Arboriculture & Urban Forestry 33(3): May 2007 223 Table 2. Oxygen production by urban forests in various cities and its relative effect compared with the city population. Trees (×1,000) City Atlanta, GA Total SE Oxygen prod.z (t/year × 1,000) Net total SE Net w/d total 9,415 749 112 8 86 Baltimore, MD 2,627 570 39 5 25 Boston, MA 1,183 109 25 2 19 Calgary, Alberta 11,889 2,777 52 7 45 Freehold, NJ 48 Jersey City, NJ Minneapolis, MN 979 165 22 3 11 Moorestown, NJ 136 583 Morgantown, WV 658 6 1 0 1 22 2 0 2 53 9 1 7 79 7 1 5 New York, NY 5,212 719 102 11 55 Philadelphia, PA 2,113 211 39 4 29 San Francisco, CA 668 Syracuse, NY 98 12 2 11 876 119 13 1 11 Toronto, Ontario 7,542 889 98 10 76 Washington, DC 1,928 224 39 4 31 Woodbridge, NJ 986 97 13 1 10 SE Oxygen offsety Net no. of people (% pop) Net w/d no. of people (% pop) Net no./ha Per person offsetx Net w/d no./ha 12 366,400 (88%) 280,300 (67%) 29 22 4 127,700 (20%) 82,500 (13%) 29 19 2 82,900 (14%) 60,400 (10%) 26 19 6 169,000 (19%) 147,700 (17%) 33 28 0 4,300 (39%) 0 7,000 (3%) 6 70,200 (18%) 36,200 (9%) 18 2,800 (25%) 25 16 5,100 (2%) 16 11 9 1 29,700 (160%) 22,300 (120%) 28 21 1 22,800 (85%) 17,800 (66%) 29 22 12 333,600 (4%) 180,500 (2%) 20 11 4 127,200 (8%) 1 40,200 (5%) 93,000 (6%) 24 17 36,900 (5%) 28 26 1 42,800 (29%) 37,200 (25%) 29 25 10 318,300 (13%) 246,500 (10%) 25 19 3 127,400 (22%) 101,400 (18%) 28 22 1 43,900 (44%) 32,000 (32%) 25 18 Net no. of trees 26 21 14 70 11 19 14 20 29 16 17 17 20 24 15 22 Net w/d no. of trees 34 32 20 81 17 27 27 26 37 29 23 18 24 31 19 31 zAnnual oxygen production by the urban forest (thousands of metric tons per year). Multiply by 1.102 to convert to tons. yNumber of people and percent of city population that urban forest oxygen production offsets in terms of average adult human oxygen consumption per year. xAverage number of people whose oxygen consumption is offset by oxygen production per hectare of tree cover in the city (no./ha; divide by 2.471 to convert to no./ac) and average number of trees needed in city to offset the oxygen consumption of one adult human (no. of trees). Netnet oxygen production of population without consideration of decomposition; Net w/dnet oxygen production of population considering decomposition; net production minus estimated oxygen consumed resulting from decomposition; SE standard error. is an acre of trees (100% tree canopy) can provide enough oxygen for 18 people (e.g., American Forests 2006; Tree- People 2006), but based on this study, this estimate appears to be high by at least a factor of two. The number is more on the order of eight people per acre of tree cover (100% tree canopy). Oxygen production per acre of tree cover will vary based on tree density, diameter distribution, and tree health and growth. Oxygen production is one of many environmental benefits that trees produce, and urban trees can produce a significant amount of oxygen. However, is this oxygen production ac- tually creating a significant environmental benefit in com- parison with other environmental benefits of trees such as carbon sequestration and air pollution removal? In the cote- rminous United States, annual carbon sequestration by urban forests is estimated at 22.8 million metric tons (25.1 million tons) with a societal value of ≈$460 million per year (Nowak and Crane 2002). Air pollution removal in the coterminous United States is estimated at 711,000 metric tons (784,000 tons) with a $3.8 billion annual value (Nowak et al. 2006a). Oxygen production by U.S. urban forests is estimated at 61 million metric tons (67 million tons), but the value to society is negligible. The reason the oxygen production value of urban trees is insignificant has to do with the large amount of oxygen within the atmosphere (approximately 21% of the atmo- sphere’s volume is oxygen). As stated by Miller (1979): “We have a large number of serious ecological problems, but suf- focation from lack of oxygen is not one of them (Broecker 1970; SCEP 1970). The oxygen content of the atmosphere remains essentially constant with the oxygen consumed by all animals, bacteria, and respiration processes roughly bal- anced by the oxygen released by land and sea plants during photosynthesis. The present atmospheric oxygen content seems not to have changed since 1910 (SCEP 1970). Further- more, because air is about 20 percent oxygen, the total supply is immense (Broecker 1970).” Our atmosphere has such an enormous reserve of oxygen that even if all fossil fuel re- serves, all trees, and all organic matter in soils were burned, atmospheric oxygen would only drop a few percent (Broecker 1996). Also, waters of the world are the main oxygen gen- erators of the biosphere; their algae are estimated to replace ≈90% of all oxygen used (Encyclopaedia Britannica 1994). Thus, although urban trees do produce significant amounts of oxygen, it is not a significant ecologic benefit given the glob- al nature of oxygen and the sheer volume of oxygen in the atmosphere. A growing forest will remove carbon dioxide and produce oxygen. Conversely, a decaying or declining forest will release carbon dioxide and consume oxygen through ©2007 International Society of Arboriculture
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