UBC Theses and Dissertations
Wildfire and mountain pine beetle boundary zones : spatial pattern, boundary creation ans successional consquences McIntire, Eliot Jonathan Benet
Wildfire and mountain pine beetle (MPB) boundaries are ubiquitous in many forested landscapes, yet they have been rarely described in two-dimensions at the stand-level and their formation and effects on plant community dynamics are poorly understood. Using spatially constrained clustering, the width of seven wildfire and eleven MPB boundaries was determined to be variable and ranged from Om to 120m (means approximately 30 to 50m). Thus, natural disturbance boundaries varied along a continuum from hard to soft. Using multiple spatially explicit path hypotheses and AICC to weigh relative support for each, these boundaries were found to be generated by a combination of stand-level factors as well as unmeasured spatial factors. A key factor was the basal area of susceptible host (for mountain pine beetle) and the basal area of trees that are highly susceptible to fire mortality. The pattern of tree mortality across these boundaries showed little relationship to stand age, stand density, slope or aspect. The spatial trend of mortality across seven of eleven MPB boundaries was sufficiently accounted for by the abundance of susceptible host, indicating that, at these sites, the MPB outbreaks were likely host limited rather than dispersal limited. Using multiple spatially explicit path hypotheses, natural regeneration patterns were found to be primarily driven by a light surrogate across the boundaries. Seed bed and disturbance severity were minor factors affecting the natural regeneration patterns. Seed source and vegetation competition had very little influence on the natural regeneration patterns. Successional trajectories in the boundary zones, as measured by overstory tree abundance and mortality as well as advanced regeneration growth release and natural regeneration patterns, were found to be different than in either the disturbance cores or the intact forest immediately adjacent to the boundary zones. These differences demonstrate that natural disturbances create spatial variation in the boundaries that translates into diverse successional sequences. Forests that have simplified edges, such as the majority of those in a harvested landscape, may have reduced spatial and temporal heterogeneity compared to those with produced by natural disturbance, such as fires and MPB.
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