Functional role of fire-derived charcoal in boreal forest ecosystem processes

摘要

Charcoal is a natural product of wildfires which operate as a major natural disturbance in boreal forested ecosystems. This carbon (C)-rich material is present in most forest soils but its effects on ecosystem processes remain poorly understood. This thesis explores how charcoal, through its characteristics or traits, affects above- and belowground processes in the Swedish boreal forest by using laboratory mesocosm and glasshouse studies and a large field experiment. The relative importance of charring condition and species identity in determining charcoal traits was also investigated. These experiments covered a wide range of humus types, charcoal types and plant species in order to better understand the factors that determine the functional role of charcoal. With regard to aboveground processes, fire-derived charcoal promoted tree seedling growth but had only a minimal effect on seed germination, and plant community characteristics. Belowground processes such as humus decomposition and N mineralization rate were enhanced by the presence of charcoal, even though charcoal had minimal effect on microbial biomass and composition. Charcoal traits were shown to be affected primarily by species identity and to a lesser extent by charring conditions. The magnitude of charcoal effects was influenced by humus type, charcoal type and plant species identity. The mechanisms by which fire-derived charcoal affect ecosystem processes differed between above- and belowground processes; notably, while the effects of charcoal on aboveground processes were linked mostly to the direct input of phosphorus and especially PO43- from charcoal, its effect on belowground processes were mostly determined indirectly through its impact on microbial specific activity. These findings suggest that charcoal is likely to play a role in boreal forest succession, plant-soil feedbacks and ecosystem C dynamics. Moreover, the impacts of charcoal in boreal ecosystems are relevant to better understanding the ecological consequences of forest management practices such as site preparation, prescribed burning, fire suppression and biochar addition. Overall, the findings described in this thesis show that charcoal is a significant component of the C cycle and one that can have strong impacts on boreal ecosystem processes.