban area.
This is the perspective taken in a recent paper by T.R. Oke titled, "The Micrometeorology of the Urban Forest," where he examines the ability of the urban forest to create microclimates that directly affect the residents of a city and the spaces they inhabit. Oke argues that the microclimate created by street trees is imperative to good urban design. But in order to implement it in our designs, we need to better understand the implications of scale. How are these effects impacted by scale, and how can they be maximized and controlled through planning?
Take, for example, an urban park. Temperatures inside the park used in Oke's research study were rarely more than 1° or 2°C (1.8° or 3.6° F) lower than surrounding areas. However, a zone of larger influence extended well beyond the park, cooling adjacent areas as well – and extending the total impact of the park across other areas of the city. In this case, the micro-scale effects of the urban canopy layer in the park (UCL) were reductions in pollution and slight reductions in temperature, but the macro-scale effect was cooling over a larger area. Trees are important at both of these scales for different reasons.
The impact of the urban canopy layer can vary by climate and location. In a webinar titled, "The Effect of Urban Tree Canopy on Microclimate and Heat Islands," Austin Troy of the University of Denver shared research that demonstrated that the heat island effect differs in a humid, temperate city (in this case, Baltimore, MD) from that of a semi-arid city (Denver, CO). Baltimore has a classic temperature pattern that gets cooler at the outskirts of town. Denver, on the other hand, does not see this effect. Instead, the pattern is nearly flat, with city temperatures and outer ring temperatures being surprisingly close in value.
Why? Baltimore is a city surrounded by woods and agricultural areas, therefore the outskirts cool down at night considerably. Denver, on the other hand, was more of a prairie before it was a city – and had few trees. When it was urbanized, a large number of trees were brought in, which counteracted the heat of the native grasslands. Today, the heat-trapping tendency of the city is counteracted by the large preponderance of trees present in the city center relative to the more barren outskirts. This results in the nighttime temperature of the outskirts of the city and the center of the city center being very similar. The benefits urban trees bring to those living and working in central Denver are still desirable, but this example illustrates that basic readings of temperature alone do not always show the whole picture, or demonstrate the myriad benefits the UCL brings to the city.
Tree canopy arrangement and shape also impact heat mitigation. Trees produce the most shade when they are arranged in large patch sizes with tight spacing. This effect is reduced the more the trees are spread out, creating multiple edge conditions. Large patches with area larger core and fewer edge conditions are better for heat mitigation than scattered, isolated trees (up to a certain size). However, more concentrated forests come at the cost of less direct shade on the building, which requires distribution of trees. A project may receive cooling from a large patch of trees nearby but may lose some out on a larger percentage of shade provided by more widely distributed trees. These are some of the scales designers need to consider as they think about what is best for a given project.
There is a water (hydrometeorological) component to the climactic impact of the urban forest as well. The fundamental problem of urban hydrology is storm runoff. Urban watersheds, which are comprised of many impervious surfaces and materials, generate extremely large volumes of water very rapidly. Trees and other vegetation provide some short-term interception storage on their foliage, and the soil they grow in absorbs precipitation and creates an avenue for ground water storage. During daily rain events and even in extreme weather, parks and other stands of trees may provide temporary retention of floodwater. Urban trees also act as conduits for water loss to the air. This act of transpiration is an important control of the moisture and thermal climate of the urban built landscape.
Since the benefits of urban trees differ depending on density, placement, and geographic location, designers must identify their priorities for the site, and use those to guide decisions about where to place trees on a given project. Large patches would be effective in providing heat advection, the movement of hot air, and also pollution filtration, whereas a large quantity of evenly distributed trees would be a benefit for reducing the overall heat island. For each problem, trees can provide an answer if you know how to use them most effectively.
Designing an urban forest is a balancing act for designers, who are often considering everything from hardiness zone, to form, to local availability when specifying trees. Understanding the impacts of trees on weather and climate at various scales – from the individual block to the surrounding neighborhood – may be equally as important to our larger urban forest strategy. Urban trees affect air cooling, shade, pollution control, hydrology, prevailing winds, geographic location, and native vegetation. The deeper our understanding of the interplay of these factors, the richer the return we can make from investing in our urban forest.
Oke, T. R., J. M. Crowther, K. G. Mcnaughton, J. L. Monteith, and B. Gardiner. "The Micrometeorology of the Urban Forest [and Discussion]."Philosophical Transactions of the Royal Society B: Biological Sciences324.1223 (1989): 335-49. JSTOR [JSTOR]. Web. 27 May 2016.
Troy, Austin, and Sara Davis. "The Effect of Urban Tree Canopy on Microclimate and Heat Islands." US Forest Service Research & Development. US Forest Service, 13 Jan. 2016. Web. 24 Sept. 2016.