UBC Theses and Dissertations
Evaluating and modeling long-term climate and disturbance impacts on evapotranspiration and canopy conductance in boreal aspen and black spruce stands Stephens, Jilmarie
Nearly one tenth of the world’s forest is located in Canada, with one third being boreal forest. Understanding how this biome is responding to climate change is important to global carbon (C) and water balances. Long-term climate and eddy-covariance (EC) measurements of C and water vapour fluxes were made on a mature deciduous aspen stand (old aspen, OA) and a mature coniferous black spruce stand (old black spruce, OBS) to determine the impact of climate variations and disturbances on the C and water fluxes, and test if commonly utilized models can successfully model these fluxes. The impact of a defoliation event at OA in the summer of 2016, due to forest tent caterpillar infestation, was investigated and annual GEP was found to be reduced by ~20% that year, leading to the most negative annual NEP (-72 g C m⁻² year⁻¹) over the observation period. Long-term trends in annual climate variables, along with trends in gross ecosystem production (GEP), ecosystem respiration (Re), net ecosystem production (NEP), evapotranspiration (E), P – E, and water use efficiency (WUE = GEP/E) were investigated using 22 and 19 years of continuous data at OA and OBS, respectively. The impacts of growing season (GS) metrics on GS and annual NEP, GEP and E were investigated. Trends were also investigated at GS and monthly scales for climate variables and E, along with the impact of GS metrics on E at GS scale. The two stands showed varied responses to climatic variability, including responses to a multi-year drought that affected both sites but had a larger impact on the C and water fluxes at OA than OBS. The dependence of the canopy conductance (Gc) and E on their controlling variables was investigated at GS and monthly scales for both sites. The Jarvis-Stewart (JS) and modified Ball-Woodrow-Berry (MBWB) models were tested to estimate half-hourly Gc at each site and compute both Gc and E at seasonal to annual scales. This thesis demonstrates the importance of long-term observations in capturing variations in climate and disturbances forests experience, that need to be further studied and modelled for improved understanding of their impacts.
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