Host and site factors affecting tree mortality caused by the spruce bark beetle (Ips typographus) in mountainous conditions

Highlights

• We analysed the entire course of an outbreak of Ips typographus.

• We examined two processes: the initiation and the severity of spruce mortality.

• The initiation of tree mortality was primarily related to host factors.

• The severity of mortality was dependent on host size and insolation.

Abstract

To better understand the initiation and severity of tree mortality caused by the spruce bark beetle (Ips typographus (L.)) during an outbreak, we analysed the entire course of an outbreak from 1990 to 2000 in the Tatra Mountains (Western Carpathians, Central Europe). This time period represents the last complete bark beetle gradation in this area. We distinguished three outbreak phases: the incipient epidemic, epidemic and post-epidemic stages. The sampling unit was the forest subcompartment. We analysed a total of 315 forest subcompartments over more than 2000 ha. We investigated the influence of 11 environmental and stand variables on two processes in different phases of the outbreak: the initiation and the severity of spruce mortality. We used factor analysis, discriminant analysis, multiple linear regressions and boosted regression trees for the statistical analyses. The results showed that the roles of host and site factors in the initiation and severity of spruce mortality caused by the spruce bark beetle differed during the outbreak according to the exploitation of available host resources. The initiation of tree mortality was primarily related to host factors, and the severity of mortality was dependent on host size and insolation.

Introduction

Bark beetle outbreaks are significant forest disturbances that affect wildlife, watershed quality and recreational uses and cause extensive timber losses. Because of these dramatic effects, ecologists and forest managers must understand the mechanisms that drive outbreaks, to better predict and mitigate them (Walter and Platt, 2013).

Many authors have studied the factors that influence spruce mortality caused by bark beetles (Ips typographus) (Dutilleul et al., 2000, Becker and Schröter, 2001, Gilbert et al., 2005, Klopcic et al., 2009, Ogris and Jurc, 2010, Lausch et al., 2011, Marini et al., 2012, Overbeck and Schmidt, 2012, Albrecht et al., 2012, Thom et al., 2013, Mezei et al., 2014). The results of these investigations indicate that the mortality caused by I. typographus can be explained by characteristics such as forest age, potential solar radiation, and disturbance history. In these studies, favourable forest characteristics are understood to set the stage for outbreaks that are then triggered by climate and weather. An I. typographus infestation on standing trees usually begins after wind damage. However, the tree has to first reach a size that is suitable for an I. typographus attack (Baier, 1996). The role of insolation, or sun effects (Kautz et al., 2013), is also an important factor in the spread of bark beetle infestations once they have been initiated.

A substantial portion of the research on susceptibility to bark beetles describes how likely a tree or stand is to become infested if an outbreak occurs (Walter and Platt, 2013). Some authors, such as Boone et al. (2011), have highlighted the need for additional attention to research on the different phases of forest herbivore outbreaks.

Most of the studies on this topic do not distinguish between the initial conditions leading to an outbreak and those that facilitate the spread of an outbreak, especially for I. typographus. According to Coulson et al., 1985, Jakuš et al., 2003a, the mortality caused by bark beetles in mature coniferous stands is related to two main processes: the initiation and spread (expansion) of an infestation. The initiation of an infestation corresponds to the first spruce bark beetle attack on the living trees in a forest stand. These processes can be driven by different factors according to the stage of the outbreak. The process of initiation is not yet completely understood. Undisturbed spruce stands are well protected against direct insolation (sun effects) by individual and/or collective shading and are usually not attacked by the spruce bark beetle (Jakuš et al., 2011a), but natural disturbances such as windthrows or severe droughts can create conditions that favour a bark beetle attack (Jakuš et al., 2011b, Kautz et al., 2013). An approach in which disturbances were studied according to their initiation and severity was applied in a previous study by our group (Mezei et al., 2014), where we examined the wind – bark beetle disturbance regime. In that study, tree mortality caused by wind or by wind and spruce bark beetles together was examined.

In the first stage of an outbreak in mountainous conditions (incipient epidemic population), I. typographus predominantly colonises wind-thrown trees or trees on fresh forest edges (damage initiation), and the beetles are more likely to attack trees adjacent to the initial infestation (Jakuš et al., 2003a, Mezei et al., 2011). The subsequent spread of the infestation is caused by bark beetle attacks on trees neighbouring the primary attacked tree or group of trees. In the broader sense, spreading represents any further attack on trees after the initiation of infestation (during the years after the initial attack). The key factor influencing the spread of an infestation is the resistance of trees to bark beetle attack (Lieutier, 2004).

In this study, we analysed the entire course of a spruce bark beetle outbreak from 1990 to 2000 in the Tatra Mountains, as this time period represents the last completed bark beetle outbreak in this area. Data from this study area have also been analysed by Grodzki et al., 2006, Jakuš et al., 2003a, Mezei et al., 2014. Grodzki et al. (2006) compared different management strategies on both sides of the Tatra Mountains and found that there was no difference in the course and collapse of an outbreak. Jakuš et al. (2003a) studied the spread of an outbreak using terrestrial and remote sensing techniques. They found that the course and spread of the outbreak were related to the phase of the outbreak and to insolation. Neither of these studies considered environmental variables as factors affecting the initiation and spread of an outbreak. Mezei et al. (2014) found that factors affecting the initiation and intensity of tree mortality (caused by wind and bark beetles) changed during the outbreak period according to the phase of the outbreak. In the current study, we only examined the factors underlying the initiation and severity of tree mortality caused by the spruce bark beetle. Therefore, this work focus more on the population dynamics of I. typographus itself than on the interactions between disturbances. We find this approach reasonable because the findings of the present study can be used for modelling of the population dynamics of I. typographus. In addition, if the environmental characteristics of infested areas tend to change from the beginning of an outbreak cycle through the final crash of the outbreak, then understanding those dynamics may improve predictions regarding the spread of outbreaks (Walter and Platt, 2013). Additionally, by examining the environmental characteristics associated with bark beetle infestations through time, we can improve our understanding of how bark beetles use resources across the landscape (Nelson et al., 2007).

Following Coulson et al., 1985, Jakuš et al., 2003a we describe tree mortality using two related metrics: the initiation and the severity of tree mortality. According to our experience, mature stands tend to be completely attacked (destroyed) by bark beetles after such initial damage. Because we worked with database data without any spatial considerations, we were able to analyse the severity of spruce bark beetle-caused tree mortality as a variable related to the spread of bark beetle damage or to spot expansion. A more detailed analysis of the spread of damage (infestation) would require a different type of data (detailed infestation maps) and a different type of analysis than we present in this paper. The advantages of the dataset used here are the length of the time series and the precise quantification of tree morality caused by bark beetles.

Our working hypothesis was that the role of host and site factors in the initiation and severity of spruce mortality caused by bark beetles differs through the course of an outbreak. The initiation of spruce mortality due to bark beetles is mostly related to the age or size of the host trees, while the expansion (growth) or severity of tree mortality is related to factors associated with the age or dimensions of the host trees and to factors associated with insolation (Jakuš et al., 2003a, Jakuš et al., 2003b). We hypothesised that host factors would be most important in the early stages of the outbreak and that the role of insolation would be more important in later stages.