An important function of trees and forests both in and out of urban areas is carbon sequestration. Nowak and Crane (2002) have estimated that urban trees in the U.S. hold about 774 million tons of carbon; this is approximately 2.84 billion tons of CO2e (equivalent). Forest product research has demonstrated that over their usable lives wood products continue to store carbon (C), reducing the build-up of atmospheric CO2. This area of research has focused on products manufactured from hardwood and softwood trees harvested only from rural timberlands. There has been no comparable research on the sequestration potential of wood products made from urban trees.
The purpose of this report is to estimate within several different scenarios the net cumulative total amounts of CO2e that could be sequestrated in urban hardwood products. The three products investigated are landscape mulch (chips), biomass for fuel (chips) and solid wood products.
Chips used for landscape mulch have many benefits including moisture conservation, aesthetics, and soil enrichment. However, from a CO2e sequestration perspective, landscape chips should be classified as a short-term product (lifetime of 5 years or less). Consequently, we conclude that chips used for landscape purposes (ground cover) have zero long-term CO2e sequestration.
Chips used as a biomass fuel source for heat and/or power were evaluated from a fossil fuel displacement (substitution) perspective. Using an LCA-type approach for both urban trees and coal (by calculating CO2e emissions from harvesting or mining through combustion), we found the following: 1 ton of urban wood at 50 percent moisture content displaces approximately 0.41 tons of coal and results in approximately 0.92 tons of avoided fossil fuel emissions (CO2e). From a national perspective, combusting 10 percent of the annual urban tree removals (at 50 percent moisture content) in place of coal results in avoided fossil fuel emissions of roughly 2.1 million tons. This is the equivalent of about 1% of annual energy-related CO2e emissions attributable to coal; the value is also comparable to annually removing over 367,000 passenger cars from U.S. highways.
The solid wood product portion of this study (making up the majority of this report) focuses only on hardwood products made from urban trees and excludes both paper products and all products made from urban softwoods.
An Excel spreadsheet model was created to generate net cumulative CO2e estimates for urban forest hardwood products over a 30-year period. Projections in the model are based on assumptions about the growth rate in the capacity of the nation's urban forests to sequester C and on the potential sequestration amount of C that could end up in urban hardwood products. We also examined estimated growth capacity and utilization in two regions of the U.S. (North East and West). We found that strong nationwide growth in urban hardwood product sequestration over three decades is possible even when growth does not occur in all or a majority of regions.
From this study, forest products manufactured from felled urban trees have significant CO2e sequestration benefits with conservative estimates between 124 and 472 million tons over a 30-year period. And, although urban tree chips used for landscape purposes have no long-term carbon storage benefits, chips combusted for heat and/or power have the potential to annually displace 2.1 million tons of fossil fuel emissions. Thus, a practice of diverting urban hardwoods to solid products and utilizing other urban tree material as an alternative to fossil fuels would contribute to the reduction of CO2 in the earth's atmosphere and move our nation closer to making the highest and best use of urban trees.
The work upon which this publication is based was funded in whole or in part through a grant awarded by the Wood Education and Resource Center, Northeastern Area State and Private Forestry, U.S. Forest Service.
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