Hardwoods better than conifers at sequestering carbon dioxide

Posted Dec. 25, 2009, at 4:06 p.m.

Planting trees, one at a time, is the most significant act one person or group of people can perform to mitigate global climate disruption. A tree planted today will absorb carbon dioxide from the atmosphere and sequester it in the wood of branches and trunk for decades, until it dies.

Burned or allowed to rot, a dead tree returns its carbon to the atmosphere, making a single tree’s life a carbon-neutral event. To reduce atmospheric carbon dioxide levels, we must plant many more trees each year than we cut down, since the annual carbon sequestration of an old oak can be matched only by several newly planted oaks.

Recent events have shifted our focus to the economic value of living trees and, it is hoped, put the brakes on the worldwide deforestation accountable for 20 percent of annual global carbon emissions. Now, we must plant millions of trees to replace what has been lost.

What tree species should we plant?

According to the U.S. Forest Service, large-diameter, long-lived, leafy species are twice as effective at sequestering carbon than conifers. Atlanta’s urban hardwood forest sequesters more than twice as much carbon (46,345 tons each year) as Calgary, Alberta’s, conifer forest (21,422 tons annually).

The message: Plant native hardwoods.

By focusing on native species, we will create community forests that not only sequester carbon but also nourish wildlife. Native tree species provide food for native insects and thus food for birds. Plantings of native trees also provide corridors of safe passage for wildlife.

Take a look at what native tree species are growing in nearby natural areas and “consult the genius of the place,” as Alexander Pope advised the gardener. Keep in mind, however, that the future planting site may have been altered substantially from the natural.

Also, examine what your neighbors have planted and, if at all possible, plant something else. Tree species diversity will determine the overall biodiversity of the community forest.

In general, I would lean toward those species with proven tolerance to the environmental stresses of modern communities, including the heat stress that accompanies a planting site surrounded by asphalt and concrete, salt stress from deicing salts, drought stress, and soil compaction stress associated with heavy equipment operation and frequent foot traffic. There are, surprisingly, several trees that can handle life in the “urban” landscape.

I mentioned white ash, Fraxinus americana, in last week’s column. Several trees of this species grace the landscape of the University of Maine in Orono, in spite of the recent droughty summers and the cow paths that pass through and around them, rewarding campus residents with the unique beauty of their autumn foliage.

I often recommended yellow birch, Betula alleghaniensis, an adaptable species capable of growing in damp and dry sites, for its brilliant gold fall foliage and its shredding bark. Two broad-spreading trees grow in Marjorie’s garden where the glow of honey-colored trunks warms my spirit on a sunny winter day. From spring through fall, the high branches are filled with flitting chickadees feeding on insects.

Although not quite native to Maine, the river birch, B. nigra, is the most stress-tolerant of all birches.

Valued for its salmon-colored exfoliating bark, the cultivar ‘Heritage’ grows splendidly in parking lot islands where few other trees could thrive.

Plant red oaks, Quercus rubra, in drier landscape sites; red maples, Acer rubrum, in wetter sites. Learn to identify our native trees so that you can consult the genius of the place where you live and garden.

When you run out of tree sites in your own garden, plant a tree at a school, church or community park. Dedicate every tree that you plant to your grandchildren, and the generations that follow.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@shead.org. Include name, address and telephone number.

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