Long-term risk assessment of uprooting and stem breakage under drought conditions and at high N deposition in beech and Norway spruce

Abstract

Damage by wind in forests has increased in recent decades, counteracting possible growth increases by nitrogen (N) deposition. This is also shown by the long-term forest monitoring data from this study. The main factors affecting uprooting and stem breakage in Fagus sylvatica (L.) (9′846 individual beech trees in 102 plots) and Picea abies (Karst.) (9′168 Norway spruce trees in 72 plots) have been analysed by binomial mixed regression models. Our long-term forest observations showed that drought increased uprooting of beech and increased both uprooting and stem breakage of Norway spruce. In beech, high N deposition interacted with drought, resulting in increased uprooting under drought. In Norway spruce, N deposition increased stem breakage directly. Foliar potassium (K) deficiency increased uprooting of both beech and Norway spruce. Moreover, we found an indirect effect of soil acidification in increasing uprooting of beech by decreasing foliar calcium (Ca) concentrations. In Norway spruce, the soil acidification effect was confounded by foliar K concentrations. In beech, stem breakage increased with higher temperature which can be explained by a temperature-mediated growth increase of white rot fungi. In contrast, temperature was negatively related with stem breakage in Norway spruce, which may be interpreted as an inhibition of the heartrot fungus Heterobasidion at higher temperatures.In summary, drought and high N deposition increase the risk of mechanical damage to both beech and Norway spruce and thus endanger sustainable growth rates. We therefore suggest that wind damage must be taken in account in productivity calculations and carbon budgets of temperate forests.