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Ice storms cause significant damage to forests and increase trees’ vulnerability to secondary biotic factors. Thus, in their article, “The Effects of a Large-Scale Ice Storm Event on the Drivers of Bark Beetle Outbreaks and Associated Management Practices,” Groot, Ogris, and Kobler (2018) aim to research the patterns of insect outbreaks after a storm-induced disturbance and evaluate the effectiveness of current management methods. The researchers used a sample of damaged Norway spruce located across Slovenia after a catastrophic event that took place there in 2014. Only half of the fallen trees was removed afterward, making European spruce bark beetle spread around large areas. Groot et al. (2018) used the storm damage data collected through a field survey and the sanitary felling data on the number of species damaged to identify the links between variables: spruce damage vs. geological factors, damage vs. number of attacked trees, number of trees removed vs. spatial patterns of insect outbreaks, etc.
The data collected by Groot et al. (2018) reveals that the studied population of spruce bark beetle significantly grew between 2014 and the beginning of 2015 resulting in bigger patches to be attacked. They mainly attribute this finding to the extent of ice storm damage (i.e., amount of fallen and weakened trees). They also mention that the insect population could be developed because of favorable weather factors. However, this essential variable was not included in the analysis. It means that the acquired findings on links between the damage and the insect spreading can be biased to some extent. A comparison with a control sample (e.g., located in similar climatic conditions, yet non-affected by a storm) could help address the given issue more effectively.
The consideration of the intensity of a storm and tree damage as significant predictors of insect outbreaks can be regarded as a positive point in the study. However, the researchers draw their conclusion regarding the growing scope of pest attacks based on the evidence obtained by other researchers. The lack of sufficient longitudinal data did not allow Groot et al. (2018) empirically identify the behavioral features of spruce bark beetle. Although their explanation for a large-scale outbreak due to incomplete removal of fallen trees seems logical, the investigation of beetle dynamics throughout the time could help understand why the links between tree damage and insect spread became evident merely in the second year after the storm.
As for the selected methodology, the implementation of the stratified random sampling is appropriate for the given study as the administration of simple random sampling across the forest would not be feasible. Groot et al. (2018) collected the data from a sufficient number of homogeneous forest plots (n=810). In this way, the researchers managed to capture the major common characteristics across nine strata and to calculate more precise average numbers.
Based on the critique results, it is possible to suggest further research of various outbreak management and prevention techniques in the context of beetle population dynamics. To do so, the samples should be studied before the event and a few years after the event. Additionally, a more detailed evaluation of reactive measures should be conducted. The investigation of two different sample groups, i.e., partial removal of trees and complete removal of trees, can provide substantiated evidence upon which managerial recommendations can be developed.
Groot, M. D., Ogris, N., & Kobler, A. (2018). The effects of a large-scale ice storm event on the drivers of bark beetle outbreaks and associated management practices. Forest Ecology and Management, 408, 195-201.