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Tree Carbon in Forests and Urban Environments…Concepts and Trajectories

by Dr. Christopher Woodall

There are three important words when it comes to the current status and future of the greenhouse gas induced warming of our planet and interaction with our terrestrial systems: carbon, carbon, and carbon. Over twenty years ago, over 150 Nations were signatories to an agreement (the United Nations Framework Convention on Climate Change) to annually report their carbon sources and sinks. This agreement across continents and cultures to monitor one element is unprecedented. Decades removed from that original agreement where do we stand in regards to the trajectories of carbon sources and sinks, especially trees/forests in the United States?

In the US, CO2 emissions from fossil fuels have largely stabilized with expectations of future declines. The US land sink continues to sequester additional carbon at increasing rates, of which the urban land use is an essential component. The term “sink” is often used to refer to the ability of trees to soak up atmospheric carbon and store it for many years in tissues and associated carbon pools (such as litter fall and dead wood). The amalgam of these two dynamics of emission and sequestration suggests that the US land sink will be of ever increasing focus and concern into the future.

The components of the U.S. forest carbon cycle (2016 estimates, pools, and flow) are shown in the context of fossil fuel emissions (values from USEPA 2015 report). In a manner similar to carbon stored in fossil fuel reserves for millennia, forests store carbon in long-lived pools such as soils. On a daily basis there is sequestration and emission of carbon occurring in forests of the United States through tree growth and wildfires.¹

Forests account for nearly 90 percent of this land sink and offset nearly 15 percent of United States’ CO2 fossil fuel emissions. Given the threats of global change, such as invasive pests, climate change, and urbanization, we should expect increased future focus on the condition of trees in rural and urban environments across the country. In the US, the rate of forest fossil fuel emissions offset by the forest sink could far surpass 15 percent or conversely contribute to emissions. Indeed, in nations such as Finland, the forest sink currently offsets more than 100 percent of fossil fuel emissions. Objective information on forest/tree carbon dynamics in concert with land use change trajectories will be of utmost importance to fully inform future concerns of landowners, citizens, and policy makers.


1 Figure taking from Woodall, C.W., Coulston, J.W., Domke, G.M., Walters, B.F., Wear, D.N., Smith, J.E., Anderson, H.-E., Clough, B.J., Cohen, W.B., Griffith, D.M., Hagan, S.C., Hanou, I.S.; Nichols, M.C., Perry, C.H., Russell, M.B., Westfall, J.A., Wilson, B.T.  2015. The US Forest Carbon Accounting Framework: Stocks and Stock change 1990-2016. Gen. Tech. Rep. NRS-154. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station. 49 pp.

About the Author

Christopher Woodall, PhD, is the Project Leader of the US Forest Service’s Center for Research on Ecosystem Change. The Center, which works in concert with dozens of state, local, industry, and university partners, gathers data in order to develop tools and techniques to better maintain forests in the northern region of the US. His work focuses on forest inventory analysis and research. Dr. Woodall has worked on various national and regional forest inventory programs, and has been involved with research and coordination activities for the US Forest Service and other federal agencies.


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