Laurent Bremond

The Geological, Isotopic, Botanical, Invertebrate, and Lower Vertebrate Surroundings of Ardipithecus ramidus

Sediments containing Ardipithecus ramidus were deposited 4.4 million years ago on an alluvial floodplain in Ethiopia’s western Afar rift. The Lower Aramis Member hominid-bearing unit, now exposed across a >9-kilometer structural arc, is sandwiched between two volcanic tuffs that have nearly identical 40Ar/39Ar ages. Geological data presented here, along with floral, invertebrate, and vertebrate paleontological and taphonomic evidence associated with the hominids, suggest that they occupied a wooded biotope over the western three-fourths of the paleotransect. Phytoliths and oxygen and carbon stable isotopes of pedogenic carbonates provide evidence of humid cool woodlands with a grassy substrate.

Control of the multimillennial wildfire size in boreal North America by spring climatic conditions

Wildfire activity in North American boreal forests increased during the last decades of the 20th century, partly owing to ongoing human-caused climatic changes. How these changes affect regional fire regimes (annual area burned, seasonality, and number, size, and severity of fires) remains uncertain as data available to explore fire–climate–vegetation interactions have limited temporal depth. Here we present a Holocene reconstruction of fire regime, combining lacustrine charcoal analyses with past drought and fire-season length simulations to elucidate the mechanisms linking long-term fire regime and climatic changes. We decomposed fire regime into fire frequency (FF) and biomass burned (BB) and recombined these into a new index to assess fire size (FS) fluctuations. Results indicated that an earlier termination of the fire season, due to decreasing summer radiative insolation and increasing precipitation over the last 7.0 ky, induced a sharp decrease in FF and BB ca. 3.0 kyBP toward the present. In contrast, a progressive increase of FS was recorded, which is most likely related to a gradual increase in temperatures during the spring fire season. Continuing climatic warming could lead to a change in the fire regime toward larger spring wildfires in eastern boreal North America.

Spatial Variability of Fire History in Subalpine Forests: From Natural to Cultural Regimes

Abstract: The goal of this study was to determine the effect of local and large-scale processes on fire frequency during the postglacial period in a subalpine ecosystem (Alps, France). Large-scale processes should produce homogeneous distribution of fire-free intervals and synchronicity of fire series, and dominance of local-scale processes, such as those triggered by differences in relief, slope aspect, human history, etc. should create heterogeneous fire regimes. Four ponds and peat were sampled at different elevations and exposures. Sedimentary charcoal was used as a fire proxy, and plant macroremains were used as a vegetation proxy. Synchronicity analysis was based on a transformed Ripleys K-function. Similar fire-free intervals during the early Holocene suggest that fire regimes were controlled at that time by large-scale natural processes such as climate and vegetation patterns and establishment. No fire was reconstructed before 9000 y cal BP. Infrequent fires occurred following establishment of the subalpine bio-climate belt. However, local-scale processes have dominated the pattern of fire intervals during the late Holocene, with more fires at lower elevation and on south-facing slopes. Although altitude, topography, and slope aspect certainly drove between-site differences during the early Holocene, these differences disappeared during the late Holocene, when fire frequency was related not to ecological features of the natural landscape but likely to human population density and activities, e.g., need for pastures (woody fuel suppression). Fires were certainly controlled at first by climate and vegetation (Pinus cembra), but human practices have affected the fire regime for centuries. A new fire epoch might result from both the current global warming and on-going land-use abandonment, which has led to a significant fuel build-up in the Alps. Nomenclature: Tutin et al., 1968–1993.

Trees in the subalpine belt since 11 700 cal. BP: origin, expansion and alteration of the modern forest

High altitude alpine ecosystems are likely to be highly sensitive to future climate change. Understanding long-term tree stand dynamics may be a key requirement for forecasting such changes. Here, we present a high resolution record of paleobotanical macroremains covering the last 11 700 years, from a small subalpine pond situated in the inner French Alps, at 2035 m a.s.l. The early presence of larch (Larix decidua), arolla pine (Pinus cembra) and birch (Betula) at this elevation, just after the end of the Younger Dryas cold transition, suggests the occurrence of either glacial tree-refugia located nearby in the northwestern Alps, or a previously unrecorded early and rapid tree migration. The 8200 cal. BP cooling event is characterized by a rapid and limited expansion of mountain pine (Pinus mugo/uncinata type). Mixed stands of larch, birch and arolla pine established at 8300 cal. BP and were present through the mid Holocene. After the Holocene climatic optimum, at 5600 cal. BP, recurrent fires led to the development of highly dynamic and more diversified forests, with larch, birch, arolla pine, mountain pine and fir (Abies alba). Natural and anthropogenic disturbances, e.g., fires, avalanches, slash-and-burn and other agricultural practices, influenced subsequent vegetation until the last millennium when tree-pasture established around the lake. The data indicate that the vegetation was progressively dominated by open larch woodland from 4000 years ago, and was clearly established during the Middle Ages (1250 cal. BP) up to the nineteenth century, when land began to be abandoned. The modern vegetation, dominated by larch and arolla pine and resulting from land abandonment, tends to resemble the communities that occurred from 8300 to 4000 cal. BP, before the postulated anthropogenic alteration of subalpine forest ecosystems. The plant macroremains analysis provides a unique and precise record of stand-to-local vegetation composition and dynamics that can bridge paleoecology and forest management.

Tracking land-cover changes with sedimentary charcoal in the Afrotropics

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.

New Evidence of Human Activities during the Holocene in the Lowland Forests of the Northern Congo Basin

In the last decade, the myth of the pristine tropical forest has been seriously challenged. In central Africa, there is a growing body of evidence for past human settlements along the Atlantic forests, but very little information is available about human activities further inland. Therefore, this study aimed at determining the temporal and spatial patterns of human activities in an archaeologically unexplored area of 110,000 km 2 located in the northern Congo Basin and currently covered by dense forest. Fieldwork involving archaeology as well as archaeobotany was undertaken in 36 sites located in southeastern Cameroon and in the northern Republic of Congo. Evidence of past human activities through either artifacts or charred bo- tanical remains was observed in all excavated test pits across the study area. The set of 43 radiocarbon dates extending from 15,000 BP to the present time showed a bimodal distribution in the Late Holocene, which was interpreted as two phases of human expansion with an intermediate phase of depopulation. The 2300-1300 BP phase is correlated with the migrations of supposed farming populations from northwestern Cameroon. Between 1300 and 670 BP, less material could be dated. This is in agreement with the population collapse already reported for central Africa. Following this, the 670-20 BP phase cor- responds to a new period of human expansion known as the Late Iron Age. These results bring new and extensive evidence of human activities in the northern Congo Basin and support the established chronology for human history in central Africa.

Evidence of moist niches in the Bolivian Andes during the mid-Holocene arid period

To examine the climate of the mid-Holocene and early human settings in the Andes when the Altiplano was recording the most arid phase of the Holocene, we analyzed plant-related proxies (pollen, phytoliths, diatoms, stable isotopes) from a sediment core sampled at high elevation in the Eastern Cordillera of Bolivia. Our study was carried out in the wetland of Tiquimani (16°12′06.8″S; 68°3′51.5″W; 3760 m), on a well-known pathway between Amazonia and Altiplano. The 7000-year old record shows a two-step mid-Holocene with a dry climate between 6800 and 5800, followed by a wetter period that lasted until 3200 cal. yr BP. In the Central Andes of Bolivia, a widespread aridity was observed on the Altiplano during the mid-Holocene. However, here, we show that moisture was maintained locally by convective activity from the Amazon lowlands. During the arid interval between 5000 and 4000 yr BP, these niches of moisture produced specific grasslands that may have enabled the survival of an archaic culture of hunter–gatherers on the Puna. This development occurred 2000 years before expansion of quinoa cultivation on the Puna.

Effects of vegetation zones and climatic changes on fire-induced atmospheric carbon emissions: a model based on paleodata

An original method is proposed for estimating past carbon emissions from fires in order to understand long-term changes in the biomass burning that, together with vegetation cover, act on the global carbon cycle and climate. The past carbon release resulting from paleo-fires during the Holocene is examined using a simple linear model between measured carbon emissions from modern fires and sedimentary charcoal records of biomass burning within boreal and cold temperate forests in eastern Canada (Quebec, Ontario). Direct carbon emissions are estimated for each ecozone for the present period and the fire anomaly per kilo annum (ka) v. present day (0 ka) deduced from charcoal series of 46 lakes and peats. Over the postglacial, the Taiga Shield ecozone does not match the pattern of fire history and carbon release of Boreal Shield, Atlantic Maritime, and Mixedwood Plains ecozones. This feature results from different air mass influences and the timing of vegetation dynamics. Our estimations show, first, that the contribution of the Mixedwood Plains and the Atlantic Maritime ecozones on the total carbon emissions by fires remains negligible compared with the Boreal Shield. Second, the Taiga Shield plays a key role by maintaining important carbon emissions, given it is today a lower contributor.

Present-day central African forest is a legacy of the 19th century human history

The populations of light-demanding trees that dominate the canopy of central African forests are now aging. Here, we show that the lack of regeneration of these populations began ca. 165 ya (around 1850) after major anthropogenic disturbances ceased. Since 1885, less itinerancy and disturbance in the forest has occurred because the colonial administrations concentrated people and villages along the primary communication axes. Local populations formerly gardened the forest by creating scattered openings, which were sufficiently large for the establishment of light-demanding trees. Currently, common logging operations do not create suitable openings for the regeneration of these species, whereas deforestation degrades landscapes. Using an interdisciplinary approach, which included paleoecological, archaeological, historical, and dendrological data, we highlight the long-term history of human activities across central African forests and assess the contribution of these activities to present-day forest structure and composition. The conclusions of this sobering analysis present challenges to current silvicultural practices and to those of the future. DOI: http://dx.doi.org/10.7554/eLife.20343.001

Tree cover in Central Africa: Determinants and sensitivity under contrasted scenarios of global change

Tree cover is a key variable for ecosystem functioning, and is widely used to study tropical ecosystems. But its determinants and their relative importance are still a matter of debate, especially because most regional and global analyses have not considered the influence of agricultural practices. More information is urgently needed regarding how human practices influence vegetation structure. Here we focused in Central Africa, a region still subjected to traditional agricultural practices with a clear vegetation gradient. Using remote sensing data and global databases, we calibrated a Random Forest model to correlatively link tree cover with climatic, edaphic, fire and agricultural practices data. We showed that annual rainfall and accumulated water deficit were the main drivers of the distribution of tree cover and vegetation classes (defined by the modes of tree cover density), but agricultural practices, especially pastoralism, were also important in determining tree cover. We simulated future tree cover with our model using different scenarios of climate and land-use (agriculture and population) changes. Our simulations suggest that tree cover may respond differently regarding the type of scenarios, but land-use change was an important driver of vegetation change even able to counterbalance the effect of climate change in Central Africa.