Spatiotemporal heterogeneity of larch budmoth outbreaks in the French Alps over the last 500 years

Abstract

In the subalpine forest ecosystems of the French Alps, European larch trees (Larix decidua Mill.) are periodically affected by outbreaks of a defoliating insect, the larch budmoth (Zeiraphera griseana (Hubner, 1799); LBM). To assess the long-term dynamics of LBM populations, we propose a spatiotemporal analysis of a long outbreak chronology reconstruction for the entire French Alps covering the period 1414-2009. This chronology was obtained by analyzing tree ring width (TRW) chronologies collected from 44 larch populations. The evidence of a latitudinal gradient in LBM is an original result that we have related to the "travelling waves" and "epicenter" theory. Wavelet analyses revealed a strong explicit continuous signal for periodicities of 4, 8, and 16 years throughout the entire 1500-2003 time series, except for a loss of power from 1690 to 1790 and since the early 1980s. We hypothesize that these abrupt changes could reflect a physiological response of LBM to past climatic variations. The spatial and temporal variability of LBM outbreaks and the propagation phenomenon in the French Alps highlighted by this study raises questions regarding its future dynamics in response to the expected climate change.Dans les écosystèmes forestiers subalpins des Alpes françaises, le mélèze (Larix decidua Mill.) subit périodiquement les épidémies d’un insecte défoliateur, la tordeuse du mélèze (Zeiraphera griseana (Hübner, 1799); LBM). Afin de mieux comprendre les dynamiques à long terme des populations de tordeuse, nous proposons dans cet article une analyse spatio-temporelle d’une très longue reconstruction des épidémies pour la période 1414–2009 sur l’ensemble des Alpes françaises. Cette chronologie a pu être reconstruite à partir des cernes de croissance de 44 populations de mélèze. Les résultats ont permis de mettre en évidence le rôle d’un gradient latitudinal que nous avons attribué au phénomène théorique des « épicentres » et « vagues de dissémination ». D’autre part, les analyses temporelles, réalisées à l’aide de la méthode des ondelettes, ont révélé l’existence d’un fort signal continu pour des fréquences périodiques de 4, 8 et 16 ans sur l’ensemble de la chronologie à l’exception d’une perte d’énergie entre 1690 et 1790 et depuis le début des années 1980. Ces changements pourraient refléter une réponse physiologique de la tordeuse à des variations climatiques passées. La variabilité temporelle et spatiale des épidémies de tordeuse ainsi que le mode de propagation à travers les Alpes françaises nous ont conduits à nous interroger sur les dynamiques futures de l’insecte et de son hôte en réponse au réchauffement climatique rapide dans les écosystèmes de haute élévation

Forest Insects and Climate Change

“…Dendrochronological analyses of Z. griseana outbreaks over 500 years reveal periodicities of 4, 8, and 16 years throughout the time series, except during 1690-1790, and since 1980, suggesting that disruption of periodicity may be related to changes in climate [93]. Although still peaking at 8-10-year intervals, present outbreaks show a ca.…”

“…For example, variation in precipitation negatively affects caterpillar parasitism [106]. Also, outbreaks of Z. griseana across the Alps have collapsed since the 1980s [93], even though recent temperatures are warmer than those of the past.…”

Tree-ring-based reconstruction of larch budmoth outbreaks in the Central Italian Alps since 1774 CE

“…Thus, they cannot distinguish between a climatically induced reduction in tree growth and a signal caused by an LBM outbreak. The synchronicity between the hatching of LBM eggs and larch needle sprouting is fundamental for the development of the larvae, and the survival of LBM eggs is negatively related to winter and early spring temperatures (Johnson et al 2010, Saulnier et al 2017. However, low spring and summer temperatures also induce narrow rings in larch, which could be interpreted as LBM outbreaks by statistical methods.…”

“…(Baltensweiler & Rubli 1999, Johnson et al 2010. In the French Alps, the history of LBM outbreaks from 1750 to 1994 was reconstructed (Rolland et al 2001) and compared with climate (Saulnier et al 2017) and land use changes (Battipaglia et al 2014). Tree-ring analyses conducted at the border between the Italian and French Alps were used to reconstruct LBM outbreaks that occurred from 1760 to 1999 (Nola et al 2006), and in the Lötschental, Switzerland, a 1200-year tree-ring density chronology was used to reconstruct LBM outbreaks well back into medieval times (Esper et al 2007).…”

New insights on the chronology of medieval mining activity in the small polymetallic district of Faravel (Massif des Écrins, Southern French Alps) derived from dendrochronological and archaeological approaches

“…At around 2000 m a.s.l., ring growth extends from mid-May to mid-October. Early wood is formed between the end of May and late June whereas latewood is produced in summer and at the beginning of fall ( Moser et al, 2009;Saulnier et al, 2017). Based on this seasonal growth development, we can demonstrate that 75% of trees used in the Faravel mines were felled during their dormancy period, in autumn or winter.…”