The inter- and intra-specific variability of charcoal traits in boreal ecosystems
Abstract: Wildfire-produced charcoal has been shown to influence key soil ecological processes. However, few studies have considered the role of interacting variables in describing the wide variability of charcoal ecological traits under natural und semi-natural conditions. Our study tested how different chemical and structural properties of charcoal are affected by single and interaction effects of 1) the original wood species, 2) fire temperature, and 3) flame exposure time. We produced charcoal from three common boreal tree species (Pinus, Betula and Sorbus) at three different temperatures 450°C, 700°C and 900°C. The wood was exposed to 45 min at 450°C, 10 and 15 min at 700°C, and 5, 10 and 15min at 900°C. Further, at low temperature (450°C), we compared our newly developed charring method (isolated gas flame in a barrel) with a more conventional charring procedure (muffle furnace). We also collected charcoal produced at a prescribed fire in central Västerbotten, Sweden, in order to compare natural fire conditions with our controlled high temperature charring method. Our results show that key ecological traits, such as electric conductivity (EC), transversal porosity and bulk density are dependent on temperature, species and, importantly, their corresponding interaction. Chemical charcoal properties, such as pH, NO3- and NH4+ were temperature-dependent, while pH was also influenced by the flame exposure time. Structural charcoal traits, such as pore size distribution, were strongly dependent on the original wood species, but were largely unaffected by the fire conditions. We did not detect any significant differences in charcoal properties between our barrel method and to the muffle furnace method, indicating that the barrel method successfully isolates the wood from outside oxygen during the charring process. The collected wildfire-produced charcoal showed lower pH and EC values, but higher PO43- concentration than barrel-produced charcoal. The results from this experiment suggested that the most determining temperature in wildfire is not solely the peak temperature. The longer residence time at the lower temperatures is also a large contributing factor to the observed variation in charcoal traits. These findings of interaction effects open up the possibility to fully explain the trait variability in wood produced charcoal.
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