Business as usual: macrolichen community response to the resilience of spruce–fir forests to beetle disturbance in northwestern Colorado

Over the past few decades, spruce beetles (Dendroctonus rufipennis Kirby) have drastically altered spruce–fir forests in western North America. Past research has shown that forest conditions (i.e., canopy openings, amounts of coarse woody material) change after spruce beetle disturbance, but little work has studied the impacts of these changes on the resident biotic communities. Even less research has investigated how spruce beetle disturbance affects lichen communities, which we anticipated as benefiting them due to canopy opening and increased woody material availability. We studied macrolichen community structure, including abundance and community composition, and habitat characteristics in areas with significant spruce beetle damage, as first detected between 1996 and 2017 in northwestern Colorado. We found few habitat differences among beetle disturbance classes that reflected varying time since infestation, and only the most recently affected plots (2012–2017) had significantly different lichen community structure relative to other sampled areas. We observed that the spruce–fir forests within our study exhibited ecological resilience to beetle outbreaks, and we did not measure differences in the amount of canopy openings or amounts of coarse woody material. Subsequently, the lack of differential lichen community response was not surprising.

Toxicity of two Engelmann spruce (Pinaceae) monoterpene chemotypes from the southern Rocky Mountains to North American spruce beetle (Coleoptera: Scolytidae)

Engelmann spruce, Picea engelmannii Parry ex Engelm. (Pinaceae), in the southern Rocky Mountains is composed of two distinct phloem monoterpene chemotypes that differ in relative abundances of multiple monoterpenes, particularly α-pinene and Δ3-carene (hereafter, the “α-pinene chemotype” and the “Δ3-carene chemotype”). Here, relative toxicity of these chemotypes is tested on spruce beetle (Dendroctonus rufipennis Kirby) (Coleoptera: Scolytinae), a phloeophagous herbivore that colonises trees of both types. Synthetic monoterpene blends representing each chemotype were tested across a range of concentrations (0, 10, 50, 100, 200, and 500 µg/L) in the lab, and probability of survival of adult beetles exposed to each blend was modelled using a logit function. Logit curves were solved to determine LC25, LC50, and LC75 of each monoterpene blend. On average, probability of beetle survival was lower when exposed to the Δ3-carene chemotype than when exposed to the α-pinene chemotype. However, both chemotypes were completely lethal to beetles at concentrations exceeding 100 µg/L. Adult body mass did not affect survival probability. It is concluded that spruce phloem chemotypes may differ in their toxicity to spruce beetles, with potential consequences for patterns of host-tree colonisation by spruce beetle.