Effects of fire and stolen felling on RTB damage
In line with our first hypothesis and many previous studies (Komonen and Kouki, 2008; Toivanen et al., 2009; Westlind and Kelsey, 2019), we found that fire and stolen felling can significantly promote RTB invasion. The aggressive behavior of RTB under disturbance conditions can be attributed largely to their sensitivity to ethanol and monoterpenes (such as 3-carene) produced in stressed woody phloem (Kelsey and Westlind, 2017, 2018).
Hekkala et al. (2020) has shown that the number of recently attacked spruce in gap-cut stands increased significantly compared with that of burnt stands. This finding contrasts with our study in which no significant difference was observed between felled and burnt stands. This phenomenon may be related to the sampling time after fire and fire intensity. In general, after three years of fire, the burnt forest stands are less attractive to bark beetles due to the decrease in the concentration of attractive volatiles (Westlind and Kelsey, 2019). Moreover, the stands burnt by high intensity fire are not favored by bark beetles due to the destruction of stem phloem (Kulakowski and Jarvis, 2013). The sample plots in our study were located at the edge of burnt stands within one to two years after fire, which were characterized as low-intensity fire. In addition, a small amount of fallen trees in plots disturbed by stolen felling could still attract a large number of RTBs to harm the fallen and surrounding healthy pine trees. It is worth noting that the current forest management strategies to deal with RTB damage mostly focus on hanging traps or clearing burnt woods in forests after fire, while ignoring the strong attraction of fallen woods after stolen felling to RTB. The new control strategy should simultaneously consider the attraction of woods in forests under natural (e.g., fire) and human disturbances (e.g., stolen felling) to RTB.
Impacts of stand characteristics on RTB damage in the absence of disturbances
Previous studies have shown that bark beetle damage is affected by both beetle pressure and forest susceptibility (Nelson et al., 2006; Raffa et al., 2008). Similar to the studies of Nelson (2006) and Bone (2013), we only considered forest susceptibility (without considering beetle pressure) in our research, which may also be the attribution of the low R2 values obtained from generalized models. The purpose of our study was to explore the role of landscape structure and stand-scale factors in forest susceptibility assessment during the early stage of RTB invasion. The obtained results supported our hypothesis that stand characteristics were the main factors affecting RTB invasion in this stage.
The factors affecting bark beetle damage are associated with the stage of outbreak (Simard et al., 2012). In the endemic population phase, stand-scale attributes, such as tree vigor, tree size, and canopy density, determine the outcome of bark beetle damage (Jakus et al., 2011; Lausch et al., 2011; Mezei et al., 2014). When bark beetle population increases, landscape-level factors, such as host connectivity and forest structure, outperform in predicting stand sensitivity (Simard et al., 2012; Seidl et al., 2015). The connectivity of host patches is the key for bark beetles to locate their hosts quickly in the outbreak period when the population density is high, while in the period when the population density is low, bark beetles prefer to damage stands at a small scale due to the widespread of hosts (Raffa et al., 2008; Seidl et al., 2015). Bone et al. (2013) found that forest structure (AI index) played different roles in different stages of the mountain pine beetle outbreak, and forests with low fragmentation were more seriously damaged in the late stage of outbreak. All the above results are consistent with our findings. In the early stage of RTB invasion, the factors affecting RTB damage were mainly small-scale stand factors, such as aspect and canopy cover, while large-scale landscape factors did not play a role in the early stage of RTB invasion (Table 2).
We found that aspect played an important role in predicting RTB damage (Table 2). South-facing stands showed a higher degree of RTB damage compared to stands facing other directions. South-facing stands are more sensitive to bark beetles, because higher levels of solar radiation may reduce tree vitality and increase beetle survival (Coops et al., 2006; Lausch et al., 2011). Stands with lower canopy closure are more likely to be attacked by bark beetles, because relatively higher temperatures in such stands are more attractive to bark beetles (Netherer and Nopp-Mayr, 2005; Pasztor et al., 2014). Therefore, it is believed that aspect and canopy density are associated with solar radiation and stand temperature, respectively, and the significant role solar radiation plays during bark beetle invasion has also been revealed in many other studies (Kautz et al., 2013; Chen et al., 2014; Mezei et al., 2019). However, we did not observe a high relative importance in solar radiation in our study, which was present in the optimal model but was not significant. A possible explanation may be that the effect of extremely high radiation on RTB invasion is neutralized by the long-term cumulative radiation calculation (April to September).
Effects of forest proportion and elevation on RTB damage in the presence of disturbance
In line with our third hypothesis, forest proportion could significantly affect RTB invasion in the presence of disturbance conditions compared with the non-disturbance condition. The average distance of RTB dispersal flight in forests is beyond 500 m (Jones et al., 2019). Within a radius of 250 m, the forest structure with a high proportion of open habitats (non-forest areas) was conducive to RTB dispersal from the surrounding areas to disturbed forests rich in ethanol and monoterpenoid volatiles. However, under non-disturbance condition, RTB prefers to randomly select weak host trees in situ for feeding and mating, which is also the reason why landscape indicators could not alter the preference of RTB in selecting the hosts in the early stage of outbreak (Raffa et al., 2008; Walter and Platt, 2013). Hekkala et al. (2020) explored the effects of the proportion of broad-leaved trees and spruce forests within a radius of 500 m around disturbed plots on the abundance of Ips typographus. Although the above landscape indicators did not perform well in regression models, they showed a trend (negative correlation) consistent with that observed in our study. Therefore, the present study, which emphasizes the potential importance of disturbance and landscape structure during RTB spread, not only contributes to expanding the understanding of the roles large-scale factors play under different disturbance conditions but also assists in the implementation of pest management programs.
Low-altitude areas are susceptible to bark beetle invasion in the early stage of beetle outbreak, while the low temperature accompanying high-altitude stands could slow down the development of larvae, thus reducing the generation number of bark beetles. (Lausch et al., 2011; Logan & Powell, 2001). However, during the outbreak period, the activity range of bark beetles shifts to high-altitude areas due to the limitation of host resources in the low-altitude areas (Gregory et al., 2017). In this study, elevation did not exhibit significant effect on RTB damage in the absence of fire or stolen felling, which may possibly be attributed to the relatively small range of elevations of our study areas. However, forest stands with an elevation above 1000 m were more seriously damaged by RTB in the presence of fire or stolen felling. The possible reason may be that the attractive volatiles released by disturbed stands in high-altitude areas could spread around more easily.