Remote sensing applications to assess the impact of environmental risk factors to Norway spruce mortality by Ips typographus in Alpine and Hemiboreal regions

The growing frequency and magnitude of climatic anomalies associated with ongoing climate change are increasing the susceptibility of European forests to stress factors and creating favourable conditions for pest infestations. A clear example is represented by the current widespread epidemic outbreaks of the European spruce bark beetle Ips typographus (L.), the most relevant forest pest in Europe. In recent years, spruce forests across central and northern Europe have been going through a chronic and progressive increase in the frequency and intensity of bark beetle outbreaks. The underlying ecological context is often the result of several factors that act simultaneously on both insect populations and host tree species stands. Besides abiotic and biotic forest disturbances triggering eruptive insect proliferations, it is still unclear which environmental factors are responsible for the severity of bark beetle outbreaks, hence for the spatial extent of spruce mortality. Additionally, there is a significant lack of research on the role of the same risk factors in the outbreak spatio-temporal dynamics across different geographical and environmental contexts. This study aims at shedding light on these knowledge gaps, by making use of remote sensing as a main tool and focusing on two remarkably different climatic regions, the Alpine and the Hemiboreal forest ecosystems. The type of triggering factor (wind vs. drought) seems to affect outbreak dynamics, since infestation spots tend to be of the expansion type under drought conditions, thereby producing growth spots in the following year, while of the extinction type under wind circumstances, thus either stopping and dying out or generating proliferation spots. Furthermore, growth spots remain limited under wind conditions, whereas they undergo a considerable increase under drought ones. The presence of non-hosts appears to reduce overall damage risk, as infestation spots in a mixed stand tend to be fewer and smaller over time. Nevertheless, when considering single years, in a pure stand both expansion and extinction spots are unexpectedly smaller compared with those in a mixed stand, whereas both growth and proliferation spots appear to be much larger, indicating that a mixed forest causes fragmentation of infestation spots, which is an expected result. These conflicting outcomes likely explain the overarching lack of a significant effect of non-host abundance on damage risk by Ips typographus.