by A Morani, DJ Nowak, S Hirabayashi, C Calfapietra
Environmental Pollution 159:1040-10472011
Tree planting programs, such as MillionTreesNYC, often plant trees with the primary goal of improving air quality. However, because the majority of urban emissions are generated at the ground level via automobiles, increasing canopy cover in populated areas could potentially trap these emissions near the ground and increase pollutant concentrations. This paper assesses the design of tree plantings near human populations while considering the negative attributes of trees in relation to air quality. Dispersion modeling techniques were utilized along with data on human population density, tree cover, and pollution concentration to determine the best locations to enhance pollution removal in New York City. The results showed that an increased tree mortality rate from 4% to 8% per year produces a significant decrease in the total pollution removed, from 11,000 tons to 3,000 tons over a 100-year period. The authors also found that cumulative carbon storage in planted trees will gradually increase through time and then begin to decline as loss of carbon from tree mortality exceeds the carbon gained through the growth of existing trees. Based on the results, planting campaigns can find greater success when enhancing growth rates, reducing mortality rates, and refining species selection. Consequently, the benefits produced from the trees can extend for years far beyond the length of the MillionTreesNYC initiative.