Climate‐induced changes in host tree–insect phenology may drive ecological state‐shift in boreal forests

Abstract

Climate change is altering insect disturbance regimes via temperature-mediated phenological changes and trophic interactions among host trees, herbivorous insects, and their natural enemies in boreal forests. Range expansion and increase in outbreak severity of forest insects are occurring in Europe and North America. The degree to which northern forest ecosystems are resilient to novel disturbance regimes will have direct consequences for the provisioning of goods and services from these forests and for long-term forest management planning. Among major ecological disturbance agents in the boreal forests of North America is a tortricid moth, the eastern spruce budworm, which defoliates fir (Abies spp.) and spruce (Picea spp.). Northern expansion of this defoliator in eastern North America and climateinduced narrowing of the phenological mismatch between the insect and its secondary host, black spruce (Picea mariana), may permit greater defoliation and mortality in extensive northern black spruce forests. Although spruce budworm outbreak centers have appeared in the boreal black spruce zone historically, defoliation and mortality were minor. Potential increases in outbreak severity and tree mortality raise concerns about the future state of this northern ecosystem. Severe spruce budworm outbreaks could decrease stand productivity compared with their occurrence in more diverse, southern balsam fir forest landscapes that have coevolved with outbreaks. Furthermore, depending on the proportion of balsam fir and deciduous species present and fire recurrence, changes in regeneration patterns and in nutrient cycling could alter ecosystem dynamics and replace black spruce by more productive mixedwood forest, or by less productive ericaceous shrublands. Long-term monitoring, manipulative experiments, and process modeling of climate-induced phenological changes on herbivorous insect pests, their host tree species, and natural enemies in northern forests are therefore crucial to predicting species range shifts and assessing ecological and economic impacts.

No growth stimulation of Canada’s boreal forest under half-century of combined warming and CO 2 fertilization

“…Furthermore, the complex interplay of different biotic and abiotic drivers of boreal forest productivity, including unpredictable wildfires, insect population outbreaks, and other disturbances, may yield counterbalancing effects on the overall net carbon sequestration (6,42,43). Notably, insectinduced collapses in growth in western and eastern Canadian forests played a major role in defining growth trajectories at the turn of the 21st century (3,44,45), but their relative effects on growth are only partially, if at all, captured by this current NFI tree-ring analysis. Although remote-sensing products can provide insights into the impacts of these phenomena and of climate change, our results suggested extensive areas of disagreement between forest growth trajectories and remotely sensed NDVI trends: the accelerated growth over large regions was not necessarily correlated with greening and, inversely, with browning where trees experienced a slower growth.…”

Forest Insects and Climate Change

“…An interesting indirect effect involving the host plant has been hypothesized and modeled for C. fumiferana in Canada, which is associated with balsam fir as its main host and black spruce as a secondary host. Climate change is predicted to advance the phenology of the secondary host that is more abundant at the upper latitudinal edge, making it more susceptible to defoliation during outbreaks, and thus facilitating expansion of the outbreak area into higher latitudes [17,18,44,45,70] (Fig. 1).…”

“…Bentz et al [16] predict that spruce bark beetle, Dendroctonus rufipennis, outbreaks may occur throughout the range of spruce in North America in the future. Outbreak dynamics of the spruce budworm, Choristoneura fumiferana, are predicted to change, move further north, and on to secondary host species [17,18]. Similarly, cyclical outbreaks of larch bud moth, Zeiraphera griseana, are predicted to decrease in magnitude in optimal mid-elevation zones of the Alps, and to shift toward higher altitudes [19].…”

Population cycles: generalities, exceptions and remaining mysteries

“…Whether a period of low density persists ( figure 1, 1981-1986 for snowshoe hares) or if change is continual as seen in figure 3 for voles or figure 4 for tent caterpillars, influences the interpretation of mechanisms involved in the population dynamics. For example, if spruce budworm populations go through a 'predator pit' that maintains a low-density equilibrium or if the oscillation is continual and the result of a second-order density-dependent process, remains controversial [97].…”