How does tree mortality caused by bark beetle (Trypophloeus klimeschi) outbreaks affect changes in soil fungal communities?

Abstract

Forest disturbances such as fires, windthrows, and insect outbreaks have a serious influence on how these ecosystems function. In recent years, an outbreak of an endemic bark beetle species (Trypophloeus klimeschi Eggers) has caused a major disturbance in the Populus alba var. pyramidalis northwest shelter forest in China. The rapid and widespread death of P. alba var. pyramidalis caused by this beetle has cascading effects on the soil properties and fungal communities’ diversity that remain uncertain. We evaluated the mortality of P. alba var. pyramidalis forest attacked by T. klimeschi in this area and set four different levels of tree mortality: T1 level (0–10%), T2 level (11–40%), T3 level (41–70%) and T4 level (71–100%). We simultaneously measured rhizosphere soil properties and fungal community composition (using high-throughput sequencing) at different tree mortality levels. The increase in tree mortality resulted in a steep increase in the soil concentrations of ammonium nitrogen (NH4+), nitrate nitrogen (NO3–) and available phosphorus, as well as a reduction in soil pH. Meanwhile, the fungal communities showed profound changes, the relative abundance of ectomycorrhizal fungi decreased significantly by 71%, and the relative abundance of pathogenic fungi increased significantly by 73%. The combined effects of the changes in the soil fungal communities’ composition and available nitrogen content resulted in a significant 26% reduction in the richness of soil fungi. Our findings suggest that the mortality of poplar species due to the rapid range expansion of T. klimeschi will have significant impacts on crucial soil fungal communities, which may have sustained and potentially important implications for the future restoration of forest ecology in this area. The composition and changes of soil fungal communities should be considered in the subsequent forest ecological restoration process in this area.