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

Knowing the historical variation in fire regimes is instrumental in managing forests today and in predicting what may happen in the future. By cross-dating 745 fire scars in 378 samples of remnant Scots pines, we delineated 254 individual forest fires during the past 700 years in a 74-km2 section of Trillemarka-Rollagsfjell Nature Reserve in south-central Norway. Fire sizes, numbers, burn rates, and frequencies were compared with historical climate proxies, vegetation maps, and written sources. The results revealed patterns consistent with a predominantly climate-driven fire regime up to 1625, followed by periods of strong anthropogenic influence that increased fire frequency during 1600–1700s and diminished fires during 1800–1900s. This was documented by an abrupt increase in number of small fires from the early 1600s that markedly shortened fire intervals from a median of 73 to 37 yr. This shift in fire frequency coincided with a sudden appearance of early-season fires from 1625 and onward. Whereas late-season burn rate increased with summer temperature, no such relationship was found for early-season fires. These results were corroborated by written sources that describe anthropogenic forest fires and slash-and-burn cultivation expanding with the increasing population from the late 1500s and subsequently diminishing due to increasing timber values during 1700–1800s. Whereas human activity strongly influenced the fire regime at multidecadal to centennial scales, it was the interannual variability in climate that triggered large fire events, especially during the pre-1625 period. Prior to 1625, the percentage of years with fire tripled from 7% during cold summers (10–12°C) to 21% during warm summers (14–16°C). Burn rate increased even more, from 0.01% to 1.3% for the same temperature intervals. Ecologically, the post-1625 period is remarkable in such a way that human activity, first by greatly increasing fire frequency and subsequently almost eradicating fires, possibly influenced the fire regime to such an extent that it may be unprecedented for millennia.

了解火灾制度（fire regimes）的历史变化，对于当今的森林管理以及未来情景预测均具有重要指导意义。本研究通过对挪威中南部特里勒马拉-罗拉格斯菲耶尔自然保护区74平方千米区域内378个残存欧洲赤松（Scots pines）样本中的745个火灾疤痕开展交叉定年（cross-dating），重建了过去700年间该区域内的254次独立森林火灾事件。研究将火灾规模、数量、过火率与发生频率与历史气候代用资料（climate proxies）、植被图及文字记载资料进行了对比分析。结果显示，1625年之前的火灾制度主要受气候驱动；1600至1700年间，人类活动的强烈影响导致火灾频率上升，而1800至1900年间人类活动则抑制了火灾发生。17世纪早期小型火灾数量的突然增加，将火灾间隔期从中位数73年显著缩短至37年，这一现象印证了上述变化。火灾频率的这一转变与1625年之后早季火灾的突然出现相吻合。晚季过火率随夏季气温升高而上升，但早季火灾未表现出此类相关性。上述结果得到了文字记载资料的佐证：16世纪晚期以来，随着人口增长，人为森林火灾与刀耕火种（slash-and-burn cultivation）活动逐渐扩张；而18至19世纪随着木材价值提升，此类活动逐渐减少。尽管人类活动在数十年至百年尺度上对火灾制度产生了强烈影响，但触发大型火灾事件的关键因素仍是气候的年际变率，尤其在1625年之前的时期。1625年之前，火灾发生年份占比从寒冷夏季（10–12℃）的7%增至温暖夏季（14–16℃）的21%，增长至原来的三倍。对应温度区间内，过火率增幅更为显著，从0.01%升至1.3%。从生态学角度而言，1625年之后的时期尤为特殊：人类活动先是大幅提升了火灾频率，随后又几乎彻底消除了火灾，其对火灾制度的影响程度可能达到数千年来前所未有的水平。