Age-dependent sensitivity of trees disturbed by debris flows - Implications for dendrogeomorphic reconstructions

Dating of geomorphic events using dendrogeomorphic methods has increased rapidly during the last years to provide a systematic overview on event frequencies, spatial reach and magnitudes. Despite the recent advances in methodology, multiple questions remain when it comes to the age-dependent sensitivity of trees which may further influence the quality of dating. Yet, the impacts of ageing effects on dendrogeomorphic reconstructions have not been assessed in detail so far. This study clearly shows the evolution of numbers, types, and intensities of growth disturbances (GDs) across the lifespan of three different tree species (Larix decidua Mill., Picea abies (L.) Karst., and Pinus sylvestris L.) affected by debris flows in the European Alps. Based on the re-analyses of datasets from past dendrogeomorphic reconstructions and calculation of statistical tests we investigated decreased ability of all studied tree species to record the debris-flow impact with increasing age. The great abundance of GDs occurred between 31 and 60 year of a tree life. By contrast, below-average number of GDs started around the 10th decade. L. decidua and P. abies show almost twofold number of GD per tree and decade compared to P. sylvestris. Injuries and compression wood are the most frequent GDs at rather young ages of L. decidua and P. abies specimens while tangential rows of traumatic resin ducts and abrupt growth increases are much more scattered. The most balanced distribution of GD is obvious across the lifespan of P.sylvestris. Unequivocal signals (GD of intensity reactions: 5, 4, and 3) of all tree species occur at significantly younger ages than uncertain signals of debris flow impacts (GD of intensity reactions: 2, 1, and 0). Based on the findings of this study and with the aim of improving detection of debris flows we proposed options of sampling strategy and laboratory steps (namely identification of an event) for forest stands with different age structure. Whereas the study sites across the world may not provide the “ideal” age composition of forests due to, for instance, forest management interventions or presence of high-magnitude disasters, the list of recommendations provided here seems to be a good alternative when the conditions are somewhat limited.