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UBC Theses and Dissertations

Assessing and monitoring the stability of stream detrital dynamics under forest disturbances Yeung, Alex Chee Yu


Disturbances affect ecosystem stability and the delivery of ecosystem services. Stability responses to disturbances involve multiple components, including changes in the levels (indicating resistance) and recovery time (resilience) of state variables. The stability of stream ecosystems is an active research area, however, empirical studies commonly measure single components of stability, and the basis of its spatio-temporal variability is often unclear. Bivariate and multi-dimensional frameworks were proposed to facilitate stability comparisons within and across ecosystems. I recommended adjusting two bivariate frameworks to better address diverse disturbance-response trajectories, shifting baselines, and broaden their applications for streams and other ecosystems. In my dissertation, I quantified multiple stability components of terrestrial-derived particulate organic matter (POM) availability and breakdown in small, temperate streams under forest harvesting. I focussed on stream POM dynamics, as they underpin the stability of consumer productivity (e.g., fish) and nutrient cycling, which are important aquatic ecosystem services. First, I investigated whether the natural variability of leaf litter breakdown – critical for detecting disturbance impacts – was affected by natural, weather-driven discharge variations across years, and decomposer preference of local versus exotic litter species. I showed that inter-annual hydrologic variations poorly explained litter breakdown, whose natural range of variation exceeded the benchmarks set by a popular bioassessment framework. Accordingly, I recalibrated reference conditions of litter breakdown to allow more robust bioassessments. Decomposer preference did not differ significantly between high-quality native and exotic litter, supporting their standardised use for disturbance studies across geographic regions. Secondly, I demonstrated that the resilience of litter breakdown to forest harvesting was likely greater in streams affected by thinning (with 50% basal area removal of riparian trees; 2-9 years post-harvest) than those affected by clear-cutting with or without riparian buffers (8-15 years). Thirdly, I modelled the resistance of stream POM quantity under variable, realistic harvesting impacts. Logging-induced changes in litterfall were more influential than peak discharge and stream temperature alterations in regulating POM quantity. Management strategies minimising riparian forest disturbances would more likely sustain detrital resource availability and productivity in small streams. My results indicated that riparian vegetation, through litterfall and shading, importantly controlled the stability of stream POM dynamics.

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