A 400-year history of fires on lake islands in south-east Sweden

Island-lake ecosystems are suitable for testing scale dependence in forests disturbance theories thanks to differences in the potential for fire spread on islands and the mainland. We investigated past fire regime on the mainland and on islands in a large lake in south-east Sweden. We used dendrochronological methods to reconstruct fire disturbances on 18 small islands (0.04–24.1 ha) and in 43 sites in the surrounding 75-km2 landscape over the last 400 years. In the past, fires were frequent on both islands and mainland but not synchronised on an annual scale. Significant temporal changes occurred around the middle of the 18th century. Before 1750, fires were less frequent on islands than on the mainland (median fire return interval 58 v. 25 years respectively). However, an inversion of this pattern was observed during 1750–1860: islands showed even shorter fire intervals than mainland locations, suggesting additional and likely human-related source of ignitions (median fire return interval 15 v. 29 years respectively). A substantial decrease in fire activity in both islands and mainland was apparent in 1860–1890. We suggest that the present fire regime (the last 100 years) on the small islands is largely natural as fire suppression is not present there. The dynamic nature of the fire regime on islands still requires further studies: islands may, at times, attract lightning, humans with fire, or both.