The changing ability of Norway spruce (P. abies) to record hydro-geomorphic processes based on the age and diameter of the tree stem – A dendrogeomorphic approach

Abstract

Dendrogeomorphic reconstructions of debris flows and related hydro-geomorphic processes (i.e. debris floods and flash floods) are widely used in territories with incomplete chronologies and potential future hazards. In recent years, the tree-ring analysis of disturbed trees affected by dangerous geomorphic processes has developed into a discipline with sophisticated methodical approaches and with annual up to monthly accuracy of dating. Field sampling strategies are based on various ranges of sample depth (i.e., different amounts of sampled trees of different age structures). The age and diameter of trees influence the ability of trees to respond to geomorphic processes; therefore, we investigate age-dependent and diameter-dependent tree sensitivity. Our study, for the first time, addresses the sensitivity to hydro-geomorphic processes in detail. We tested 462 individuals of Norway spruce (Picea abies (L.) Karst.) – a very common species used in the dendrogeomorphic reconstructions – in the apical part of the medium-high-mountains where there is widespread occurrence of debris flows and debris floods. The tree sensitivity analysis was performed based on the position of growth disturbances within the increment cores of trees (original age and original diameter of the tree before the tree was affected). P. abies shows very high tree sensitivity in the first two decades of the tree lifespan. With increasing age and diameter, trees are less sensitive recorders of debris flows/floods. Moreover, the intensity of growth disturbances significantly decreases with tree age. Mature trees (51–120years) record the geomorphic events mainly through abrupt growth changes (suppression and release) and tangential rows of traumatic resin ducts. In contrast, scars, the onset of reaction wood and traumatic resin ducts are dominant growth disturbances in young, thin trees (11–30years; diameter 0–15cm). Different occurrences of particular growth disturbances as well as the different age- and diameter-dependent sensitivities emphasize the need for the inclusion of new variable that will take into account the amount of sensitive trees for each year. After that, adequate mixture of sampled tree (i.e. mixture of age classes), which is necessary for the completeness of debris flow/flood chronologies, could be balanced regarding their sensitivity.