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.

明晰火干扰制度（fire regime）的历史变化，对于当前森林经营及未来情景预测均具有重要指导意义。本研究通过对378份残存欧洲赤松（Scots pine）样本中的745个火疤进行交叉定年，在挪威中南部特列尔马克-罗拉格斯菲尔自然保护区74平方千米的研究区内，重建了过去700年间的254次独立森林火灾事件。研究将火灾规模、数量、过火速率及发生频率与历史气候代用指标、植被图及文字史料进行对比分析。结果显示，1625年以前的火干扰制度主要受气候驱动；17至18世纪受人类活动强烈影响，火灾频率显著上升，而19世纪则出现火灾活动减弱的阶段。17世纪初期小型火灾数量的骤然增加，将火灾间隔期从中位数73年显著缩短至37年，这一现象佐证了上述变化。火灾频率的这一转变，与1625年起首次出现的早季火灾事件相吻合。晚季火灾的过火速率随夏季气温升高而上升，但早季火灾未呈现此类关联。上述研究结果得到文字史料的佐证：史料记载，16世纪末期起，随着人口增长，人为森林火灾与刀耕火种活动逐渐扩张；而18世纪因木材价值攀升，此类活动逐步减少。尽管人类活动在数十年至百年尺度上对火干扰制度产生了强烈影响，但引发大规模火灾事件的核心驱动因素仍是气候的年际变异，尤其在1625年以前的阶段。1625年以前，火灾发生年占比从冷夏时期（10–12℃）的7%增至暖夏时期（14–16℃）的21%，增幅达两倍。同期过火速率的增幅更为显著，从0.01%升至1.3%。从生态学视角来看，1625年后的阶段具有显著特殊性：人类活动先是大幅提升火灾频率，随后近乎彻底消除火灾，其对火干扰制度的影响程度可能在数千年间均属罕见。