Urban carbon: a look behind the research.

Atmospheric carbon dioxide levels have risen swiftly and steadily over the last several decades, due to a combination of increased fossil fuel burning and deforestation. The 1992 Rio Summit opened the world's eyes and ears to a growing concern about global climate change, which occurs when carbon dioxide gas traps heat in the atmosphere and raises temperatures worldwide. While scientists disagree on the ultimate effect on our climate, countries including the United States agreed to reduce these greenhouse gases. The U.S. is committed to voluntarily reducing greenhouse gases to 1990 levels by the year 2000.

The USDA Forest Service has conducted research on urban forests for more than 20 years, including trees' ability to store atmospheric carbon. Since the ecological study is complicated, several staff scientists selected areas in which they had expertise, and Chicago was chosen as the focus.

There is a direct link between the amount of fossil fuels used in heating and cooling buildings and the carbon emitted in that process. Thus, each scientist studied an interlocking piece of the energy/carbon puzzle. For example, David Nowak, who conducted his doctoral research dissertation on the structure and function of the urban forest in Oakland, California, focused on air quality, specifically trees' direct uptake and storage of carbon. Greg McPherson, who had conducted several Arizona studies on how trees save energy, considered Chicago's urban forest from an energy savings and avoidance (of building power plants) perspective. Gordon Heisler focused on trees and microclimate issues.

Carbon research from the Chicago Urban Forest Climate Project revealed that urban residential greenspaces can act as net carbon sinks. Forest Service scientists measured carbon storage and uptake from trees in a residential block in Chicago and recorded management practices - such as pruning, watering, and fertilizing - to determine net carbon gain or loss. The study reported that, as with rural forests, the largest proportion of carbon in urban forests is found in the soil - 78 percent compared to 63 percent in the average rural forest. Trees and shrubs accounted for 21 percent of total carbon storage and less than 1 percent of all carbon was found in herbaceous plants and grasses.

The researchers suspect that carbon is stored in urban trees in proportion to their rural counterparts: 79 percent in trunk and stems, 18 percent in roots, and 3 percent in foliage. Carbon found in the soil is stored primarily through root growth, fallen leaves, and mulch. On the output side, most of the carbon (79 percent) was found in decomposed woody material and litter, and 21 percent accounted for maintenance activities.

Nowak studied carbon release associated with different tree care practices. His work revealed that trees and shrubs released 15 percent of the carbon they sequestered through pruning. When comparing carbon inputs and outputs in a greenspace, trees and shrubs contribute nearly 70 percent of the trees' final carbon storage - thus they are a carbon sink.

Of course, once a tree or shrub dies and is removed, all its carbon will be released into the system. However, by maintaining a healthy urban forest, by prolonging a mature tree's life, and continually renewing the forest with new trees, communities can increase their net carbon storage over the long term. The Forest Service study estimates that, "if all available sites were planted, the total annual carbon uptake by trees could be increased by about 1.8 times the present sequestration amount."

Since half of all land use in cities is residential, homeowners can maximize an urban forest's carbon storage potential by changing their landscape design and maintenance practices as follows:

* Create or retain impervious surfaces, since soil and woody plants store the most carbon.

* Maintain species and age diversity to ensure a continuous forest canopy cover.

* Select trees that are adapted to the local conditions, so they will thrive with a minimum of maintenance.

* Compost tree litter and reapply as mulch, minimizing the need for fertilizer and irrigation, which release carbon.

* Reduce residential gas- and electric-powered maintenance tools such as mowers, chainsaws, and leaf blowers, all of which produce carbon.