Jane U. Jepsen

Ecosystem Impacts of a Range Expanding Forest Defoliator at the Forest-Tundra Ecotone

Insect outbreaks in northern-boreal forests are expected to intensify owing to climate warming, but our understanding of direct and cascading impacts of insect outbreaks on forest ecosystem functioning is deficient. The duration and severity of outbreaks by geometrid moths in northern Fennoscandian mountain birch forests have been shown to be accentuated by a recent climate-mediated range expansion, in particular of winter moth (Operophtera brumata). Here, we assess the effect of moth outbreak severity, quantified from satellite-based defoliation maps, on the state of understory vegetation and the abundance of key vertebrate herbivores in mountain birch forest in northern Norway. We show that the most recent moth outbreak caused a regional-scale state change to the understory vegetation, mainly due to a shift in dominance from the allelopathic and unpalatable dwarf-shrub Empetrum nigrum to the productive and palatable grass Avenella flexuosa. Both these central understory plant species responded significantly and nonlinearly to increasing outbreak severity. We further provide evidence that the effects of the outbreak on understory vegetation cascaded to cause strong but opposite impacts on the abundance of the two most common herbivore groups. Rodents increased with defoliation, largely mirroring the increase in A. flexuosa, whereas ungulate abundance instead showed a decreasing trend. Our analyses also suggest that the response of understory vegetation to defoliation may depend on the initial state of the forest, with poorer forest types potentially allowing stronger responses to defoliation.

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.

Record-low primary productivity and high plant damage in the Nordic Arctic Region in 2012 caused by multiple weather events and pest outbreaks

The release of cold temperature constraints on photosynthesis has led to increased productivity (greening) in significant parts (32–39%) of the Arctic, but much of the Arctic shows stable (57–64%) or reduced productivity (browning, <4%). Summer drought and wildfires are the best-documented drivers causing browning of continental areas, but factors dampening the greening effect of more maritime regions have remained elusive. Here we show how multiple anomalous weather events severely affected the terrestrial productivity during one water year (October 2011–September 2012) in a maritime region north of the Arctic Circle, the Nordic Arctic Region, and contributed to the lowest mean vegetation greenness (normalized difference vegetation index) recorded this century. Procedures for field data sampling were designed during or shortly after the events in order to assess both the variability in effects and the maximum effects of the stressors. Outbreaks of insect and fungal pests also contributed to low greenness. Vegetation greenness in 2012 was 6.8% lower than the 2000–11 average and 58% lower in the worst affected areas that were under multiple stressors. These results indicate the importance of events (some being mostly neglected in climate change effect studies and monitoring) for primary productivity in a high-latitude maritime region, and highlight the importance of monitoring plant damage in the field and including frequencies of stress events in models of carbon economy and ecosystem change in the Arctic. Fourteen weather events and anomalies and 32 hypothesized impacts on plant productivity are summarized as an aid for directing future research.

Outbreaks by canopy-feeding geometrid moth cause state-dependent shifts in understorey plant communities

The increased spread of insect outbreaks is among the most severe impacts of climate warming predicted for northern boreal forest ecosystems. Compound disturbances by insect herbivores can cause sharp transitions between vegetation states with implications for ecosystem productivity and climate feedbacks. By analysing vegetation plots prior to and immediately after a severe and widespread outbreak by geometrid moths in the birch forest-tundra ecotone, we document a shift in forest understorey community composition in response to the moth outbreak. Prior to the moth outbreak, the plots divided into two oligotrophic and one eutrophic plant community. The moth outbreak caused a vegetation state shift in the two oligotrophic communities, but only minor changes in the eutrophic community. In the spatially most widespread communities, oligotrophic dwarf shrub birch forest, dominance by the allelopathic dwarf shrub Empetrum nigrum ssp. hermaphroditum, was effectively broken and replaced by a community dominated by the graminoid Avenella flexuosa, in a manner qualitatively similar to the effect of wild fires in E. nigrum communities in coniferous boreal forest further south. As dominance by E. nigrum is associated with retrogressive succession the observed vegetation state shift has widespread implications for ecosystem productivity on a regional scale. Our findings reveal that the impact of moth outbreaks on the northern boreal birch forest system is highly initial-state dependent, and that the widespread oligotrophic communities have a low resistance to such disturbances. This provides a case for the notion that climate impacts on arctic and northern boreal vegetation may take place most abruptly when conveyed by changed dynamics of irruptive herbivores.

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.

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.

Phenological diversity in the interactions between winter moth (Operophtera brumata) larvae and parasitoid wasps in sub-arctic mountain birch forest.

Population cycles of the winter moth (Operophtera brumata) in sub-arctic coastal birch forests show high spatiotemporal variation in amplitude. Peak larval densities range from levels causing little foliage damage to outbreaks causing spatially extensive defoliation. Moreover, outbreaks typically occur at or near the altitudinal treeline. It has been hypothesized that spatiotemporal variation in O. brumata cycle amplitude results from climate-induced variation in the degree of phenological matching between trophic levels, possibly between moth larvae and parasitoids. The likelihood of mismatching phenologies between larvae and parasitoids is expected to depend on how specialized parasitoids are, both as individual species and as a guild, to attacking specific larval developmental stages (i.e. instars). To investigate the larval instar-specificity of parasitoids, we studied the timing of parasitoid attacks relative to larval phenology. We employed an observational study design, with sequential sampling over the larval period, along an altitudinal gradient harbouring a pronounced treeline outbreak of O. brumata. Within the larval parasitoid guild, containing seven species groups, the timing of attack by different groups followed a successional sequence throughout the moths larval period and each group attacked 1-2 instars. Such phenological diversity within parasitoid guilds may lower the likelihood of climate-induced trophic mismatches between victim populations and many/all of their enemies. Parasitism rates declined with increasing altitude for most parasitoid groups and for the parasitoid guild as a whole. However, the observed spatiotemporal parasitism patterns provided no clear evidence for or against altitudinal mismatch between larval and parasitoid phenology.

Long-term Impacts of Contrasting Management of Large Ungulates in the Arctic Tundra-Forest Ecotone: Ecosystem Structure and Climate Feedback

The arctic forest-tundra ecotone (FTE) represents a major transition zone between contrasting ecosystems, which can be strongly affected by climatic and biotic factors. Expected northward expansion and encroachment on arctic tundra in response to climate warming may be counteracted by natural and anthropogenic processes such as defoliating insect outbreaks and grazing/browsing regimes. Such natural and anthropogenic changes in land cover can substantially affect FTE dynamics, alter ground albedo (index of the amount of solar energy reflected back into the atmosphere) and provide important feedbacks into the climate system. We took advantage of a naturally occurring contrast between reindeer grazing regimes in a border region between northern Finland and Norway which was recently defoliated by an outbreak of the geometrid moth. We examined ecosystem-wide contrasts between potentially year-round (but mainly summer) grazed (YRG) regions in Finland and mainly winter grazed (WG) regions in Norway. We also used a remotely sensed vegetation index and albedo to quantify effects on local energy balance and potential climate feedbacks. Although differences in soil characteristics and ground vegetation cover were small, we found dramatic differences in the tree layer component of the ecosystem. Regeneration of mountain birch stands appears to have been severely hampered in the YRG regime, by limiting regeneration from basal shoots and reestablishment of individual trees from saplings. This has led to a more open forest structure and a significant 5% increase in spring albedo in the summer grazed compared to the winter grazed regions. This supports recent suggestions that ecosystem processes in the Arctic can significantly influence the climate system, and that such processes must be taken into account when developing climate change scenarios and adaptation strategies.

Rock Ptarmigan (Lagopus muta) breeding habitat use in northern Sweden

AbstractAlpine and arctic tundra regions are likely to retract as a result of climate warming and concerns have been raised over the status of the Rock Ptarmigan (Lagopus muta). In Fennoscandia, the Rock Ptarmigan has low population abundance, and predictions based on harvest statistics show population declines throughout the range. In this study, we used a long-term opportunistic dataset of Rock Ptarmigan observations, environmental predictors derived from a digital vegetation map and a digital elevation model to describe the breeding distribution at three different ecological scales. Patterns of spatial distribution were similar across all the three study scales. The presence of permanent snow-fields positively influenced the occurrence of Rock Ptarmigan at the territory and landscape scale. Open vegetation, rock-dominated areas and, in particular, dry heath influenced Rock Ptarmigan presence positively at all scales. Altitude and terrain heterogeneity were important variables at all scales, with higher probabilities of Rock Ptarmigan being present at intermediate altitude ranges, with a high degree of terrain heterogeneity. This is the first study to describe Rock Ptarmigan breeding distribution in Fennoscandia and our findings yield new insights into the environmental variables that are important for the spatial distribution of Rock Ptarmigan during the breeding season. When planning conservation efforts, this information should be used to inform management regarding the protection of core areas and buffer zones related to the conservation and harvest management of the Rock Ptarmigan.ZusammenfassungBruthabitatnutzung beim Alpenschneehuhn (Lagopus muta) in Nordschweden Es ist zu erwarten, dass alpine und arktische Tundralebensräume in Folge der Klimaerwärmung schrumpfen, und es wurden bereits Bedenken bezüglich des Status des Alpenschneehuhns (Lagopus muta) laut. In Fennoskandinavien kommt das Alpenschneehuhn in geringer Populationsdichte vor, und Prognosen auf der Grundlage von Jagdstatistiken zeigen Bestandsrückgänge im gesamten Verbreitungsgebiet. Für diese Studie verwendeten wir einen über einen längeren Zeitraum gesammelten Datensatz mit Zufallsbeobachtungen von Alpenschneehühnern, dazu unabhängige Umweltvariablen aus einer digitalen Vegetationskarte und ein digitales Höhenmodell, um die Brutverbreitung auf drei verschiedenen ökologischen Ebenen zu beschreiben. Die räumlichen Verbreitungsmuster waren für alle drei in der Studie betrachteten Ebenen ähnlich. Das Vorhandensein dauerhafter Schneefelder hatte einen positiven Einfluss auf das Vorkommen von Alpenschneehühnern, sowohl auf Revier- als auch auf Landschaftsebene. Offene Vegetation, felsenreiche Flächen und speziell trockenes Heideland beeinflussten das Vorkommen von Alpenschneehühnern auf allen drei Ebenen positiv. Die Variablen „Höhenstufe“ und „Heterogenität des Geländes“ waren auf jeder Ebene wichtig; die Wahrscheinlichkeit eines Alpenschneehuhn-Vorkommens war in mittleren Höhenlagen mit starker Geländeheterogenität höher. Dies ist die erste Untersuchung, die sich mit der Brutverbreitung des Alpenschneehuhns in Fennoskandinavien befasst, und unsere Befunde liefern neue Erkenntnisse darüber, welche Umweltvariablen für die räumliche Verteilung der Alpenschneehühner während der Brutzeit von Bedeutung sind. Bei der Planung von Naturschutzmaßnahmen sollten den für das Schutz- und Jagdmanagement Verantwortlichen die für die Gewährleistung von Kernund Pufferzonen relevanten Informationen zur Verfügung gestellt werden.

Spatial patterns of goose grubbing suggest elevated grubbing in dry habitats linked to early snowmelt

The western Palaearctic tundra is a breeding habitat for large populations of European geese. After their arrival in spring, pink-footed geese (Anser brachyrhynchus) forage extensively on below-ground plant parts, using a feeding technique called grubbing that has substantial impact on the tundra vegetation. Previous studies have shown a high frequency of grubbing in lowland fen vegetation. In the present study, we examined the occurrence of grubbing in other habitat types on Spitsbergen, in the Arctic archipelago of Svalbard. Goose grubbing was surveyed along 19 altitudinal transects, going from the valley bottom to altitudes dominated by scree. Grubbing was more frequent in the wet habitat type at low altitudes compared to the drier habitat type at higher altitudes. For the dry habitat type, a higher frequency of grubbing was found in study plots with a south-east facing exposure where snowmelt is expected to be early. This suggests that pink-footed geese primarily use dry vegetation types for grubbing when they are snow-free in early spring and the availability of snow-free patches of the preferred wet vegetation types in the lowlands is limited. Dry vegetation types have poorer recovery rates from disturbance than wet ones. Sites with early snowmelt and dry vegetation types may therefore be at greater risk of long-term habitat degradation. We conclude that the high growth rate of the Svalbard-breeding pink-footed goose population suggests that increasing impacts of grubbing can be expected and argue that a responsible monitoring of the effects on the tundra ecosystem is crucial.