UBC Theses and Dissertations
Understanding disturbance, facilitation, and competition for conservation of whitebark pine in the Canadian Rockies Wong, Carmen Monica
Understanding forest resilience to novel disturbances and how tree interactions will be affected by global change is critical for predicting future forest composition. The widespread decline of the endangered whitebark pine (Pinus albicaulis) in the Canadian Rockies due to non-native white pine blister rust (Cronartium ribicola) and native mountain pine beetle (Dendroctonus ponderosae) and Ips sp. permitted examination of interactions between disturbances and tree responses in high-elevation forests. Disturbance severity was high with 20-90% whitebark pine mortality over 50 years in 16 stands. Basal annual increment (BAI) of whitebark pines prior to mortality from mountain pine beetles declined 46%, but only by 25% for those subsequently killed by blister rust and Ips sp.. Climate-growth relationships suggest blister rust increased sensitivity of whitebark pines to variation in summer precipitation, reducing resistance to beetles. The mortality of whitebark pine was used as an in-situ experiment simulating neighbour removal to test the stress-gradient hypothesis of tree interactions. Facilitation intensity, determined by comparing subalpine fir (Abies lasiocarpa) regeneration around live, top-killed, or dead adult whitebark pines, increased with elevation but depended on benefactor size and neighbourhood density. Large-diameter, top-killed whitebark pines were more facilitative than live trees, indicating thresholds in benefactor size, below which live, healthy trees were facilitators and above which they were competitors. Size thresholds were also found in interactions between adult trees where competition intensity increased between trees of greater diameter differences as indicated in BAI releases of subalpine fir after the death of neighbouring trees. Conversely, the importance of competition relative to other factors influencing growth increased between trees more similar in size and with abiotic stress. My results refine the stress-gradient hypothesis by demonstrating hierarchical influences on tree interactions. The predominant release from competition doubled subalpine fir’s BAI from the landscape average pre- disturbances compensating for the decline predicted by climate-growth relationships. Lack of regeneration and growth release in surviving whitebark pines and an abrupt shift in key variables suggest a regime shift to fir dominance and whitebark pine extirpation. Whitebark pine resilience was higher at sites of low abiotic stress and disturbance severity, relationships useful for conservation.
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