Numbers and Sizes of Fires: Long-Term Spatially Explicit Fire History in a Swedish Boreal Landscape

• Published in: Ecology (Durham) 2000-06, Vol.81 (6), p.1484-1499
• Main Authors: Niklasson, Mats, Granstrom, Anders
• Format: Article
• Language: English
• Subjects: anthropogenic fires; Biological and medical sciences; Boreal forests; boreal landscape; dendrochronology; Ecology; Fire ecology; fire frequency; Fire hazards; Fire history; fire history, spatially explicit; fire regime; Fire regimes; fire size; Fires; Forest & brush fires; Forest and land fires; Forest dynamics; Forest ecology; Forest fires; Fundamental and applied biological sciences. Psychology; Ignition; landscape ecology; Landscapes; Phytopathology. Animal pests. Plant and forest protection; Pinus sylvestris; Sweden; Swedish boreal forest; Weather damages. Fires
• ISSN: 0012-9658; 1939-9170
• DOI: 10.1890/0012-9658(2000)081[1484:NASOFL]2.0.CO;2

The spatial display of fire over time on the landscape is ecologically important, and spatially explicit analyses offer a possibility of revealing anthropogenic influence on fire regimes. Nonetheless few such analyses have been attempted for longer time frames. We identified past fires in a northern Swedish boreal landscape using fire scars on Pinus sylvestris trees. Within a 19 × 32 km area, local fire chronologies were established at 203 points by cross-dating fire scars on 1133 wood samples, the earliest dating back to the 1100s. A total of 349 separate fires were identified to location and size. The estimated number of fires per unit area and time (after correcting for varying sample density over time) was relatively constant at 0.095 fires· (104ha)-1· yr-1from 1350 to 1650. It increased gradually thereafter, except for a low period in the early 1700s, peaked at 1.17 fires· (104ha)-1· yr-1in the mid-1800s, and then dropped dramatically after 1860. The proportion of the area burned per unit time also increased after 1650, in parallel with the increase in the number of fires (although much less strongly due to a counteracting trend in fire size), from an annual rate of 0.8% prior to 1650 to 2.8% in the mid-1800s. Prior to 1650, 90% of the total burned area was due to fires larger than 1000 ha, compared to 55% after 1650. This decrease in fire size with increasing number of fires may be an intrinsic property of the system: a negative feedback caused by lack of fuel in early succession. Fire intervals shorter than 15 yr were rare, and there was an increase in the hazard of burning during the first 3-5 decades after fire, suggesting an effect of fuel accumulation. Thus, the proportion of the area burned per unit time does not increase linearly with the number of fires in the landscape, because the probability that fires will stop at boundaries with recently burned areas increases over fires. The changes in the number of fires per unit time mirror changes in the cultural use of the land, i.e., the gradual expansion of permanent settlements in the area after the late 1600s. They are not explained by changes in climate records. This suggests that the increase in fire numbers from the second half of the 1600s represents an increase in anthropogenic fires. Before 1650, the number of fires detected per unit area and time was only marginally different from the present-day density of lightning ignitions in the region (∼ 0.1 fires· (104ha)-1· yr-1), whereas during t