Trees are Air Purifiers and Carbon Bankers

By DAVID BROWN/ecoRI News contributor

Trees are often seen as part of the answer for improving the environment and slowing climate change. But can such benefits add up to much in semi-urban settings such as Newport’s?

Perhaps. But much depends on how well we and our younger generations manage our “urban forests.”

Most of us know something about how trees grow. Their leaves utilize sunlight and absorb carbon dioxide from the air. Photosynthesis produces liquid sugars that go down into the root system to combine with soil water and minerals. Some of this goes back up the tree to produce more leaves, woody tissue, flowers and seeds or nuts. Some is stored as starch in the roots themselves, to be used during the winter or when the tree is stressed. The process of respiration converts these starches back into usable sugars, and also releases some carbon dioxide back into the air.

The pace of photosynthesis, carbon dioxide removal and carbon banking is affected by a tree’s age, health and species. A newly planted tree puts most of its energy into trunk, branch and leaf growth. A well-established tree is at its peak of removing carbon dioxide from the air and sequestering surplus carbon as carbohydrates in its woody tissues. An older tree that has begun to decline will capture and sequester carbon at a slower rate.

The life process of a really old tree or one that has been badly treated or damaged will start to shut down, decay may set in and carbon gas may go back into the atmosphere.

Soil conditions also are important. Studies are finding that much carbon banking takes place below ground in healthy roots and nearby organic material. The need for good soil structure, proper acidity (pH), minerals, water and air in the soil has long been known.

Soil organisms are now seen as important too. Some can become pathogens, but most are helpful. Earthworms and other invertebrates open passageways, and digest and stir materials. Fungi have tiny filaments that attach themselves to root hairs and enable the tree to absorb more of the nutrients and moisture that are near its roots. In return, the tree provides the fungi with some of its nutrient flows. Subsurface bacteria, algae, protozoa and nematodes have mutual relationships that help to process organic compounds and inorganic elements into forms that the tree can use.

Climate also affects the vigor of a tree’s growth and carbon storage processes. In general, these are most active when it is warm, sunny and moist. But a lot depends on tree species. Some species can grow in cold places. Others need tropical settings. Many tree species, like those that are common in Rhode Island, thrive in the non-arid mid-latitudes. Small changes in climate can affect trees’ vigor.

Given the long lives that we hope our trees will have, looking ahead to the future can be important when deciding which species to plant now.

What about the carbon-banking benefits of specific trees near our own homes and workplaces? That can be estimated with a web tool called the National Tree Benefit Calculator. The calculator also estimates other tree benefits such as air quality, energy, stormwater and property value. It’s being applied to community tree inventories and other official uses.

For instance, suppose you live in Newport next to a large European beech tree that helps to shade your building. Plug in European beech, 45-inch diameter and residential. The tree calculator estimates that this beech will reduce atmospheric carbon by 695 pounds. But it shows the tree sequestering little or nothing any more to the carbon already stored in its branches, trunk and roots. The energy used to keep this big old tree going — respiration — puts just about as much carbon dioxide back into the air as its photosynthesis removes.

The calculator shows carbon benefits of this mature beech to come mainly from indirect effects — CO2 emissions avoided. Foresters have found that the shade from a large tree like this can greatly help reduce heating and air conditioning needs of nearby residences. This in turn reduces CO2 emissions from the power sources of that heating and cooling.

You also can use the calculator to guesstimate what the effects would be of having a more youthful beech in the same situation, say with a 25-inch trunk. It is judged to reduce more total atmospheric carbon annually — 854 pounds — and would provide somewhat less shade than the older beech. But a beech this size would be near the peak of its ability to sequester carbon.

An even younger beech, say with a 5-inch diameter, would provide little shade and sequester little carbon; most of its energy would be going into growth.

The tree calculator is based on detailed study results for a sample of cities and adapted to traits of major U.S. regions. When considering how long to keep an old tree and which species to replace it with, environmental impacts may not be the only key consideration.

For example, it seems important to sustain the unique vistas provided by European beeches and other magnificent species along Bellevue Avenue and other streets that are part of the new series of “Newport Arboretum” tree walks. Other considerations may well be proneness to damage from storms and serious tree pests.

The calculator provides a good starting point for assessing tree retention, replacement and location options.

Newport, like many Rhode Island cities and towns, doesn’t have an up-to-date tree inventory for both public and private holdings. But satellite imaging, digitalized overlay maps and census data can be meshed with local information to provide a general picture of our trees as a land-cover component.

The 2008 USDA Forest Service publication, “Urban and Community Forests of New England,” by David Nowak and Eric Greenfield, provides information of this kind. It shows, for example, how the benefits of urban trees in Rhode Island can really add up:

In 2000, 37 percent of the state’s land was classified as urban and 91 percent of Rhode Islanders lived in those urban areas.

Thirty-four percent of that urban land had tree canopy cover — that’s like each urban Rhode Islander benefiting from a tree canopy nearly 70 feet in diameter.

The report also estimated that these urban Rhode Island trees had stored about $68.4 million worth of carbon and were removing about $19.6 million of air pollutants a year.

Trees in Rhode Island can be important in reducing, or at least delaying, emissions of carbon and other unwanted gasses — especially if they are placed to help shade buildings, and we use “greener” tree and soil-care practices, and don’t let old leaves and woody materials go to waste.

This won’t replace steps we can take to reduce emissions from fossil fuels — less car gasoline use, more energy-efficient devices, better home insulation, etc. But we have plenty of spaces for new trees, and need to replace those in serious old-age decline.