Modelling Dynamic Predisposition to Ips typographus

The proposed study is based on the hypothesis that Norway spruce is increasingly susceptible to mass attack by the Eurasian spruce bark beetle, Ips typographus, beyond a critical threshold of tree water stress. Actual water shortage instantly acts upon tree disposition to bark beetle attack, implicating changes at the level of tree physiology. Looking at the interactions between soil, tree and herbivore (bark beetle) species, the aim of the project is to identify "attack thresholds" subject to "water deficits" accumulated at study plots, and specific tree physiological parameters indicating water stress of potential host trees. Rain-out shelters are established at the experimental site situated in the Forest Demonstration Centre of BOKU in order to artificially induce severe and moderate tree water deficiency. Investigations during the 3-year project period involve the examination of microclimatic, soil, hydrological, tree physiological and biometric parameters, as well as entomological experiments on stressed and non-manipulated control trees. The results of field and laboratory analyses are incorporated into a "Water deficit model" based on the hydrological model BROOK90, which describes soil-water-plant interactions and is suited to determine stress thresholds and model water stress on a stand scale. Main objective of the project is to develop a dynamic predisposition assessment system (DPAS) accounting for spatial and temporal variability of tree and stand susceptibility to bark beetle infestation. A unique, dynamic monitoring system is to be set-up for day-to-day assessment of attack probabilities subject to water supply. The quality of proposed research lies in the availability and advancement of already well-established models and monitoring systems (Brook90, PAS, PHENIPS) and in the direct practical applicability of project outputs for pest risk assessment in forest management. DPAS shall allow for an improved assessment of tree and stand susceptibility to outbreaks of the Eurasian spruce bark beetle by considering seasonal and spatial variations in predisposition, especially due to actual occurring host tree stressors (drought). The newly developed monitoring instrument shall serve for an evaluation of actual stand susceptibility as affected by short-term water deficits, modelled in a dynamic way similar to the accumulation of day-degrees for predicting bark beetle development (PHENIPS) and for short-term prospective analyses, e.g. on the basis of weather forecasts. We expect improved understanding of landscape scale outbreak dynamics and more reliable evaluation of risk by incorporating retrospective analyses and drawing of model scenarios for stand susceptibility subject to changes of external factors (e.g. due to climate change) and stand internal processes (e.g. aging processes, changes in stand management). There is high potential for the implementation of an advanced, dynamic predisposition assessment system given the increasing incidence of elevated summer temperatures and short-term water deficiencies of Norway spruce dominated forests.

Drought as a main stress factor renders trees susceptible to attacks by biotic agents! - We could prove this assumption with respect to bark beetles within the scope of a drought manipulation experiment established in a mature Norway spruce forest. From both an ecological and economical view, Norway spruce is one of the most important tree species in Central Europe; yet, disturbance events increasingly put at risk the vitality and intactness of conifer stands. Besides storm throw and snow breakage, bark beetles constitute the main agents of forest damage. However, only a few bark beetle species, such as the European spruce bark beetle, Ips typographus, have the potential to kill trees or ultimately cause forest dieback in case of mass attacks. Epidemics are due to mass propagation of bark beetles in weakened or dying trees e.g. extensively supplied after storm events. In a changing climate, increased spring and summer temperatures as well as drought episodes further trigger beetle development but also weaken the host trees. To uncover some of these complex mechanisms and improve tools for predicting potential bark beetle damage, we realised an ambitious study design in a 90-year-old Norway spruce forest. The physiological status of sample trees, some of them highly stressed by the deprivation of precipitation by a roof construction, some of them intermediately and some non-stressed, was monitored over a 3-years period. As important indicators of tree health we measured sap flow, daily and seasonal changes in tree diameter, bark water content and osmotic potential, as well as twig water potential. We permanently logged diverse weather variables, such as air temperature and precipitation by means of a weather station, and soil water content at our study plots. Our main objective was to investigate whether and to what extent drought affects bark beetle attack of Norway spruce. For this purpose, we followed a novel experimental approach by repeatedly confronting a specific number of bark beetles with our sample trees by means of specifically designed attack boxes fixed to the stems. We found that tree attack was highly driven by variations in the stress status of individual trees over time. The capability of the trees to defend themselves from being attacked was impaired by lowered resin exudation and decreased with increasing stress status as for instance indicated by more negative twig water potentials. Yet, at the same time, our data also point to reduced host acceptance of the beetles given extreme tree drought stress. Our results are incorporated into a risk assessment framework consisting of several components: sub-models to evaluate water budgets and seasonal water deficits of a forest stand, respectively; a system for assessing the predisposition of Norway spruce forests to bark beetle attack based on site and stand characteristics; and a model for predicting bark beetle development based on seasonal temperature conditions.