Fires have played an important role in creating and maintaining savannas over the centuries and are also one of the main natural disturbances in forests. The functional role of fires in savannas and forests can be investigated through examining sedimentary charcoal in order to reconstruct long-term fire history. However, the relationship between charcoal and vegetation structure in tropical grassy ecosystems remains to be elucidated. Here, we compared recent charcoal records from lake sediments in three tropical ecosystems (forest, savanna, and forest-savanna mosaic) with land cover inferred from remote-sensing images. Charcoal width-to-length (W/L) ratio is a good proxy for changes in fuel type. At one of the lakes, a significant W/L modification from values >0.5 (mainly wood) to <0.5 (¿grass) was recorded simultaneously with changes in land cover. Indeed, a significant deforestation was recorded around this lake in the remote-sensing imagery between 1984 and 1994. The results also indicate that a riparian forest around a lake could act as a physical filter for charcoal accumulation; we used the mean charcoal size as a proxy to evaluate this process. Charcoal Accumulation Rates (CHAR), a burned biomass proxy, were combined with W/L ratio and the mean charcoal size to investigate the land-use history of the landscapes surrounding the study sites. This combined approach allowed us to distinguish between episodic slash-and-burn practices in the forest and managed fields or pastures burning frequently. (Résumé d'auteur

In many boreal regions of Russia the past natural variability of forest fire activity remains largely undocumented, preventing accurate assessment of the impact of current climate warming on forest ecosystem dynamics. This study aims to reconstruct the Holocene fire history of the northern Ural mountain foothills, in the Komi Republic, based on analyses of charcoal particles from peatland deposits and coupled with dendrochronological investigations. The results show that there was a gradual increase in forest fire activity during the past 11,000 years. Between 11,000 and 5100 cal. yr BP, the mean fire return interval (FRI) oscillated between 600 and 200 years. During this period, regional data showed that cold temperature, humid climatic conditions, combined with steppe vegetation between 11,000–9000 cal. BP, and then the development of spruce-dominated forest between 9300 and 4600 cal. yr BP, were less conducive to fires. After 5100 cal. yr BP, a gradual increase in drought conditions through reduced precipitations, associated with the establishment of a Scots pine forest favored fire frequency, with a mean FRI under 200 years (range, 200–40 years). Nowadays (since CE 1500), human activity induces an unprecedented fire activity with a mean FRI below 100 years (range, 100–40 years).

We compared fire episodes over the past 150 years reconstructed using charcoal particles retrieved from well-dated sediment deposits from two small lakes in the eastern Canadian boreal forest, with dendrochronological reconstructions of fire events from the corresponding watersheds. Fire scars and age structure of living trees highlighted three fire events (ad 1890, 1941, and 1989). To explore the ability to detect these fire events based on sedimentary charcoal records, we explored the influence of two user-determined parameters of a widely used peak-detection algorithm (the CharAnalysis software): (1) the temporal resolution used to interpolate charcoal series and (2) the width of the smoothing window used to model background noise. The signal-to-noise index (SNI) is often used to evaluate the ability to detect peaks in sedimentary charcoal records, which can be related to fire events. SNI values >3 identify records appropriate for peak detection. Selecting standard settings in paleoecological studies (median temporal resolution of the entire sequence and 500- to 1000-year window width) yielded higher global SNI values but failed to detect most recent fire events. Instead, selecting a shorter reference period (the past ~150 years) to determine the temporal resolution to interpolate the charcoal series and a narrower smoothing window (100 years) best matched the tree-ring data despite lower SNI values (often <3.0). However, Holocene fire history differed markedly when reconstructed using different smoothing window widths (100–150 years vs >300 years). Consequently, we suggest using the smallest window width yielding a SNI >3. Practitioners must not necessarily focus on obtaining the highest possible SNI, usually related to wide smoothing windows. We also suggest that fire history reconstructions should focus on core sections presenting fairly constant sedimentation rates. Alternatively, sediments could be subsampled after age–depth models have been obtained.

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We present a comparative analysis of fire reconstructions from tree rings and from wood charcoal preserved in forest soils, peat and lake sediments. Our objective is to highlight the benefits and limits of different archives and proxies to reconstruct fire histories. We propose guidelines to optimize proxy and archive choice in terms of spatial and temporal scales of interest. Comparisons were performed for two sites in the boreal forest of northeastern North America. Compared to others archives, tree-ring analysis remains the best choice to reconstruct recent fires (<1000 years). For longer periods (from several centuries to millennia), lake charcoal can be used to reconstruct regional or local fire histories depending on the method used, but the focus should be on historical trends rather than on the identification of individual fire events. Charcoal preserved in peat and soils can be used to identify individual fire, but sometimes cover shorter time periods than lake archives.

Summary Macroscopic sedimentary charcoal and plant macroremains from two lakes, 50 km apart, in north‐western Ontario, Canada, were analysed to investigate fire frequency and tree abundance in the central boreal forest. These records were used to examine the controls over the long‐term fire regime, and vegetative dynamics associated with fire return intervals (FRIs). There were 52 fire events at Lake Ben (surrounded by a xeric landscape) between 10 174 calibrated years before present (cal. year bp) and the present with an average FRI of 186 years with values oscillating between 40 and 820 years. Forty‐three fire events were recorded at Lake Small (surrounded by a mesic landscape) between 9972 cal. year bp and the present with an average FRI of 229 years and a range of 60–660 years. FRIs at Lake Small decreased significantly after c. 4500 cal. year bp, whereas at Lake Ben FRIs remained similar throughout the Holocene. Different FRI distributions and independence in the occurrence of fire events were detected between 10 000 and 4500 cal. year bp for the two sites. Between 4500 cal. year bp and the present, similar FRIs were observed, but fires continued to occur independently. Longer FRIs resulted in declining abundance of Larix laricina in both landscapes. Longer FRIs resulted in a decline in the abundance of Picea mariana in the xeric landscape, but a marginal increase in the mesic landscape. Abundances of Pinus banksiana, Pinus strobus and Betula papyrifera were unrelated to FRI, underlying that these species maintain their local abundance irrespective of fire frequency. Synthesis. Our results show contrasting fire regime dynamics between a xeric and mesic landscape in central boreal forests, Canada. These results highlight the influence of local factors as important drivers of fire frequency at centennial to millennial scales. Local site factors, especially soil moisture, need to be incorporated into predictive models of vegetation response to climate change.

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Severe windthrows often require salvage operations that can lead to increased costs. Given these extra costs, it is of paramount importance to make sure that wood degradation does not become so advanced that significant value loss is incurred. The rate at which wood deteriorates is a function of many factors, including species and climate. The study was conducted in a northern area affected by two partial windthrows. Logs from the damaged area were collected for two species, balsam fir (Abies balsamea) and black spruce (Picea mariana). Logs were classified into one of three degradation classes based on visual assessments. A sample of logs from standing trees was also collected. In total, 167 logs were sampled. Each log was sawn and one piece of lumber was selected from each to determine the bending strength and stiffness and the visual grade. The time since tree death, as determined from dendrochronology, ranged from 1 to 31 years. The visual grade of the lumber was not affected after 1 year but severe downgrades were observed after 4 years. Moisture content decreased rapidly for both species during the first year and continued to decrease until 4 years after mortality. No clear decrease in bending stiffness was identified even though such a tendency was noticed for older black spruce windthrows. Bending strength became variable after 4 years for balsam fir and was reduced after 4 years for black spruce. Windthrows older than 7 years will produce low visual grade timber of reduced bending strength and possibly of lower bending stiffness.