Spatiotemporal dynamics of forest geometrid outbreaks.

We highlight recent developments and avenues for advancement, which can improve insight into the causes of changes in the spatiotemporal dynamics of forest Geometridea moth species (hereafter 'geometrids'). Some forest geometrids possess fundamental biological traits, which make them particularly liable to outbreak range expansions and host shifts mitigated by climate change. Indeed, recently observed changes in geometrid spatiotemporal dynamics represent both new research opportunities and challenges for empirically testing drivers of intra- and interspecific spatial synchrony, including the role of trophic interactions and biological traits (e.g. dispersal ability). We advocate that the emerging field of near-term ecological forecasting holds promise for studies of the spatiotemporal dynamics of forest geometrids and could be tailored to give both accurate predictions at management-relevant timescales and new insights into the mechanisms that underlie spatiotemporal population dynamics.

Seasonal and elevational variability in the induction of specialized compounds from mountain birch (Betula pubescens var. pumila) by winter moth larvae (Operophtera brumata).

The mountain birch [Betula pubescens var. pumila (L.)] forest in the Subarctic is periodically exposed to insect outbreaks, which are expected to intensify due to climate change. To mitigate abiotic and biotic stresses, plants have evolved chemical defenses, including volatile organic compounds (VOCs) and non-volatile specialized compounds (NVSCs). Constitutive and induced production of these compounds, however, are poorly studied in Subarctic populations of mountain birch. Here, we assessed the joint effects of insect herbivory, elevation and season on foliar VOC emissions and NVSC contents of mountain birch. The VOCs were sampled in situ by an enclosure technique and analyzed by gas chromatography-mass spectrometry. NVSCs were analyzed by liquid chromatography-mass spectrometry using an untargeted approach. At low elevation, experimental herbivory by winter moth larvae (Operophtera brumata) increased emissions of monoterpenes and homoterpenes over the 3-week feeding period, and sesquiterpenes and green leaf volatiles at the end of the feeding period. At high elevation, however, herbivory augmented only homoterpene emissions. The more pronounced herbivory effects at low elevation were likely due to higher herbivory intensity. Of the individual compounds, linalool, ocimene, 4,8-dimethylnona-1,3,7-triene, 2-methyl butanenitrile and benzyl nitrile were among the most responsive compounds in herbivory treatments. Herbivory also altered foliar NVSC profiles at both low and high elevations, with the most responsive compounds likely belonging to fatty acyl glycosides and terpene glycosides. Additionally, VOC emissions from non-infested branches were higher at high than low elevation, particularly during the early season, which was mainly driven by phenological differences. The VOC emissions varied substantially over the season, largely reflecting the seasonal variations in temperature and light levels. Our results suggest that if insect herbivory pressure continues to rise in the mountain birch forest with ongoing climate change, it will significantly increase VOC emissions with important consequences for local trophic interactions and climate.

Insect Herbivory Strongly Modifies Mountain Birch Volatile Emissions.

Insect herbivory is known to augment emissions of biogenic volatile organic compounds (BVOCs). Yet few studies have quantified BVOC responses to insect herbivory in natural populations in pan-Arctic regions. Here, we assess how quantitative and qualitative BVOC emissions change with increasing herbivore feeding intensity in the Subarctic mountain birch (Betula pubescens var pumila (L.)) forest. We conducted three field experiments in which we manipulated the larval density of geometrid moths (Operophtera brumata and Epirrita autumnata), on branches of mountain birch and measured BVOC emissions using the branch enclosure method and gas chromatography-mass spectrometry. Our study showed that herbivory significantly increased BVOC emissions from the branches damaged by larvae. BVOC emissions increased due to insect herbivory at relatively low larvae densities, causing up to 10% of leaf area loss. Insect herbivory also changed the blend composition of BVOCs, with damaged plants producing less intercorrelated BVOC blends than undamaged ones. Our results provide a quantitative understanding of the relationship between the severity of insect herbivore damage and emissions of BVOCs at larvae densities corresponding to background herbivory levels in the Subarctic mountain birch. The results have important and practical implications for modeling induced and constitutive BVOC emissions and their feedbacks to atmospheric chemistry.

Continental-scale travelling waves in forest geometrids in Europe: an evaluation of the evidence.

A recent paper claims the existence of one of the most large-scale travelling waves ever recorded for any animal population. Here we address why conceptual and methodological pitfalls may have served to exaggerate or even impose the spatial patterns reported. Photo credit: Jane U. Jepsen.

Spatially mismatched trophic dynamics: cyclically outbreaking geometrids and their larval parasitoids.

For trophic interactions to generate population cycles and complex spatio-temporal patterns, like travelling waves, the spatial dynamics must be matched across trophic levels. Here, we propose a spatial methodological approach for detecting such spatial match-mismatch and apply it to geometrid moths and their larval parasitoids in northern Norway, where outbreak cycles and travelling waves occur. We found clear evidence of spatial mismatch, suggesting that the spatially patterned moth cycles in this system are probably ruled by trophic interactions involving other agents than larval parasitoids.

Phase-dependent outbreak dynamics of geometrid moth linked to host plant phenology.

Climatically driven Moran effects have often been invoked as the most likely cause of regionally synchronized outbreaks of insect herbivores without identifying the exact mechanism. However, the degree of match between host plant and larval phenology is crucial for the growth and survival of many spring-feeding pest insects, suggesting that a phenological match/mismatch-driven Moran effect may act as a synchronizing agent. We analyse the phase-dependent spatial dynamics of defoliation caused by cyclically outbreaking geometrid moths in northern boreal birch forest in Fennoscandia through the most recent massive outbreak (2000-2008). We use satellite-derived time series of the prevalence of moth defoliation and the onset of the growing season for the entire region to investigate the link between the patterns of defoliation and outbreak spread. In addition, we examine whether a phase-dependent coherence in the pattern of spatial synchrony exists between defoliation and onset of the growing season, in order to evaluate if the degree of matching phenology between the moth and their host plant could be the mechanism behind a Moran effect. The strength of regional spatial synchrony in defoliation and the pattern of defoliation spread were both highly phase-dependent. The incipient phase of the outbreak was characterized by high regional synchrony in defoliation and long spread distances, compared with the epidemic and crash phase. Defoliation spread was best described using a two-scale stratified spread model, suggesting that defoliation spread is governed by two processes operating at different spatial scale. The pattern of phase-dependent spatial synchrony was coherent in both defoliation and onset of the growing season. This suggests that the timing of spring phenology plays a role in the large-scale synchronization of birch forest moth outbreaks.

Anisotropic patterned population synchrony in climatic gradients indicates nonlinear climatic forcing.

Although climatic forcing has been suspected to be the most common cause of spatial population synchrony owing to the Moran effect, it has proved difficult to disentangle the impact of climate from other possible causes of synchrony based on population survey data. Nonlinear population responses to climatic variation may be a part of this difficulty, but they can also provide an opportunity to highlight the climate impacts through targeted survey designs. In particular, when species distribution ranges encompass consistent spatial gradients in climate (e.g. according to latitude or altitude), such gradients can be strategically included in the spatial design of population surveys as to facilitate comparisons of spatial synchrony patterns across and along the gradient. In that case, we predict that nonlinear impacts of climatic variation on population growth rates will result in anisotropic (direction specific) synchrony patterns in the sense that synchrony will drop faster with distance along the climatic gradient than across it. We provide an empirical case study to exemplify survey design and analyses. Of two sympatric species of geometrids, inhabiting an altitudinal gradient in subarctic birch forest, one (Operophtera brumata L.) showed anisotropic synchrony consistent with a strongly nonlinear sensitivity to climatic variation, whereas the other (Epirrita autumnata Bkh.) did not. These results are interpreted in light of the biological characteristics of the species.

Climate change and outbreaks of the geometrids Operophtera brumata and Epirrita autumnata in subarctic birch forest: evidence of a recent outbreak range expansion.

1. Range expansions mediated by recent climate warming have been documented for many insect species, including some important forest pests. However, whether climate change also influences the eruptive dynamics of forest pest insects, and hence the ecological and economical consequences of outbreaks, is largely unresolved. 2. Using historical outbreak records covering more than a century, we document recent outbreak range expansions of two species of cyclic geometrid moth, Operophtera brumata Bkh. (winter moth) and Epirrita autumnata L. (autumnal moth), in subarctic birch forest of northern Fennoscandia. The two species differ with respect to cold tolerance, and show strikingly different patterns in their recent outbreak range expansion. 3. We show that, during the past 15-20 years, the less cold-tolerant species O. brumata has experienced a pronounced north-eastern expansion into areas previously dominated by E. autumnata outbreaks. Epirrita autumnata, on the other hand, has expanded the region in which it exhibits regular outbreaks into the coldest, most continental areas. Our findings support the suggestion that recent climate warming in the region is the most parsimonious explanation for the observed patterns. 4. The presence of O. brumata outbreaks in regions previously affected solely by E. autumnata outbreaks is likely to increase the effective duration of local outbreaks, and hence have profound implications for the subarctic birch forest ecosystem.