Dendrochronological Indication of Phyllophages’ Outbreaks by Larch Radial Growth in the Forest-steppe Zone of the Republic of Tyva

One of the possible consequences of climate change is a change in the demographic dynamics of phyllophagous insects. For a retrospective analysis of this dynamics, tree rings are used, especially in regions with limited documentary data. Due to the complex nature of the factors determining tree growth, in order to more clearly identify pest-induced defoliation in tree-ring chronologies one suppress the climatic signal expressed directly or indirectly (through chronologies of non-host tree species). However, in South Siberia, the choice of non-host species is hampered by the wide distribution of polyphages, like the gypsy moth (Lymantria dispar Linnaeus) and the Siberian silk moth (Dendrolimus sibiricus Tsch.). Therefore, the analysis of pest outbreaks in larch forests of the Republic of Tuva low mountains was started by removing the climatic response based on instrumental data, identifying depressions in the residual time-series of individual tree radial growth at several sites, and comparing them with available actual data on forest damage by phyllofages. Dendroclimatic analysis showed that the model including the precipitation-to-maximum-temperature ratio for June-September of the previous season and June of the current year, i.e. the degree of aridity during the previous and current growing season, describes 43.7% of the regional variation in the growth of Siberian larch (Larix sibirica Ledeb.). After removal of this component, several periods of larch growth depression were revealed during confirmed outbreaks of the Siberian silk moth, gypsy moth, and larch tortrix (Zeiraphera diniana Gn.). The use of documented data over 1998–2016 allowed to clarify the threshold values of the portion of affected trees for intensities of the growth depression ranging 1–1.5 standard deviations, providing the required reliability of the outbreak reconstruction. The dependence of the spatio-temporal patterns of growth depression on the pest species was revealed, reflecting their migration in the affected area. It has been shown that growth depressions may be delayed compared with the actual damage by pests due to the long recovery after recurrent defoliation.