Abstract
The increasing effects of storms are considered the main abiotic disturbance affecting forest ecosystems. Bark-stripping damage from the growing ungulate populations, in turn, are among the main biotic risks, which might burden the stability of trees and stands. Therefore, the aim of our study is to estimate the effect of cervid bark-stripping on the mechanical stability of Norway spruce using a static tree-pulling test. For the test, eight damaged and 11 undamaged canopy trees were selected from a 40-year old stand (plantation with 1 × 3 m spacing) growing on mineral mesotrophic soil. The selected trees were bark-stripped 7–9 years prior to the experiment. Uprooting was the most frequent type of failure; only two trees broke at the stem. For the damaged trees, the resistance to pulling was significantly reduced (p-value < 0.001). Stem volume and presence of bark-stripping were the best linear predictors of the basal bending moment at the primary failure (irreversible deformation of wood structure) and secondary failure (collapse of the tree). A significant (p-value < 0.001) interaction between stem–wood volume and presence of bark-stripping was observed for primary failure, indicating a size-dependent reduction of stability of the damaged trees. Such interaction lacked significance (p-value = 0.43) for the secondary failure (mostly uprooting), indicating a decrease in stability irrespectively of tree size. Somewhat surprisingly, the decrease in the overall mechanical stability of the bark-stripped trees appeared not to be related to a direct reduction of the strength of the stems, but rather to physiological effects such as altered allocation of carbon, increased drought stress because of interfered hydraulic conductance of wood, or secondary infestation. The reduced stability also suggests that bark-stripped trees can act as the weak spots decreasing the collective stability of stands in the long term, thus increasing the susceptibility to storms.
Highlights
In European forests, storms are the main abiotic disturbance [1], which have caused more than half of the losses in standing stock during the last 50 years [1,2]
The aim of the study is to evaluate the effect of bark-stripping on the mechanical stability of middle-aged Norway spruce growing in a pure stand
The critical bending moments were comparable to the results found for Norway spruce in other
Summary
In European forests, storms are the main abiotic disturbance [1], which have caused more than half of the losses in standing stock during the last 50 years [1,2]. The impact of storms is expected to further increase due to climatic change [3]. In Northern Europe, increasing damage of storms is related to unfrozen soil conditions under warming winters, the mechanical stability of individual trees is essential for the sustainability of stands [4]. The impact of storms is intensified by the accompanying legacy effects, such as pest outbreaks and predisposal to pathogens as the trees are weakened [1,2]. Storms are projected to further intensify the negative effects of climate. Karst), which is already associated with lower mechanical stability against wind, in pure stands [5,6,7]
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