Outbreak dynamics of the spruce bark beetle Ips typographus in time and space

The European spruce bark beetle Ips typographus (L.) is one of the most important pests of mature Norway spruce Picea abies (Karst.) in Europe. Outbreaks (periods with large-scale beetle-caused tree mortality) are often triggered by large-scale stormfellings or drought, which provide a large surplus of suitable breeding material. The overall aim of the studies in this thesis was to increase knowledge about how forest characteristics and local beetle populations influence tree mortality and bark beetle outbreak dynamics in time and space. We found that the size of infestation spots in general were small (<10 killed trees), that they had high extinction probabilities in the next beetle generation. The main factors increasing the probability of infestation spots in new locations (i.e. colonisations) and decreasing the probability of spot extinction were volume of spruce per ha and to some extent connectivity (i.e. distance and spot size) to neighbouring infestation spots from the previous year. The total number of killed trees during the outbreak in stands surrounding storm gaps increased with the size of the local population of I. typographus initially produced in the storm-felled trees in the storm gaps (measured as number of colonised stormfelled trees). The effect of the previous year’s local population declined as the outbreak progressed due to host tree depletion in the areas with the largest storm gaps. The reproductive success of beetles at the tree level scale was negatively influenced by the colonisation density (as a result of intraspecific competition), which in turn was affected by the trees’ diameter. The reproductive success and colonisation density differed strongly between the outbreak years. A low reproductive success in the final years may have contributed to the ultimate collapse of the outbreak. Increases in the density of natural enemies were lower than expected but may also have contributed somewhat to the outbreak collapse. The results from the different studies demonstrate a large complexity in the bark beetle-host tree interactions that influence the outbreak dynamics of I. typographus in time and space

Climate change-induced shifts in survival and size of the worlds' northernmost oviparous snake: A 68-year study

Because of their dependence on ambient temperature ectothermic animals can serve as sentinels of conservation problems related to global warming. Reptiles in temperate areas are especially well suited to study such effects, as their annual and daily activity patterns directly depend on ambient temperature. This study is based on annual data spanning 68 years from a fringe population of Grass Snakes (Natrix natrix), which is the world's northernmost oviparous (egg-laying) reptile, and known to be constrained by temperature for reproduction, morphology, and behavior. Mark-recapture analyses showed that survival probability was generally higher in males than in females, and that it increased with body length. Body condition (scaled mass index) and body length increased over time, indicative of a longer annual activity period. Monthly survival was generally higher during winter (i.e., hibernation) than over the summer season. Summer survival increased over time, whilst winter survival decreased, especially during recent decades. Winter survival was lower when annual maximum snow depth was less than 15 cm, implying a negative effect of milder winters with less insulating snow cover. Our study demonstrates long-term shifts in body length, body condition and seasonal survival associated with a warming climate. Although the seasonal changes in survival ran in opposite directions and though changes were small in absolute terms, the trends did not cancel out, but total annual survival decreased. We conclude that effects of a warming climate can be diverse and pose a threat for thermophilic species in temperate regions, and that future studies should consider survival change by season, preferably in a long-term approach

Large-scale wall-to-wall mapping of bark beetle damage and forest practices using the distance red swir index and operational harvester data

Satellite-based inventories of bark beetle attacks are increasingly used for detecting and monitoring infested forest at the landscape scale. The Normalized Distance Red & SWIR index is one of few indices that have shown higher accuracies than commonly used vegetation indices. In this study, the temporal changes of the distance red swir (Delta DRS) index were analyzed, validated and applied to multi-temporal Sentinel-2 images covering one tile of 110 x 110 km(2). The main purpose was to assess the applicability of a new Delta DRS vegetation index to detect spruce forest after bark beetle (Ips typographus) attacks. Harvester data from a private forest company were used to validate the method. The normalized DRS index has previously been developed and tested at test site level, while this study explored and demonstrated the use of Delta DRS in an applied context on a larger scale. Water and chlorophyll induced changes and different disturbances were effectively identified across the landscape. A linear-discriminant analysis was used to classify 274 clusters as attacked and healthy forest, with an overall accuracy of 78%. The largest Delta DRS values in our study (>0.06) corresponded well to clear-cuts, and all 172 clear-cuts were correctly classified. We conclude that the Delta DRS index has a potential to map vegetation changes related to water and chlorophyll changes in the Scandinavian forests and that it can be useful to identify bark beetle-infested forest within 1 year after the attacks and clear-cuts

Mitochondrial DNA haplotypes indicate two postglacial re-colonization routes of the spruce bark beetle Ips typographus through northern Europe to Scandinavia

Species in northern Europe re-colonized the region after the last glacial maximum via several routes, which could have lingering signatures in current intraspecific trait variation. The spruce bark beetle, Ips typographus, occurs across Europe, and biological differences have been found between southern and northern Scandinavian populations. However, the postglacial history of I. typographus in Scandinavia has not been previously studied at a fine geographical scale. Therefore, we collected specimens across northern Europe and analysed the genetic variation in a quite large mitochondrial fragment (698 bp). A high genetic diversity was found in some of the most northern populations, in the Baltic States, Gotland and central Europe. Detected genetic and phylogeographic structures suggest that I. typographus re-colonized Scandinavia via two pathways, one from the northeast and one from the south. These findings are consistent with the re-colonization history of its host plant, Picea abies. However, we observed low haplotype and nucleotide diversity in southern Scandinavian populations of I. typographus, indicating that (unlike P. abies) it did not disperse across the Baltic Sea in multiple events. Further, the divergence among Scandinavian populations was shallow, conflicting with a scenario where I. typographus expanded concurrently with its host plant from a 'cryptic refugium' in the northwest

Population dynamics of tree-killing bark beetles

During outbreak periods, the European spruce bark beetle and the North American mountain pine beetle are able to kill millions of coniferous trees. Throughout the 20th century, six outbreaks have occurred in Sweden and four in British Columbia, with about 20-year intervals in both regions. The outbreaks of the mountain pine beetles seem to grow much larger and last longer compared to the outbreaks of the spruce bark beetles. Over the years, the mountain pine beetle has killed about 60 million ha forest or 550 million m3 trees in British Columbia, which is at least one hundred times more than for the Spruce bark beetle in Sweden. Damages of both species have increased markedly in the last forty years. About 750 spruce bark beetles per m2 are necessary to kill a healthy spruce, whereas seven times fewer, i.e., about 110 mountain pine beetles per m2, are needed to kill a healthy pine. Furthermore, twice as many offspring per m2 bark are produced by the spruce bark beetle compared to the mountain pine beetle. An explanation for the large differences in population dynamics between these two beetle species may spring from differences in (1) the availability of host trees, (2) number of specimens required to kill a tree, and (3) reproductive success. The latter is in turn affected by the intraspecific competition, nutrient content, and occurrence of fungi

A comparison of outbreak dynamics of the spruce bark beetle in Sweden and the mountain pine beetle in Canada (Curculionidae: Scolytinae)

The European spruce bark beetle (Ips typographus) and the North American mountain pine beetle (Dendroctonus ponderosae) may kill millions of trees during outbreak periods. Both species have also experienced large outbreaks in recent years. But the magnitude of the outbreaks of D. ponderosae is much larger. In this review we compare the outbreak history of I. typographus in Sweden with D. ponderosae in British Columbia in Canada. We also discuss some possible explanations for the difference in outbreak magnitude between the two species. During the last fifty years (1960-2009), three outbreaks of I. typographus have occurred in Sweden which resulted in a volume of about 9 million m3 of killed Norway spruces (Picea abies). During the same period D. ponderosae has killed about 600 million m3 of lodgepole pine (Pinus contorta) in British Columbia. Based on a literature review we suggest two factors that may contribute to the much more severe outbreaks caused by D. ponderosae: (1) a lower colonisation density needed by D. ponderosae to overcome tree defences and (2) a higher reproductive success of D. ponderosae in killed trees. In addition, the proportion of old stands, susceptible to bark beetle attacks, is much higher in British Columbia than in Sweden

Local colonization-extinction dynamics of a tree-killing bark beetle during a large-scale outbreak

Forest pest insects may cause large-scale tree growth reductions and tree mortality during outbreaks. The large-scale development of outbreaks has frequently been studied, while the colonization-extinction dynamics during outbreaks is less known. We study the colonization-extinction dynamics of a severe tree-killing bark beetle, Ips typographus, during an outbreak across a 130000ha forest landscape in southern Sweden. We recorded annual colonization and extinction events in 1ha pixels across the landscape by helicopter surveys during three consecutive years, and modeled colonization and extinction probabilities based on focal pixel quality, local population size, connectivity to surrounding beetle populations, and tree composition in the surrounding landscape. The local populations had a high turnover; 81-93% of all occupied pixels were the result of colonizations in the same year and the annual extinction rates were 84-90%. The colonization probability increased and the extinction probability decreased with increasing spruce (host-tree) volume in focal pixels and the connectivity. The fitted spatial scaling parameter of the connectivity measure suggested that the colonization probability was mainly affected by the number of surrounding occupied pixels within a few hundred meters. The colonization probability also decreased with increasing volume of birch (a nonhost) in the focal pixel, while the extinction probability decreased with increasing local population size. In conclusion, the local population size and quality of the focal pixel explained more of the colonization and extinction probability than the connectivity and composition of surrounding forest. The distribution of tree-killing bark beetles during outbreaks can be patchy and highly dynamic. Two reasons for this are the increased probability of successful attacks when exceeding a critical attack density and the fragmented distribution of large spruce volumes throughout the forest landscape

Habitat heterogeneity is a good predictor of boreal forest biodiversity

Reliable assessment measures are crucial for tracking changes in biodiversity and for evaluating the state of biodiversity. Two of the main drivers of biodiversity are habitat heterogeneity and resource amount. These drivers are used as proxies of biodiversity but assessing both is costly, limiting their practical use. To test which of the drivers best predicts the number and abundance of sessile species of conservation concern (including macrofungi, lichens, bryophytes, and vascular plants), we assessed forest stand heterogeneity using a method developed in Sweden ('Habitat Heterogeneity Score HHS'), and quantified the resource amount and quality of ecologically important structural variables (deadwood volume, basal area of living trees, proportion of broadleaved trees, and the age of the oldest tree in the stand). We conducted the assessments in 77 boreal coniferdominated forest stands in two regions of Sweden. Despite some group-specific organism differences, HHS was the best predictor of both number and abundance of all species of conservation concern, regardless of the region. Further, HHS was the best predictor of red-listed species number and abundance in the southern region, while a model including the volume of deadwood and the age of the oldest tree performed best in the northern region. Deadwood (CWD) volume was the single best resource amount predictor of the number and abundance of species of conservation concern, emphasizing the critical role that dead trees have for biodiversity. In addition, we calculated threshold values for deadwood volume and HHS depicting the level above which the number of red-listed species is significantly higher, and found this value to be higher in the southern region (22.4 m(3) ha(-1) deadwood and a HSS value of 17) than in the north (20.0 m(3) ha(-1) and 16). These values can be used as guidance when identifying coniferous forests with high enough qualities to support red-listed species. To conclude, the method of assessing habitat heterogeneity presented in this study is a practical and reliable way to identify forests of high biological diversity, and can therefore be part of the toolbox for sustainable forestry in boreal forests

Different triggers, different stories: Bark-beetle infestation patterns after storm and drought-induced outbreaks

In the recent decades, Norway spruce forests (Picea abies Karst.) in Europe have been subject to large-scale tree mortality caused by the spruce bark beetle (Ips typographus L.). The outbreaks were induced by storm-felling events and periods of drought, which are becoming increasingly frequent due to climate change. Because storms and droughts spatially affect forests differently, the infestation patterns and configuration of the bark beetles might differ between storms and drought. In this study, we examined local and landscape factors associated with bark beetle-caused tree mortality after one storm (2005) and one drought-induced spruce bark beetle outbreak (2018), both occurring in southern Sweden. A total of 13,192 infested one-ha pixels after the storm and 6,425 one-ha pixels after the drought (in total 19,617) were compared regarding differences in infestation occurrence and size and associated forest structures and climate between the two different outbreaks, using a generalized linear model (GLM) approach. Based on our findings, we discovered that the allocation of infestation patch sizes (including four classes: 5-10, 11-25, 26-50 and >50 infested trees) for the two outbreaks were quite similar with a large proportion (>0.6) of small groups (& LE;10 trees). However, the outcomes from this study demonstrate that the drivers behind the spatial configuration of bark beetle infestations can differ considerably between outbreaks triggered by storms and droughts, and the main cause seems to be linked to the spatial distribution of susceptible trees. The most consistent differences for both occurrence and infestation size were that storm-induced infestations increased more with spruce volumes and area of protected forests (nature reserves) in the landscape; whereas for the drought-induced infestations, occurrence and size increased more with clear-cuts in the landscape and spruce heights across spatial scales. Soil moisture and mean drought index (SPEI; May-July) were important for both outbreaks, but generally more important for the infestation sizes after droughts than after storms and may involve a time-lagged effect. The reasoning behind the differences between storms and droughts may be that during storm-induced outbreaks, when the wind-felled trees are removed or not suitable anymore, bark beetles need to find specific susceptible standing trees, while after drought all trees are more or less stressed, which results in a selection of large trees in dry and warm landscapes as they have more resources and favorable reproduction conditions. Finally, we show that the previous infestation size influenced the later infestation size negatively within landscapes of 25 ha and this seems to be related to depletion of susceptible host trees. These results are important for the assessment of more specific outbreak predictions, which should be integrated in future risk mapping of bark beetle outbreaks