The role of larch budmoth (Zeiraphera diniana Gn.) on forest succession in a larch (Larix decidua Mill.) and Swiss stone pine (Pinus cembra L.) stand in the Susa Valley (Piedmont, Italy)

In the Alps, larch (Larix decidua Mill.) is severely affected by larch budmoth (Zeiraphera diniana Guenee) (LBM) attacks. The impact of these outbreaks on the Swiss stone pine (Pinus cembra L.) and on the dynamic processes acting in subalpine forest stands are still not well known. Dendroecological methods were used in this study to reconstruct past LBM outbreaks in Susa Valley, Piedmont, Italy. The analysis was carried out on 62 cores from larch and 101 cores from stone pine. The length and severity of each outbreak was quantified for both species and for each tree by means of the programme OUTBREAK. The frequency of the outbreaks was determined using singular spectral analysis and superposed epoch analysis was used to test the significance of the associations between outbreaks and tree-ring growth. In order to verify if trees belonging to different age classes are differently affected by LBM, the reconstructed outbreaks are then classified taking into account the cambial age of the tree at the moment of the outbreak. From 1760 to 1999, 19 outbreaks were recorded in the larch chronologies, while only three outbreaks in the stone pine chronologies. The larch growth is strongly influenced by LBM and the identified outbreaks are equally distributed in all the three age classes. On the stone pine the sporadic occurrence of the identified events made difficult any interpretation of the eventual effect of LBM. Our results lead us to argue that LBM has not played an important role both in determining the stone pine growth rate and in influencing the present observed succession from the stage dominated by larch, to a stage dominated by stone pine or by a mixed stone pine-larch forest.