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Forest Carbon export via head-water streams

The ability of forests to fix carbon through photosynthesis is well recognized, but forests are simultaneously losing carbon via a range of processes, including respiration and fire.  The loss of forest carbon through the export of dissolved organic carbon (DOC) in head-water streams may also be a significant component of the forest carbon budget.  The red/brown colour of streams in many parts of Tasmania indicates that DOC is present.  In collaboration with the University of Tasmania, we have recently quantified the amount of DOC exported via three small streams located in native forest in the Warra Long-Term Ecological Research site. 

Quantifying carbon export in streams is difficult as the concentration of DOC varies significantly in response to variation in stream-flow as well as seasonally, and it is impractical to make continuous measurements of stream DOC.  After a period of intensive monitoring of DOC concentrations, we were able to develop a model that predicts DOC concentration based on stream-flow, turbidity, temperature and electrical conductivity.   This model was then used to predict total flux of DOC from the three catchments over a period in 2009. 

DOC export from Warra Creek, the largest of the three streams, was estimated at 93 t C yr-1, or approximately 0.2 t C ha-1 yr-1.  Average annual carbon sequestration for mature Australian forests is approximately 2.4 t C ha-1 yr-1, suggesting that mature forests may be losing through export of stream DOC some 8% of the carbon they sequester, an amount which to date has not been included in carbon accounts. 

Further studies are underway to determine the interaction between DOC and stream bacterial communities.  More research is also required to understand the particular origins of DOC within the forest, as well as its fate downstream and in the ocean. 

Red/brown water is visual evidence of dissolved organic carbon export in head-water streams in Tasmania’s temperate forests.

Ian RIley

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