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    This study seeks to answer the question, “Will the Million Trees LA (Million Trees Los Angeles, MTLA) program be a carbon dioxide (CO2) sink or source?” Because there has never been a full accounting of CO2 emissions, it is unclear if urban tree planting initiatives (TPIs) are likely to be effective means for reaching local reduction targets.


    Using surveys, interviews, field sampling, and computer simulation of tree growth and survival over a 40-year time period, we developed the first process-based life cycle inventory of CO2 for a large TPI. CO2 emissions and reductions from storage and avoided emissions from energy savings were simulated for 91,786 trees planted from 2006 to 2010, of which only 30,813 (33.6 %) were estimated to survive.

    Results and discussion

    The MTLA program was estimated to release 17,048 and 66,360 t of fossil and biogenic CO2 over the 40-year period, respectively. The total amount emitted (83,408 t) was slightly more than the −77,942 t CO2 that trees were projected to store in their biomass. The MTLA program will be a CO2 sink if projected 40-year-avoided fossil fuel CO2 emissions from energy savings (−101,679 t) and biopower (−1,939 t) are realized. The largest sources of CO2 emissions were mulch decomposition (65.1 %), wood combustion (14.5 %), and irrigation water (9.7 %).


    Although trees planted by the MTLA program are likely to be a net CO2 sink, there is ample opportunity to reduce emissions. Examples of these opportunities include selecting drought-tolerant trees and utilizing wood residue to generate electricity rather than producing mulch.

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    McPherson, E. Gregory; Kendall, Alissa. 2014. A life cycle carbon dioxide inventory of the Million Trees Los Angeles program. The International Journal of Life Cycle Assessment. 19(9): 1653-1665.


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    Carbon footprint, Carbon sequestration, Climate change, Life cycle inventory, Tree planting, Urban forestry, Urban trees

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