This post begins an occasional series that will be dedicated to exploring how the physical characteristics of neighborhoods interact with the regional meteorological conditions to produce "the sidewalk experience" — that is, the climate on the spatiotemporal scale of the pedestrian.
Trees have long been recognized as a pleasant addition to roads for travelers, going back many centuries. And their use simply for ornamentation goes back even farther. The book "Our Heritage of Community Trees" quotes a 1700 Philadelphia ordinance decreeing that house owners "should plant one or more trees before the door that the town may be well-shaded from the violence of the sun [...] and thereby rendered more healthy." [This is a classic example of an externality.] Despite this early awareness of the benefits of trees in the countryside, many city streets were largely devoid of vegetation in the Early Modern period— perhaps because of the focus instead on public parks, gardens, and plazas in addition to the traditional village common [see "The Handbook of Urban and Community Forestry in the Northeast", ch. 1 for a good historical discussion, from which the following information is drawn]. In Europe especially, tree-lined boulevards in newer parts of cities became the standard for urban elegance and healthfulness in the 18th and 19th centuries, while city centers, with their narrow crooked streets, generally remained treeless. Even villages did not see much greenery. The development and concept of the sidewalk as a distinct entity helped set aside the space in the streetscape for vegetation, while its usefulness as a provider of shade became more appreciated, as did its filtering capabilities (at a time when 'bad air' was still considered the cause of many diseases) [Our Heritage, p. 16]. Consequently and subsequently, in the 19th and early 20th centuries, the American "City Beautiful" and "Good Roads" movements demanded more shade trees in addition to paved surfaces; in New York State in the 1930s, landowners could receive a bonus of $1 for every 3 living roadside trees. Unfortunately, crowding due to the intense economic pressures associated with rapid industrialization, in combination with street widening, installation of underground utilities, and like pressures resulted in large sections of cities continuing to have few or no trees, especially in working-class neighborhoods; indeed, in some places the industry actually caused the vegetation to die. This pattern played out in the United States as well as Europe, with lingering economic disparities in street-tree abundance. And though the perception is that street trees are on the rebound in the U.S., mostly the opposite is true. Although science cannot entirely explain how a tree could be valued at over a million dollars, it can help us understand the roles vegetation plays in the broader urban setting.
Physiologically equivalent temperatures through the course of a day in Freiburg, Germany: the data in red were recorded on a treeless section of street, while those in green were near (but not directly underneath) trees. Source: "Human Thermal Comfort Below the Canopy of Street Trees on a Typical Central European Summer Day." Mayer et al., 2008.
Most noticeably, trees intercept insolation and provide shade. They have an overall cooling effect as well that in the Northern subtropics averages nearly 10 K, due in part to being more reflective (having a higher albedo) than the surfaces that they are shading; in undeveloped or agricultural areas, this situation is reversed and non-forested areas like grasslands and fields have lower annual-mean temperatures than adjacent forested ones. Cooling also results from the water stored in vegetation; rather than going toward heating the pavement (sensible heat), insolation striking a tree is partially devoted to evaporating the water. This results in stress on the tree, to be sure, especially in concert with the effects of wintertime salt spreading and uptake of CO2 from nearby anthropogenic sources; but their raison d'être is functionality, and their watchword is sacrifice. On the other hand, average wind speeds (which cool hot streets by mixing) decrease in the presence of obstructions like trees — but this cost is outweighed by the moderating effects. And of course it is a benefit to the pedestrian on windy winter days. Analogously to this partial reversal of costs and benefits on a seasonal basis, because the above discussion is predicated on greater abundances of trees in rural areas versus urban, the moisture budget would have to be reversed in places where the opposite is true, such as in arid regions. In the tropics, meanwhile, tree health is determined in large part by the tree's sensitivity to vapor-pressure deficits caused by high temperatures and drought. Vegetation also helps filter the air, though in terms of mortality from air pollution alone, the effect of street trees is quite modest.
The below map subset comes from a census conducted in 2005 of New York City street trees, color-coded by species. Lower Manhattan is shown, with a rather abrupt line of separation roughly along Houston St (connecting the Williamsburg Bridge and the Holland Tunnel). Comparison with the satellite view (bottom) shows that the substantial amount of vegetation in the government housing developments (near the west end of the Williamsburg Bridge) built according to the 'Tower in the Park' model is not counted in this assessment. The comparison also gives a sense of how many street trees are so small as to barely be visible from space in a densely populated city, limiting their impact mostly to their immediate vicinity rather than being a moderating influence on neighborhood or larger scales. Maps like this help planners understand not just where trees are and are not, but also the sensitivity of trees in neighborhoods given species distributions and characteristics. The full map is here.