Abstract
Research Highlights: Bark beetles are important agents of disturbance regimes in temperate forests, and specifically in a connected wind-bark beetle disturbance system. Large-scale windthrows trigger population growth of the European spruce bark beetle (Ips typographus L.) from endemic to epidemic levels, thereby allowing the killing of Norway spruce trees over several consecutive years. Background and Objectives: There is a lack of evidence to differentiate how outbreaks are promoted by the effects of environmental variables versus beetle preferences of trees from endemic to outbreak. However, little is known about how individual downed-tree characteristics and local conditions such as tree orientation and solar radiation affect beetle colonization of downed trees. Materials and Methods: To answer this question, we investigated the infestation rates and determined tree death categories (uprooted, broken, and stump) in wind-damaged areas in Western Tatra Mts. in Carpathians (Slovakia) from 2014–2016, following a windthrow in May 2014. In total, we investigated 225 trees over eight transects. For every tree, we measured its morphological (tree height, crown characteristics), environmental (solar radiation, terrain conditions, trunk zenith), temporal (time since wind damage), and beetle infestation (presence, location of attack, bark desiccation) parameters. We applied Generalized Additive Mixed Models (GAMM) to unravel the main drivers of I. typographus infestations. Results: Over the first year, beetles preferred to attack broken trees and sun-exposed trunk sides over uprooted trees; the infestation on shaded sides started in the second year along with the infestation of uprooted trees with lower desiccation rates. We found that time since wind damage, stem length, and incident solar radiation increased the probability of beetle infestation, although both solar radiation and trunk zenith exhibited nonlinear variability. Our novel variable trunk zenith appeared to be an important predictor of bark beetle infestation probability. We conclude that trunk zenith as a simple measure defining the position of downed trees over the terrain can anticipate beetle infestation. Conclusions: Our findings contribute to understanding of the bark beetle’s preferences to colonize windthrown trees in the initial years after the primary wind damage. Further, our findings can help to identify trees that are most susceptible to beetle infestation and to prioritize management actions to control beetle population while maintaining biodiversity.
Highlights
Forests worldwide are shaped by natural disturbances [1,2]
Broken trees were less susceptible to I. typographus infestations, mainly due to higher summarizes all recorded characteristics
We found that the proportion of I. typographus infested trees desiccation rates (29% of trees vs. 1% of uprooted trees) and larger numbers of successful attacks by increased over three consecutive years
Summary
Forests worldwide are shaped by natural disturbances [1,2]. Eruptive insect species are considered as one of the main disturbance agents causing changes in ecosystems and affecting people’s livelihoods, the economy, and hydrology [3,4,5,6]. In Europe, one of the main tree species affected by large-scale insect outbreaks is the Norway spruce (Picea abies, [L.] Karst) [7], whose natural habitat belt spans. The transition period between a windfall and subsequent outbreak may take from one to three years [12] and can be regarded as a “wind-bark beetle disturbance regime” [13,14]. Such disturbances are considered calamities in managed forests, the newly formed dead wood enhances biodiversity [15].
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