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Larch Budmoth Outbreaks in Forests of the European Alps
Volume 10, Number 11: 14 March 2007

In an impressive new study, Esper et al. (2007) reconstruct an annually-resolved history of population cycles of a foliage-feeding Lepidopteran commonly known as the larch budmoth (Zeiraphera diniana Gn.) - or LBM for short - within the European Alps in the southern part of Switzerland. As is typical of many such insect pests, they report that "during peak activity, populations may reach very high densities over large areas," resulting in "episodes of massive defoliation and/or tree mortality." Consequently, it can be appreciated that any new light they might be able to shed on the subject could be of great ecological and economic significance.

The first thing the team of Swiss and US researchers did was develop a history of LBM outbreaks over the 1173-year period AD 832-2004, which they describe as "the longest continuous time period over which any population cycle has ever been documented." This they did using radiodensitometric techniques to characterize the tree-ring density profiles of 180 larch (Larix deciduas Mill.) samples, where "LBM outbreaks were identified based upon characteristic maximum latewood density (MXD) patterns in wood samples, and verified using more traditional techniques of comparison with tree-ring chronologies from non-host species," i.e., fir and spruce. Then, they developed a matching temperature history for the same area, which was accomplished by combining "a tree-ring width-based reconstruction from AD 951 to 2002 integrating 1527 pine and larch samples (Buntgen et al., 2005) and a MXD-based reconstruction from AD 755 to 2004 based upon the same 180 larch samples used in the current study for LBM signal detection (Buntgen et al., 2006)."

Over almost the entire period studied, i.e., from its start in AD 832 to 1981, there were a total of 123 LBM outbreaks with a mean reoccurrence time of 9.3 years. In addition, the researchers say "there was never a gap that lasted longer than two decades." From 1981 to the present, however, there have been no LBM outbreaks; and since there had never before (within their record) been such a long outbreak hiatus, they concluded that "the absence of mass outbreaks since the 1980s is truly exceptional."

To what do Esper et al. attribute this truly unprecedented recent development? Noting that "conditions during the late twentieth century represent the warmest period of the past millennium" - as per their temperature reconstruction for the region of the Swiss Alps within which they worked - they point to "the role of extraordinary climatic conditions as the cause of outbreak failure," and they discuss what they call the "probable hypothesis" of Baltensweiler (1993), who described a scenario by which local warmth may well lead to reduced LBM populations.

Such may well be the case; but we hasten to add that atmospheric CO2 concentrations since 1980 have also been unprecedented over the 1173-year period of Esper et al.'s study. In fact, they have been even more unprecedented than have air temperatures. Hence, the suppression of LBM outbreaks over the past quarter-century may possibly have been the result of some synergistic consequence of the two factors (temperature and CO2) acting in unison, while a third possibility may involve only the increase in the air's CO2 content.

Whatever the case may be, and in further discussing their remarkable findings, Esper et al. say their observations suggest the "vulnerability of an otherwise stable ecological system in a warming environment," in what would appear to be an attempt to attach an undesirable connotation to the observed outcome. This wording seems strange indeed, for it is clear that the "recent disruption of a major disturbance regime," as Esper et al. refer to the suppression of LBM outbreaks elsewhere in their paper, would be considered by most people to be a positive outcome, and something to actually be welcomed.

Sherwood, Keith and Craig Idso

References
Baltensweiler, W. 1993. Why the larch bud moth cycle collapsed in the subalpine larch-cembran pine forests in the year 1990 for the first time since 1850. Oecologia 94: 62-66.

Buntgen, U., Esper, J., Frank, D.C., Nicolussi, K. and Schmidhalter, M. 2005. A 1052-year tree-ring proxy for alpine summer temperatures. Climate Dynamics 25: 141-153.

Buntgen, U., Frank, D.C., Nievergelt, D. and Esper, J. 2006. Alpine summer temperature variations, AD 755-2004. Journal of Climate 19: 5606-5623.

Esper, J., Buntgen, U., Frank, D.C., Nievergelt, D. and Liebhold, A. 2007. 1200 years of regular outbreaks in alpine insects. Proceedings of the Royal Society B 274: 671-679.