Aki Pitkänen

Carbon accumulation in West Siberian Mires, Russia Sphagnum peatland distribution in North America and Eurasia during the past 21,000 years

The rates of carbon (C) accumulation and the role of fires in the C dynamics of the major types of boreal West Siberian mires were investigated. Detailed analysis of dry bulk density, C, and N concentration and age of the peat layers were used to determine C accumulation rates throughout the Holocene. The average long-term apparent rate of carbon accumulation (LORCA) at 11 studied sites was 17.2±1.0 (SE) g m−2 yr−1, ranging from 12.1 to 23.7 g m−2 yr−1, and the total apparent carbon sink 11.8 Tg yr−1 (1 Tg = 1012 g) for Russian raised string bogs (68.5 million hectares). These estimates of C accumulation in West Siberian mires are roughly a half of the earlier estimates for these boreal mires. Differences in LORCA for three major mire types in the study area, the ridge-hollow pine bogs, Sphagnum fuscum pine bogs, and dwarf-shrub pine bogs, were not significant. The age versus depth (measured as cumulative carbon from the surface downward) curve was slightly convex, indicating a general declining trend in LORCA with decreasing age. About 55% of the present carbon store was already accumulated about 6000 cal. BP. The most intensive expansion phase of the study area occurred between 7000 and 8000 cal. BP. The subsequent lateral expansion has been very slow in the later Holocene. The charcoal data indicated that these mires have burned only 2–3 times during the past 7000–8000 cal. BP, and only a strip of a few meters along the mire margins has burned relatively frequently. No evidence of significant carbon losses due to fires could be found. The charcoal layers at the mire margins suggest a declining trend in burning rates during the later Holocene.

The role of fire in the carbon dynamics of a mire, eastern Finland

The fire history and the effect of fires on the long-term (apparent) rate of carbon accumulation (LORCA) in the Patvinsuo mire complex, eastern Finland, were determined through stratigraphic and pollen analyses of 98 peat cores. The peat cores were characterized by a large number of charcoal layers and the age of the basal peat varied between 57 and 10 500 years. Mire fires slowed down the progress of vertical peat accumulation and resulted in great carbon losses. The average LORCA in the Patvinsuo cores was clearly lower, 9.2 ± 1.0 (SE) g m-2 yr-1, compared to the average value for all mire sites in the southern half of Finland, 17.7 ± 0.6 (SE) g m-2 yr-1. The average rate of carbon loss in Patvinsuo mire complex was 9.5 ± 1.0 (SE) g m-2 yr-1 and the mean carbon loss in an individual fire was estimated to be 2.5 kg m-2.

Long-term fire frequency in the spruce-dominated forests of the Ulvinsalo strict nature reserve, Finland

Abstract The long-term fire history of the spruce-dominated forests of the Ulvinsalo strict nature reserve in Kuhmo, eastern Finland (63°58′N, 30°22′E), was studied by means of charcoal particle layer records and macroscopic charcoal particles in peat and by pollen analysis. The data suggest that spruce forest sites in the area burned at a mean interval of a few centuries over a period of almost 6000 years prior to the beginning of slash-and-burn cultivation in the area. This low forest fire frequency can be attributed to the fragmented landscape pattern, which restricted the spread of fires ignited by lightning. Because a similar landscape pattern predominates over large areas of boreal Fennoscandia, it is possible that natural fire frequencies may also have been considerably lower than expected in these areas. It may well have been the case that, at least in spruce forests located in fragmented landscapes, small-scale disturbances were more important for the forest dynamics than fires.

A basin-based approach to the long-term history of forest fires as determined from peat strata

Valid reconstruction of long-term forest fire histories from individual peat cores usually fails, for a number of reasons, even though visible charcoal horizons in peat are indisputable evidence of local orin-situ fires. Both the dating and reliability of fire records from peat can be improved by the means of a ‘basin-based approach’, in which the gradual lateral growth of the peat is studied carefully by means of numerous dated basal peat samples. Applying this principle, the frequency of forest fires in an esker landscape in southern Finland was elucidated over the past 7000 years. The results indicate that a period of active land use, but undeveloped fire control in the area, reduced the mean fire interval from the natural background fromc. 130 years toc. 40 years.

Fire frequency and forest structure at a dry site between AD 440 and 1110 based on charcoal and pollen records from a laminated lake sediment in eastern Finland

The fire history of a dry Boreal forest site in eastern Finland before settlement in the area was studied by means of charcoal and pollen analysis of annually laminated lake sediments. The average estimated local fire interval of 70-80 years corresponds to values quoted in a few earlier Finnish studies based on annually laminated lake sediments from more mesic sites. The fire interval is longer than expected from dendrochronological results obtained for dry sites in Fennoscandian forests, suggesting human influence during the periods studied by the dendrochronological method. The high representation of Picea pollen preceding the period of slash-and-burn cultivation at this and some other dry sites suggests a mixed coniferous forest structure at dry forest sites before significant human influence. The charcoal influx reflected estimated climatic shifts, suggesting that the fires increased during the warmer climatic periods. A small test comparing charcoal records on pollen slides with thin sections was included in this study. Large macroscopic charcoal particles were so scarce in the sediment of Lake Laukunlampi that two local fires discernible in the pollen and microscopic charcoal records could not be detected in the thin sections.