The Carbon Quandary: Essential and Detrimental - Sustainable Design: The Science of Sustainability and Green Engineering - page 288

of weeds. For example, planting leguminous cover crops under sycamore
trees ( Platanus occidentalis ) in plantations during the second growing season has
been shown to increase tree growth during subsequent year. * This particular
study indicates that the use of cover crops during the establishment phase is
a workable alternative for SRWC production. The researchers have indicated
that more study is needed to see which cover plant best reduces erosion in
SRWC plantations while causing the least growth reduction.
The tree is also part of the two most important biogeochemical processes on
earth: ion exchange and photosynthesis (see the discussion box: Photosynthesis:
Nature's Green Chemistry). Ion exchange is actually an example of sorption:
that is, movement of a chemical species from the liquid or gas phase to the
solid phase. (Movement of a chemical species from the solid to, liquid phase is
dissolution . Movement from the solid phase to the gas phase is volatilization .) So
the tree grows and thrives as a function of available nutrients and other cycles
within the forest ecosystem. But it can also be part of systems other than a
forest, such as your yard. As in the forest, the tree is part of a complex balance
among grass, shrubs, annuals, and compost and other decomposing materials
in the soil.
* S. G. Haines, L. W. Haines, and G. White, “Leguminous plants increase sycamore growth in
northern Alabama,” Soil Science Society of America Journal , 42; 130-132, 1978.
“GREEN” TREES
The tree is a central feature of green design. For example, the choice of wood
as a material affects the sustainability of a structure. Standards such as LEED
recognize that the life-cycle costs for local genera are preferable to distant
species since trees are heavy and expensive to ship. Also, certain species are
rapid growers and replenish the biomass much faster than do others. Bamboo
is an example of a quick-growing, easily harvested genus. Decisions about
trees must also consider greenhouse gas balances. Trees' extensive root systems
account for most of the biomass of many tree species. Many coniferous trees
(e.g., pines) cannot survive if they are cut too far down the trunk, whereas
many deciduous trees will grow back readily after top-harvesting. So, for
example, a maple stand may be harvested repeatedly for wood, whereas pines
must be replanted.
“Active” approaches include the application of technologies to send carbon
to the sinks, including deep rock formations and the oceans. Such technology
can be applied directly to sources. For example, fires from China's coal mines
presently release about 1 billion metric tons of CO 2 to the atmosphere every