Knowledge of past forest fire regimes is important for developing management plans for conservation areas and for predicting the probable effects of forest management and climate change on the structure and dynamics of forests. In this study, fire scars on living and dead trees were systematically sampled on 256 study plots in three landscapes in northeastern Finland dominated by Scots pine ( Pinus sylvestris L.). A total of 1030 disks or partial cross sections from different trees, including scars from 98 distinct forest fires, were dendrochronologically dated with an accuracy of 1 year or better. The extraordinarily well-preserved old Pinus snags and stumps allowed us to reconstruct annual tree-ring and fire chronologies beginning from the year AD 653. The fire cycles in the studied landscapes were exceptionally long for a boreal region dominated by Pinus, on average 350 years during the last millennium. This demonstrates that the fire regimes of poorly studied remote regions cannot be extrapolated from fire regimes of sites examined in more detail. Based on statistics on lightning-ignition densities, we suggest that most of the fires detected in this study were ignited by humans. The reconstructed past fire cycles were probably shortened by human influence.

Abstract:We performed climatic assessments of Scots pine growth variations in Finland over recent decades by their tree-ring series and comparing growth variations to those observed in meteorological records. Tree-ring indices showed non-significant growth trends over roughly the past four decades . The observed growth variability was explained by connexions to the mid-summer (July) climate (temperature and, likely, cloudiness) in the north and to the spring (April) soil temperatures in the south. Tree growth was negatively associated with snow depth at the beginning of dormancy (in November, previous to the corresponding growth year) in the south and during the growing season start (May) in the north. Thus, these analyses emphasized the relevance of snow and soil data, which have usually been overlooked in studies of associations between climate and tree-ring changes.

The northern boreal forests in Fennoscandia have been managed for decades using clear-cut harvesting and subsequent mechanical site preparation (MSP). MSP is intended to reduce competing vegetation, improve soil temperature and water conditions as well as mobilize nutrients for the use by outplanted seedlings. The use of heavy machinery, particularly bulldozerdriven Marttiini-plough, has raised concerns about soil quality and therefore sustainability of forest productivity and forest floor diversity. We applied soil physical measurements and inventories of naturally established Norway spruce (Picea abies (L.) Karst.) to investigate these concerns 16-18 years after intensive forest management in Finnish Lapland. MSP with Marttiiniplough had inverted Podzol soil profile and created the following micro-sites: trench, tilt and untreated (UTR) soil between the ploughed tracks at former spruce sites. The soil electrical conductivity (EC a ) and soil solution conductivity (EC w ) at micro-sites of spruce seedling/ saplings, differed significantly (p<0.001). The majority of spruce individuals (60%) were established on UTR (mean EC a =0.54±0.34 mSm −1 ), while the proportion on tilt was minor (34%, EC a =0.35±0.18 mSm −1 ) and almost absent on trench (6%, EC a =0.12±0.11 mSm −1 ). In a similar way, the soil quality decreased such that EC wUTR = 11.7 ± 9.1mSm −1 > EC wTILT = 10.7 ± 9.2mSm −1 > EC wTRENCH = 2.2 ± 2.9mSm −1 . Spruce individuals at UTR (mean height 127±89 cm) and tilt (111±74 cm) were significantly (p<0.001) taller compared to those in trench (32±44 cm). The five study sites were surrounded by mature Norway spruce stands, yet only 0-14% of spruce seedlings/ saplings were found on the MSP-exposed mineral soil. These results imply that intensive forest practices result in losses in soil nutrient supply, hence endangering sustainability of the soil quality, and therefore occasionally hamper natural regeneration of Norway spruce.

Tree-ring records constitute excellent high-resolution data and provide valuable information for climate science and paleoclimatology. Tree-ring reconstructions of past temperature variations agree to show evidence for annual-to-centennial anomalies in past climate and place the industrial-era warming in the context of the late Holocene climate patterns and regimes. Despite their wide use in paleoclimate research, however, tree rings have also been deemed unsuitable as low-frequency indicators of past climate. The arising debate concerns whether the millennia-long tree-ring records show signals of orbital forcing due to the Milankovitch cycles. Here, we produce a summer-temperature reconstruction from tree-ring chronology running through mid- and late-Holocene times (since 5486 BCE) comprising minimum blue channel light intensity (BI). The BI reconstruction correlates with existing and new tree-ring chronologies built from maximum latewood density (MXD) and, unlike the MXD data, shows temperature trends on Milankovitch scales comparable to various types of sedimentary proxy across the circumpolar Arctic. Our results demonstrate an unrevealed potential of novel, unconventional tree-ring variables to contribute to geoscience and climate research by their capability to provide paleoclimate estimates from inter-annual scales up to those relevant to orbital forcing.

High altitude and latitude findings of subfossil peatland pine trees were unearthed from the region of NW Finnish Lapland and dated by 14C and tree‐ring methods. The depositional history of the trees illustrated two distinct peatland pine phases dated to Middle Holocene intervals 4900–4400 and 4100–3400 cal. a BC. It seems evident that both thermal and hydroclimatic fluctuations have played roles of varying importance in the establishment of this pine population and its demise. The presence of these pines, from a site ~60 km north of the coniferous timberline and conditions ~1 °C and 100 degree‐days colder than those at the present‐day timberline, concurs with previous studies demonstrating the association between the high‐latitude summer‐temperature cooling and circumpolar timberline retreat since the Middle Holocene due to Milankovitch forcing. On the other hand, the peatland pine recruitment was made possible by drier than present surface conditions during the previously reconstructed Middle Holocene drought anomaly (Hyvärinen‐Alhonen event). Our data suggest this event was not continuous but reached its two‐phase climax during the peatland pine phases, with an interruption of several centuries with moister surface conditions between 4400 and 4100 cal. a BC. The findings highlight the sensitivity of well‐dated peatland tree assemblages in terms of recording past climatic evolution and events and the need for new collections from north and south Fennoscandia and the Baltic region, for more detailed analyses over extended time intervals and regions.