UBC Theses and Dissertations
Fire-resilient ecosystems : fire exclusion and selective harvesting degrade dry forests in British Columbia Greene, Gregory Allen
In dry forests of southeastern British Columbia (BC) dense stands may be legacies of past high-severity fires and exist within the historical range of variability, or they may result from disruptions to historical fire regimes and indicate lost resilience. I conducted three dendrochronological studies that reconstructed the historical fire regimes and dynamics of these forests to discern the origin of high tree densities and guide ecosystem restoration to enhance forest resilience to fire and climate change. Historically, all 20 study stands were under an Indigenous-influenced, frequent, lower-severity fire regime. Moderate-severity fires initiated contemporary subcanopy cohorts, but ensuing fires, harvesting, and climate interacted to facilitate high tree densities. Fire exclusion prevented subsequent fires and allowed high densities to persist through time. In contrast to contemporary dense forests, historical stands were low-density and comprised of large, fire-tolerant trees; shade-intolerant ponderosa pine (Pinus ponderosa) and western larch (Larix occidentalis) dominated stand basal area. Historical selective harvesting removed the largest trees and favored shade-intolerant species. Contemporary stands are dominated by shade-tolerant Douglas-fir (Pseudotsuga menziesii var. glauca), with 1407% more trees, 143% more basal area, and 63% smaller quadratic mean diameter than historical stands. Western larch regeneration is absent and ponderosa pine regeneration is negligible. All contemporary trees are stressed: growth rates have declined, missing rings have increased, and many trees are dying. Canopy-dominant trees are more stressed than trees in lower canopy positions, most likely caused by competition with suppressed trees for soil moisture. Western larch was most stressed while Douglas-fir was least stressed, owing to differences in life history attributes. Dense stands are artefacts of human exclusion of fire and alterations to historical stand structures and composition, and represent degraded components of the dry forest matrix. To enhance resilience to fire and climate change, proactive forest management by thinning subcanopy trees will alleviate intense competition for soil moisture. Stands containing western larch should be prioritized to ensure its long-term persistence. Reintroducing fire provides necessary ecological feedbacks that will maintain resilience through time. Prescribed fires must be consistent with the reconstructed variation in historical frequency and severity, and will be enhanced by Indigenous knowledge.
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