Multimillennial fire history of northern Finland along a latitude/elevation gradient

In boreal environments, wildfires are expected to decrease in frequency and/or size with latitude/ elevation, mainly in response to climate, as well as fuel availability and type. Furthermore, fire frequency and biomass burned are supposed to have been higher during warm and dry periods of the Holocene (last ~11,000 years). We tested these assumptions in northern Finland by using charcoal analysis to reconstruct Holocene regional fire regimes from eight lake sediment sequences sampled within four different environments in terms of elevation, latitude and vegetation type: (1) low latitude/mid elevation coniferous forests (Pinus sylvestris and Picea abies); (2) mid latitude/low elevation pine forests (Pinus sylvestris); (3) mid-high latitude/elevation transitional mixed forests (Pinus sylvestris and Betula pubescens subsp. tortuosa); and (4) high latitude/elevation birch woodlands (Betula pubescens subsp. tortuosa). As expected, fire frequency decreased with increasing latitude/elevation. Contrary to expectations, fire frequency was not significantly higher in the warm and dry mid-Holocene than in the cool and wet late Holocene, except in transitional mixed forests. Biomass burned was higher in the mid-than in the late Holocene, but only in pine and pine-spruce forests. The opposite was observed in transitional mixed forests and birch woodlands, suggesting that factors other than climate controlled fire regimes in these environments. While climate warming will likely result in more biomass burned in low latitude/elevation coniferous forests, other disturbances (such as insect outbreaks) might play a more important role in high latitude/elevation birch-pine forests and birch woodlands.