Tree species identity in high-latitude forests determines fire spread through fuel ladders from branches to soil and vice versa

- Fuel ladders are potentially important determinants of future boreal fire regimes.
- Tree species identity determines the ignition probability of organic soils.
- Flames from smoldering soils ignite conifer branches more easily than birch branches.
- Chemical branch traits contribute to fire ladder behavior in both directions.

Peat fires in boreal and tundra regions can potentially cause a high CO2 release, because of their large soil carbon stocks. Under current and future climate warming the frequency and intensity of droughts are increasing and will cause the plant community and organic soil to become more susceptible to fire. The organic soil consumption by fire is commonly used as a proxy for fire severity and is a large source of carbon release. However, the role of organic soils in both above- and belowground fire behavior has only rarely been studied. In this study we collected soil and branches from Betula pubescens, Pinus sylvestris and Picea abies/obovata from the taiga/tundra ecotone across a large spatial scale. In laboratory fire experiments we burned different fuel type combinations to examine the fire spread through fuel ladders both from branches to soil and vice versa. We found that the tree species identity influences the fire spread from branches to soil and vice versa. The combination of chemical and structural plant traits could explain the stronger interaction between soil and coniferous spruce and pine fuels in a fire ladder compared to the deciduous birch. Therefore, total carbon emission from a boreal forest fire may not only depend on burned plant fuel, but also on the species-specific potential of the trees to ignite the soil. Carbon emission models and forest management could be improved if not only the aboveground plant fuel consumption is considered, but also the interaction between fuels in a fuel ladder and the probability of soil ignition by a forest crown fire and vice versa.