Long-term effects of climate and land-use change on larch budmoth outbreaks in the French Alps

The intensity of cyclic larch budmoth (Zeiraphera diniana Guenee; LBM) outbreaks across the European Alps has been reported to have weakened since the early 1980s. In addition to a warmer climate, changes in land-use cover over modern and historical times may have affected the LBM system. Here, we present tree-ring-based reconstructions of LBM outbreaks from a mixed subalpine larch-pine forest in the French Alps for the period 1700-2010. Temporal variation in LBM outbreak severity was mainly driven by land-use changes, including varying forest structure and species composition. Human population pressure and associated resource demands for fuel wood and construction timber not only resulted in a reduction of larch and subsequent suppression of pine, but also supported an overall grassland expansion for livestock. Superimposed on modern land abandonment and pine re-colonization is a strong warming trend, which may also contribute to the observed late 20th-century weakening of Alpine-wide cyclic LBM outbreaks. Our results suggest that a complex interplay of different factors triggered less synchronized LBM outbreaks at broader scales, with overall significantly lower intensities at local scales.