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It's Not Just the Number of Trees; It's How Long They Live


How to and how not to mulch trees.                                                                                             
Submitted photo

Evanstonians pride ourselves on the City’s forward-looking forestry program, which has won the Tree City USA designation for 35 years running.

So it’s natural that the Climate Action and Resilience Plan adopted in December by City Council makes it a priority to maintain and expand the urban tree canopy. Trees contribute both to reducing carbon pollution and adapting to the inevitable consequences of climate change.

Putting the plan into action means stepping up the pace of tree planting, as reported in the last issue. But to really start paying dividends, a tree needs to survive and thrive – and that’s where we all come in.

It Is Not Easy Being a City Tree
Far too few city trees make it past a few years in the ground. In New York, more than a quarter of street trees die within nine years of planting. That’s because city trees face challenges that trees growing in rural areas don’t have to deal with, like compacted soil, small planting spaces and vandalism.

To really deliver all of the benefits we expect from them – cleaning and cooling the air, slowing stormwater runoff and removing carbon pollution from the atmosphere, to name a few – trees have to stay healthy and live long enough to grow big.

Researchers at Boston University recently concluded that to even reach the point when the carbon they store exceeds the carbon released in growing, moving, and planting them, city trees have to live for more than 25 years.

Big is Beautiful
The good news is that City trees appear to grow faster than forest trees. That’s likely because cities are warmer and carbon dioxide levels higher than in rural areas. Boston’s trees, the BU researchers found, grew at nearly four times the rate of forest trees and stored carbon faster.

But to really deliver, a tree needs to keep on growing. Another study, published in 2014 in the journal Nature, found that “a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree.”

Here’s what this means for a single swamp white oak, a type of tree found in Evanston’s remnant landscapes (like Perkins Woods) and also planted along streets. A swamp white oak along a residential street that’s just three inches in diameter will store 67 pounds of carbon every year. But if it reaches 31 inches in diameter, it will store 1,500 pounds of carbon every year. (An oak that big, by the way, is likely 200 years old.)

Other benefits that trees provide also increase with age. For example, the big tree in this example will keep more than 5,000 gallons of stormwater out of our storm sewers every year compared with 236 gallons of stormwater for the smaller tree.

It’s intuitive that a bigger tree makes more shade, and shaded surfaces can be more than 40 degrees  Fahrenheit cooler than surfaces in direct sunlight. As they photosynthesize, trees also release moisture, and evapotranspiration can make the area around a tree feel even cooler.

This means that big trees can reduce electricity use for cooling during hot months – and they can make it much more comfortable to get around by walking or riding a bike, which in turn cuts down on emissions from cars.

As the Chicago Region Trees Initiative points out, “Trees are the only infrastructure that increases in value with age.”

The problem, the Boston University researchers conclude, is that city trees “live fast and die young.”

So what to do?

Take Care of the Big Old Trees – All of Them
The City of Evanston does an outstanding job of caring for the stately trees along our streets, parkways, and in public parks, including inoculating American elms against Dutch elm disease and maintaining a regular cycle of preventive pruning. Volunteer TreeKeepers trained by Openlands also help, especially with winter pruning of young elms and oaks. Taking care of public trees pays big dividends not only by preserving all of the benefits these trees bring us, but by saving in the short term the cost of removing a dead tree and planting a replacement.

But trees don’t pay attention to property lines when it comes to storing carbon and cleaning the air or soaking up excess stormwater. For that reason, there is a strong public interest in protecting all of our big old trees. If you are fortunate enough to have big old trees in your own yard, by preserving and maintaining them, you are making an important contribution to the overall urban forest.

Working at Street Level to Help Young Trees Survive
Meanwhile, we can all work to help young trees survive and thrive. While the professionals are in a bucket truck pruning large elms or responding to emergency calls after a storm, we can be working to get our young trees past those crucial early years of life.

If you have a young parkway tree in front of your house, take a look at the base of the tree. Is mulch or dirt piled up around the trunk? Are grass or weeds growing right up to the trunk? Is the bark damaged from a mower or weed whip? Are there signs that dogs have been visiting (white marks from urine)?

Over time, these problems can kill a tree. Piled-up mulch and dirt hold moisture against the bark and start decay that can eat away at the heart of the tree. Damage to the bark from mowers or dogs can also give disease and decay a place to start

Take Care of the Trees
It’s easy to take big trees for granted – until they’re gone. But while they are standing there silently, they are providing vast benefits, and many of these benefits, or “ecosystem services,” are of tangible value to humans.

We aren’t going to have many more 200-year-old trees any time soon. But as a Kenyan friend told me, quoting the Nobel Prize winner Wangari Maathai, “If we take care of the trees, they will really, really take care of us.”

Ms. Pollock is a TreeKeeper and Steward at Ladd Arboretum Natural Area.




 

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