Defoliation-induced changes in foliage quality may trigger broad-scale insect outbreaks

Progression of a spruce budworm outbreak over seven years is associated with increased soil nutrient fluxes and availability and improved foliage quality in surviving host trees. This could create a bottom-up feedback that sustains an insect outbreak. Top-down effects, like predation, are drivers of insect outbreaks, but bottom-up effects, like host nutritional quality, also influence outbreaks and could in turn be altered by insect-caused defoliation. We evaluated the prediction that herbivory leads to a positive feedback on outbreak severity as nutrient concentration in plant tissues increases through improved soil nutrient availability from frass and litter deposition. Over seven years of a spruce budworm outbreak, we quantified litter nutrient fluxes, soil nitrogen availability, and host tree foliar nutrient status along a forest susceptibility gradient. As the outbreak progressed, both soil nutrient fluxes and availability increased which, in turn, improved foliage quality in surviving host trees. This is consistent with boosted insect fitness and increased population density and defoliation as outbreaks grow. Our results suggest that a positive bottom-up feedback to forest ecosystems from defoliation may result in conditions favorable to self-amplifying population dynamics in insect herbivores that can contribute to driving broad-scale outbreaks.

Automated summary

The progression of a spruce budworm outbreak over seven years is associated with increased soil nutrient fluxes and availability, as well as improved foliage quality in surviving host trees. This positive bottom-up feedback sustains the insect outbreak. Both predation and host nutritional quality influence insect outbreaks, and defoliation caused by insects can also alter the bottom-up effects. Our study demonstrates that herbivory leads to a positive feedback on outbreak severity through improved soil nutrient availability and increased nutrient concentration in plant tissues.