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.

 

Phenology has become a field of growing importance due to the increasingly apparent impacts of climate change. However, the time-consuming, subjective and tedious nature of traditional human field observations have hindered the development of large-scale phenology networks. Such networks are rare and rely on time-lapse cameras and simplistic color indexes to monitor phenology. To automatize rapid, detailed and repeatable analyzes, we propose an Artificial Intelligence (AI) framework based on machine learning and computer vision techniques. Our approach extracts multiple ecologically-relevant indicators from time-lapse digital photography datasets. The proposed framework consists of three main components: (i) a random forest model to automatically select relevant images based on color information; (ii) a convolutional neural network (CNN) to identify and localize open tree buds; and (iii) a density-based spatial clustering algorithm to cluster open bud detections across the time-series. We tested this framework on a dataset including thousands of black spruce and balsam fir tree images captured using our phenological camera network. The performed experiments showed the efficiency of the proposed approach under challenging perturbation factors, such as significant image noise. Our framework is exceedingly faster and more accurate than human analysts, reducing the time-series processing time from multiple days to under an hour. The proposed methodology is particularly appropriate for large-scale and long-term analyzes of ecological imagery datasets. Our work demonstrates that the use of computer vision and machine learning methods represents a promising direction for the implementation of national, continental, or even global plant phenology networks.

1. As major alterations are occurring in climate and pest ranges, it is imperative to evaluate their combined contribution to tree mortality in order to propose mitigation measures and limit losses in forest productivity. The objective of this study was to explore the association between declines in tree growth resulting from climatic and biotic (spruce budworm) disturbances, and their interactions on tree mortality of two dominant tree species, Abies balsamea and Picea mariana, of the eastern North?American boreal forest.
2. We disentangle the influences of abiotic and biotic components on growth through a combination of model?data comparison techniques. First, we characterized the variability in tree growth and mortality in the study area using a network of tree?ring width measurements collected from living and dead trees. Subsequently, a bioclimatic simulation model was used to estimate the past annual, nonlinear, responses of stand?level net primary production (NPP) to climate variability (period 1902–2012). From these two data sources, we defined the biotic stress events as the variance in the tree?ring data unexplained by the bioclimatic forest growth simulation.
3. Throughout the 20th century, two periods of adverse climatic conditions preceded spruce budworm outbreaks episodes and induced tree mortality. Climatic stress events were associated with cold springs, warmer than average summers. We found that past stress history in interaction with tree characteristics and species predisposed trees to mortality. In addition, co?occurring events (climatic and biotic) increased the severity of mortality episodes.
4. Synthesis. Our study challenges the belief that spruce budworm outbreak is the primary driver of broad?scale tree mortality in eastern boreal forest. Rather, tree mortality is the result of cumulative events that combine unfavourable conditions for growth, resulting in loss of tree vigour and subsequently, mortality. Co?occurrence of stresses in the future may lead to more severe episodes of mortality, as extreme climatic events become more frequent.

Climate change is predicted to alter relationships between trophic levels by changing the phenology of interacting species. We tested whether synchrony between two critical phenological events, budburst of host species and larval emergence from diapause of eastern spruce budworm, increased at warmer temperatures in the boreal forest in northeastern Canada. Budburst was up to 4.6 ± 0.7 days earlier in balsam fir and up to 2.8 ± 0.8 days earlier in black spruce per degree increase in temperature, in naturally occurring microclimates. Larval emergence from diapause did not exhibit a similar response. Instead, larvae emerged once average ambient temperatures reached 10°C, regardless of differences in microclimate. Phenological synchrony increased with warmer microclimates, tightening the relationship between spruce budworm and its host species. Synchrony increased by up to 4.5 ± 0.7 days for balsam fir and up to 2.8 ± 0.8 days for black spruce per degree increase in temperature. Under a warmer climate, defoliation could potentially begin earlier in the season, in which case, damage on the primary host, balsam fir may increase. Black spruce, which escapes severe herbivory because of a 2?week delay in budburst, would become more suitable as a resource for the spruce budworm. The northern boreal forest could become more vulnerable to outbreaks in the future.

1.As major alterations are occurring in climate and pest ranges, it is imperative to evaluate their combined contribution to tree mortality in order to propose mitigation measures and limit losses in forest productivity. The objective of this study was to explore the association between declines in tree growth resulting from climatic and biotic (spruce budworm) disturbances, and their interactions on tree mortality of two dominant tree species, Abies balsamea and Picea mariana, of the eastern North?American boreal forest.

2.We disentangle the influences of abiotic and biotic components on growth through a combination of model?data comparison techniques. First, we characterized the variability in tree growth and mortality in the study area using a network of tree?ring width measurements collected from living and dead trees. Subsequently, a bioclimatic simulation model was used to estimate the past annual, non?linear, responses of stand?level net primary production (NPP) to climate variability (period 1902?2012). From these two data sources, we defined the biotic stress events as the variance in the tree?ring data unexplained by the bioclimatic forest growth simulation.

3.Throughout the 20th century, two periods of adverse climatic conditions preceded spruce budworm outbreaks episodes and induced tree mortality. Climatic stress events were associated with cold springs, warmer than average summers. We found that past stress history in interaction with tree characteristics and species predisposed trees to mortality. In addition, co?occurring events (climatic and biotic) increased the severity of mortality episodes.

4.Synthesis: Our study challenges the belief that spruce budworm outbreak is the primary driver of broad?scale tree mortality in eastern boreal forest. Rather, tree mortality is the result of cumulative events that combine unfavourable conditions for growth, resulting in loss of tree vigour and subsequently, mortality. Co?occurrence of stresses in the future may lead to more severe episodes of mortality, as extreme climatic events become more frequent.

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Climate change is causing northward shifts in species ranges. For mobile species such as insects, this will increase their access to forest ecosystems where in the past their presence and impact was limited. Range expansion and increases in outbreak severity of forest pests have been documented in Europe and North America (Jepsen et al. 2008; Bentz et al. 2010). Temperature-mediated phenological changes and trophic interactions among host trees, herbivorous insects, and their natural enemies are linked to the long-term effects of range expansion on boreal ecosystems. The degree to which temperate and boreal forest ecosystems are resilient to novel disturbance regimes will have direct consequences on the provisioning of goods and services from these forests and on long-term forest management planning. These concerns were the impetus for the organization of a workshop on climate-induced range shifts in boreal forest pests. Contributions to this special feature are selected papers from the International Union of Forest Research Organizations (IUFRO) workshop held in July 2016 in northeastern Quebec. The workshop was organized around five themes related to the consequences of range shifts of boreal forest insect pests: (1) plant–insect phenology, (2) species range expansions, (3) ecosystem response to changes in disturbance regimes, (4) interactions among disturbances, and (5) forest management and adaptation to change.

Plant neighborhood effects on herbivore damage have been observed in many systems although few studies have assessed the different component effects of the neighborhood (conspecific neighbors vs. heterospecific neighbors) on defoliation. No earlier studies have monitored how temporal scale influences neighborhood effects. We tested hypotheses on resource concentration/dilution and associational effects in the eastern spruce budworm (Choristoneura fumiferana)–forest system over a 10-yr period across different stands including stands dominated by highly vulnerable hosts (balsam fir), stands dominated by a species with low vulnerability (black spruce), and mixed composition stands (fir and spruce). We observed persistent resource concentration effects on the primary host (balsam fir) during the increasing phase of the outbreak in balsam fir-dominated and mixed stands, while both resource dilution and associational susceptibility effects were observed on the secondary host (black spruce) and the strength of associational susceptibility increased with an increase in resource dilution. We did not observe associational effects on the primary host in mixed and secondary host-dominated stands but in stands dominated by the primary host, we observed associational resistance when the resource was highly depleted. Therefore, the complexity of neighborhood effects suggests that future studies should consider the separate effects of conspecific and heterospecific neighbors, as well as changes through time in order to predict herbivore damage in different systems, and provide better preventive and reactive strategies to manage herbivore outbreaks.

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 on the provisioning of goods and services from these forests and on 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 climate-induced 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. While spruce budworm outbreak centres 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 mixed-wood 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.