Spruce bark beetle disturbance in the forest-tundra ecotones of southwest Yukon: impacts and predisposing factors
As climate warms, phytophagous forest insects are expected to advance into previously unoccupied regions and/or experience population eruptions within their extant ranges. Already stressed by abiotic factors, arctic and alpine treeline environments will therefore become increasingly vulnerable to insect disturbances. Using vegetation surveys and dendrochronological techniques, we examined factors that may have predisposed the forest-tundra ecotones of southwest Yukon to spruce bark beetle (Dendroctonus rufipennis) infestation, and investigated the subsequent effects of insect disturbance on the growth and establishment of white spruce (Picea glauca). Specific objectives were to evaluate (i) the relationships between stand structure and variance in beetle-induced mortality and (ii) growth patterns between stands affected and relatively unaffected by the outbreak. Information on spruce size, reproduction and health, shrub cover, seedling density and size, stand density, and basal area was collected from three elevations at six sites, divided equally into three mortality classes. A subset of mature spruce individuals – both living and deceased – was sampled from one low-mortality and one medium-mortality site. Results demonstrated that hosts selected by spruce bark beetle were typically large individuals growing in dense stands, with spruce density and basal area dictating the extent of stand mortality. The majority of mortality was concentrated at – though not exclusive to – forestline elevations, though incidence of attack was sporadic across the region. At forestline, deceased individuals shared a period of accelerated growth when trees were between 75 and 150 years old, not present in their surviving counterparts, and had stronger temperature-growth correlations in early summer of the growing season than for still-living trees. Post-outbreak growth releases were observed in several surviving forestline mature spruce, but this accelerated growth was restricted to stands that experienced high levels of spruce mortality. The results indicate that host selection mechanisms and competitive restrictions in forest-tundra ecotones are similar to those in low-elevation, closed-canopy forests, but suggest that additional factors may increase susceptibility to attack in treeline environments. Insect disturbance has the capacity to significantly alter treeline positions and dynamics, and is therefore an important factor to consider when assessing the future of forest-tundra ecotones.
dendrochronology, forest dynamics, southwest Yukon, Kluane region, forest-tundra ecotone, spruce bark beetle
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