•Field survey, phenology model, airborne hyperspectral remote sensing were combined.•Remote sensing method using VNIR identified infested trees later than field survey.•Red-edge based indices (REIP, ...ANCB) were more sensitive during early infestation.
Detection in the early phase of bark beetle infestation is a vital task for proactive management strategies, as practiced in most Central European forests, to minimize economic losses due to bark beetle infestation and to mitigate their further spreading. For this work, remote sensing methods are coming to be in great demand as an objective approach to enable monitoring bark beetle infestation even at individual tree level.
This case study monitored bark beetle (Ips typographus) activity at local level in Norway spruce forest in the Czech Republic. The main aim of this study was to compare the remote sensing methods against classical field survey conducted by forest workers in detecting newly infested trees.
To compare these two methods, an extensive field and aerial campaign was conducted in the southern part of the Czech Republic during 2020. Bark beetle infestation was monitored by traditional methods (i.e. field survey) on a weekly basis from mid-March to mid-September. During the same period, aerial scans were performed once per month (seven in total) using a CASI-1500 hyperspectral sensor (visible and near-infrared, 400–1000 nm) with spatial resolution of 0.5 m. This work mapped transition from healthy up to red attack of 75 Norway spruce trees that were infested during the same week. The same number of healthy trees were added to the data set for hyperspectral data analysis. Both groups were analysed by vegetation indices, with emphasis on effect caused in the canopy by bark beetles.
The success rate for bark beetle detection is always associated with acquisition time. In order to define the optimal time for data acquisition, we employed a phenology model for I. typographus (RITY 2.0) to take into consideration bark beetle development.
The results of the experiment showed that classic field survey detected infested trees earlier than did analysis using remote sensing data from the visible and near-infrared region. The difference was 23 days for the most successful indices (i.e. REIP, PRI, and ANCB650–720) in our test. Nevertheless, both methods detected the infested trees within 6 weeks after infestation, which is the recommended period for taking measures to prevent bark beetles from spreading further, and thus hyperspectral imagery can be used as a valid information source for bark beetle detection.
•Windstorms and bark beetle outbreaks restored natural forests in the Bohemian Forest.•Increased dead wood amount and insolation contributed to increasing biodiversity.•Retention of legacies ...maintains biodiversity in salvage logging.
Severe natural disturbances are common in many forest ecosystems, particularly in the Northern Hemisphere. Attempts to minimize their effects through forest management include salvage logging. In the Bohemian Forest, one of Central Europe’s largest continuous forests, windstorms and bark beetle outbreaks have affected stands of Norway Spruce for centuries. Over the past decades, these natural disturbances and their management in the Bavarian Forest National Park and the adjacent Šumava National Park in the central part of the Bohemian Forest have been scientifically studied. Owing to a benign-neglect strategy, both windstorms and bark beetle outbreaks have increased stand structural heterogeneity, the amount of dead wood and light availability, which contribute to increased populations of nearly-extinct forest specialists. However, the response of a particular taxonomic group or species strongly depends on its relationship to specific legacies that persist after disturbances. Stand climate but not dead wood appears to greatly influence the diversity of epigeal bryophytes, whereas both factors determine the diversity of epixylic bryophytes. Both the amount and heterogeneity of dead wood seems to be more important than stand climate in determining assemblages of wood-inhabiting fungi and lichens. To reduce the population density of bark beetles in the management zones of both national parks, storm-felled spruces are salvage logged, which alters a variety of these legacies and natural successional pathways. Consequently, the numbers of species of wood-inhabiting fungi, saproxylic beetles and epixylic lichens are reduced. Natural levels of biodiversity in salvage-logged areas can be preserved by (1) preserving root plates of storm-felled trees with partly retained trunks; (2) avoiding soil disturbance by using cable yarding instead of harvesters; (3) retaining sun-exposed dry branches of storm-felled trees and snags of beetle-killed spruces; (4) avoiding logging damage of natural regeneration and of large trees that survive disturbances; and (5) bark scratching instead of debarking to avoid bark beetle outbreaks while maintaining biodiversity. Windstorms and bark beetle outbreaks could be utilized to restore intensely managed forests of Central Europe to their natural composition and structure. Furthermore, experimentally mimicked natural disturbances might help in gaining a mechanistic understanding of how natural disturbances affect biodiversity.
•Seasonal trajectories of nine bands and six indices from Sentinel-2 were analysed.•Seasonal change values were better for early detection than absolute reflectance.•Tasselled Cap Wetness and ...SWIR-based NDVI were most successful.•Separation of green-attack trees was best in autumn before next beetle generation.
In the past decade, massive outbreaks of bark beetles (Ips spp.) have caused large-scale decline of coniferous-dominated, prevailingly managed forests of Central Europe. Timely detection of newly infested trees is important for minimizing economic losses and effectively planning forest management activities to stop or at least slow outbreaks. With the advancement of Copernicus services, a pair of Sentinel-2 satellites provides a unique remote sensing data source of multi-temporal observations in high spatial resolution on the scale of individual forest stands (although not allowing for individual tree detection). This study investigates the potential for using seasonal trajectories of Sentinel-2 bands and selected vegetation indices in early detection of bark beetle infestation (so–called green-attack stage detection) in Norway spruce monoculture forests in the Czech Republic. Spectral trajectories of nine bands and six vegetation indices were constructed for the 2018 vegetation season using 14 satellite observations from April to November to distinguish four infestation classes. We used a random forest algorithm to classify healthy (i.e., stands not infested) and infested trees with various trajectories of decay. The seasonal trajectories of vegetation indices separated the infestation classes better than did the individual bands. Among the most promising vegetation indices we identified the tasselled cap wetness (TCW) component and normalized difference index constructed from near and shortwave infrared bands. Analysing the inter-annual change of the indices was more promising for early detection than is single-date classification. It achieved 96% classification accuracy on day of year 291 for the tested data set.The algorithm for early detection of tree infestation based on the assessment of seasonal changes in TCW was applied on the time series of Sentinel-2 observations from 2019 and its outputs were verified using field observations of forest conditions conducted on 80 spruce forest plots (located in spruce monoculture stands). The overall accuracy of 78% was achieved for the separation of healthy and green-attack classes. Our study highlights the great potential of multi-temporal remote sensing and the use of shortwave infrared wavelengths for early detection of spruce forest decline caused by bark beetle infestation.
•Tree mortality due to Ips typographus was explored over a 26-year time series.•Annual tree loss change was most affected by maximum temperature sums and wind throw.•Tree mortality increased after ...years allowing for the development of sister broods.
Norway spruce forests (Picea abies Karst.) of the Carpathian High Tatra Mountains have been subject to unprecedented tree mortality caused by attacks of the Eurasian spruce bark beetle (Ips typographus L.) in recent decades. The outbreaks were preceded and accompanied by wind throw events and periods of increased seasonal temperature.
We obtained climatic data and data on tree mortality due to storm throw and bark beetles by using a combination of direct dead tree inventory and remote sensing techniques, and examined annual tree loss change over a 26-year period in a forest district of the Slovak High Tatra National Park. In particular, we found that the combination of the previous year’s maximum daily temperature sum, tree mortality caused by wind, and bark beetle-caused tree mortality best explained the annual tree loss change in nonlinear regression models. The number of trees infested by bark beetles clearly increased with maximum air temperature sums ranging from 2850 to 3150° days, but declined below or above this thermal optimum. Annual tree mortality clearly increased subsequent to years associated with seasonal temperature sums allowing for the development of sister broods. Given consistently favourable future climatic conditions for development of I. typographus populations in the study area, a possible shift from the now predominantly uni-voltine to multi-voltine bark beetle populations might increase attack pressure also at high elevation sites.
•An insect epidemic occurred in North-Western Russia from 2001 to 2014.•We analyzed the subsequent annual tree loss caused by bark beetles.•Summer temperatures (June) were the most important ...driver.•High previous-year temperature increased tree mortality.•A longer sunshine duration also contributed to tree loss.
Acute or chronic drought stress caused by climate change can contribute to the weakening of forest ecosystems and lead to extensive bark beetle infestations. Siberian spruce (Picea obovata Ledeb.) forests of the Dvinsko-Pinegskiy, a natural reserve in the Arkhangelsk region, Russia, have been subject to unprecedented tree cover loss caused by the Eurasian spruce bark beetle (Ips typographus L.) in the last two decades. This is the first recorded case of such an extensive outbreak of Ips typographus occurring at higher latitudes. We used remote sensing and climate data to model and compute annual tree-loss change due to natural factors, with a focus on bark beetle outbreaks, over a 14-year period (2001–2014). Usinglinear regression models, we found a combination of average annual temperature and precipitation, temperature and precipitation in June, to be the most important drivers of annual tree-loss.
•Dynamics of spruce stand mortality varied across the study area.•Presence of trees other than spruce in stands reduces the risk of tree mortality.•Spruce height influence significantly mortality ...dynamics.•The most intensive outbreak progressed in old stands dominated by spruce.•Mixed young forests or pine stands were the most outbreak-resistant areas.
Dealing with tree mortality caused by forest pests is one of the most demanding challenges in forest protection and management. The current spruce bark beetle infestation in the Polish part of the Białowieża Forest (BF), which started in 2012, is considered the largest in history and has nearly eliminated Norway spruce (Picea abies (L.) Karst) as a major forest tree species there. This research analyzed the dynamics and spatial extent of the BF outbreak from 2015 to 2019. The extensive beetle epidemic caused a more than six times higher number of infested trees in 2019 as compared to 2015. Under the conditions of the unprecedented ongoing I. typographus outbreak, the dynamics of spruce stand mortality varied across the study area. A lower spruce mortality was observed in Białowieża National Park as compared to other forest districts. Host and environmental factors influenced the timing of spruce mortality. Spruce height, stand age, the proportion of spruce, the share of area covered by tree crowns, excluding spruce and dominant species in the stands, were the key factors of bark beetle infestation. The most intensive outbreak progressed in stands older than 100 years dominated by spruce, and areas with the lowest dynamics of spruce dieback were localized in mixed young forests or pine stands. The methods and results presented in this study serve as baseline information, supporting the efforts to model the spread of bark beetles and future decision-making.
•The structure of forest with bark-beetle-induced spruce die-back changed over time.•The recovering bark beetle stand became more diverse than the former dense forest.•Snags delayed wood decay and ...thus the establishment of seedlings on deadwood.•The bark beetle stand’s protective function reached a minimum after 10–15 years.•Bark beetle stands may provide protection againstsmall- to medium-scale avalanches.
Large-scale bark beetle outbreaks in spruce dominated mountain forests have increased in recent decades, and this trend is expected to continue in the future. These outbreaks have immediate and major effects on forest structure and ecosystem services. However, it remains unclear how forests recover from bark beetle infestations over the long term, and how different recovery stages fulfil the capacity of forests to protect infrastructures and human lives from natural hazards.
The aim of this study was to investigate how a bark beetle infestation (1992–1997) in a spruce dominated forest in the Swiss Alps changed the forest structure and its protective function against snow avalanches. In 2020, i.e. 27 years after the peak of the outbreak, we re-surveyed the composition and height of new trees, as well as the deadwood height and degree of decay in an area that had been surveyed 20 years earlier. With the help of remote sensing data and avalanche simulations, we assessed the protective effect against avalanches before the disturbances (in 1985) and in 1997, 2007, 2014 and 2019 for a frequent (30-year return period) and an extreme (300-year return period) avalanche scenario.
Post-disturbance regeneration led to a young forest that was again dominated by spruce 27 years after the outbreak, with median tree heights of 3–4 m and a crown cover of 10–30%. Deadwood covered 20–25% of the forest floor and was mainly in decay stages two and three out of five. Snags had median heights of 1.4 m, leaning logs 0.5 m and lying logs 0.3 m. The protective effect of the forest was high before the bark beetle outbreak and decreased during the first years of infestation (until 1997), mainly in the case of extreme avalanche events. The protective capacity reached an overall minimum in 2007 as a result of many forest openings. It partially recovered by 2014 and further increased by 2019, thanks to forest regeneration. Simulation results and a lack of avalanche releases since the infestation indicate that the protective capacity of post-disturbance forest stands affected by bark beetle may often be underestimated.
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