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Paying attention to the carbon footprint behind the wood curtain

“Determining the Carbon Footprint of Wood” (PCA R&D 3287) is a valuable work based on Portland Cement Association-sponsored research. The timely report tracks trees and forests’ carbon dioxide-sequestering capacity, one of the value propositions wood building product interests channel to green-building practitioners; paints rationale for the wood industry to document carbon accounting methods transparently, especially when product is not sourced from a sustainably managed forest; and, helps cast-in-place concrete, precast and masonry construction practitioners respond to competitors’ claims of environmental mettle.

Two of 115-page report’s five co-authors are familiar to concrete circles through their time at CTLGroup: ASTM Committee E60 on Sustainability Vice Chair and ACI Committee 130 on Sustainability member Emily Lorenz, who consulted on life cycle assessments (LCA) and environmental product declarations (EPD) prior to her appointment earlier this year as Precast/Prestressed Concrete Institute sustainability manager; and, ASHRAE Committee189.1 on Sustainability member Martha Van Geem, a consultant in green buildings, energy efficiency, energy codes and thermal mass.

Reviewing wood industry metrics essential to credible environmental product or material attribute reporting, they call out failure to account for carbon from the five Intergovernmental Panel on Climate Change-designated pools. Those include above-ground, “roundwood” biomass and organic soil; the latter is key to mature forests, whose carbon storage capacity significantly eclipses that of younger stands. One cited report (Samson and Hair, 1996) draws carbon dioxide storage in stark terms: A 25-year-old maple, beech and birch forest in the Northeast sequesters about 2.5 lbs. of CO2 per year against a 120-year-old forest’s 6-lbs./tree annual rate.

To underscore carbon sequestration variations not fully accounted in wood industry documents, “Determining the Carbon Footprint of Wood” offers as an example the harvesting of a Southeast forest at 25-year intervals, which eliminates “the opportunity for [each] tree to sequester an additional 380 lbs. of carbon if harvested at 120 years. If harvested at 220 years, well within the natural lifespan of the tree, an additional 940 lbs. of carbon could be sequestered. Using this approach, each tree harvested causes a negative carbon sequestration of over one-half ton.”

“Part of the reasons old trees still absorb carbon is that they have to send carbon below ground [to organic soil] to maintain the ecosystems that support them,” Lorenz, Van Geem and co-authors observe. “Only a small percentage of wood removed from forests ends up as durable goods or construction products. According to one study [Heiken, Jelen and Stevens, 2008], as little as 15 percent of the initial carbon stored in a live tree is retained in forest products. Most wood ends up as waste or non-durable goods. Logging kills trees, which stops them from storing carbon and transferring carbon to the soil … Logging debris is frequently burned, which immediately releases carbon to the atmosphere.”

The authors also point to environmental documents’ absence of verification that wood is sourced from a forest certified as sustainably managed—a label applicable to only one-quarter of U.S. forest acreage. After challenging wood industry environmental accounting, they list areas warranting action, research or scrutiny: Wood source and deforestation rate disclosure; age and rotation of trees, whose ages and species sequester carbon at different rates; and, forest management modeling. They also suggest reporting sequestered carbon separately from CO2 emissions, which would allow LCA users to know how much of the greenhouse gas is released back to atmosphere when wood reaches the end-of-life stage.

Joining Lorenz and Van Geem in “Determining the Carbon Footprint of Wood” research and writing were University of Georgia College of Engineering Associate Professor Ke Li, Ph.D., and Mechanical Engineering Program Coordinator Thomas Lawrence, Ph.D., P.E.; plus, Rita Schenck, Ph.D., executive director of Institute for Environmental Research and Education, an EPD developer in Washington State. The authors reveal how wood building product interests assume carbon emissions are balanced without actually measuring or calculating them. One of their report’s most important underlying messages: The wood industry should be bound by the same principles that compel green-building hawks to cackle about CO2 emitted in portland cement production.