Siberian silkmoth outbreaks surpassed geoclimatic barrier in Siberian Mountains

Siberian silkmoth(SSM,Dendrolimus sibiricus Tschetv.) is the most important defoliator of Siberian pine(Pinus sibirica Du Tour) and fir(Abies sibirica Ledeb.) stands.Warming-induced SSM outbreaks are one of the major driving factors of successions within the taiga zone.It is suggested that climate change impacted the SSM range and life cycle.We analyzed the migration of alpine and northerly SSM outbreak boundaries in Siberia and the impact of the climate variables and topography on the outbreak dynamics.We used time-series scenes(multispectral data,and vegetation indexes EVI and NDII) in combination with field studies,climate variables,and GIS techniques.We found that SSM outbreaks in the area of alpine boundary shifted about 370 m uphill since the mid of 1950.The outbreak onset was promoted by increased dryness and active temperatures and decreased root zone moisture content in the spring-early summer period.The terrain topography strongly affected SSM outbreak onset and dynamics.Initially,the outbreak was located at the middle elevations on the gentle concave southeastern slopes,which are the favorable insect habitats between outbreaks.Then the outbreak expanded uphill and downhill,to steeper slopes,and both concave and convex terrains.Alongside with elevation range expansion,SSM surpassed its northern historical outbreak boundary:the potential outbreaks' boundary moved about 300 km northward.Climate warming contributes to SSM migration into former outbreak free conifer stands located in highlands and at northern latitudes.