Site influences on net ecosystem productivity in two Abies balsamea stands in southwestern New Brunswick monitored by eddy covariance

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University of New Brunswick
This thesis provides an initial study of environmental control on net ecosystem productivity (NEP) in two balsam fir-dominated stands in southwest New Brunswick. Fundamental to this study is (i) numerical optimisation of an existing ecosystem-based NEP-model, initially formulated to fill gaps in NEP-data streams, and (ii) an analysis of model-generated values of environmental controls (modifiers) on NEP. Optimal estimates of model parameters and associated environmental modifiers are obtained by minimizing the weighted differences between target and model-predicted NEP-flux densities with least median squares regression. The model performed reasonably well in simulating half-hourly values of mean net CO₂ exchange for the two stands, yielding modelling efficiencies ≥ 93% and r²-values > 0 .75. NEP was fairly high for the two sites, with annual means ranging from 0.77-3.70 and 1.11-1.68 g C m⁻² day⁻¹ at Charlie Lake (CL) and Nashwaak Lake (NWL), respectively. Between the two sites, mean growing-season NEP and stand growth was consistently highest at CL. Differences in NEP resulted from differences in the effects of site conditions on gross primary productivity (GPP) and ecosystem respiration (RE). Intersite variation in GPP was mainly attributed to variation in local topography. Growing-period RE was associated with previous-winter snow cover depth and air temperature and growing-period soil biophysical conditions, including state of decomposer communities. Decomposition of NEP into its flux components is extremely useful in the study of ecosystem response to changing site conditions. Keywords: ecosystem respiration, eddy covariance, flux measurement, gross ecosystem productivity, incident solar radiation