Climate change and elevational shifts in the spatial pattern of larch budmoth outbreak: Evidence from 300 years of tree ring data

Background/Question/Methods

Climate change since the onset of the industrial revolution has had numerous effects on population dynamics and distributions. These effects are perhaps the most evident across latitudinal and elevational gradients. The effects of climate change on non-random spatial patterns in population dynamics are less studied. Here we report the novel finding of an upward elevational shift in the epicenter of traveling waves of outbreak by larch budmoth populations in the Alps. Traveling waves are a form of partial synchrony where there is a spatial gradient in the timing of outbreak. Larch budmoth outbreaks can be detected in larch tree rings, which reveal consistent outbreaks at a periodicity of nine years over at least the past 1,200 years.  We analyzed 300 years of larch tree ring data from 67 sites across the Alps to test for an elevational shift in the pattern of larch budmoth outbreak with the goal of relating the shift to warming temperature trends in the Alps.  

Results/Conclusions

Multiple regression analysis revealed that the dominant trend from 1700-1900 was traveling waves of outbreaks spreading from epicenters at low elevations to high elevations. However, from 1900-1950 this pattern disappeared, and from 1950-1983 the pattern reversed to traveling waves of outbreak spreading downward from epicenters at high elevations. A simulation model of larch budmoth outbreak pattern confirms that climate forcing through warming temperatures would cause this reversal in traveling waves, to downward spread from high elevations. The model further predicts that climatic warming beyond a certain threshold would result in a cessation of periodic outbreaks. Larch budmoth populations have indeed uncharacteristically failed to outbreak in the last 25 years, supporting the model prediction. Recent speculation is that climate forcing is the cause of the collapse of population cycles in a diversity of taxa including voles, grouse, and multiple insect species including the larch budmoth.  This study provides convincing evidence that the cessation of larch budmoth population cycles has resulted from an upward elevational shift in outbreak epicenters to above the treeline of larch. Rising temperatures in the Alps region is implicated as the causal force.