City trees grow faster and die younger than trees in rural forests, a new study finds. Over their lifetimes, then, urban trees will likely absorb less carbon dioxide, or CO2, from the air than will forest trees. If confirmed, this trend would be important information for people charged with finding ways to slow global warming.
CO2 is a greenhouse gas, meaning it traps energy from the sun as heat. That makes temperatures near the ground rise. Human activities, especially the widespread burning of fossil fuels, have been releasing extra greenhouse gases into the air. This has led to a rise in average temperatures across the globe.
Trees and other plants remove CO2 from the air as they grow. They store its carbon in their tissues. To absorb CO2 and slow the warming, many cities have been planting trees.
Studies had shown forests readily sop up this gas. But there hadn’t been much data on whether city trees grow, die and absorb CO2 at the same rate that forest trees do. So Lucy Hutyra and her colleagues at Boston University in Massachusetts decided to find out.
Hutyra is an environmental scientist who studies CO2 in the environment. Scientists had assumed city trees were no different from country trees, she says. In fact, her data now show, “Young city trees grow like gangbusters compared to country trees.”
And they die sooner.
The researchers described their findings May 8 in PLOS ONE.
City living — and dying
To figure out how quickly trees were growing, the researchers tracked their diameters (the width of their trunks) between 2005 and 2014. A tree’s diameter increases as it grows, much as a person’s waist size increases as they gain weight. About half the weight of a tree is carbon, research has shown. Most of the rest is water.
The team focused on red oaks and red maples growing on Boston streets. These grew four times faster than did the same species in a nearby forest.
Faster-growing trees absorb more CO2. Over the nine years the researchers tracked these trees, city ones absorbed four times as much carbon from the air as did the forest trees. The city trees also, however, were twice as likely to die. So over the lifetime of each type of tree, forest trees actually absorbed more CO2.
City trees grew faster because they had less competition for light from their neighbors, Hutyra says. In a forest, trees tend to grow close together, shading their neighbors. So few may get as much light as they want.
Street trees also benefit from higher levels of nitrogen in rainwater, says Hutyra. Nitrogen is an element that helps plants grow. It’s also an ingredient of the exhausts that gas-burning cars spew from their tailpipes. That tailpipe pollution enriches city air with nitrogen, she says. Later, rainwater may wash much of it to the ground.
Some street trees also may have better access to water than trees in the country, notes Hutyra. That’s because the underground water pipes can leak.
Still, she cautions that her team’s findings may not hold for arid cities. “Our story is about New England trees,” she says. “In [dry] locations like Phoenix or Los Angeles, city trees might respond differently because it’s extra hot in the city and they have so little water.” At such sites, she says, “Trees might grow slower.”
So why were Boston’s street trees twice as likely as their country cousins to die young?
Roads and sidewalks can leave big-tree roots less room to grow. Cars plow into some trees during fender benders. Plus, homeowners and city planners often decide to take down trees that are sick, unsightly or in the way of some desired structure or view.
Shaun Watmough is an environmental scientist at Trent University in Peterborough, Canada. He says it’s important to keep in mind that people don’t plant city trees along city streets just to sop up carbon. Trees also help clean the air, provide shade and make a city more beautiful, he says — even if it’s only for decades, not centuries.
arid A description of dry areas of the world, where the climate brings too little rainfall or other precipitation to support much plant growth.
average (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
colleague Someone who works with another; a co-worker or team member.
data (singular: datum) Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.
diameter The length of a straight line that runs through the center of a circle or spherical object, starting at the edge on one side and ending at the edge on the far side.
element (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).
exhaust (in engineering) The gases and fine particles emitted — often at high speed and/or pressure — by combustion (burning) or by the heating of air. Exhaust gases are usually a form of waste.
forest An area of land covered mostly with trees and other woody plants.
fossil fuel Any fuel — such as coal, petroleum (crude oil) or natural gas — that has developed within the Earth over millions of years from the decayed remains of bacteria, plants or animals.
global warming The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.
greenhouse gas A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.
New England A term for states of the U.S. Northeast: Connecticut, Rhode Island, Massachusetts, New Hampshire, Vermont and Maine.
nitrogen A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth's atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn. It comes in two stable forms. Both have 14 protons in the nucleus. But one has 14 neutrons in that nucleus; the other has 15. For that difference, they are known, respectively, as nitrogen-14 and nitrogen-15 (or 14N and 15N).
species A group of similar organisms capable of producing offspring that can survive and reproduce.
tissue Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.
urban Of or related to cities, especially densely populated ones or regions where lots of traffic and industrial activity occurs. The development or buildup of urban areas is a phenomenon known as urbanization.
Journal: I.A. Smith, V.K. Dearborn and L.R. Hutyra. Live fast, die young: Accelerated growth, mortality, and turnover in street trees. PLOS ONE. Vol.14, May 8, 2019. doi: 10.1371/journal.pone.0215846.