This tree in a plastic wrapping is being used to collect and measure gases—oxygen, carbon dioxide, and water vapor. This experiment is to measure water loss to develop a model for irrigating apple trees in New York State.
In the ongoing debate about carbon footprints and global warming and how human activities contribute to or mitigate it, how might fruit growers fare? Are they heroes or villains?
It is known that carbon dioxide is generated through the use of tractors and equipment, but few estimates are available about the trees and cover crops in our orchards, said Dr. Alan Lakso, a fruit physiologist at Cornell University. He has devoted his career to studying and modeling how apple trees take up and use carbon dioxide—the key greenhouse gas involved in global warming. He has made some estimates and came to the following conclusion:
“The environmental footprint of an apple orchard is both positive and negative. Some CO2 is released to the atmosphere in the production of the crop (making and using fuel, fertilizers, and pesticides), but we estimate that an acre of orchard fixes about 20 tons of CO2 from the air each season, releases 15 tons of oxygen, and provides over 5 billion BTUs of cooling power. In addition, some carbon is sequestered in the new wood and roots of the trees.”
Lakso published this assessment in the New York Horticultural Society’s Fruit Quarterly last spring (available at www.nyshs.org/fq.php), along with an explanation of the math involved. He used weather and sunlight data for western New York, but said the results were quite similar for all locations across New York and would be similar for most of the Northeast and Great Lakes regions.
In the process of living and growing, apple trees take up carbon dioxide and release oxygen during photosynthesis, and in the process, they release moisture and evaporatively cool the air.
He estimates the cooling effects of the trees and the grass in the alleys at about 5 billion BTUs per acre per year, equivalent to about 125 window-size air conditioners running day and night for six months. The trees and grass take up about 20 tons of carbon dioxide and release 15 tons of oxygen.
The uptake and release of oxygen and carbon dioxide are part of a natural cycle of things, so the apple crop only traps and holds (sequesters) carbon for a brief period—until fruit is eaten or wood and leaves are burned or rot, Lakso said. Roots, leaves, and wood tie up carbon for longer periods than does fruit, but often more than half the carbon is put into fruit in modern, productive orchards. The carbon held in wood can be lost if the stumps are burned when the orchard is grubbed, he said.
Methods for a more complete accounting of the carbon footprint for orchards are being developed, he said.
Fossil fuels, which are not part of the usual carbon cycle, contribute carbon dioxide and heat to the atmosphere. A barrel of oil is mostly carbon and weighs about 300 pounds. Burning 300 pounds of carbon removes 800 pounds of oxygen from the atmosphere and releases 1,100 pounds of carbon dioxide. The combustion releases nearly six million BTUs of energy.