Dominique Schwartz

Phytoliths : indicators of grassland dynamics during the late Holocene in intertropical Africa

Abstract The reconstruction of African tropical grassland history during the late Holocene can be carried out using phytolith analysis. Fossil phytolith assemblages from Lake Guiers, in the Sahelian region of Senegal, and from Lake Sinnda, in the Guineo-Congolian region of Congo were investigated. The results are interpreted on the basis of modern phytolith assemblages from the same regions and compared to pollen data previously obtained. Tall or short grass associations are discriminated by their phytolith index Iph(%) = saddle/(cross + dumbel + saddle), while the density of shrubs and trees is indicated by relative proportions of the dicotyledon phytoliths. The phytolith data emphasize that, in the Guineo-Congolian region around Lake Sinnda, the driest phase of the late Holocene occurred between 4000 and 1200 yr B.P., commencing with the opening of the dense forest and its replacement by a short grass savanna. From ca. 1000 yr B.P., wetter climatic conditions developed, as represented by the setting up of a tall grass savanna woodland. The modern shrub and tall grass savanna was developed ca. 700 yr B.P. In the Sahelian region around Lake Guiers, the driest phase occurred after about 2000 yr B.P. and has not been followed by moister conditions. A tall grass savanna woodland was gradually replaced by a shrub and short grass savanna which still occurs. A short period of development of swampy vegetation, which can be related to a lake level change, interrupted the semi-arid adaptation of the vegetation, between about 2000 yr B.P. and the present.

Pollen evidence of late Quaternary vegetation and inferred climate changes in Congo

A detailed palynological analysis of two cores recovered from a swamp in the Southern Bateke Plateaux (Congo), provides information on the botanical history of this region during the last 24,000 yr B.P. Rich and well-diversified pollen counts exhibit changes in the vegetational communities related to hydrological and climatic fluctuations. Around 24,000 yr B.P., the occurrence of hydromorphous forest elements gives evidence of a humid phase. Between 24,000 and around 13,000 yr B.P., swampy herbaceous communities expanded, indicating slightly drier climatic conditions. From approximately 13,000 yr B.P. onward, the beginning of a humid episode is reflected by the development of mesophilous forests. The decrease of these forests is documented since 3000 yr B.P. Grasslands are found to extend locally whereas Elaies guineensis (oil palm) spreads out, indicating both climatic change but also anthropogenic activities.

Present dynamics of the savanna-forest boundary in the Congolese Mayombe: a pedological, botanical and isotopic (13C and 14C) study

Isolated savannas enclosed by forest are especially abundant in the eastern part of the Congolese Mayombe. They are about 3000 years old, and were more extensive some centuries ago. The boundary between forest and savanna is very abrupt, as a consequence of the numerous savanna fires lit by hunters. Floristic composition and vegetation structure data, organic carbon ratios, Δ14C and δ13C measurements presented here show that forest is spreading over savanna at the present time and suggest that the rate of forest encroachment is is currently between 14 and 75 m per century, and more probably about 20–50 m per century. As most savannas are less than 1 km across, such rates mean, assuming there are no changes in environmental conditions, that enclosed savannas could completely disappear in the Mayombe in about 1000–2000 years.

SOIL ORGANIC CARBON DYNAMICS UNDER EUCALYPTUS AND PINUS PLANTED ON SAVANNAS IN THE CONGO

Abstract In the Congo, Eucalyptus and Pinus have been planted commercially for 30 yr on savanna. Large differences in the abundance of carbon isotopes of the savanna Gramineae and the trees enabled a study to be made of the soil organic matter (SOM) turnover by means of its carbon isotope composition. The organic carbon content slightly increased with plantation age without any significant differences (90% confidence limit) between plantation types. The δ13C of SOM varies with the plantation age from −14.4%, in the savanna reference plot to values close to −26.5% in the oldest tree plantations. The distribution of δ13C as a function of plantation age was modelled by an exponential function and exhibited no significant variation (90% confidence level) between plantation type. As the relative contribution of organic matter inherited from the savanna and that derived from the tree species was proportional to the δ13C value, the fraction of each organic compartment was evaluated for plantations grouped together without distinguishing between the different tree species. With time, the SOM inherited from the savanna tended to disappear. The half-life was estimated to be 16.5 yr, clearly illustrating the rapid SOM turnover which characterizes the savanna ecosystems in psammitic ferrallitic soils. The fraction of organic C of tree origin progressively introduced into the soils was linearly related to time. The annual increment of organic C was found to be equal to 0.3 mg C g−1 yr−1, but it was clear that the linear evolution of carbon with time could not be extrapolated over more than 30 yr. The various models predicted that after 30 yr there would remain in the soil, 1.8 and 0.8 mg C g−1 of savanna origin and 9.4 mg C g−1 of newly introduced organic C of tree origin respectively.

Agreement between floristic and soil organic carbon isotope (13C/12C,14C) indicators of forest invasion of savannas during the last century in Cameroon

In an area of savanna-forest (S-F) mosaic of Cameroon, at Kandara near Bertoua, an enclosed savanna bordered by young semi-deciduous forests was selected for detailed studies of vegetation and soil carbon isotope compositions with a view to estimating the rate of forest advance into savannas. Forest floristic composition and structure were analysed in small plots along two S-F transects and within two large stands. Tree species counts and basal area (BA) measurements gave convergent results defining (1) an edge forest with low BA values that forms an irregular strip parallel to the S-F border, (2) a large colonization zone zone composed of pioneer species ( Albizia species, with individuals of very large diameter) and (3) a mature forest composed of abundant Rinorea individuals and large individuals of Triplochiton scleroxylon and Piptadeniastrum africanum . Carbon stable isotopes were deter mined from organic matter of soil profiles sampled at various depths in savanna, colonization-zone and mature forest. In the deep soil horizons (40-50 cm) of the colonization-zone profiles, δ 13 C values similar to those of the present savanna reveal the past existence of a large-tree savanna. In subsurface horizons (15-20 cm), δ 13 C values intermediate between those of savanna and mature forest prove the encroachment of the forest ecosystem on savanna. Using 14 C measurements, the mean residence time (MRT) of soil organic matter of these last horizons was determined with precision owing to the atmospheric 14 C pulse from nuclear bomb tests prior to 1964. By assuming an exponential age distribution of organic compounds and by taking account of MRT and remaining carbon from the savanna, the coloniza tion zone was found to be 60-80 y old. The age of the colonization zone being the same near the present savanna and near the mature forest, it seems that the forest advance was probably not a linear process but would result from the coalescence of Albizia thickets born in savanna.

Use of plots to define pollen–vegetation relationships in densely forested ecosystems of Tropical Africa

Modern soil samples from South Congo were analyzed for pollen content and compared to forest inventories to define modern pollen-vegetation relationships. A correspondence analysis (CA) was applied independently to botanical and pollen data and a hierarchical cluster analysis to pollen data only. Subsequently, a CA using a presence-absence approach has been made to directly compare the two types of data. Results show that the pollen rain and floristic composition of the sampled sites are not directly linked to altitudinal or precipitation gradients, but clear evidence of variation in relation to hygromorphy and soil type is detected. The forests occurring in swampy environments are well differentiated from the forests developed on well-drained soils by pollen and floristic data. Among forests on well-drained soils, a good distinction can be made between those growing on sandy soils and those growing on ferralitic soils. The comparison between pollen spectra and vegetation shows site-to-site variations in pollen assemblages in relation to the floristic heterogeneity of forests, and it appears that few taxa show a good correlation between plant cover and pollen abundance.

Change in lignin content during litter decomposition in tropical forest soils (Congo): comparison of exotic plantations and native stands

Abstract Fast-growing tree plantations are being extended in tropical countries resulting in new forest ecosystems, the functioning of which is yet not well known. In particular, few data are available concerning lignin decay rate. Lignin, nitrogen and tannin contents of fresh and decaying litter were measured in natural rain forest and in planted stands ofEucalyptus hybrids. Acacia mangium and A. auriculiformisin Congo, together with litter-fall and forest-floor accumulation. Lignin evolution in aging litter exhibited different patterns. Lignin was accumulated under Eucalyptus plantation, but disappeared under natural forest, and was intermediate underAcaciaplantations. The relationships with decomposition rates and lignin degradation factors, such as white rot fungi and termites, are also discussed.

Vegetation history of chernozems in the Czech Republic

Chernozem is a soil type which can be characterised by a thick dark surface horizon, which consists of organic matter and tends to change into a carbonate horizon or more often into a loess horizon. Chernozem is defined as a zonal soil that has developed under steppe vegetation in a dry continental climate. Nevertheless, chernozems can also be found in central Europe, where there are no climatic conditions for the existence of any steppe. This study is focused on the vegetational aspect of the pedogenesis of chernozems. We have examined three sorts of chernozems for their charcoal and pedological characteristics: the functional chernozems, the chernozems buried in Holocene material and the chernozems buried in Pleistocene material. The charcoal examination has proved the presence of woodland taxa in the areas of chernozems at different periods of time. The results of this study reveal that the high stability of soil organic matter has caused the persistence of chernozems in the areas with prevalent woodland vegetation.

CHERNOZEM. FROM CONCEPT TO CLASSIFICATION: A REVIEW

In this paper, we put together the most important facts that lead to the research on chernozem. Thanks to the work of V. V. Dokuchaev (1846–1903), chernozem stands at the forefront of pedology. In 1883, Dokuchaev introduced the first concept of chernozem: he defined chernozem as a steppe soil with pedogenesis dominated by a dry continental climate and steppe vegetation, with calcareous parent material. Chernozem is a soil well known for its high agronomical potential; therefore the perception of chernozem as something extraordinary valuable goes back far into history. Our review presents the key factors of the pedogenesis of chernozem by explaining the causes of the high stability of its organic matter and the role of vegetation and fauna in this process. Moreover, it shows that chernozem can have many aspects due to various (a) textures, (b) chemical compositions, (c) influence of water, (d) fauna, or (e) anthropogenic factors. We examine the position of chernozem in numerous national and international soil classifications over time. We have found out that chernozem had been classified, according to its properties, as a “steppe soil”, as a “calcic soil” or as an “isohumic soil”.

A comparative review of soil charcoal data: Spatiotemporal patterns of origin and long-term dynamics of Western European nutrient-poor grasslands

The nutrient-poor grasslands of Western Europe are of major conservation concern because land use changes threaten their high biodiversity. Studies assessing their characteristics show that their past and on-going dynamics are strongly related to human activities. Yet, the initial development patterns of this specific ecosystem remain unclear. Here, we examine findings from previous paleoecological investigations performed at local level on European grassland areas ranging from several hundred square meters to several square kilometers. Comparing data from these locally relevant studies at a regional scale, we investigate these grasslands’ spatiotemporal patterns of origin and long-term dynamics. The study is based on taxonomic identification and radiocarbon AMS dating of charcoal pieces from soil/soil sediment archives of nutrient-poor grasslands in Mediterranean and temperate Western Europe (La Crau plain, Mont Lozère, Grands Causses, Vosges Mountains, Franconian Alb, and Upper-Normandy region). We address the following questions: (1) What are the key determinants of the establishment of these nutrient-poor grasslands? (2) What temporal synchronicities might there be? and (3) What is the spatial scale of these grasslands’ past dynamics? The nutrient-poor grasslands in temperate Western Europe are found to result from the first anthropogenic woodland clearings during the late Neolithic, revealed by fire events in mesophilious mature forests. In contrast, the sites with Mediterranean affinities appear to have developed at earlier plant successional stages (pine forest, matorral), established before the first human impacts in the same period. However, no general pattern of establishment and dynamics of the nutrient-poor grasslands could be identified. Local mechanisms appear to be the key determinants of the dynamics of these ecosystems. Nevertheless, this paleoecological synthesis provides insights into past climate or human impacts on present-day vegetation.