Insect and storm disturbance in boreal forests — predisposing site factors and impacts on ecosystem carbon

The importance of forests and soil in carbon (C) sequestration and storage is continually increasing with climate change. Disturbances, such as storms and insect outbreaks, are the drivers of forest functioning, composition and structure, and many of them are predicted to become more common in the future. However, environmental factors that predispose forests to disturbance as well as the diverse effects of disturbances on forest C cycling are not fully known. In this dissertation, stand, site and soil characteristics predisposing forest stands to outbreaks of two common insect species that can cause tree damage and mortality—the common pine sawfly (Diprion pini L.) and the European spruce bark beetle (Ips typographus L.)—were examined, and the impacts of storm and I. typographus disturbance on soil respiration, tree and soil C stocks, and microbial community composition and associated C contents were investigated in forests located in eastern and southern Finland. The level of tree damage by D. pini and I. typographus in managed Scots pine (Pinus sylvestris L.) and urban Norway spruce (Picea abies (L.) Karst.) forests, respectively, were associated with various site and soil characteristics. Defoliation of P. sylvestris by D. pini was more severe on sites with soil properties indicating greater fertility (e.g. lower soil C/N ratio and finer textured). Highest cumulative probabilities for severe I. typographus infestation of P. abies were associated with trees growing on sites having an east-facing aspect and the most fertile site types combined with either moderately steep slopes, shallow till soil or high soil C/N ratio. The effects of storm and I. typographus (5–7 years and circa 1–4 years after tree mortality, respectively) disturbance on forest C were studied in P. abies dominated forests that had been left unmanaged after disturbance. Soil surface total and heterotrophic CO2 effluxes, and topsoil C stocks of storm and I. typographus disturbed and undisturbed sites differed little, despite the shift in tree C stocks from living to dead after both disturbances and greater litter detritus C stocks on the I. typographus disturbed sites. Soil surface autotrophic CO2 effluxes were mostly lower at the disturbed sites than at undisturbed ones. The most distinct differences in the humus layer microbiology were the lower abundances of tree-symbiotic ectomycorrhizal fungi, and consequently slightly lower microbial and fungal biomasses in the storm and I. typographus disturbed sites in comparison to the undisturbed sites. The remaining living trees on or in close proximity to the disturbed sites probably mitigated the belowground response to disturbance to some extent. This dissertation shows that certain site and soil characteristics predispose trees and forest stands to D. pini and I. typographus infestations, which could help in identifying sites that are susceptible to insect disturbance. Furthermore, it provides new information about the short-term effects of natural disturbance on boreal forest C cycling and soil microbiology, which is important for improving understanding of the complexity of the possible impacts of climate change on forest C sequestration.