The effects of larch budmoth (Zeiraphera diniana Gn.) in the radial growth of Larix decidua (Mill) in Susa valley, Piemont, Italy.

MARIANO S. MORALES; PAOLA NOLA; RENZO MOTTA; RICARDO VILLALBA

Quebec, Canada

Conferencia; 6th International Conference on Dendrochronology. Dendrochronology, environmental change and human history.; 2002

Centre d`études nordiques, Université Laval

Insects outbreaks are an important agent of forest disturbance, affecting the stand composition, species dominance, tree age, size structure, radial growth and successional patterns. In the Alps, Larix decidua Mill. is severely affected by larch budmoth (Zeiraphera diniana Guénée) attacks. Larch budmoth outbreaks on L. decidua are easily recognized by the discoloration of the larch crowns, the abrupt reduction of radial growth, and the decrease in latewood density. Dendroecological techniques were used in this study to reconstruct past larch budmoth outbreaks in Susa Valley, Piedmont, Italy. The analysis was carried out on cores from 48 L. decidua and 85 Pinus cembra  in the Susa Valley. The length and severity of each outbreak is quantified for each tree and for the mean of all trees by means of the program OUTBREAK which compares host and non-host chronologies to identify the occurrence of insect attacks. The frequency of the outbreaks was determined using spectral analysis. A superposed epoch analysis (SEA) was conducted in order to verify if the larch budmoth outbreaks also affected the radial growth of P. cembra. As far as L. decidua was concerned, the results pointed out several outbreaks with variable intensity. The outbreaks had a cyclical recurrence of 9 years since 1718, however, the most severe attacks occurred approximately every 24 to 28 years. The most evident effect of these outbreaks was a significant decrease in radial growth. Conversely, in P. cembra the analysis did not reveal any significant peak at a wavelength of 9 years, nor any significant decrease in the radial growth, so that the species may be considered as non host tree. In order to provide a more detailed ecological interpretation of the larch budmoth outbreaks, the relationships between climate and intensity of the attacks were also evaluated.