Today, stories of ecosystem devastation and industrial excesses bring the carbon crisis into troubling focus.  It’s easy to despair about balancing even our own “carbon footprint”.  Where to begin?  Do trees really absorb greenhouse gases, and if so, are all trees created equal?  How do I plant a tree that will thrive and maximize its contribution?

What is carbon sequestration?

The element known as carbon is essential for all plant and animal life on earth.   The total amount of carbon on the planet is constant, but it moves around and changes form with relative ease.  Burning of fossil fuels, which have stored huge amounts of carbon below the earth’s surface for millennia, converts the carbon to carbon dioxide (CO2).  In the atmosphere, CO2 absorbs and emits infrared radiation, contributing to planetary warming.  The current level of CO2 is thought to be the highest in 20 million years, and scientists are working on solutions to capture and safely contain atmospheric carbon.  One approach, “terrestrial sequestration”, involves the simple planting of trees.  A tree absorbs carbon during photosynthesis and stores it in the wood for the life of the tree.  The massive trunk of an ancient oak or redwood represents many tons of sequestered carbon.

Which trees should I plant?

Studies have identified several optimal tree species for carbon storage, and botanists continue to experiment with new hybrids.  Surprisingly, we should avoid trees such as the willow, which store comparably little carbon and emit more harmful volatile organic compounds.  When choosing trees to plant, consider:

• Fast growing trees store the most carbon during their first decades, often a tree’s most productive period.
• Long-lived trees can keep carbon stored for generations without releasing it in decomposition.
• Large leaves and wide crowns enable maximum photosynthesis.
• Native species will thrive in your soil and best support local wildlife.
• Low-maintenance, disease-resistant species will do better without greenhouse-gas-producing fertilizers and equipment.

Consider these reliable and versatile star-performers.  The “best trees” vary by region, so look around local parks to see what’s hardy in your climate zone.

1.   Yellow Poplar (or Tulip Tree), the top carbon-storer in one New York City study, works hard under rough conditions.

2.  Silver Maple can trap nearly 25,000 pounds of CO2 in a 55 year period, according to the Center for Urban Forests.

3.  Oak (White Oak, Willow Oak, Laurel Oak and Scarlet Oak) has adapted to thrive in many climates, provides food and shelter to wildlife.

4.  Horse Chestnut grows well in cities; its domed top provides exceptional shade which offers passive cooling benefits.

5.  Red Mulberry provides the added benefit of delicious seasonal fruit. 

6.  London Plane is an excellent choice for urban planning, very tolerant of pollution and root-cramping, resistant to cold and disease.

7.  American Sweetgum has brilliant fall colors, is large and long-lived. In the north, consider American Linden instead.

8. Dogwood offers lovely seasonal flowers; this and other particularly dense trees like Black Walnut can store more carbon in a smaller tree.

9.  Blue Spruce, widely introduced as an ornamental, thrives in most northern regions; in the Pacific Northwest, Douglas Fir also excels.

10. Pines (White, Red, Ponderosa and Hispaniola) are the most carbon-effective conifer; find out which is right for your zone.

Where trees are most needed?

Cities and suburbs.
In urban “heat islands,” vast stretches of asphalt magnify and reflect sun, sending CO2 directly skyward and creating “dead zones” below.  A tree forms an oasis of shade, provides wildlife habitat, and improves air quality.  Adding street trees can actually lower summer temperatures through evaporative cooling.

Clear cuts.
While sustainable logging may be necessary to support human systems, large-scale clear cuts represent an environmental disaster.  The forest floor sequesters enormous amounts of carbon, accumulated over centuries by natural decay.  Soil disruption and erosion caused by poor logging practices cause tremendous carbon release.  Prompt replanting can help.

Tropical regions.
Carbon-offset studies have shown that the perpetual growing season and fertile soil of equatorial regions enables trees to multiply their carbon storage capacity.  Forests in some of these regions have been devastated by industrial development and are in critical need of healing.

Your back yard or neighborhood.
Build a grass-roots movement by raising awareness of the urgency of carbon storage.  Small local efforts create positive ripples and, yes, actual carbon benefits.  You can spread the word by involving some neighborhood kids, a group of coworkers, or friends.  They will walk away with practical know-how and pass it on.

Some practical advice when planting:

Allow plenty of room.  Find out how large your tree will be at maturity, and plan accordingly.  Keep in mind that roots can spread even wider than branch span, and some disrupt sidewalks or even house foundations.  Call local utility companies to locate underground pipes and wires, or you may be liable for damages.

Optimize energy benefits.  Trees planted to the south, east, or west of a building can actually reduce energy consumption for air-conditioning in summer.  Evergreens planted along the north side create a wind-break in winter which can lower heating fuel consumption.

Minimize fertilizers.  Many of the best carbon-offset trees flourish in marginal soil.  Commercial fertilizers placed directly in the planting hole can injure roots, and release damaging nitrous-oxide which may cancel out the carbon benefits of the tree.  The best low-impact soil amendment is your own kitchen compost.

When to plant?  Try to transplant in early spring, before leaf buds begin to form, especially with any tree that comes “bare root” (with no protective ball of soil).  In temperate climates, plant metabolism slows almost to a standstill in winter, and the dormant tree is less prone to transplant shock.  Nursery trees with plastic or burlap-wrapped “root balls” can be planted in other seasons with caution, but summer planting requires more intensive watering which reduces the overall environmental benefit.

Ready to dig?  Mark your chosen site with a circle about three times the diameter of the tree’s rootball.  Don’t dig too deep: roots need oxygen from the surface.  In high-clay soil, make your hole at least two inches shallower than your rootball.  In sandy soil, dig to the same depth as the rootball.  Gently separate any compressed or “root-bound” tendrils.  If using compost or another amendment, mix it with some of the soil from the hole before back-filling around the tree.  Water gently, preferably using a drip system or trickling hose, and let the water settle the soil rather than compressing it manually.  Mulch with fallen leaves or grass clippings to conserve water and soil nutrients.

Take responsibility for your tree.  Feel the soil periodically to a depth of 2 inches: it should be moist, but not soggy. For its first three seasons, the tree is vulnerable and may need watering if rainfall is less than one inch per week.  Overwatering can “drown” the roots.  Choose a location you can access without burning gas (walk, bike, or public transit), and use hand tools, such as a rake or handsaw over a leaf-blower or chainsaw.  And finally, give some thought to what will happen when your tree’s life is over.  Use the wood if you can.  Carpentry can prolong its carbon storage indefinitely; burning the wood for home heating reduces use of fossil fuels.

Alternative Options

Find a program to participate in.  Many US cities now have urban tree planting initiatives.  In San Francisco, for example, Friends of the Urban Forest holds neighborhood tree planting events, and helps residents find the “greenest” solutions for their own streets.  Try a web search to find out if your city has a similar program, or call your local Parks & Recreation office.

 No room to plant?  Donate for worldwide reforestation.  If you’d like to support a non-profit to plant trees in critical areas, the choice can be confusing.  Those nonprofits that keep overhead low, commit to maintaining their trees, and engage the local people in the work and benefits of the plantings are more likely to create lasting carbon storage.  Two with modest administrative costs and sustainable practices are Trees for the Future, which trains indigenous people to steward ecological plantings, and Plant-It 2020, which guarantees that their trees will not be logged in the future.