Factors Contributing to the Outbreaks of Small Spruce Bark Beetle Ips amitinus (Eichhoff) (Coleoptera: Curculionidae, Scolytinae) in Siberian Stone Pine Forests of the West Siberian Plain
摘要
The influence of the local environmental conditions on invasive small spruce bark beetle Ips amitinus (Eichhoff) (Coleoptera: Curculionidae: Scolytinae) populations and on the vitality of Siberian stone pine (Pinus sibirica Du Tour) trees has been investigated. The field measurements were carried out in the Siberian stone pine forests near settlements (semi-artificial high-productive stands) of Tomsk region damaged by I. amitinus outbreaks. On eleven plots, the population characteristics and reproductive success of the bark beetle, Siberian stone pine vitality, attributes of the stand (e.g, age, height, and diameter), presence of rot and canker diseases, phytosociological characteristics (as a proxy of soil moisture and fertility), and dendrochronological features (as a proxy of weather impact) were measured. By principal component analysis for mixed data and by linear regression with or without mixed effects, we have analyzed the structure of relationships between the measured features. It was found that I. amitinus populations and Siberian stone pine vitality during the bark beetle outbreak were influenced by several parallel processes. The long-term decrease in the resistance of Siberian stone pine stands is provoked by aging and low soil fertility. This decrease leads both to more frequent small spruce bark beetles attacks and to higher attack density (nuptial chambers per dm–2). Another cause of an increase in attack density is high variance of Siberian stone pine height along with short-term stresses triggered by a deficit of precipitation early in the growing season. In such a situation, the number of suitable microstations of I. amitinus, i.e., stressed trees with a well-lit crown, increased. The abundance of the trees attacked by the small spruce bark beetle (m3 ha–1) is positively related to the total volume of Siberian stone pine per hectare. The review of the literature about natural Siberian stone pine forests in the taiga zone allow us to propose a low likelihood of I. amitinus outbreaks under these conditions. Such forests in the taiga have a lot of differences from P. sibirica semi-artificial forests near settlements, and these differences are unfavorable for the small spruce bark beetle. In the native south- and middle-taiga forests, the risk of bark beetle outbreaks is high with the correspondence of several criteria. First, the Siberian stone pine stands must be old-grown with a high variance of age. This is possible only if the stand is undisturbed. Second, the soil must be moderately dry. The possible exception is forests under acute stress, like defoliation or fire. I. amitinus may impact the undisturbed Siberian stone pine stands as a consequence of migration from such stressed forests.