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UBC Theses and Dissertations

Vertical distribution of carbon dioxide, water vapour, momentum and energy exchange within and above a forest stand affected by the mountain pine beetle Emmel, Carmen


The mountain pine beetle (MPB) has killed vast areas of pine forest in British Columbia, Canada converting forests from carbon sinks to sources. Different management options for these forests exist ranging from no treatment to complete removal of the infested forest (clearcut). The MPB attack and the following management alter the microclimate and carbon balance of affected stands. An intensive field campaign was conducted in the summer of 2010 in an affected forest without treatment in the interior of British Columbia. Eddy covariance, radiation, temperature, humidity and carbon dioxide (CO₂) measurements were made at seven heights on a tower within and above the canopy. This dissertation assessed the impact of the MPB attack and the structure of the disturbed canopy on the contribution of various vegetation layers (ground, secondary structure, overstory) to exchanges of CO₂, water vapour (H₂O), momentum and sensible heat. Previous research has shown that forests without treatment can return to being carbon sinks faster than clearcut sites. It was hypothesized that the rich secondary structure (mostly immature trees that survived the beetle attack) was responsible for this fast recovery. The current dissertation showed that canopy layers in this sparse and open-canopy stand were aerodynamically well coupled with the atmosphere above and allowed 60 % of photosynthetically active radiation to reach the ground. Given these favorable light conditions, the secondary structure was indeed responsible for a large proportion of the CO₂ uptake; however, the understory (< 1 m high) contributed at least equally to the CO₂ uptake. A dissimilarity in the vertical distribution of sources and sinks of CO₂, H₂O and sensible heat was found. The dissimilarity between CO₂ and H₂O was caused by the differences in water use efficiency of the different vegetation layers. Gradient-diffusion theory (K-theory) applicability was examined in order to guide modeling of stand microclimates and growth conditions. Momentum flux (shear stress) could be adequately determined using K-theory and an adjusted length scale. In the case of the other scalars, the use of K-theory was found to be problematic due to counter-gradient fluxes, the inability to resolve gradients and fluxes and/or source scales.

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