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

Abstract

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·(104 ha)−1·yr−1 from 1350 to 1650. It increased gradually thereafter, except for a low period in the early 1700s, peaked at 1.17 fires·(104 ha)−1·yr−1 in 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·(104 ha)−1·yr−1), whereas during the mid 1800s it was 11.7 times higher. These results show that large alterations in the fire regime can occur without substantial changes in the proportion of area burned per unit time, as exemplified by the trend after 1650, when there were concurrent changes in the number of fires and in average fire size. Therefore, the number of fire events per unit area and time should be an important variable in the analysis of fire history and its underlying causes.

Natural stand structures, disturbance regimes and successional dynamics in the Eurasian boreal forests: a review with special reference to Russian studies

“…Fire has long been acknowledged as a crucial factor shaping boreal forest structure and development (Melekhov, 1947;Niklasson and Granström, 2000;Saari, 1923;Sirén, 1955). There is wide variability in fire behaviour and regimes, from infrequent high-severity stand-replacing fires to frequent low-severity surface fires.…”

A continuous record of fire covering the last 10,500 calendar years from southern Sweden — The role of climate and human activities

“…5500 and 2000 BC, with the lowest CHARs between 3500 and 2000 BC. Such fire intervals can be compared to 50-110 years intervals that were common in forests of northern Sweden during the last centuries (Zackrisson, 1977;Granström et al, 1995;Niklasson and Granström, 2000). Long fire intervals are usually characteristic of natural fires, while short fire intervals are often related to human-induced fires (e.g.…”

“…These characteristics were interpreted as indicating anthropogenic, intentional fires to improve the quality of the grazing land. Natural fires would occur less frequently, later in the season (mid-June-August) (Granström, 1993), and cover larger areas (Niklasson and Granström, 2000). The last fire dated by dendrochronology in the Storasjö area occurred AD 1793 and seems to mark the end of a period with high fire frequency (Wäglind, 2004).…”

“…The last fire dated by dendrochronology in the Storasjö area occurred AD 1793 and seems to mark the end of a period with high fire frequency (Wäglind, 2004). The sudden change from very frequent fires to complete absence of fires in the 18th-19th centuries is a consequent trend found in Sweden (Zackrisson, 1977;Niklasson and Granström, 2000;Niklasson and Drakenberg, 2001;Niklasson, 2002;Lindbladh et al, 2003). (Digerfeldt, 1988) and mountain glaciers fluctuations (Nesje et al, 2001) are presented as in Hammarlund et al (2003), with the addition of the pollen-based annual temperature reconstruction from Seppä et al (2005).…”

Holocene fire in Fennoscandia and Denmark

“…Fire has not always been so rare throughout Fennoscandia and Denmark: fire scars record a significantly more intensive fire regime in the recent past (e.g. Niklasson and Granström 2000;Power et al 2013;Storaunet et al 2013). Fire scars are valuable for understanding past human fire activity (Neolithic to present day slash and burn activity).…”