Abstract
Studies showed that over the past few decades, the frequency and intensity of insect caused forest disturbances (e.g., by spruce bark beetles) have dramatically increased under the impacts of climate change, leading to extensive tree mortality worldwide, among others in alpine regions. The temporal dynamics of the physiological changes in trees in response to attack have not been sufficiently addressed. Moreover, the identification of forest stands at risk, as well as the understanding of how insect populations interact with their surrounding landscape in time, is of high value for mitigation and for prevention of mass outbreaks. Another important aspect regards the assessment of affected above-ground biomass as well as the identification of parameters that describe the spatial dynamics of bark beetle outbreaks. To address these aspects, the first aim of this research was to disentangle the earliest signs of spruce bark beetle (Ips typographus L.) induced stress in Norway spruce (Picea abies L. Karst). For this purpose, a manipulation experiment was carried out in the Latemar Forest of South Tyrol. We assumed that the mechanism of tree mortality is mediated by cessation of phloem transport and simulated bark beetle attack by either compressing or girdling the phloem of trees and compared our simulated attack with responses of infested trees. We measured sap flux density and tree stem growth as indicative physiological parameters of tree stress response. One third of the girdled trees were attacked by bark beetles in the season following the experimental start. These trees had smaller diameters (DBH < 40 cm) and a high crown base height and died < 6 months. Cessation of stem growth was the first stress signal after bark beetle infestation. Naturally infested trees with large diameters (DBH > 45 cm) and a low crown base height survived > 1 year after infestation. Bark beetle infested trees showed an anticipated decrease in sap flux density under increasing vapor pressure deficit. The outer xylem vessels maintained their function until complete cessation of hydraulic transport in the trunk sections closer to the ground in all bark beetle infested trees. Neither compressed nor girdled trees showed statistically significant differences neither in sap flux density nor in stem growth compared to control and have not yet experienced mortality since the experimental start. An approach to assess the predisposition status of forest stands to bark beetle infestations in complex terrain at the regional scale at a 30 m cell size by relying on satellite derived data is proposed. We identified drivers of bark beetle host selection related to topography using a high-resolution Digital Terrain Model-derived downslope index and to canopy water stress or canopy closure before attack by relying on the Normalized Difference Water Index calculated from Sentinel-2 data. Canopy openings were characterized by GEDI derived products and bark beetle infestation probability was found to be higher in open stands in the beginning of the epidemic phase. The testing of the inter-year variability of these bark beetle drivers of infestation revealed that the predictors generally remained stable from one year to the next. The models showed consistent behavior in spatial patterns of predisposition of bark beetle infestation suggesting an overall suitability of using models with information extracted from one part of the region to predict the probability of disturbance over the entire region. The proposed methodology for the prediction of bark beetle attack predisposition at a broad scale can guide forest management in complex terrain and focus precision forestry intervention to specific areas. The last part of the research regards the characterization of a disturbance regime based on a unique database including more than 50000 records of ground-based bark beetle disturbance observations in the Eastern Alps for the years 2020 to 2023. The dataset was used to extract precise temporal and spatial information on disturbance events in terms of sizes, distances, intensity and frequency. Disturbance events were modeled as spatial point processes based on scale dependency (landscape-regional) and their deviation from random distributions was assessed. Parameters typically used in forest disturbance models such as clustering degree, intensity slope and probability scale were retrieved. Additionally, above-ground biomass (AGB) loss was estimated. The disturbance metrics and parameters can help for the correct parameterization of forest disturbance models, and thus supporting our capability of predicting future patterns of beetle dispersal and effects on carbon stocks in the alpine region. Overall, the objectives of this research involved the testing of combined technologies for the estimation of tree response in relation to spruce bark beetle infestations, the prediction of forest stands’ vulnerability and the spatial analysis of a bark beetle outbreak as well as impacts on AGB at the regional scale.