While disturbances in European P. abies forests are on the rise, there is also an increasing need for the preservation of biodiversity through means such as retention forestry. Thus, we endorse bark gouging as an economic and conservation friendly pest control intervention to combine the regulation of I. typographus populations with the retention of dead wood biomass, particularly in inaccessible terrain or at small scales. Bark gouging has an economic advantage to manual debarking, since double the amount of P. abies logs can be treated to control I. typographus populations (Fig. 2). When compared to natural emergence densities, the pest control efficiency of bark gouging is even with a low accuracy (less than 50% of gouging stripes continuously cut) very high (Fig. 4). This will allow forest workers to work also under challenging conditions and process a higher amount of breeding material. The number of beetle species is reduced when the gouging stripes are more consistently removed and less bark remains intact (Fig. 3), confirming bark gouging as the conservation friendly alternative to complete (manual) debarking.
Bark gouging cost half of manual debarking
We highly recommend the conversion of pest management of I. typographus from manual debarking with spud to machinery bark gouging. To prevent outbreaks also without necessity of removal, with bark gouging breeding material can be made unsuitable more efficiently. Solely for forests in the alpine region this accounts for annually 25.000 m3 of manually debarked timber in the administration of the Bavarian state forests (BaySf, 2022). Together with the concomitant benefits of bark gouging in protected areas and in forest mires (Hagge et al., 2019a; Thorn et al., 2016), the economic benefits of bark gouging compared to manual debarking is relevant for a wider range of applications. In general, acquisition and maintenance cost are higher for the bark gouging device. Nevertheless, labor costs take up the main share for (motor-) manual work assignments in forestry. Initial and regular expenses for the gouging device will recoup quickly due to the higher output per work hour. While accurate delimbing is essential for manual debarking, with the bark gouging device small remnants of branches can also be milled away mechanically. Otherwise, the debarking spud is light and flexible which is particularly beneficial in challenging or steep terrain and could oppose the economic advantage of the gouging device in for example mountain forests. However, the overall physical demand on forest workers is higher when the debarking is done manually. If the wood is not fresh the strength needed to separate the bark from the wood is increasing (Duchesne and Nylinder, 1996), which in the case of the bark gouging device is carried out motorized. Yet, work conditions like different work experience, challenging terrain and contract work can lead to a decrease in gouging accuracy with this relatively new device. When less focus is put on the accuracy the amount of wood that can be treated will increase, improving the economic advantages even further. Nonetheless, with enough bark intact to support high densities of bark beetles emerging, pest control efficiency could be reduced.
Bark gouging controls Ips typographus outbreaks also with low accuracy
We demonstrate with our results that even when the phloem is not cut on the full length and gouging stripes are not continuous I. typographus populations can be reduced sufficiently. When compared to natural emergence densities, we were able to determine that the number of emerging I. typographus is reduced by at least 95.7% even with a gouging accuracy below 50% (Fig. 3). With salvage logging after i.e. wind disturbance I. typographus populations are reduced by about 90% when compared to unlogged areas (Thorn et al., 2014). Consequently, beetles can still emerge in high numbers from the logging residues, i.e. remaining stumps and crowns. Since pest control efficiency is also sufficient with low accuracy and more wood can be processed than with manual debarking, with gouging additional capacities to increase focus on the treatment of residues can be expected. The logs with gouged bark can then remain in the forest without threatening commercial P. abies forests nearby. Therefore, we can recommend bark gouging under field conditions where accuracies can be expected to variegate. We also suggest that the continuity of phloem disruption is not the most important factor for successful pest control, but rather the amount of potential breeding material treated.
Bark gouging destroys the majority of the existing beetle larvae and galleries mechanically. The remaining bark stripes dry out quickly and are thus unsuitable as food source for surviving larvae or further colonization (Hagge et al., 2019a). If bigger pieces of infested bark remain sufficiently intact (i.e. gouging with low accuracy) the ability to expand remaining breeding galleries is limited. Subsequently, this leads to a high colonization density and competition for the leftover sugar reserves in the phloem before it dries out. With increasing colonization density the impact of pathogens intensifies (Wegensteiner and Weiser, 1996), while the size and fitness of the offspring can also be negatively affected from the intraspecific competition for the limited foraging resources (Anderbrant et al., 1985; Salle et al., 2005). Consequently, with the reduction of breeding area not only the number of emerging beetles is reduced, but also their fitness and ability to spread might be compromised. Henceforward, research also needs to study the possible impairment of relevant traits in bark beetle populations emerging from trees with partly removed bark.
Besides, we confirmed the applicability of the traditional method of manual debarking with a pest control efficiency of 100% for I. typographus in P. abies logs. Nonetheless, it is important to consider, that early intervention (maximum five weeks after colonization) is imperative for this method to assure desiccation of larvae. In case the larval stage is already surpassed the fully developed beetles can finish their lifecycle also in the detached bark pieces. Thus, guidelines recommend to burn, chip, cover, or remove the residues (Kautz et al., 2021) or use the debarking device, which is milling the bark in small pieces and mechanically decimates also fully developed beetles. In the case of late intervention we can therefore advise to preferably use bark gouging over manual debarking. Given that bark gouging is based on the debarking device, yet with less area of contact, bark beetle populations can be reduced correspondingly, but to a lower extent.
Bark gouging biodiversity and dead wood retention
Our results for the impact on the non-target beetle species diversity indicate that increasing gouging accuracy and consequently a decrease of intact bark has a compromising effect on the number of species. Dead wood from bark beetle outbreaks is an important resource for many species and a crucial element for forest biodiversity and nature conservation (Müller et al., 2010). To reduce the impact of pest control on natural processes and to maximize habitat functionality it is vital to only apply the minimum required intensity of intervention. Manual debarking, in contrast, is removing the bark very accurately and, accordingly, only one beetle species emerged from the completely debarked segments (Fig. 2, Table 2). Even when the bark was gouged precisely and less than 50% of the gouging stripes were interrupted, on average two beetle species emerged from the experimental log segments.
Especially in early decay stages the bark is an important source for nutrients supporting a wide variety of specialized saproxylic species (Parisi et al., 2018; Ulyshen, 2018). Due to the phloem, the tissue to transport photosynthetic products in life trees, carbon and nutrient concentration are higher in the bark than xylem (Martin et al., 2015). Fallowing the species energy theory (Wright, 1983), this implements that the increased nutritional capacity can support a wider range of species when bark remains on the log. Additionally, as a protective outer layer the bark regulates the moisture content as well as the decomposition of the underlying biomass. Thus, it acts as an environmental filter in the early stages of species assembly in decaying wood (Dossa et al., 2018; Zuo et al., 2016) which in turn affects the later community composition and decomposition rates. Therefore, we are further endorsing the importance of bark as a structural component of dead wood supporting biodiversity in forests. Previous studies already indicated that bark gouging maintains natural levels of biodiversity in dead wood, while the complete removal of bark is significantly reducing it (Hagge et al., 2019a; Thorn et al., 2016). This reduction in biodiversity and structural integrity of the dead wood can also negatively influence higher trophic levels like i.e. foraging woodpeckers (Thorn et al., 2016).
However, first and foremost it is imperative to retain and accumulate dead wood biomass in protected as well as commercial forests (Müller and Bütler, 2010). On the one hand insect outbreaks and the resulting dieback are inherent parts in coniferous forest dynamics (Franklin et al., 2002). On the other hand salvage logging after wind disturbance or bark beetle infestations is even in protected areas a common management response (Lindenmayer et al., 2017; Müller et al., 2019). Here, the effectiveness of pest control interventions is currently debated due to the concurrent reduction in natural control agents (i.e. pathogens and predators), which usually limit pest outbreaks (Vanická et al., 2020). In fact, due to salvage logging after disturbances important forest functions as well as services can be degraded (Leverkus et al., 2020; Lindenmayer and Noss, 2006; Mayer et al., 2022). To uphold specified species diversity in disturbed forests it is beneficial to sustain unlogged areas with dead wood biomass (Thorn et al., 2020). Saproxylic diversity does not depend solely on the availability of dead wood, but also its dimensions, exposition and quality (Müller et al., 2015; Seibold et al., 2015; Thorn et al., 2020). In accordance to our results, the quality of the wood for beetle biodiversity is increasing the more bark remains intact. Thus, with bark gouging important structural components of dead wood can be uphold after pest control. The integration into pest control operations will support biodiversity conservation efforts, without subsequent risks for timber production.
Management recommendations
This is particularly applicable for the 39% of European protected areas which are overlapping with the range of Norway spruce (Hagge et al., 2019a) and hence susceptible to natural disturbances, such as bark beetle outbreaks following windstorms (Thorn et al., 2016). These areas include national parks, where a potential pest management must fulfil the additional requirements of biodiversity protection, environmental education, and recreation (IUCN primary objective of a national park; www.iucn.org). Consequently, bark beetle control measures which are poorly perceived by national park visitors may potentially lower the recreational value of the respective protected area (Berto, 2005). However, pest control interventions within protected areas are a common response to mitigate their spread and protect economic interests (Hlásny et al., 2021; Lindenmayer et al., 2017; Müller et al., 2019). To avoid large scale salvage logging in protected areas with all its negative effects we endorse to also integrate bark gouging to slow down the spread and establish pest free buffer zones. In commercial forests bark gouging can particularly provide advantages in small-scale disturbances where transport and harvesting cost exceed profits from the sale of timber. In mountain forests, where accessibility for machinery is not given or very costly, pest control treatments based on man power are often the only economic option, while the retention of dead wood provides valuable protection for infrastructure and people (Caduff et al., 2022; Ringenbach et al., 2022). The Bavarian state forestry already endorses mechanical bark gouging as a forest protection measure not only due to the economic, but also ecological, advantages. Logs designated for biomass retention are recommended to be gouged instead of debarked for preventative as well as early therapeutic treatment (BaySf, 2022). Completely mechanized methods (i.e. modified heads of harvesters for debarking) for the treatment of breeding material are a possible forest protection measures for large scales (Heppelmann et al., 2019; Mergl et al., 2021). But, pest control efficiency has only been investigated at small scales (FVA, 2022) and needs yet to be confirmed.