Sustainable+Landscape+Construction

When plant cover is removed, or its density is reduced, several things occur, all trending toward warming.

• No longer shaded by vegetation, soil bakes in direct sun, holding enough extra heat to raise local temperatures.

• Heated soils kill carbon-storing micro-organisms and speed decomposition of organic matter, releasing CO2.

• With less vegetation to protect it from rain, runoff, and wind, exposed soil erodes; this further releases organic matter and emits CO2.

• Heating and erosion of soil kills more plants, leading to more heating and erosion in a vicious cycle.

• If removed plants are burned or eaten, CO2 stored in them is released.

Loss of soil and vegetative cover is well known to historians under a different name: deforestation. Many of the world’s deserts are the direct result of human deforestation practices. Land clearance, for whatever purpose, almost always tends to increase hot-season temperatures, drought, and wildfire.

What is less commonly understood is that landscapes with sparse vegetation and dead or dying soils are also typically colder and windier in winter, less capable of infiltrating precipitation and more prone to intense runoff and flooding. In short, removal of any significant percentage of vegetative cover from a large area, or from many small areas cumulatively, contributes to the extremes of heat and cold, drought, and flooding that are part of global climate change.

Is Construction to Blame?

Construction almost always involves some land clearance. This is nearly unavoidable. In some regions, cleared areas regrow rapidly if left alone. Most projects, however, create impervious surfaces, from which vegetation and soil are permanently excluded.

Even when a cleared landscape is replanted, this usually reduces the density and biodiversity of vegetative cover. As the Intergovernmental Panel on Climate Change states, “Conversion of natural ecosystems to croplands and pastures has resulted in. . . agro-ecosystems [that] continue to take up carbon, but at levels generally inferior to the previously forested ecosystems.” Ornamental landscapes are clearly agro-ecosystems in this sense and do not replace the CO2-uptake of established regional vegetation. Most plantings also provide less shade, soil stabilization, and runoff prevention than mature forest cover.

Agriculture has been the main reason for land clearance historically, and remains so in developing countries today. In the tropics, 500,000 trees are cut every hour, primarily for forestry and new agriculture. In industrialized countries like the United States and Europe, however, clearance for buildings, infrastructure, and landscapes may be outpacing new agricultural clearance. The cumulative effect of clearing 1.39 million sites (a low estimate of annual new US housing starts) is directly linked to global problems. This puts landscape professionals and landuse planners in a position of serious influence and responsibility.

Estimating the extent of US land clearance is not easy. Something like 500,000 to 1.5 million acres are probably cleared per year; 3 million acres are “lost to agriculture” annually. The smallest Figure equals an area half the size of Rhode Island. A great deal of this cleared land remains as “landscape” of some sort; many architectural or engineering structures are surrounded by landscapes covering three to five times the area of the facility itself. Thus, what landscape professionals do about clearance, revegetation, soil protection, paving, and water management cumulatively influences huge areas. Areas, in fact, that are more than large enough to affect climate.

What the Landscape Professions Can Do

This is good news and bad news. The bad news is that landscape business-as-usual contributes significantly to what many believe is humanity’s single greatest challenge. The good news is that the strategies advocated in this book offer practical contributions toward reversing climate change if we act now.

One hundred trees can remove five tons of CO2 and half a ton of other pollutants from the air each year. The same hundred trees will also capture 250,000 gallons (or 61/4 acre-feet) of stormwater per year in temperate climates. Those one hundred trees, carefully located for shade, would cut airconditioning usage in half for thirty-three houses (three trees per house). These effects have direct impact on climate locally and globally.

Among the things landscape professionals can do:

• Collaborate with architects to achieve Architecture 2030’s fossil-fuel-reduction goals for buildings; many landscape measures contribute directly.

• Avoid unnecessary vegetation clearing (using methods discussed in Principle 1)

• Lobby against “preclearing” of real estate prior to sale. (See p. 39.)

• Aim for canopy cover and density similar to regional plant communities, both in “restoration” projects and in planting design. (See Principles 2 and 3.)

• Find better methods of wildfire protection; especially, resist land clearance wrongly promoted as fire prevention. (See p. 106.)

• Use greenwalls and greenroofs to reinstate partial vegetative cover on structures. (See p. 118 and 125.)

• Manage stormwater with vegetation, infiltrating it to benefit soils and plants. (See Principle 4.)

• Minimize paving to avoid soil and vegetation loss through erosion. (See Principle 5.)

• Cut down fossil-fuel use for transportation of materials and workers and for construction machinery. (See Principle 7.) Bio-based fuels reduce (but do not eliminate) CO2 from combustion.

• Learn about “carbon sequestration,” by which CO2 is locked up in trees, wood, and other materials.

• Don’t buy the desperate or silly “solutions” proposed by industrial eccentrics. These have included giant mirrors in space, aerial spreading of tinfoil confetti, and even deliberately increasing opaque air pollutants, all to cut sunlight. The unintended consequences of such actions would almost certainly worsen climate problems.

Sequestration (discussed above) may well become the main economic reason for protecting and planting trees, surpassing even timber production and providing unheard-of funding for planted landscapes. Sequestration also gives wood construction a new justification: keeping carbon out of circulation until the wood rots or burns.

A Forest Service research center specializing in urban trees is testing which species sequester CO2 most effectively. Regional variation and age of trees are critical, but the following trees were found highly effective in a 2002 study: horse chestnut, black walnut, sweet gum, bald cypress, Douglas fir, and London plane; scarlet, red, and Virginia live oaks; and ponderosa, red, Hispaniola, and white pines.

Carbon sequestration is also the basis for “carbon trading” schemes, such as the Chicago Climate Exchange. In theory, polluters in rich countries fund sustainable developments in poor countries through these trades. There is considerable controversy over this concept, with charges of conflict of interest, falsified reports, and lack of oversight. Other “mitigation banking” schemes—for example, wetlands banking—have had poor results. Pollution and cleanup affect specific places—can they be made portable? Under carbon-trading procedures, a Texas 18 Sustainable Landscape Construction coal-fired plant whose pollutant output was obscuring the Grand Canyon could buy carbon credits from a forest in India. Although this would positively affect global carbon levels, it simply excuses rather than helping the pollution problem at the Grand Canyon.

Until recently, only large brokerages and corporations could trade carbon futures. Individuals and small firms, however, now use a growing number of trading services. These allow a person or firm to buy enough carbon credits to offset their car or truck’s annual output (about 5.5 metric tons worth) for around $50, or a house’s worth (23 metric tons) for $99.54 Many committed environmentalists do so. We question whether this is merely paying for convenient absolution. Surely fixing the car or house, or one’s own behavior, to generate less actual pollutants is more important than shuffling paper credits for them.

Sustainable Landscape Construction, Second Edition Green Building Reader