222 Nowak et al.: Oxygen Production by Urban Trees are often burned or mulched); therefore, they will most likely release their carbon relatively soon after removal. If dead trees are not removed annually, they have an in- creased probability of being measured in the tree sample, and decomposition rates must reflect this difference. All trees on vacant, transportation, and agriculture land uses, and 50% of trees in parks, were assumed to be left standing (i.e., not removed) because these trees are likely within forest stands and/or away from intensively maintained sites. These trees were assumed to decompose over a period of 20 years. Data on tree decomposition rates are limited. However, using de- composition rates from 10 to 50 years had little effect on overall net decomposition within a single year. Trees on all other land uses were assumed to be removed within 1 year of tree death. For removed trees, aboveground biomass was as- sumed to be mulched with a decomposition rate of 3 years; below-ground biomass was assumed to decompose in 20 years. Although no mulch decomposition studies could be found, studies on decomposition reveal that 37% to 56% of carbon in tree roots and 48% to 67% of carbon in twigs is released within the first 3 years (Scheu and Schauermann 1994). Estimates of carbon emissions resulting from decomposi- tion were based on the probability of the tree dying within the next year and the probability of the tree being removed using the formula: Emission = C ×Mc ×∑pi((Dremove)+(Dstand)) Dremove =(pabyi)(1dm)+((1−pab)yi)(1dr) Dstand = ((yi−1)yi)(1dr) where emission individual tree contribution to carbon emissions; Ccarbon storage in the next year; Mcprob- ability of mortality based on condition class; i decompo- sition class (based on number of years left standing before removal); pi proportion of the land use tree population in decomposition class i; pab proportion of tree biomass aboveground; yi number of years left standing before re- moval (yi → for dead trees that will never be cut down (natural decomposition)); dm decomposition rates for mulched aboveground biomass (3 years); and dr decom- position rate for standing trees and tree roots (20 years). Individual tree estimates of mortality probability and de- composition rates were aggregated upward to yield total es- timates of decomposition for the tree population. The amount of carbon sequestered as a result of tree growth was reduced by the amount lost resulting from tree mortality to estimate a net carbon sequestration rate that accounts for carbon loss resulting from decomposition. Human Oxygen Consumption An average adult human oxygen consumption rate of 0.84 kg/day (1.85 lb/day) (Perry and LeVan c. 2003) was used to estimate how much human oxygen consumption would be ©2007 International Society of Arboriculture offset by urban forest oxygen production annually. To esti- mate how much human oxygen consumption would be offset, oxygen production was divided by average annual oxygen consumption per person. RESULTS Net annual oxygen production by urban forests (after ac- counting for decomposition) in selected cities ranged from 1,000 metric tons (1,100 tons) in Freehold, New Jersey, U.S. to 86,000 metric tons (94,800 tons) in Atlanta, Georgia (Table 2). This net oxygen production offsets oxygen con- sumption from between 2% of the human population in Jer- sey City, New Jersey, and New York, New York, to greater than 100% in Moorestown, New Jersey. Mean net annual oxygen production (after accounting for decomposition) per hectare of trees (100% tree canopy) offsets oxygen consump- tion of 19 people per year (eight people per acre of tree cover), but ranges from nine people per hectare of canopy cover (four people/ac cover) in Minneapolis, Minnesota, to 28 people/ha cover (12 people/ac cover) in Calgary, Alberta. The average number of trees needed to offset the annual oxygen consumption of one adult was 30 trees (net oxygen production after accounting for decomposition) but ranged from 17 trees in Freehold, New Jersey, to 81 trees in Calgary, Alberta. This difference is a reflection of different tree sizes, conditions, and growth rates among these cities. Tree oxygen production varies by tree size. Based on data from Minneapolis, Minnesota (Nowak et al. 2006b), trees 1–3 dbh produced ≈2.9 kg O2/year (6.4 lb O2/year); trees 9–12 dbh: 22.6 kg O2/year (49.9 lb O2/year); 18–21 dbh: 45.6 kg O2/year (100.5 lb O2/year); 27–30 dbh: 91.1 kg O2/year (200.8 lb O2/year); and greater than 30 dbh: 110.3 kg O2/year (243.2 lb O2/year). Based on the national estimate of net carbon sequestration in the coterminous United States of 22.8 million metric tonsC/year (25.1 million tonsC/year) (Nowak and Crane 2002), urban forests in the United States produce ≈61 million metric tons (67 million tons) of oxygen annually, which is enough oxygen to offset human oxygen consumption for ap- proximately two-thirds of the U.S. population. DISCUSSION Oxygen production by trees varies among cities based on differences in number of healthy trees, growth rates, and di- ameter distributions. Cities with mostly small trees would require more trees on average to offset the oxygen consump- tion of one person. Percent of the population’s oxygen con- sumption offset by urban forests varies depending on popu- lation density and total oxygen production. Cities with high human population density (e.g., Jersey City and New York) tend to have the lowest proportion of their oxygen consump- tion offset by their urban forest. 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