Strong anthropogenic signals in historic forest fire regime: a detailed spatiotemporal case study from south-central Norway

Abstract

To better understand the historic range of variability in the fire regime of Fennoscandian boreal forests we cross-dated 736 fire scars of remnant Scots pine (Pinus sylvestris L.) wood samples in a 3.6 km2 section of the Trillemarka-Rollagsfjell Reserve of south-central Norway. Using a kernel range application in GIS we spatially delineated 57 individual forest fires between 1350 and the present. We found a strong anthropogenic signal in the fire regime from 1600 and onwards: (i) infrequent variably sized fires prior to 1600 shifted to frequent fires gradually decreasing in size during the 1600s and 1700s, with only a few small fires after 1800; (ii) time intervals between fires and the hazard of burning showed substantial differences pre- and post-1600; (iii) fire seasonality changed from late- to early-season fires from the 1626 fire and onwards; and (iv) fire severity decreased gradually over time. Written sources corroborated our results, narrating a history where anthropogenic forest fires and slash-and-burn cultivation expanded with the increasing population from the late 1500s. Concurrently, timber resources increased in value, gradually forcing slash-and-burn cultivators to abandon fires on forest land. Our results strengthen and expand previous Fennoscandian findings on the anthropogenic influence of historic fire regimes.

Fire history in a western Fennoscandian boreal forest as influenced by human land use and climate

“…However, in the small‐scale study that was encompassed by the present one, fire severity displayed a decreasing trend over time (Storaunet et al. ). Although this change in fire severity was rather gradual, it is consistent with the changes in number and seasonality of fires.…”

“…Previously, we have reported a detailed account of historical fires in a small 3.6‐km 2 section of the Trillemarka‐Rollagsfjell Nature Reserve in south‐central Norway, documenting a strong anthropogenic signal in the fire chronology from ca 1600 (Storaunet et al. ). However, the small size of this study area prevented unbiased estimates of fire sizes and meaningful assessment of possible influence of climate.…”

“…S1) of the fire history emphasized that anthropogenic forest fires were common during the 1600–1700s (Storaunet et al. ). Today, the study area bears no signs of recent large fires, the last recorded fire >0.1 km 2 occurred more than 200 years ago in 1795.…”

“…Taking advantage of the previously sampled 3.6 km 2 subsection of our study area, we supplemented our material with these already dated fire‐scarred samples (Storaunet et al. ). To conform with the present sampling effort, this was done by randomly drawing 49 samples from a total of 321, giving a sample density (13–14 per km 2 ) equaling those parts of the present study area with highest sample density.…”

“…The analyses were done pre and post AD 1625 based on the timing of the anthropogenic influence on the fire regime (see Results and Storaunet et al. ). For intervals crossing the year 1625, these were grouped to pre‐1625 if more than half of the interval occurred pre 1625 (same for post 1625).…”