Vertical distribution of carbon dioxide sources and sinks in a recovering mountain pine beetle-attacked lodgepole pine stand

•In-canopy eddy flux measurements allowed determination of CO2 source and sink layers.•An ecophysiological approach agreed with the eddy flux source and sink distribution.•Overall photosynthesis of the living vegetation exceeded respiration in this stand.•Living plants in the understory and secondary structure were responsible for CO2 uptake.•The overstory with only dead attacked pine trees was a weak source of CO2.

Large-scale insect outbreaks can substantially alter the carbon exchange of forests. Currently, British Columbia, Canada, is experiencing the largest mountain pine beetle (MPB, Dendroctonus ponderosae) outbreak ever registered, leaving large areas of forest killed or partially killed. It has been found that some unharvested attacked stands have returned to being a carbon sink within a relatively short time. A three-week field campaign was conducted during the summer of 2010 in one of these MPB-attacked stands, to determine the contributions of different vegetation layers to the overall net ecosystem exchange (NEE) of the stand. Almost all of the mature pine trees in this stand were dead, while there was a relatively rich secondary structure (younger conifer trees not killed by the beetle) and a broadleaf dominated understory. Two independent approaches, the eddy-covariance (EC) approach using seven EC systems distributed vertically on a tower of which six were installed within the canopy and one above the canopy and an ecophysiological (EP) approach based on carbon dioxide (CO2) chamber measurements on foliage and boles were employed in this study. Air-column storage corrected CO2 flux below each measurement level was partitioned into photosynthesis and respiration components. The climatic conditions during the field campaign were dry, and water use efficiency was analyzed while going from moderate to dry conditions. Due to the open-stand structure, approximately 60% of the photosynthetically active radiation reached the 1-m height. The photosynthesis of the living vegetation exceeded the respiration in this stand. The understory was found to have the largest CO2 sink strength in the canopy, while the secondary structure also contributed substantially to the overall CO2 uptake. The overstory, consisting of mainly dead trees, was a weak CO2 source. The EC and EP approaches agreed regarding the vertical distribution of sources and sinks of CO2 in the canopy; however, the magnitude varied between the two approaches. Water use efficiency was relatively high with the daytime average value between 2.3 g C kg−1 H2O close to the ground where the broadleaf vegetation was dominant and 6.5 g C kg−1 H2O where the conifer trees of the secondary structure were located. The secondary structure exhibited greater adjustment to increasingly dry conditions than the broadleaf vegetation.

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