Mats Niklasson, Igor Drobyshev, Tomasz Zielonka. A 400-year history of fires on lake islands in SE Sweden. International Journal of Wildland Fire 2010, 19, 1050–1058.
Lake islands can be viewed as miniature ecosystems where theories concerning biogeography ecological functioning, biological diversity, and its relation to disturbance and stability can be tested empirically. A fundamental property of an island is its size influencing the probabilities for species to migrate on this island and for certain ecological processes to occur. An example of such a process is lightning strike occurrence and associated with it natural fire activity. The probability of a lightning strike to hit an island is a function of the island area. Since fire spread to a distant island from the surrounding landscape (another island or the mainland) is uncommon, it is the lightning strike probability which represents the main source of fire-initiating ignitions in these ecosystems under natural conditions. Given the lightning strikes being the only sources of ignitions and variation in local weather or fuels among islands or between islands and the mainland is insignificant (see however Drobyshev et al., 2010), properties of fire regime (e.g. fire frequency and fire occurrence) should therefore be closely related to the corresponding island sizes.
Rationale & Hypotheses
To the best of our knowledge, comparative analyses of fire history in island-mainland systems are currently missing in the Eurasian temperate zone. To get a better insight into such systems we chose an area in the hemi-boreal zone of SE Sweden with a long fire history documented in the sediment records and with a naturally high level of lightning caused fires (Lindbladh et al. 2003). In this study we focused on fire return intervals (FRI, time between fire events within a site) dendrochronologically reconstructed on islands of a relatively large (3130 ha) lake and the surrounding mainland. Calculation of FRI does not require reconstruction of past fire sizes and, as a result, FRI cannot be used to evaluate the overall fire impact on the studied landscape. By deliberately selecting FRI as the main statistic of interest we avoided a discussion of fire cycles in this paper due to the difficulty with defining a reasonable spatial unit ("the study area") for islands. Our main objectives were to reconstruct FRI on the islands, analyse its temporal variation over 400 years, and compare them with intervals reconstructed on nearby mainland locations. Humans have been actively using forest resources in this part of Scandinavia since early middle ages (Goldammer et al. 1997; Frödin 1952) and the time period covered by the current study partly coincided with use of slash-and-burn agriculture and pastoral activities. It was therefore of interest to discuss changes in fire regimes based on available evidence on historical land use patterns in the study area.
We addressed four partly competing hypotheses concerning the past fire regime:
(1) Changes in fire regime are consistent with historical record of forest use and onset of different forest use patterns in Fennoscandia.
(2) Fire intervals on islands should be generally longer, as compared to the mainland. This effect should arise due to fires on islands being a product of fire ignition frequency solely, whereas on the mainland it is a product of both ignition frequency within a study area and probability of fire spread from the area surrounding the study area;
(3) Island size should positively correlate with fire frequency due to increasing lightning ignition probability with larger surface.
(4) Spread of fire from the mainland to island or between the islands should be unlikely. This would result in empirically observed frequency of such events (a fire recorded on an island and the mainland or at neighbouring islands at the same year) not being different from statistical expectation of such events, based on the assumption of independent (at annual scale) nature of fire histories on these two site types.
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 synchronized on the annual scale. Significant temporal changes occurred around the middle of 18th century. Before 1750, fires were less frequent on islands than on mainland (median fire return interval, FRI, 58 vs. 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 FRI 15 vs. 29 years, respectively). A substantial decrease in fire activity in both islands and mainland was apparent since 1860-1890. We suggest that the present fire regime (the last 100 years) on the small islands is largely natural since fire suppression is not present there. The dynamic nature of fire regime on islands still requires further studies: islands may, at times, attract ignitions, humans with fire, or both.
Fig. 1. Relationship between island area and the number of fires detected on the islands (full dots) and theoretically expected numbers, based on fire ignition frequency data (empty dots) (Granström 1993). R2 and significance of regression coefficients are given for the island data.