Resilience of southern Yukon boreal forests to spruce beetle outbreaks

Abstract

Changed disturbance regimes could drive biome-level shifts in vegetation structure and have cascading societal consequences. Expanding and intensifying bark beetle outbreaks pose a risk of major changes to large regions of boreal forests in North America. We evaluated the resilience of boreal forests in southwestern Yukon to an unprecedented spruce beetle (Dendroctonus rufipennis Kirby [Coleoptera: Curculionidae) outbreak using data collected from permanent plots, which were measured repeatedly, in 21 stands. We determined basal area (m2/ha) and density (stems/ha) of canopy trees (which are most vulnerable to beetles), density of advance regeneration, and growth response to the outbreak using ring widths from over 800 trees ranging in size from small advance regeneration to canopy trees. Beetle-related reductions in tree canopy basal area averaged 52% across all stands but there was considerable variability among stands. About 68% of variation in basal area reductions among stands was explained by variability in pre-outbreak canopy tree basal area (β = 1.1; p = 0.0021) and a climatic moisture deficit index (β = 0.5; p = 0.0015). Although the percentage of white spruce (Picea glauca (Moench) Voss) in the canopy was reduced in some stands, by 2–29%, white spruce remained the dominant canopy tree species. Almost all stands had more than adequate advance regeneration density (average = 5 448 stems/ha) in 2016 to replace canopy trees killed by beetles. Variability in advance regeneration among stands could partly be explained by pre-outbreak tree canopy basal area (β = −0.4; p = 0.0018), a climate moisture deficit index (β = −0.3; p = 0.0004), and shrub per cent cover (β = −0.3; p = 0.0003), as well as, humus depth (β = 544; p = 0.0186). Over the last two decades, spruce trees, of all size classes, exhibited substantial increases in mean annual radial growth increment, beginning 4–5 years after the start of the outbreak. The proportion of trees in a stand with at least a 50% increase in radial growth increment varied significantly with canopy tree basal area at the end of the outbreak (β = −0.12; p < 0.0001), climatic moisture deficit (β = −0.02; p < 0.0196), and humus depth (β = 0.26, p < 0.0001). Our findings indicate high response diversity to disturbance and suggest that forests of southwestern Yukon have high resilience to the recent spruce beetle outbreak because of the large number of surviving canopy trees, the abundant advance regeneration of spruce that can replace beetle-killed trees, and the increased growth of surviving trees.