Drought and stand susceptibility to attacks by the European spruce bark beetle: A remote sensing approach

1. Several time-series analyses have demonstrated that after extreme summer drought bark beetle damage increased. However, studies predicting stand susceptibility over large spatial extents are limited by technical constraints in obtaining detailed, spatially-explicit data on infestation spot occurrence.
2. Using a unique dataset of georeferenced bark beetle infestation data, we tested whether the spatial variation of local growing conditions of forest stands, topography, and landscape variables modified the local occurrence of Ips typographus infestations after a severe hot drought in Central Europe.
3. Bark beetle infestation occurrence depended on soil-related aridity intensity, elevation, slope, and soil conditions. We showed that elevation interacted with growing conditions and topography. At low elevations, spruce forests growing on flat areas and wetter soils were more sensitive to the infestations. On the contrary, forests on steep slopes and soils with low water availability were rarely attacked. At the landscape scale, bark beetle damage increased with host tree cover but decreased with compositional diversity.
4. Our findings are generally consistent with the growth-differentiation balance hypothesis that predicts that trees growing under chronic dry conditions tend to be more resistant against biotic disturbances.
5. Spruce stands at low elevations located in homogeneous landscapes dominated by spruce were those more exposed to bark beetles in the initial phase of a drought-induced outbreak.

     

Impacts of silvicultural thinning treatments on beetle trap captures and tree attacks during low bark beetle populations in ponderosa pine forests of northern Arizona

Our research used a combination of passive traps, funnel traps with lures, baited trees, and surveys of long-term thinning plots to assess the impacts of different levels of stand basal area (BA) on bark beetle tree attack and on trap captures of Ips spp., Dendroctonus spp., and their predators. The study occurred at two sites in ponderosa pine, Pinus ponderosa Dougl. ex Laws., forests, from 2004 to 2007 during low bark beetle populations. Residual stand BA ranged from 9.0 to 37.0 m2/ha. More predators and bark beetles were collected in passive traps in stands of lower BA than in stands of higher BA; however, significance varied by species and site, and total number of beetles collected was low. Height of the clear panel passive traps affected trap catches for some species at some sites and years. When pheromone lures were used with funnel traps [Ips pini (Say) lure: lanierone, +03/-97 ipsdienol], we found no significant difference in trap catches among basal area treatments for bark beetles and their predators. Similarly, when trees were baited (Dendroctonus brevicomis LeConte lure: myrcene, exo-brevicomin and frontalin), we found no significant difference for days to first bark beetle attack. Surveys of long-term thinning treatments found evidence of bark beetle attacks only in unthinned plots (approximately 37 m2/ha basal area). We discuss our results in terms of management implications for bark beetle trapping and control.     

A predictive framework to assess spatio‐temporal variability of infestations by the European spruce bark beetle

Natural disturbances are key factors for the development of forest ecosystems. In forests of central Europe and Scandinavia, the European spruce bark beetle Ips typographus is the most devastating biotic disturbance agent in Norway spruce Picea abies, but our understanding of the factors determining its spatio‐temporal dynamics is still quite limited. To quantify the drivers of bark beetle dynamics, we analyzed a survey dataset with annual resolution that covers 17 yr and 469 forest districts (10 860 km² of forest area) all over Switzerland. We used Poisson log‐normal models in a Bayesian framework to analyze the spatio‐temporal dynamics of bark beetle infestation spots at the forest district level. Bark beetle infestations increased with increasing heat sum (> 8.3°C), volume of standing Norway spruce stock, and the number of infestation spots of the previous year. Precipitation tended to slightly affect the risk of bark beetle infestations. Two major storm events further increased the spatio‐temporal variability of bark beetle infestations. Spruce abundance, storm damage and temperature are known to be important factors influencing the population dynamics of the European spruce bark beetle. Our study is the first to quantify the combined effects of spruce abundance and heat sum, whereby the heat sum turned out to be the most important and consistent predictor. Because our study area encompasses large ecological and climatological gradients, our model is likely to be applicable to Norway spruce forests in other regions of central Europe and Scandinavia.     

Field response of non‐target beetles to Ips sexdentatus aggregation pheromone and pine volatiles

Although mass trapping cannot be a definitive control measure, it is one of the few ones available to contain the destruction of millions of cubic metres of conifer forests perpetrated every year worldwide by bark beetles. However, using bark beetle aggregation pheromones during both monitoring and control programs may negatively affect other saproxylic insects. The aim of this study was to describe the response of both Ips sexdentatus and its saproxylic beetle associates, especially predators, to traps baited with a commercial blend of I. sexdentatus aggregation pheromone. Furthermore, the usefulness of adding pine volatiles, such as (?)‐α‐pinene and ethanol, to the pheromone was discussed. The commercial blend proved to be attractive to I. sexdentatus adults, both when used alone and together with pine volatiles. Pheromone attractiveness, however, was lessened by the addition of the volatiles. The pheromone blend proved to be attractive to Thanasimus formicarius, as well as to other predator species. Overall, although during our study, traps baited only with (?)‐α‐pinene and ethanol attracted some predator specimens, I. sexdentatus pheromone traps were more attractive. Our study confirms that calendar differences in flight activity between the bark beetle and its predators are substantial; therefore, they should be taken into account when planning control measures. According to our data, the commercial blend of I. sexdentatus pheromone seems to be the most effective, among the baits used, in catching I. sexdentatus adults, while reducing the impact on T. formicarius.     

Kairomonal Responses of Natural Enemies and Associates of the Southern Ips (Coleoptera: Curculionidae: Scolytinae) to Ipsdienol, Ipsenol and Cis-Verbenol

Bark beetle infested pines are an ephemeral habitat utilized by a diverse assemblage of insects. Although many bark beetle insect associates have little or no measurable impact on bark beetle brood production, some reduce brood production by either competing with brood for the limited phloem tissue or by feeding on brood. Several studies have observed synchrony between the colonization of hosts by bark beetles and the arrival of insect associates. Some insect associates mediate synchrony with bark beetle mass attacks with kairomonal responses to bark beetle aggregation pheromones. The objectives of this study were to document the community of Coleoptera associated with the southern Ips (Ips avulsus, Ips calligraphus and Ips grandicollis) and to test the hypothesis that synchrony of insect associates with the southern Ips is mediated by kairomonal responses to aggregation pheromones. A large community of Coleoptera (109 species) was recorded from traps baited with southern Ips pheromones. A significant treatment effect was observed for the guilds of meristem feeders, natural enemies and woodborers. The southern Ips pheromone ipsenol was broadly attractive to meristem feeders, natural enemies and woodborers and in general blends were more attractive than individual compounds. These results demonstrate that a diverse community of Coleoptera is associated with the southern Ips and that several members of this community facilitate synchrony with kairomonal responses to southern Ips aggregation pheromones.     

Log colonization by Ips sexdentatus prevented by increasing host unsuitability signaled by verbenone

Sudden increments of breeding material after windstorms, forest fires, or inappropriate management practices help bark beetles such as Ips sexdentatus Boerner (Coleoptera: Curculionidae: Scolytinae) increase in numbers and colonize standing healthy pine trees. Preventing bark beetles from arriving to susceptible trees or logs may have great relevance for bark beetle management. Recent studies have reported inhibition of the aggregation response of I. sexdentatus using verbenone. Two field experiments were conducted to examine the effect of verbenone on the colonization pattern of this beetle. The first experiment tested the combined effect of trans‐conophthorin, a non‐host bark volatile with known repellent effect, and verbenone on Pinus sylvestris L. (Pinaceae) log piles of two sizes, but failed to protect them against I. sexdentatus attack when these two infochemicals were released at low rates. The results of this experiment suggested an interaction with the associated secondary bark beetle Orthotomicus erosus (Wollaston). A second experiment examined the response of I. sexdentatus and O. erosus to log piles that released verbenone at 0, 2, 10, or 40 mg day^?1. Although I. sexdentatus colonization of Pinus nigra Arnold logs was completely prevented at 40 mg day^?1, O. erosus could be found at all tested verbenone release rates. Besides verbenone, O. erosus colonization density and the height from which logs originated were the variables that best explained I. sexdentatus log colonization pattern. In addition, I. sexdentatus and O. erosus were rarely recorded colonizing the same log, and niche breadth analyses suggested that they excluded each other. The role of verbenone in the colonization process and its potential use in the prevention of population buildups of damaging bark beetles such as I. sexdentatus are discussed.     

Evaluation of funnel traps for characterizing the bark beetle (Coleoptera: Scolytidae) communities in ponderosa pine forests of north-central Arizona

Lindgren funnel traps baited with aggregation pheromones are widely used to monitor and manage populations of economically important bark beetles (Coleoptera: Scolytidae). This study was designed to advance our understanding of how funnel trap catches assess bark beetle communities and relative abundance of individual species. In the second year (2005) of a 3-yr study of the bark beetle community structure in north-central Arizona pine (Pinus spp.) forests, we collected data on stand structure, site conditions, and local bark beetle-induced tree mortality at each trap site. We also collected samples of bark from infested (brood) trees near trap sites to identify and determine the population density of bark beetles that were attacking ponderosa pine, Pinus ponderosa Douglas ex Lawson, in the area surrounding the traps. Multiple regression models indicated that the number of Dendroctonus and Ips beetles captured in 2005 was inversely related to elevation of the trap site, and positively associated with the amount of ponderosa pine in the stand surrounding the site. Traps located closer to brood trees also captured more beetles. The relationship between trap catches and host tree mortality was weak and inconsistent in forest stands surrounding the funnel traps, suggesting that trap catches do not provide a good estimate of local beetle-induced tree mortality. However, pheromone-baited funnel trap data and data from gallery identification in bark samples produced statistically similar relative abundance profiles for the five species of bark beetles that we examined, indicating that funnel trap data provided a good assessment of species presence and relative abundance.     

Characterization of fungal communities associated with the bark beetle Ips typographus varies depending on detection method, location, and beetle population levels

The Eurasian spruce bark beetle Ips typographus and their fungal associates can cause severe damage to Norway spruce forests. In this paper, by using both molecular and cultural methods, we compared fungal assemblages on bark beetles from different locations, characterized by different beetle population levels. Ips typographus was trapped in the western Alps in two outbreak and in two control areas. Sequencing of clone libraries of Internal Transcribed Spacer (ITS) identified 31 fungal Operational Taxonomic Units (OTUs), while fungal isolations yielded 55 OTUs. Only three OTUs were detected by both molecular and cultural methods indicating that both methods are necessary to adequately describe fungal richness. Fungal assemblages on insects from these four and from an additional 12 study sites differed among stands in response to varying ecological conditions and to the limited spreading ability of I. typographus. Ophiostomatoid fungi showed higher diversity in outbreak areas; the pathogenic Ophiostoma polonicum was relatively uncommon, while O. bicolor was the most abundant species. This result was not unexpected, as insects were trapped not at the peak but at the end of the outbreaks and supports the hypothesis of a temporal succession among Ophiostoma species. Ubiquitous endophytes of trees or common airborne fungi were present both in outbreak and in control areas. Wood decaying basidiomycetes were almost never detected on beetles. Yeasts were detected only by molecular analysis, and the OTUs detected matched those reported elsewhere in Europe and in the world, suggesting a very long association between some yeasts and bark beetles.     

The Red-belted Bracket (Fomitopsis pinicola) colonizes spruce trees early after bark beetle attack and persists

The Red-belted Bracket (Fomitopsis pinicola) is one of the major decomposers of coniferous wood in Europe and can reach high densities after outbreaks of bark beetles. However, factors of dead wood type and decay stage, which determine the growth of reproductive biomass, i.e. basidiomes, remain unclear. In 2013, we surveyed 1280 dead wood objects and vital trees in spruce stands killed by the bark beetle Ips typographus in 2012, 2002, 1992 and in undisturbed stands for the presence, number, mean basidiome size and total volume of basidiomes. Living basidiomes were equally abundant on dead wood 1, 11, and 21 y after bark beetle outbreak, but were lacking on living trees. Our results indicate that F. pinicola is an effective early colonizer of the huge resource pulse of dead wood caused by the outbreak of bark beetles and basidiomes can persist for 21 y.     

Habitat loss decreases predator–prey ratios in a pine-bark beetle system

Increasing intensity of land use by humans has led to loss of natural habitats, resulting in isolation of remaining habitat fragments. Using a pine-bark beetle ecosystem as a model, we tested the hypothesis that the ratio of abundance of predators to prey should decrease with increasing habitat loss at the landscape scale. We selected ten red pine ( Pinus resinosa ) sites, representing extremes of available habitat within a 2 km radius surrounding each stand. The bark beetle, Ips pini , and its coleopteran predators were sampled using baited multiple funnel traps. Effects of stand isolation were considerable; ratios of predators to prey (mean number of predators/number of prey±SE) were significantly reduced in isolated stands (0.38±0.09) as compared to those with large amounts of surrounding conifer habitat (1.63±0.41). The decline in ratio occurred both because there was: a) a lower abundance of predators (ca 0.5–0.8×) captured in isolated stands; and b) a significantly higher number of prey (ca 2.2×) captured in isolated stands. Isolation or loss of habitat, therefore, differentially affected the two trophic levels, supporting theoretical predictions. Reductions in predator abundance and, presumably, enemy-caused mortality may lead to changes in the population dynamics of their prey species, possibly leading to increased outbreaks as habitat becomes increasingly isolated.     

Climate change alters elevational phenology patterns of the European spruce bark beetle (Ips typographus)

The European spruce bark beetle Ips typographus is the most important insect pest in Central European forests. Under climate change, its phenology is presumed to be changing and mass infestations becoming more likely. While several studies have investigated climate effects across a latitudinal gradient, it remains an open question how phenology will change depending on elevation and topology. Knowing how an altered climate is likely to affect bark beetle populations, particularly across diverse topographies and elevations, is essential for adaptive management. We developed a time‐varying distributed delay model to predict the phenology of I. typographus. This approach has the particular advantage of capturing the variability within populations and thus representing its stage structure at any time. The model is applied for three regional climate change scenarios, A1B, A2 and RCP3PD, to the diverse topography of Switzerland, covering a large range of elevations, aspects and slopes. We found a strong negative relationship between voltinism and elevation. Under climate change, the model predicts an increasing number of generations over the whole elevational gradient, which will be more pronounced at low elevations. In contrast, the pre‐shift in spring swarming is expected to be greater at higher elevations. In comparison, the general trend of faster beetle development on steep southern slopes is only of minor importance. Overall, the maximum elevation allowing a complete yearly generation will move upwards. Generally, the predicted increase in number of generations, earlier spring swarming, more aggregated swarming, together with a projected increase in drought and storm events, will result in a higher risk of mass infestations. This will increase the pressure on spruce stands particularly in the lowlands and require intensified management efforts. It calls for adapted long‐term silvicultural strategies to mitigate the loss of ecosystem services such as timber production protection against rockfall and avalanches and carbon storage.     

Thinning Jeffrey pine stands to reduce susceptibility to bark beetle infestations in California,U.S.A.

• 1 Bark beetles (Coleoptera: Curculionidae, Scolytinae) are commonly recognized as important tree mortality agents in coniferous forests of the western U.S.A.
• 2 High stand density is consistently associated with bark beetle infestations in western coniferous forests, and therefore thinning has long been advocated as a preventive measure to alleviate or reduce the amount of bark beetle‐caused tree mortality.
• 3 The present study aimed to determine the effectiveness of thinning to reduce stand susceptibility to bark beetle infestations over a 10‐year period in Pinus jeffreyi forests on the Tahoe National Forest, California, U.S.A. Four treatments were replicated three times within 1‐ha square experimental plots. Treatments included thinning from below (i.e. initiating in the smallest diameter classes) to a residual target basal area (cross‐sectional area of trees at 1.37 m in height) of: (i) 18.4 m²/ha (low density thin); (ii) 27.6 m²/ha (medium density thin); (iii) 41.3 m²/ha (high density thin); and (iv) no stand manipulation (untreated control).
• 4 Throughout the present study, 107 trees died as a result of bark beetle attacks. Of these, 71% (75 trees) were Abies concolor killed by Scolytus ventralis; 20.6% (22 trees) were Pinus ponderosa killed by Dendroctonus ponderosae; 4.7% (five trees) were P. jeffreyi killed by Dendroctonus jeffreyi; 1.8% (two trees) were P. jeffreyi killed by Ips pini; 0.9% (one tree) were P. jeffreyi killed by Orthotomicus (= Ips) latidens; 0.9% (one tree) were P. ponderosa killed by both Dendroctonus brevicomis and D. ponderosae; and 0.9% (one tree) were P. jeffreyi killed by unknown causes.
• 5 In the low density thin, no pines were killed by bark beetles during the 10‐year period. Significantly fewer trees (per ha/year) were killed in the low density thin than the high density thin or untreated control. No significant treatment effect was observed for the percentage of trees (per year) killed by bark beetles.

     

An attack of Ips amitinus (Coleoptera: Curculionidae: Scolytinae) on arboretum in West Siberia: New host of invasive bark beetle among exotic conifers

Invasive populations of small spruce bark beetle Ips amitinus were first registered in 2019 in the southeast of Western Siberia. In natural stands of Siberian pine (Pinus sibirica Du Tour), several hundred hectares of outbreak foci of the alien bark beetle were identified. In 2020, a local focus of the bark beetle was found in the conifer collection in the arboretum “Kedr” of the Institute of Monitoring Climatic and Ecological Systems SB RAS, 30 km from Tomsk. The bark beetle caused the main damage to the collection of pines. I. amitinus colonized both host plants Scotch pine (Pinus sylvestris L.) and mountain pine (Pinus mugo Turra), which were previously known to it in the native range in Europe, and the local Siberian species Siberian pine (Pinus sibirica Du Tour), Siberian spruce (Picea obovata Ledeb.) and introduced Far Eastern Korean pine (Pinus koraiensis Sieb. et Zucc.). Demographic characteristics of I. amitinus studied on damaged trees indicate its high reproduction potential in Siberia. The bark beetle outbreak focus was suppressed; however, this plantation requires further annual monitoring of pest abundance and distribution, both to preserve the scientific dendroecological field station and to study the implementation of sential plant conception in relation to the invasion of I. amitinus.     

Mapping urban growth probability in South Korea: comparison of frequency ratio, analytic hierarchy process, and logistic regression models and use of the environmental conservation value assessment

Rapid industrialization and economic growth in South Korea since the 1970s have resulted in severe environmental disturbance and pollution, problems aggravated by the imprudent expansion of urban areas. This paper analyzes and predicts urban growth patterns with the aim of contributing to more efficient urban planning. Urban growth probability index (UGPI) maps were prepared using the frequency ratio (FR), analytic hierarchy process (AHP), and logistic regression (LR) methods, with and without considering development restrictions based on the national environmental conservation value assessment map (ECVAM). Environmental and legal restrictions were associated with an average difference of 41.70% in conservation areas and an 81.32% average difference in agriculture and forest land use–land cover (LULC). Accuracy of the models was examined by area under the curve (AUC) analysis. Accuracies of UGPI maps produced with the ECVAM were higher than UGPI maps produced without the ECVAM. In addition, effectiveness and accuracy tests based on LULC showed that the UGPI maps produced with the ECVAM had a higher rate of accuracy that UGPI maps produces without the ECVAM. Using the ECVAM and assuming that urban and built-up areas will be 1.5 times greater than in 2005 and that environmental restrictions are removed, urban development can be expected to more than double in conservation areas and borderlands, increase by more than 1.5 times in developable areas, and decrease by half in old downtown areas. If legal restrictions are removed, urban development is expected to occur mostly in former conservation areas, followed by borderlands, old downtowns, and developable areas.     

Assessment of site characteristics as predictors of the vulnerability of Norway spruce (Picea abies Karst.) stands to attack by Ips typographus L. (Col., Scolytidae)

The intensity of bark beetle Ips typographus L. (Col., Scolytidae) attack on Norway spruce (Picea abies Karst.) is known to vary greatly among stands. In a control strategy approach, previous studies investigated the relationships between the variability in intensity of I. typographus attack and site characteristics such as stand age and altitude, mean tree circumference, growth rate and nearest‐neighbour distance, soil moisture, pH in H₂O and KCl, and soil contents of C, N, K, P, Mg, Ca, Fe, Cu, Zn and Mn. The data analysis method used in these studies was mainly the multiple linear regression, with the mean number of attacks per spruce tree in a stand as variable to explain. Previous results showed that the expected vulnerability of a Norway spruce stand to attack by I. typographus can be estimated on the basis of simple information of easy access to the forester, when the data on the stand in question is used with others for fitting the regression model. Prediction of the vulnerability of a stand, without including its data in the fitting of the model, was shown to be more approximate. Therefore, the objectives of this study were: (1) to improve the performance of models predicting the vulnerability of Norway spruce stands to attack by I. typographus, based on site characteristics; (2) to assess the stability of such predictive models when these are built using a moderate number of stands; and (3) to incorporate the resulting information in a global approach to control and prevention. Published data were re‐analysed for these purposes. A jackknifed multiple linear regression procedure, in which each stand in turn is discarded when fitting the model (jackknife replication), is presented. A great variability in the models fitted, depending on the stand discarded, is observed. For instance, the number of explanatory variables retained ranges from one (i.e. soil P content, for five jackknife replications) to 10 (for one jackknife replication), for R²‐values ranging from 0.5 to 1.0 and for one influential stand (i.e. the same stand characterized by an atypically low number of insect attacks compared to other stands with similar soil P content) against many influential stands. Differences between the model finally selected here using the revisited data and the models proposed earlier are discussed. A path analysis diagram is proposed for a more comprehensive modelling of Norway spruce stand vulnerability to I. typographus attack, based on site characteristics.     

Stand‐replacing fires reduce susceptibility of lodgepole pine to mountain pine beetle outbreaks in Colorado

Aim As climate change is increasing the frequency, severity and extent of wildfire and bark beetle outbreaks, it is important to understand how these disturbances interact to affect ecological patterns and processes, including susceptibility to subsequent disturbances. Stand‐replacing fires and outbreaks of mountain pine beetle (MPB), Dendroctonus ponderosae, are both important disturbances in the lodgepole pine, Pinus contorta, forests of the Rocky Mountains. In the current study we investigated how time since the last stand‐replacing fire affects the susceptibility of the stand to MPB outbreaks in these forests. We hypothesized that at a stand‐scale, young post‐fire stands (< c. 100–150 years old) are less susceptible to past and current MPB outbreaks than are older stands. Location Colorado, USA. Methods We used dendroecological methods to reconstruct stand‐origin dates and the history of outbreaks in 23 lodgepole pine stands. Results The relatively narrow range of establishment dates among the oldest trees in most sampled stands suggested that these stands originated after stand‐replacing or partially stand‐replacing fires over the past three centuries. Stands were affected by MPB outbreaks in the 1940s/1950s, 1980s and 2000s/2010s. Susceptibility to outbreaks generally increased with stand age (i.e. time since the last stand‐replacing fire). However, this reduced susceptibility of younger post‐fire stands was most pronounced for the 1940s/1950s outbreak, less so for the 1980s outbreak, and did not hold true for the 2000s/2010s outbreak. Main conclusions Younger stands may not have been less susceptible to the most recent outbreak because: (1) after stands reach a threshold age of > 100–150 years, stand age does not affect susceptibility to outbreaks, or (2) the high intensity of the most recent outbreak reduces the importance of pre‐disturbance conditions for susceptibility to disturbance. If the warm and dry conditions that contribute to MPB outbreaks concurrently increase the frequency and/or extent of severe fires, they may thereby mitigate the otherwise increased landscape‐scale susceptibility to outbreaks. Potential increases in severe fires driven by warm and dry climatic trends may lead to a negative feedback by making lodgepole pine stands less susceptible to future MPB outbreaks.     

Fungal associates of the tree-killing bark beetle,Ips typographus,vary in virulence,ability to degrade conifer phenolics and influence bark beetle tunneling behavior

The bark beetle Ips typographus carries numerous fungi that could be assisting the beetle in colonizing live Norway spruce (Picea abies) trees. Phenolic defenses in spruce phloem are degraded by the beetle's major tree-killing fungus Endoconidiophora polonica, but it is unknown if other beetle associates can also catabolize these compounds. We compared the ability of five fungi commonly associated with I. typographus to degrade phenolic compounds in Norway spruce phloem. Grosmannia penicillata and Grosmannia europhioides were able to degrade stilbenes and flavonoids faster than E. polonica and grow on minimal growth medium with spruce bark constituents as the only nutrients. Furthermore, beetles avoided medium amended with phenolics but marginally preferred medium colonized by fungi. Taken together our results show that different bark beetle-associated fungi have complementary roles in degrading host metabolites and thus might improve this insect's persistence in well defended host tissues.     

Effects of fire and spruce beetle outbreak legacies on the disturbance regime of a subalpine forest in Colorado

Aim There is increasing research attention being given to the role of interactions among natural disturbances in ecosystem processes. We studied the interactions between fire and spruce beetle (Dendroctonus rufipennis Kirkby) disturbances in a Colorado subalpine forest. The central questions of this research were: (1) How does fire history influence stand susceptibility to beetle outbreak? And conversely, (2) How does prior occurrence of a beetle outbreak influence stand susceptibility to subsequent fire? Methods We reconstructed the spatial disturbance history in a c. 4600 ha area by first identifying distinct patches in the landscape on aerial photographs. Then, in the field we determined the disturbance history of each patch by dating stand origin, fire scars, dates of mortality of dead trees, and releases on remnant trees. A geographical information system (GIS) was used to overlay disturbance by fire and spruce beetle. Results and main conclusions The majority of stands in the study area arose following large, infrequent, severe fires occurring in c. 1700, 1796 and 1880. The study area was also affected by a severe spruce beetle outbreak in the 1940s and a subsequent low‐severity fire. Stands that originated following stand‐replacing fire in the late nineteenth century were less affected by the beetle outbreak than older stands. Following the beetle outbreak, stands less affected by the outbreak were more affected by low‐severity fire than stands more severely affected by the outbreak. The reduced susceptibility to low‐severity fire possibly resulted from increased moisture on the forest floor following beetle outbreak. The landscape mosaic of this subalpine forest was strongly influenced by the interactions between fire and insect disturbances.     

Effect of temperature on the reproductive success,developmental rate and brood characteristics of Ips sexdentatus (Boern.)

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