The Cambridge Canopy Project

Trees can act as natural air filters. 

The main types of outdoor air pollutants found in cities include particulate matter (PM2.5 and PM10), ozone (O3), nitrogen oxides (NO, NO2), carbon monoxide (CO), and sulfuric dioxide (SO2).¹ They come from a variety of sources including vehicle traffic, construction sites, and fuel burning. Air pollution can increase the prevalence and severity of lung and heart diseases, such as ischemic heart disease, stroke, and lung cancer, in a population.^{2, 3} Low socioeconomic neighborhoods are particularly vulnerable to these negative effects of air pollution due to their close proximity to industrial sources. These problems are exacerbated by other factors, including increased stress and poorer nutrition.⁴ Trees can reduce the amount of pollutants in the air by absorbing them through tiny pores on the leaves called stomata. The cooling effect of trees can also help prevent the temperature-dependent formation of ozone.⁵

Massachusetts hosts over 300 different bird species each year. Below are some bird species that are commonly found around the Cambridge area.¹⁰ Press to see if you recognize some of their sounds!

Northern cardinals: The Northern Cardinals are one of the few bird species where the females also sing! Typically, only the males sing to defend their territories and attract mates. The female Northern Cardinals may sing more complex songs to signal to the males when to bring food to the nest or to tell the nestlings to leave the nest. The song of the Cardinals usually consists of one to two whistled syllables that can sound like cheer, cheer, cheer or birdie, birdie, birdie and lasts around 2-3 seconds. It’s a sound that signals the transition from late winter into early spring.^{11, 12}

Northern flickers: The Northern Flickers sing in the spring and early summer during mating season and when establishing their territories. Their song consists of a loud, rolling rattle accompanied by a high pitch tone that fluctuates in volume several times and lasts around 7-8 seconds. ¹³

Blue jays: The Blue Jays are known to be fantastic vocalists! Their song can last longer than 2 minutes and comprises a faint collection of clicks, chucks, whirrs, whines, liquid notes, and elements of other calls. ¹⁴

Baltimore orioles: Both male and female Baltimore orioles sing in the springtime. The males sing to establish and defend a breeding territory, while the females sing shorter songs to communicate with their mates. Their song consists of a series of flutelike sounds repeated 2-7 times and lasts around 1-2 seconds.¹⁵

Trees can cool our overheated cities. 

Many cities have pockets of areas called heat islands that experience higher temperatures than surrounding areas. These heat islands arise because these areas have a higher density of buildings and roads, which absorb and emit more heat than natural landscapes. This is a problem that disproportionately affects low income areas of cities, which often have a higher population density and lower percentage of tree cover.⁷ Trees are crucial in mitigating the urban heat island effect, because they can reduce the peak summer temperatures by 2-9°F (1-5°C) by providing shade from the sun’s radiation and cooling through a process called evapotranspiration.⁸

Trees can help absorb and clean stormwater. 

Because urban surfaces are mostly covered by impermeable pavement, they are unable to absorb stormwater in the same way natural landscapes do. This causes flooding and overwhelms the sewage system, resulting in overflows of untreated water into the nearby rivers and water streams. This untreated water is extremely harmful to the environment because it has been collecting pollutants, such as pesticides, bacteria, and metals, from the paved surfaces.⁶ Street trees are important in managing stormwater runoff, because their leaves can capture the rainfall and their roots can remove pollutants from it. The soil layers beneath the trees can also promote infiltration before the water reaches underground aquifers.

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Trees communicate with each other using underground fungi networks. 

Hidden underneath our feet are networks of fungi that trees use to communicate with each other. The fungi transport nutrients and minerals into the roots in exchange for food in the form of sugars.¹⁶ In addition, these fungi connect individual trees together, so that the trees can share resources with each other. For example, when a tree comes under attack by pathogens, it can warn its neighbors by sending signals through the fungi network.¹⁷ Street trees are more susceptible to pests and pathogens and often have short lifespans of around 7-10 years due to limited space and poor soil quality, which limits the formation of the fungi networks.¹⁸ Maintaining these underground networks and increasing the diversity of trees and fungi is the key to increasing both of their survival rates.

Trees serve as important connections for wildlife in cities. 

As cities expand, the natural habitats of many species become replaced by concrete buildings and paved roads. Fragmented greenspaces limit the movement between populations and decrease reproduction rates. In order to sustain the birds, insects and small mammals that live in the urban areas, it is necessary to increase tree cover and build green corridors that connect the fragmented patches.