The high cost of carbon in city trees

As the planet heats from global carbon emissions, cities have searched for ways to reverse their impact on climate change. A few have launched ambitious citywide initiatives to plant trees to temporarily absorb carbon from the atmosphere. But city trees are expensive, making them an extravagant choice for carbon storage. A new study on street trees in New York City finds that city administrators and taxpayers could store much more carbon for their buck if they paid to plant those same trees in faraway forests rather than downtown.

Kent Kovacs, an agricultural economist at University of Arkansas, gathered a team of researchers to work with the U.S. Forest Service (which funded the research) to assign a price to each ton of carbon stored in or averted by city trees. Their aim was to suss out the price difference for carbon saved by city trees as compared with the carbon saved by trees in a forest.

Just like those of a forest, city trees absorb carbon dioxide and convert that carbon into bark. But they also cast shade on nearby buildings, which can cut back on air conditioning in summer. This shade effect can reduce the carbon emissions of a nearby building by up to 31 percent over a 100-year period if the right tree is planted in the right spot, according to a 2007 study published in the journal Energy and Buildings and conducted in California.

So do these extra energy savings make up for the high cost of city trees in a carbon-to-dollars equation?

Kovacs’ team focused on the notoriously high-maintenance street trees of New York City. These trees cost roughly $3,500 to plant, prune and remove upon death according to the study, to be published in November in “Ecological Economics.”

Kovacs’ team estimated the carbon savings of these trees by factoring in average annual growth, survival rate and the cooling equipment used in nearby buildings. They found a cost of $2,848-$8,080 for each ton of carbon stored by street trees. Trees in forests or parks save carbon at a cost of only $45-$136 per ton — a rate that is at least twenty times cheaper.

A separate study in Colorado in 2007 compared the price of tree planting projects with that of carbon offsets, which are a form of credit that environmentally conscious citizens can purchase to cancel out their emissions. Their money is invested in reforestation, green energy or efficiency projects that absorb the same amount of carbon that the purchaser emits. Melissa McHale, the lead author of this study and an urban ecologist at North Carolina State University, agrees with Kovacs’ conclusions.

“When it comes down to it, planting trees for carbon storage in cities isn’t necessarily the answer to decreasing our carbon footprint,” McHale said.

None of the planting projects in McHale’s study turned out to be cost competitive, either — averaging $90 per ton more than offsets to save the same amount of carbon.

“It just depends on how much people want to pay to store carbon,” McHale said.

The results undermine sweeping initiatives to plant street trees as a viable carbon solution. Over a thousand mayors signed a formal agreement, mostly between 2005-2008, to significantly lower greenhouse gas emissions in their cities. In New York City, MillionTreesNYC was born as a public-private initiative around this same time to plant a million new trees in the span of a decade. Similar initiatives hatched in Miami and Los Angeles (while Denver has just dialed back their promise of The Mile High Million citing budget constraints and shifting priorities). A representative from MillionTreesNYC could not be reached for comment.

There is certainly more to a tree than just carbon. Though carbon storage by city trees cannot be considered cost effective, Kovacs says urban planners should still weigh benefits like reducing storm water runoff, increasing property values and improving air quality in determining the value of mass plantings.

“There’s tons of extra benefits beyond the reduction of greenhouse gas emissions that [street trees] can provide,” Kovacs said.

McHale agrees with Kovacs’ sentiment. “We need to invest in many different ways to reduce our carbon. Is it cost effective? Maybe not,” she added, “but we probably need to do everything we could at this point.”