15 - Insect Defoliators as Periodic Disturbances in Northern Forest Ecosystems
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Species distribution model identifies influence of climatic constraints on severe defoliation at the leading edge of a native insect outbreak
2023, Forest Ecology and ManagementEastern North America is in the midst of a spruce budworm (Choristoneura fumiferana Clem., SBW) epidemic. SBW is one of the most important forest insects with regard to outbreak coverage and impacts to the forest industry in this region. Numerous bioclimatic, vegetation, and spatial–temporal variables can influence the distribution and outbreak patterns of phytophagous insects such as SBW. Published models have yet to agree on consensus drivers of SBW defoliation. Spatially-explicit, fine-scale predictive models are needed, however, to aid in our understanding of current and future SBW impacts. We used a species distribution model - MaxEnt to examine the relationships between the 1972–1981 SBW defoliation and 13 predictor variables for the island of Newfoundland, Canada the leading edge of the current SBW outbreak. The optimal model for predicting the probability of severe SBW defoliation included precipitation in May and June, precipitation in July and August, and maximum temperature in June and July. The probability of severe defoliation rapidly increased when precipitation ranged at 75.0 mm – 91.0 mm in May and June but showed a complex relationship with precipitation in July and August. Probability also increased with maximum temperature in June and July until temperature reached 18.0 °C. Defoliation probability decreased with increasing distance from major roads and river channels, suggesting possible corridor effects. Counter to research in other study areas, host tree species was not an important predictor of defoliation in our analysis. We then used this MaxEnt machine learning algorithm to predict the probability of severe defoliation for the forests of the entire island on a 2 km resolution raster. Areas with high probability (>0.80) were concentrated in the northeast part of the island where black spruce (Picea mariana (Mill.) BSP) dominates the forest landscape. The predicted severe defoliation area (probability 0.4–1.0) for black spruce was 1.4 times the area for balsam fir, which is the dominant canopy forming tree species on the island and preferred host species for SBW. The higher predicted probability of severe defoliation in black spruce stands relative to balsam fir stands may arise due to the unique combination of climatic conditions and geographical distributions of two species on the island. Species distribution models for irruptive insect impacts (i.e., defoliation) are relatively rare but may be an important tool in forecasting future insect damage.
Spatial genetic structure at the leading edge of a spruce budworm outbreak: The role of dispersal in outbreak spread
2020, Forest Ecology and ManagementOutbreaking insects often undergo rapid population growth synchronously in multiple locations separated by large distances. Dispersal may play an essential role in synchronizing outbreaks over large geographic areas but its role in outbreak spread remains unclear. In our study, we used population genetics to assess how much dispersal contributes to the spread of a major forest pest, the spruce budworm (Choristoneura fumiferana Clemens). Using spatial analyses of genotypic data, we determined the extent to which dispersing individuals from epidemic populations in Quebec leave their genetic signature in adjacent, rising populations in New Brunswick and Maine. A lack of genetic differentiation between endemic and epidemic populations would indicate that outbreak spread depends on effective dispersal from epidemic to endemic regions. In contrast, endemic populations that are distinct from epidemic population would suggest that dispersal plays a reduced role in fomenting outbreak rise. An intermediate possibility with differentiation between epidemic and endemic population as a continuous spatial gradient might suggest a temporal lag in genetic differentiation that may take the form of a genetic travelling wave. PCA, cluster analyses, isolation by distance, and sPCA were used to characterize spatial genomic variation using 300 SBW larvae sampled in 2015 and genotyped at thousands of genome-wide SNPs. We found support for a genetic travelling wave pattern, matching the SBW density pattern. These results indicate that dispersal may trigger the transition of populations from the endemic to the epidemic state and thereby drive outbreak spread. By supplementing persisting endemic populations, migrants may drive populations to beyond levels that can be controlled by local biotic constraints. These results support the underlying justification of ongoing outbreak containment efforts in Atlantic Canada (i.e., the Early Intervention Strategy), although continued sampling as the outbreak progresses is needed confirm the temporal stability of the observed patterns.
Revisiting the relationship between spruce budworm outbreaks and forest dynamics over the Holocene in Eastern North America based on novel proxies
2020, Plant Disturbance Ecology: The Process and the ResponseThis chapter revisits the hypothesis put forward in 2007 that there is a close relationship between the structure (species composition and their distribution in the overstory) of mature boreal forest stands and the occurrence—as well as the periodicity, synchronicity, and amplitude—of insect outbreaks within the forest. New data from original dendroecological and paleoecological studies of spruce budworm outbreaks over the last 10 000 years strengthen this hypothesis. The variation in spatial and temporal dynamics of outbreaks is the reflection of the changes in forest structure driven by fire and insect outbreaks. There is a close inverse relationship between fire frequency and outbreak frequency over time. High fire frequency favors the presence of non-host species of the insect while low fire frequency favors the development of fire intolerant species such as balsam fir, the preferred host of the budworm, which influences outbreak frequency and impact. This would explain why the outbreaks of the 20th century were so widespread, even spreading further north due to climate warming.
Density has more influence than drought on spruce budworm (Choristoneura fumiferana) performance under outbreak conditions
2019, Forest Ecology and ManagementEruptive insect pests have significant impacts on the structure and function of forest ecosystems. Outbreaks of spruce budworm (Choristoneura fumiferana) (Clem.), for instance, occur approximately every 35–40 years, resulting in the loss of millions of hectares of spruce–fir forests in eastern North America. In addition to the density-dependent relationships that drive insect outbreaks, rising populations often coincide with drought events, which are expected to increase in both frequency and intensity in response to climate change. However, as populations approach the eruptive phase, consequences of intraspecific competition may outweigh the benefit of host water stress. The objective of our study was to quantify defoliation and insect performance responses to the interactive effects of drought and density of spruce budworm. To test for these interactions, we established a manipulative field experiment in a mature, balsam fir-dominated forest stand using a combination of single-tree rainout shelters and sleeve-caged insect larvae at four different densities: 0, 25, 50, or 100 individuals.
Defoliation of 1-year old shoots, but not current-year shoots, significantly increased in response to higher insect densities. Density also had a significant, negative effect on budworm percent survival, although the total number of recovered adults remained highest in the high density treatment. Adult female body mass was significantly reduced in response to increased density, but only on droughted trees. Lastly, male wing length was significantly decreased in response to increased density. Overall, our results demonstrate that across a broad range of outbreak densities, rain exclusion had a minor impact. Accordingly, we anticipate that as insect pest populations approach epidemic levels, the influence of density on defoliation, insect survival, and body condition is likely to outweigh the impact of moderate drought stress.
Salvage logging during spruce budworm outbreaks increases defoliation of black spruce regeneration
2018, Forest Ecology and ManagementAlthough advance regeneration abundance and vigor are critical factors determining future forest composition and productivity, very few studies have focused on how they are affected by spruce budworm (SBW) outbreaks even though they affect millions of hectares of boreal forest on a cyclical basis. Post-SBW salvage logging is often used to reduce economic losses but could interact with the outbreak to affect advance regeneration. This study aims to determine the impact of SBW outbreaks and post-outbreak salvage logging on the defoliation of advance regeneration in mixed coniferous stands of northeastern Canada. Specifically, we assessed the effect of regeneration height and species (balsam fir or black spruce), as well as canopy composition, on the defoliation of advance regeneration. We then evaluated the effect of salvage logging on defoliation sustained by advance regeneration and compared it to the one observed in stands only affected by the SBW. Regeneration height and species, canopy composition and salvage logging all significantly affected defoliation and showed multiple interactions. Taller balsam fir seedlings were three times as defoliated as smaller ones, whereas it was 2.3 times for black spruce. Balsam fir seedlings were 15% more defoliated than black spruce. Seedlings of both species located beneath a balsam fir canopy were also more defoliated (>50% defoliation) than seedlings found under black spruce trees (about 26% defoliation). Salvage logging in black spruce-dominated stands resulted in a ∼25% increase in defoliation of tall (2.5 m) black spruce regeneration when compared to non-harvested sites. We speculate that this could increase the fir component in spruce-dominated stands, leading to forests that are more susceptible to future SBW outbreaks. To protect spruce advance regeneration from increased defoliation, salvage harvesting of spruce-dominated stands may thus be delayed until the outbreak has subsided. Long-term studies are needed to determine whether a compositional change occurs or not, particularly in spruce-dominated stands. As a precautionary measure, changes in salvage logging practices may be implemented immediately to avoid potential problems such as decreased black spruce abundance and increased susceptibility to future SBW outbreaks.
Convergent herbivory on conifers by Choristoneura moths after boreal forest formation
2018, Molecular Phylogenetics and EvolutionMitogenomes are useful markers for phylogenetic studies across a range of taxonomic levels. Here, we focus on mitogenome variation across the tortricid moth genus Choristoneura and particularly the spruce budworm (Choristoneura fumiferana) species complex, a notorious pest group of North American conifer forests. Phylogenetic relationships of Tortricidae, representing two subfamilies, four tribes and nine genera, were analyzed using 21 mitogenomes. These included six newly-sequenced mitogenomes for species in the spruce budworm complex plus three additional Choristoneura species and 12 previously published mitogenomes from other tortricids and one from the Cossidae. We evaluated the phylogenetic informativeness of the mitogenomes and reconstructed a time-calibrated tree with fossil and secondary calibrations. We found that tortricid mitogenomes had conserved protein and ribosomal regions, and analysis of all protein-coding plus ribosomal genes together provided an efficient marker at any taxonomic rank. The time-calibrated phylogeny showed evolutionary convergence of conifer feeding within Choristoneura, with two independent lineages, the Nearctic spruce budworm complex and the Palearctic species Choristoneura murinana, both shifting onto conifers about 11 million years ago from angiosperms. These two host-plant shifts both occurred after the formation of boreal forest in the late Miocene. Haplotype diversification within the spruce budworm complex occurred in the last 4 million years, and is probably linked to the initial cooling cycles of the Northern Hemisphere in the Pliocene.