Dendroarchaeology and cockchafers north of the Alps: Regional patterns of a middle frequency signal in oak tree-ring series

In Europe the mature stage in the life cycles of cockchafers, 3-4 years for Melolontha melolontha L. and 4 years for Melolontha hippocastani Fabr., often causes foliage damage in deciduous forests. Within the scope of dendrochronology, secondary effects on wood production have been generally observed in tree-ring series of rather old oak trees. Flight years were detected by eye and/or sign-test (Gleichlaufigkeit) principally with reference to a 50-year long schematic model representing the cockchafer's reproduction cycles. In the SW German dendroarchaeological record, the detection of cockchafer effects in short oak tree-ring series from different periods has led to a reconsideration of the mode of calculation, which now takes into account a shorter time span which is in better agreement with the development of the cockchafer populations (similar to 30 years) and paying particular attention to the calendar distribution of the cockchafer flight systems known in the region North of the Alps. In the case of the 3-year cycle of M. melolontha, the distribution of the recorded years shows three superimposed, 1-year shifted middle frequency signals almost corresponding to the development of cockchafer populations and their three known flight systems (A: Basel; B: Berne; C: Uri). Consequently, regional patterns reflecting this development in space and time can be used as support for dating short tree-ring sequences and also for the question of timber supply and dendroprovenancing. Moreover, gradation cycles of cockchafer populations along with the frequency of mass outbreaks can be evaluated from an ecological and climatological perspective, with respect to human clearing activities and short-to middle-term changes of landscape.