Wannes Hubau

A tree-ring based comparison of Terminalia superba climate–growth relationships in West and Central Africa

Tropical lowland forests are characterized by humid climate conditions with interannual variations in amount of precipitation, length of dry season, and relative humidity. The African tree species, Terminalia superba Engl. & Diels has a large distribution area and potentially incorporates these variations in its tree rings. Tree ring analysis was performed on 60 plantation trees (increment cores) and 41 natural trees (stem disks) from Ivory Coast and the Congolese Mayombe Forest. Natural forests and old plantations (50–55xa0years) showed similar growth patterns. Regional chronologies were developed for the two sample regions and showed a long-distance relationship for the period 1959–2008. Growth in the Mayombe was associated with early rainy season precipitation, but no relation was found between tree growth and precipitation in Ivory Coast. Congolese trees possibly show a higher climate-sensitivity than Ivorian trees, because precipitation in the Mayombe is more limiting, and Congolese T. superba trees are found closer to the margins of their distribution. Likewise, tree growth in the Mayombe was also influenced by the SSTs of the Gulf of Guinea and the South Atlantic Ocean during the early rainy season. However, tree growth was influenced by ENSO in both regions. In the Mayombe, La Niña years were associated with stronger tree growth whereas in Ivory Coast, El Niño years corresponded with stronger tree growth. The presented relation between ENSO, precipitation and tree growth is original for equatorial African forests, suggesting an influence of global climate variability on tree growth.

Archaeological charcoals as archives for firewood preferences and vegetation composition during the late Holocene in the southern Mayumbe, Democratic Republic of the Congo (DRC).

Analysis of charcoal from an archaeological assemblage near the Lukula community located at the southernmost boundary of the Mayombe forest (Bas-Congo, DRC) yielded 30 taxa used as firewood between 1,200 and 700xa0cal. b.p. Local people mentioned 71 taxa preferred for use nowadays. The identified taxa belong either to mature rainforest, pioneer forest, regenerating forest or woodland savanna, indicating that ancient and current local populations gathered firewood in several different forest types. Modern firewood preferences do not seem to agree with the archaeobotanical composition. Also, linguistic evidence does not indicate a long exploitation history for all of the recorded taxa. Furthermore, no particular wood qualities such as wood density, calorific value or magical or medicinal properties seem to determine the Lukula assemblage, which was probably composed of waste material from various activities which required different specific firewood characteristics. As such, taxa composition is not biased by human selection, suggesting that it reflects the surrounding environment, which was characterised by mature rainforest with patches of regenerating forest and open vegetation types. Unlike the origin of present-day forest-savanna mosaics from human activity, fragmentation around 1,000xa0cal. b.p. may have been provoked by a well-known climatic event coinciding with the Medieval Climate Anomaly, which undoubtedly had a significant impact on Central African forest composition.

Charcoal‐inferred Holocene fire and vegetation history linked to drought periods in the Democratic Republic of Congo

The impact of Holocene drought events on the presumably stable Central African rainforest remains largely unexplored, in particular the significance of fire. High-quality sedimentary archives are scarce, and palynological records mostly integrate over large regional scales subject to different fire regimes. Here, we demonstrate a direct temporal link between Holocene droughts, palaeofire and vegetation change within present-day Central African rainforest, using records of identified charcoal fragments extracted from soil in the southern Mayumbe forest (Democratic Republic of Congo). We find three distinct periods of local palaeofire occurrence: 7.8-6.8xa0ka BP, 2.3-1.5xa0ka BP, 0.8xa0ka BP - present. These periods are linked to well-known Holocene drought anomalies: the 8.2xa0ka BP event, the 3rd millennium BP rainforest crisis and the Mediaeval Climate Anomaly. During and after these Holocene droughts, the Central African rainforest landscape was characterized by a fragmented pattern with fire-prone open patches. Some fires occurred during the drought anomalies although most fires seem to lag behind them, which suggests that the open patches remained fire-prone after the actual climate anomalies. Charcoal identifications indicate that mature rainforest patches did persist through the Early to Mid-Holocene climatic transition, the subsequent Holocene thermal optimum and the third millennium BP rainforest crisis, until 0.8xa0ka BP. However, disturbance and fragmentation were probably more prominent near the boundary of the southern Mayumbe forest. Furthermore, the dominance of pioneer and woodland savanna taxa in younger charcoal assemblages indicates that rainforest regeneration was hampered by increasingly severe drought conditions after 0.8xa0ka BP. These results support the notion of a dynamic forest ecosystem at multicentury time scales across the Central African rainforest.

Complementary imaging techniques for charcoal examination and identification

Identification of ancient charcoal fragments is a valuable tool in reconstructing past environments and determining natural and anthropogenic disturbances, and for understanding past cultures and societies. Although in Europe such studies are fairly straightforward, utilising charcoal records from the tropics is more complicated due to the species-richness of the natural vegetation. Comprehensive databases have greatly aided identification but often identification of charcoalified woods from the tropics relies on minute anatomical features that can be difficult to observe due to preservation or lack of abundance.This article illustrates the relative potential of four imaging techniques and discusses how they can provide optimal visualisation of charcoal anatomy, such that specific difficulties encountered during charcoal examination can be evaluated and fine anatomical characters can be observed enabling high-level identification of charcoal (and wood) taxa. Specifically reflected Light Microscopy is often used to quickly group large numbers of charcoal fragments into charcoal types. Scanning Electron Microscopy and High-Throughput X-ray Computed Tomography are employed to observe fine anatomical detail. More recently X-ray Computed Tomography at very high resolution has proved successful for imaging hidden or ‘veiled’ anatomical features that cannot be detected on exposed surfaces but need three-dimensional volumetric imaging.