Short-term forecasting of bark beetle outbreaks on two economically important conifer tree species

In recent years bark beetles have been shown to be an important risk factor in European forests. An early warning system is needed to mitigate bark beetle damage, and short-term forecasting models that assist efforts to identify attacked trees comprise an important part of such a system. The aim of this study was to develop short-term forecasting models of the probability of bark beetle outbreaks on two important conifer tree species: Norway spruce (Picea abies) and silver fir (Abies alba). For the development of the models, we used a time series of 20 years of sanitary felling because of bark beetles and relief data (altitude, slope and exposition), several soil variables, climate data (temperature and SPI), sanitary felling because of bark beetles, sanitary felling due to harmful abiotic factors, and amount of weakened trees due to bark beetles. The forecasting variable was sanitary felling because of bark beetles in the current year. The models were developed with a general linear model with binomial error distribution. For the probability of bark beetle outbreaks on silver fir, the amount of fir, soil base saturation percentage, sanitary felling of attacked fir, weakened fir, and sanitary felling because of abiotic factors increased the probability of sanitary felling because of fir bark beetles. Altitude, exposition, slope, phosphorus, soil depth, soil cation exchange capacity, SPI and temperature decreased the probability of sanitary felling because of fir bark beetles. For Norway spruce, the amount of Norway spruce, soil base saturation percentage, SPI, temperature, amount of sanitary felling in the previous year, amount of weakened trees in the previous year, and amount of sanitary felling because of abiotic factors in the previous year increased the probability of sanitary felling of Norway spruce because of bark beetles in the current year. Slope, soil cation exchange capacity, and precipitation decreased the probability of sanitary felling because of bark beetles in the current year. The performance of the bark beetle risk model for Norway spruce was very good. The performance of the model for silver fir was also good, but not on par with that for Norway spruce. Therefore, additional research on fir bark beetles is needed to further improve the risk model for bark beetle attacks on silver fir.