Using fallen leaves, branches and dead trees in US cities to produce commercial building and gardening products could theoretically play an important role in cutting emissions and nitrogen-related water pollution 鈥� though some researchers have questioned whether this is possible in practice.
Wood and plant debris removed from urban areas in the US often goes to landfill sites, is burned or is left to rot. But if all that tree waste were converted into useful resources, there would be significant local and national benefits, according to at Yale University and her colleagues.
US cities produce about 45 million dry tonnes of tree waste per year. Some cities and research groups have explored recycling that waste via means such as creating mulch or generating electricity through incineration. But Yao and her colleagues wondered what effect a more comprehensive recycling programme would have on the environment if it accounted for the full life cycle of the trees and of the products that such a scheme could replace, namely building materials and soil conditioners.
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To do this, they created a 鈥渃radle-to-grave鈥� life cycle assessment model 鈥� including carbon dioxide uptake by trees; emissions from the use of produced biomass; burdens related to producing and transporting fuels and materials; and end-of-life emissions from products made from recycled trees鈥� to evaluate five scenarios related to urban tree waste processing.
The scenarios varied from sending everything to landfill to recycling it all into commercial lumber and chips, composted organic fertiliser and biochar, a charcoal-like material. The researchers鈥� intermediate scenarios included various mixes of landfilling, composting, mulch production and electricity production.
They compared their findings with the emissions from the production and use of mineral fertilisers, harvested wood lumber and chips, charcoal as a soil conditioner and grid-based electricity.
Yao and her colleagues found that their optimal recycling scenario, if used across the US, could cut annual greenhouse gas emissions by up to 251.8 million tonnes of CO2 equivalent compared with the landfilling scenario. It would also result in a drop in potentially damaging eutrophication, or nutrient leakage into water, of up to 192.7 thousand tonnes of nitrogen equivalent per year.
鈥淯tilizing urban tree waste can yield nationwide environmental benefits in the US,鈥� the researchers write in their paper.
On a regional scale, New England and south-eastern states of the US, along with Texas and California, would experience the greatest emissions reductions through optimal urban tree recycling programmes, probably because of the greater quantities of urban tree waste in those regions, says the team.
The study is 鈥渂eautiful science鈥�, but ignores critical financial and practical considerations, says at the University of California, Los Angeles.
鈥淗ave they done any land analyses of cities to see where there鈥檚 space to be able to store all of that [tree waste] and process it?鈥� she asks. 鈥淎nd how do cities implement new programmes when there鈥檚 no new money? It鈥檚 this kind of endless idea, this set of expectations for cities to be able to address all these problems without any new tools.鈥�
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