Tracking land-cover changes with sedimentary charcoal in the Afrotropics

Abstract

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

Reconstructions of biomass burning from sediment-charcoal records to improve data–model comparisons

“…However, it is not possible to quantify absolute area burned in the absence of a calibration data set, and the influences of non-fire-related processes such as erosion or vegetation change on biomass burning reconstructions remain poorly understood (Aleman et al, 2013). These limitations highlight a need for more calibration studies to understand how charcoal production and taphonomy relates to the area and amount of biomass burned across a range of vegetation types and climate conditions.…”

“…As the GCD continues to expand, the expectation is that knowledge of fire histories will become more detailed. Analyzing charcoalbased fire-history records with modern data from satellites (e.g., van der Werf et al, 2010;Giglio et al, 2013), fire scars (e.g., Girardin and Sauchyn, 2008;Marlon et al, 2012), or historical records (e.g., Mouillot et al, 2006;Lamarque et al, 2010) is necessary to connect relative or qualitative variations in biomass burning from charcoal records (Aleman et al, 2013) to quantitative estimates of burned area or carbon emissions. To test hypotheses related to drivers of fire activity over longer timescales, however, research needs to integrate paleofire data with modeling approaches.…”

“…Uncertainties in charcoal records also come from the many natural processes related to charcoal production, transportation, and deposition, which interact to produce variability in charcoal accumulation over time. These processes are being studied through field experiments and calibration studies that will enable the development of higher quality fire-history reconstructions and a better understanding of uncertainties (Tinner et al, 2006;Higuera et al, 2011;Aleman et al, 2013). An important source of uncertainty in global fire models is the parameterization of the processes most directly controlling fire activity (e.g., human influence, climate influence; e.g., Pechony and Shindell, 2009;Pfeiffer et al, 2013).…”

Mid Holocene vegetation reconstruction from Vanevan peat (south-eastern shore of Lake Sevan, Armenia)

“…Figure 8 also includes length-to-width ratio against time. Ratio values < 0.5 indicate that fires mostly burned sedge and grassland vegetation (Aleman et al, 2013). Therefore, two phases are shown by concentration of charcoal and by length-to-width ratio.…”

Sparking New Opportunities for Charcoal-Based Fire History Reconstructions

“…Moreover, charcoal-based paleofire science has evolved recently toward improved information about other fire-regime parameters, including the possibility to explore area burned [17], fire severity metrics [18], and fuel types [19].…”