Losing Urban Trees—and the Wildlife that Depends on Them

Candice Gaukel Andrews December 11, 2018 1
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Living well in an increasingly urbanized world will require that we keep nature close by to every home, office and doorstep.

We need to learn to tolerate a bit of messiness when it comes to our urban neighborhoods and landscapes. And I’m not talking about litter on our sidewalks, trash in our local parks or junk in our yards.

The kind of dishevelment I’m suggesting is that we permit some fallen branches, woody debris and—especially—large, old trees to inhabit our city spaces alongside us.

Across the world and here in the U.S., the future of large, old trees is bleak. In fact, a study in the May 2018 issue of Urban Forestry and Urban Greening reports that nationwide, metropolitan areas are experiencing a net loss of about 36 million trees every year. That amounts to about 175,000 acres of tree cover, most of it in central city and suburban areas, but also on the exurban fringes.

And as cities lose their large, old trees, the native wildlife that depends on them for food and shelter are also in danger of disappearing.

Leaving leaf litter and logs

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Large, old trees are important shelters and food sources for birds and small mammals.

In order to assess changes in tree cover in the country’s cities and towns, U.S. Forest Service researchers recently examined aerial imagery taken between 2009 and 2014. Results showed that each year during that period, the nation’s metropolitan areas lost an average of 36 million trees across 45 states. The biggest losses on a percentage basis were in Rhode Island, Alabama, Georgia and the District of Columbia. Only three statesMississippi, Montana and New Mexico—saw increased metropolitan tree cover, but all by “nonsignificant” amounts.

The study’s authors say these reductions translate into an annual loss of about $96 million in benefits—based on just the few that we know about and that are relatively easy to express in dollar terms, such as the capacity of trees to improve air quality, sequester carbon, and reduce summer energy costs by shading buildings and combating the urban heat island effect.

But large, old trees also feed and shelter birds and small mammal species. Many animals also use mature trees for reproduction sites, nesting, resting and for places from which to hunt or capture prey. For example:

Barred owls nest in old tree cavities and roost on branches. ©AdA Durden, flickr

• Some eagles, hawks and owls use snags or dead branches to get a clear view of potential prey when hunting. Similarly, certain birds that engage in catching flying insects directly out of the air use these perches to launch their aerial attacks.

• Many woodpeckers use dead trees to drill for food and to nest in cavities excavated in snags (or dead parts of living trees). Other birds use the branches of mature trees for nests; as trees grow older, their branches become large and begin to grow horizontally rather than vertically, creating attractive platforms for nest construction.

• Some mammals—including opossums, squirrels and raccoons—use dead trees as nesting sites.

• Snakes use logs to sun themselves in summer to help regulate their internal temperature. Logs also provide snakes with a place to hide, find a meal or (for some) hibernate for the winter.

• Salamanders use rotting logs or stumps as both shelters and food sources.

Without big, long-lived trees, these animals could die.

Woodpeckers have evolved to be adept at climbing trees and pecking for ants and larvae from within wood. ©Paul Stein, flickr

A recent study in Australia is another case in point. Researchers at the Australian Research Council Center of Excellence for Environmental Decisions surveyed 55 bird species across Canberra and found that a quarter of them were recorded only at large trees, which are typically hundreds of years old. In fact, around 300 of Australia’s vertebrate species, such as bats, ducks, opossums and owls, have evolved to use these cavities as exclusive places to roost or nest. Mature trees also support high concentrations of food for animals that feed on nectar, such as honeyeaters, or seed, such as parrots. Even ground-dwelling animals can benefit from rocks, litter and logs that accumulate under tree canopies.

The researchers say that smaller trees simply can’t support the habitat features provided only by large trees and which are required by these species to survive over the long term. Small trees have fewer dead branches and flowers, and less nectar, peeling bark and woody debris compared with large, established trees. And it can take more than 200 years for tree hollows to form naturally.

Lightening the loss

This catastrophic loss of trees is due to development to accommodate the expanding urban population, natural aging and death of trees, storms (Hurricane Katrina knocked out a third of the shade trees in New Orleans), insect damage (the emerald ash borer has killed virtually every ash tree in southeast Michigan), individual property owners converting forests to lawns or other uses, and fire.

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Salamanders can be spotted not only on forest floors but also in trees, often climbing as high as eight feet in order to find food.

In some cases, the loss of old trees in urban landscapes is largely due to practices that are driven by negative public attitudes. Many consider trees that drop limbs, crack footpaths, appear untidy, pose a fire risk or that occupy space that could be used for housing a dangerous nuisance.

To reverse the decline, native trees need to remain standing for much longer than they are currently being tolerated in urban areas, and more young trees need to be planted. Instead of cutting down large, old trees or removing logs, landscaping techniques—such as benches, footpaths, playgrounds and surrounding dead trees with logs, native shrubs and rocks—could be used to separate people from these so-called “risks” and ensure the retention of vital wildlife habitats.

Unfortunately, urban tree-planting programs—such as the “million-tree” campaigns taking place in many U.S. cities—have not kept up with the losses. Some of these initiatives have failed because of inadequate funding or fading interest. The American Bar Association, for instance, ended its nationwide million-tree program in 2016 with only 52,693 planted. Other organizers reached the million-dollar mark, but they took no interest in caring for the trees afterward. Planting new trees can be exciting, but watering them for five years only to have them grow a few inches is not. And, as we know, American culture prefers quick fixes.

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Continuing to have trees in our cities could determine what urban wildlife species will still exist in the next century.

It will take a lot more than a few cities with million-tree programs to replace the trees that get swallowed up by big-box stores, office buildings and parking lots. The good news is that we have plenty of room for more trees. New York City can support another 200,000 street trees, according to the city’s assistant commissioner for forestry, horticulture and natural resources. And a study of urban California estimated that it has 236 million vacant tree sites.

I like trees, and I’d like to see more of them in our cities—old, young, lying down or standing up. Because having a tree—or some part of one—outside every city doorway and window may just determine what urban wildlife we’ll still have in the next 100 years.

Here’s to finding your true places and natural habitats,

Candy

One Comment »

  1. Carla Maria Valente December 13, 2018 at 10:00 am - Reply

    Maybe some individuals think this will not affect them, or don’t acknowledge the impact to our environment that this is having for livelihood of daily living. Maybe some individuals don’t think about this at all.

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