The potential for using Larix decidua ring widths in reconstructions of larch budmoth (Zeiraphera diniana) outbreak history: dendrochronological estimates compared with insect surveys

In the Alps, larch (Larix decidua Mill.) forests show periodic discolourati on due to larch budmoth (LBM) outbreaks (Zeiraphera diniana Guenee, Lepidop tera: Tortricidae). Tree defoliation causes severe reductions in radial gro wth, visible in tree rings. This paper aims at reconstructing LBM outbreak history, and critically examining the potential for using dendrochronologic al data by comparing tree-ring estimates with insect surveys. The occurrenc e of LBM outbreaks was investigated using 249 cores from larch growing near the timberline in three regions of the French Alps (Brianconnais, Maurienn e, and Tarentaise). Years with an abrupt decrease in radial growth (-40%) w ere considered as negative pointer years reflecting the potential impact of LBM. The comparison with three non-host conifers (Norway spruce, stone and mountain pines) permitted us to distinguish growth reductions in larch due to climatic effects from those due to defoliation by LBM. The dendrochrono logical data were matched with information reporting conspicuous discoloura tion in old forestry reports or recovered from systematic field observation s. Twenty-two outbreaks are discernible within the period 1800-1983. A peak reduction in larch growth occurred at intervals of 8.86 +/-1.01 years in t he following years: 1801, 1811, 1820, 1830, 1838, 1846, 1857, 1867, 1875, 1 884, 1892, 1901, 1909, 1918, 1926, 1936, 1945, 1953, 1963, 1972, 1980, and 1987. Our objective method based on ring measurements and comparison with n on-host trees was compared with qualitative techniques based on the visual detection of conspicuous latewood anomalies. Larch in the Brianconnais (whi ch experiences a continental climate) are infested first, whereas the Taren taise region exhibits a much weaker impact of LBM. Complete tree recovery w as observable 3 years after outbreak peaks.