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Carbon Dioxide Efflux from Tree Trunks
by Will Bowman
Winter 2006

Forests provide many important benefits and ecological services.  They serve as habitat for an impressive diversity of flora and wildlife, remove pollutants from rainwater before these pollutants enter rivers and estuaries, and soak up 2 billion tons of carbon dioxide (CO₂) from the atmosphere annually.  This CO₂ uptake mitigates against climate change by offsetting about one-third of the CO₂ emitted by fossil fuel combustion.

Every living surface in forests exchanges CO₂ with the atmosphere as CO₂ is taken up by canopy photosynthesis, while respiration in the cells of trunks and branches metabolizes the sugars produced by photosynthesis and returns CO₂ to the atmosphere.  I studied these exchanges of CO₂ between forests and atmosphere in Black Rock Forest.

Specifically, I aimed to quantify the amount of respiratory CO₂ that diffuses from the trunks and branches of northern red oaks (Quercus rubra) in stands of different ages (40 to 135 years old) in the Forest, and how much of the total CO₂ removed from the atmosphere by photosynthesis in these stands is returned to the atmosphere by respiration from the trunks and branches.  Most forests in the eastern United States are young; it is important to understand if their carbon uptake will change as they age.

I monitored CO₂ efflux from the trunks and branches of 20 trees from June to December in 2002 and 2004.  To determine the physiological and environmental conditions that influence this efflux, I also measured wood temperature, respiration rates of the underlying wood, sap flow rates, sap CO₂ concentrations, and tree growth rates.

These measurements showed that trunk and branch respiration annually releases 4180-6300 kilograms of CO₂ per forest hectare into the atmosphere.  This CO₂ flux is an important component of the carbon budget of these forests, accounting for 10.3 to 17.0% of total photosynthesis. Comparing respiration among stands, higher respiratory activity in the younger (40-yr-old) trees was offset by the greater biomass in the older (95-yr-old) stand, so this proportion did not vary by stand age.  The variation in trunk CO₂ efflux was strongly related to wood temperature and was correlated with wood respiratory activity during the fall and winter and with tree growth rates in the spring and summer.

This research also found that sap flow and CO₂ within the sap interfere with the efflux of respiratory CO₂ to the atmosphere, and vary with the age of the trees.  This suggests that, particularly in older trees, respiratory CO₂ may be transported by sap flow to upper regions of the tree.  Research to identify conditions in which sap flow or  CO₂ concentration interfere with trunk CO₂ efflux would improve our ability to accurately estimate this important forest CO₂ flux.

Will Bowman is about to receive his Ph.D in Ecology and Evolutionary Biology from Columbia University .