Mechanical resistance of coppice stems derived from full‐scale impact tests

Broadleaf coppice forests have the capacity to mitigate the threat posed by rockfall in many mountainous regions. Other forest types alike the rockfall protective effect is determined by the mechanical resistance of the coppice tree stems. In addition, the rockfall protective function of coppice forests is enhanced by specific stem aggregations (clumps) that have a rock interception and retention effect difficult to evaluate. The main objectives of this study are to quantify the mechanical resistance of small diameter coppice stems and to gain qualitative insight on breakage behavior. The aim is to supply data for more reliable assessments of the rockfall protective function of coppice forests with rockfall simulation models and to provide a basis for better estimating the rockfall protective effect of coppice clumps. To achieve these objectives we assessed the mechanical resistance of 73 beech (Fagus sylvatica L.) coppice stems using an impact pendulum device. We found an exponential relationship between the stem diameter at breast height (DBH) and mechanical resistance (loss of momentum or kinetic energy of the impactor during impact). Moreover, the results show that the high flexibility of the stems leads to relatively long lasting impacts and only negligible damage at the point of impact on the stem. As a result, the mechanical resistance of the stems is partly determined by impactor velocity and mass. These findings question the practicality of defining mechanical resistance by means of the change of momentum or energy of the impactor. Moreover, the results highlight the limits of upscaling or downscaling the data of this study to conclude for the mechanical resistance of beech trees of other than the tested dimensions. For the target DBH range the obtained dataset is nevertheless more reliable than data of previous studies, because the DBH specific impact process could be considered. Copyright © 2013 John Wiley & Sons, Ltd.