Urban forests - street trees, municipal parks - in the United States are estimated to span 51.5 million hectares of land and hold 800 million metric tons of carbon.
In a new study published in One Earth and funded by Yale University and the US National Science Foundation, scientists examined the economic benefits of creating circular systems for urban biomass.
"Substantial urban tree waste is generated and underutilized in the U.S ," the study authors stated. "Circular utilization of urban tree wastes has been explored in the literature, but the life-cycle environmental implications of varied utilization pathways have not been fully understood."
Researchers tackled the life-cycle environmental implications of managing and utilizing uban tree waste to determine the best means of diversion and reuse.
""Furthermore, this study contributes to the [lifestyle assessment] community by providing process-based models and life-cycle inventory (LCI) data for each tree waste utilization pathway," it said. "These models are parametric, and can be used by other researchers and LCA practitioners for different tree types and operational conditions. The stakeholders and policymakers can further use the results presented in this study to tailor their strategies or policy toward sustainable management of urban tree waste."
More than 25 million oven dry metric tons (ODMT) of leaf waste and over 20 million ODMT of tree waste per year, producing 20 Mt of carbon mass. This waste can be converted to compost, mulch, electricity, lumber and chips and biochar.
The economic potential for yard/urban waste in a circular system - such as the production of compost, electricity, firewood and pallets, is estimated to be between $89 million and $786 million per year, depending on the products produced.
Researchers focused on the mitigation of climate change and eutrophication - nutrient enrichment in aquatic ecosystems - through the conversion of forestry waste into products that can be utilized as "carbon sinks" with less environmental impact.
"Such benefits vary with state-level locations due to varied urban tree waste availability and types," the study stated. "Process-level comparisons identify the most environmentally beneficial combination as using merchantable logs for lumber and residues for biochar. The results highlight the climate change and eutrophication mitigation potential of different circular utilization pathways, supporting the development of circular bioeconomy in the urban environment."
Five scenarios were identified as "different pathways" to waste diversion. Scenario 1 explored landfilling, while others studied biochar, composting and other methods.
"Reusing or recycling most urban wood wastes for energy, products, or other purposes beyond chips or mulch is good utilization," the study stated. "Comprehensive utilization of all biomass wastes is suggested as optimal utilization"
Scenarios 2-4 examined the combination of different materials produced from urban biomass - chips, mulch and waste-to-energy practices.
"Specifically, scenario 2 incinerates removed trees for power generation, but leaf waste is still landfilled. In scenario 3, leaf waste is collected to produce compost as organic fertilizer that replaces the mineral fertilizers (i.e., nitrogen, phosphorus, and potassium fertilizers), and removed trees are incinerated. In scenario 4, leaf waste is composted, and removed trees are chipped to serve as mulch, which is a common practice,"the study stated.
The last scenario analyzed was taking an optimal approach where all urban biomass was used for products "other than energy and mulch," such as sawlogs or pulpwood.
Study authors reported that "shows high stability in soil," with more than 80 percent of the carbon retained in the soil following pyrolysis after 100 years of applications.
"Hence, biochar can be a potential stable long-term carbon pool," they continued. "Urban-tree-waste-derived biochar can substitute traditional charcoal, which is included in this analysis as substitution effects. The leaf waste is not suitable for merchantable applications or biochar due to large variations in high ash and low carbon content. Therefore, scenario 5 matches three types of urban tree wastes with different utilizations: leaf waste is composted, merchantable logs are utilized to produce lumber and chips, and residues are converted to biochar that is used as a soil amendment and substitutes for traditional charcoal."