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Le maraîchage urbain joue un rôle très important dans la sécurité alimentaire et la résorption du chômage. Cependant, l’intensification des pratiques culturales peuvent altérer les propriétés physicochimiques des sols et réduire les rendements. La présente étude avait pour but de caractériser les pratiques culturales maraîchères à Brazzaville et de déterminer leur influence sur les propriétés physicochimiques des sols. Dans les sites retenus, des enquêtes structurées ont été menées auprès des maraîchers et des échantillons de sol ont été prélevés suivant des transects. Les analyses physiques et chimiques des sols ont été réalisées suivant les procédures courantes de laboratoire. Les résultats obtenus ont montré que les pratiques culturales dans les sites étudiés sont intensives. Les sols étudiés sont sableux à plus de 95%, acides, pauvres en matière organique et ont une CEC relativement faibles. Cependant on relève une sursaturation en bases échangeables sur l’ensemble des sites. Ainsi, cette sursaturation serait liée à l’intensification des pratiques culturales. Urban market gardening plays a very important role in food security and the reduction of unemployment. However, intensification of soil use can alter soil properties and reduce yields. This study aims to characterize vegetable farming practices in Brazzaville and to determine their influence on soil properties. In the selected sites, structured questionnaire was conducted among market gardeners and soil samples were taken following transects. Soil physical and chemical analyses were carried out following lab common procedures. The results showed that the cultivation practices in the sites studied are intensive. The soils studied are more than 95% sandy, acidic, low in organic matter and have a relatively low CEC. However, there is an oversaturation in exchangeable bases on all sites. Thus, this oversaturation would be linked to the intensification of cultural practices.

Ce travail a pour objet de contribuer à la connaissance de l’orpaillage et de ses impacts dans l’environnement forestier du secteur de Souanké au Nord-Ouest de la République du Congo. La méthodologie est basée sur l’inventaire des types et des caractéristiques des exploitations, et sur une évaluation des impacts environnementaux observés à l’aide de la grille de Fecteau et des analyses au spectrophotomètre. La télédétection a servi à l’évaluation de la dégradation forestière. Occupant 18 sites sur les 20 identifiés, l’exploitation artisanale reste dominante (soit 90%) par rapport à celle semi-industrielle. L’exploitation artisanale est pratiquée par des Congolais et certains étrangers (Camerounais, Tchadiens, Ouest africains, Congolais de la RDC) organisés en équipes légères tandis que l’exploitation semi-industrielle est pratiquée par des sociétés chinoises. Les impacts négatifs relevés sur l’environnement du secteur sont : dégradation du couvert végétal, perturbation du milieu, excavations, stagnation des eaux et asphyxie des plantes, éboulements, modification du paysage, tas de graviers et morts terrains abandonnés, augmentation de la turbidité des eaux, perturbation du régime d’écoulement des cours d’eau, risques d’accident. Les impacts identifiés pour l’orpaillage semi-industriel et l’orpaillage artisanal sont respectivement : majeurs (50% vs 31%), moyens (29% vs 15%), mineurs (21% vs 54%). La turbidité est supérieure à 500 NTU du point de lavage jusqu’à 1000 m en aval dans les sites mécanisés alors qu’elle décroit rapidement en aval des sites traditionnels. La superficie forestière dégradée par les activités d’orpaillage est évaluée à 934 ha sur 13.912 ha, soit un taux de 6.7%.

Ce travail a pour objet de contribuer à la connaissance de l’orpaillage et de ses impacts dans l’environnement forestier du secteur de Souanké au Nord-Ouest de la République du Congo. La méthodologie est basée sur l’inventaire des types et des caractéristiques des exploitations, et sur une évaluation des impacts environnementaux observés à l’aide de la grille de Fecteau et des analyses au spectrophotomètre. La télédétection a servi à l’évaluation de la dégradation forestière. Occupant 18 sites sur les 20 identifiés, l’exploitation artisanale reste dominante (soit 90%) par rapport à celle semi-industrielle. L’exploitation artisanale est pratiquée par des Congolais et certains étrangers (Camerounais, Tchadiens, Ouest africains, Congolais de la RDC) organisés en équipes légères tandis que l’exploitation semi-industrielle est pratiquée par des sociétés chinoises. Les impacts négatifs relevés sur l’environnement du secteur sont : dégradation du couvert végétal, perturbation du milieu, excavations, stagnation des eaux et asphyxie des plantes, éboulements, modification du paysage, tas de graviers et morts terrains abandonnés, augmentation de la turbidité des eaux, perturbation du régime d’écoulement des cours d’eau, risques d’accident. Les impacts identifiés pour l’orpaillage semi-industriel et l’orpaillage artisanal sont respectivement : majeurs (50% vs 31%), moyens (29% vs 15%), mineurs (21% vs 54%). La turbidité est supérieure à 500 NTU du point de lavage jusqu’à 1000 m en aval dans les sites mécanisés alors qu’elle décroit rapidement en aval des sites traditionnels. La superficie forestière dégradée par les activités d’orpaillage est évaluée à 934 ha sur 13.912 ha, soit un taux de 6.7%.

The study was conducted in Brazzaville, and data collection covers the period from May to June 2017. The methodology is based on literature review and floristic and equipment inventory. Nine green spaces spread over two out of nine townships in the capital city. Four are located in Bacongo and five in Poto-Poto. According to the classification standards, 5 squares and 4 gardens were studied. Except for one square, all the others, including the gardens, are planted with trees. The equipment inventory lists 183 benches, including 63.83% permanently and/or partially in the sun, 4 playgrounds, no games for children, and 3 cultural monuments. The flora and health of the trees stands shows 186 trees and 279 shrubs, all corresponding to 26 species. An examination of the health status reveals that 57% of trees show anthropogenic injuries. Floral analysis shows that exotic plants (76.92%) predominate over local plants (23.07%). The average basal area of trees in all green spaces is 1.95 m2·ha−1. The diametric structure is erratic within all green spaces, with a dominance of large diameter subjects. This leads to poor natural regeneration of woody plants. The green spaces in Brazzaville, which are very unevenly distributed within the urban fabric, do not meet the international standards disseminated by the World Health Organization (WHO) and do not fully play their biodiversity conservation and recreational and ecological functions. History of green spaces in Brazzaville states that no creation was born after independence. The existing land has been reduced in size, and the new land has been used for other purposes.

Ce travail a pour objet de contribuer à la connaissance de l’orpaillage et de ses impacts dans les écosystèmes forestiers du secteur de Souanké au Nord-Ouest de la République du Congo. La méthodologie est basée sur l’inventaire des types et des caractéristiques des exploitations, et sur une évaluation des impacts environnementaux observés à l’aide de la grille de Fecteau et des analyses au spectrophotomètre. La télédétection a servi à l’évaluation de la dégradation forestière. Occupant 18 sites sur les 20 identifiés, l’exploitation artisanale reste dominante (soit 90%) par rapport à celle semi-industrielle. L’exploitation artisanale est pratiquée par des Congolais et certains étrangers (Camerounais, Tchadiens, Congolais de la RDC et des ressortissants des pays Ouest africains,) organisés en équipes légères tandis que l’exploitation semi-industrielle est pratiquée par des sociétés chinoises. Les impacts négatifs relevés sur l’environnement du secteur sont : dégradation du couvert végétal, perturbation du milieu faunique, excavations, stagnation des eaux et asphyxie des plantes, éboulements, modification du paysage, amoncellements de graviers et morts-terrains, augmentation de la turbidité des eaux, perturbation du régime d’écoulement des eaux, risques d’accident. Les impacts identifiés pour l’orpaillage semi-industriel et artisanal sont respectivement : majeurs (50 % vs 31%) ; moyens (29% vs 15%) ; mineurs (21% vs 54%). La turbidité est supérieure à 500 UTN du point de lavage jusqu’à 1000 m en aval dans les sites mécanisés alors qu’elle décroit rapidement en aval des sites traditionnels. La superficie forestière dégradée par les activités d’orpaillage est évaluée à 934 ha sur 13.912 ha, soit un taux de 6.7%. The aim of this work is to contribute to the knowledge of gold artisanal mining and its impacts in the forest ecosystems of Souanké sector in the northwest of the Republic of Congo. The methodology is based on an inventory of the types and characteristics of mining operations, and on an assessment of the environmental impacts observed using the Fecteau grid and spectrophotometer analyses. Remote sensing was used to assess forest degradation. Of the 20 sites identified, 18 are dominated by artisanal mining (90%) compared to semi-industrial mining. Artisanal mining is carried out by Congolese and some foreigners (Cameroonians, Chadians, West Africans, and Congolese from the DRC) and organized in small teams, whereas semi-industrial mining is carried out by Chinese companies. The negative impacts noted on the environment of the sector are degradation of the vegetation cover, environmental disturbance, excavations, stagnation of water and asphyxiation of plants, landslides, modification of the landscape, heaps of gravel and dead land abandoned, increase in water turbidity, disturbance of the flow regime of watercourses, risks of accidents. The impacts identified for semi-industrial and artisanal gold mining are respectively: major (50% vs. 31%), medium (29% vs. 15%), and minor (21% vs. 54%). Turbidity is above 500 NTU from the washing point to 1000m downstream in mechanized sites, whereas it decreases rapidly downstream in artisanal sites. The forest area degraded by gold mining activities is estimated at 934 ha out of 13,912 ha, corresponding to a rate of 6.7%.

Located in the hearth of Africa, the Congo basin is the world’s second largest rainforest ecosystem, spanning over nine countries including the Republic of the Congo. Nature-based solutions, i.e., afforestation, reforestation or agroforestry supplying wood energy, halting food insecurity, restoring land desertification and fostering mitigation and adaptation to climate warming, have been increasingly used in the past decades. Within this framework, Congolese coastal plains have been afforested using fast growing trees since the early 1950s. Due to the low forest productivity and soil fertility, sustainable management of these forest ecosystems (trees, soils and environment) have been performed. Improved germplasms, increased stand wood biomass and healthier soils have the potential to enhance wood and fuel wood energy supply, mitigation and adaptation to climate change, food security, restoration of land and ecosystem biodiversity. This meets ten out of the seventeen sustainable development goals (SDG #), specifically goals related to alleviating poverty (1) and hunger (2), improving health (3), education (4), sanitation and access to clean water (6). Other goals include providing affordable clean energy (7), sustainable production and consumption (12), action on climate change (13), life on land (15), and partnerships for goals (17). Nature-based solutions help to face important societal challenges meeting more than half of SDGs of the United Nations.

The study aimed at evaluating the natural regeneration of some characteristic species in the Loukaya peri-urban forest was carried out in Brazzaville, over three surveys of one hectare each. The botanical inventory of the 23 species monitored covers the subjects of 2 ≤ d0.20 < 10 cm and d1.30 ≥ 10 cm. The study shows 1255 individuals of which 68.27% are 2 ≤ d0.20 < 10 cm. While being tropophilic and mesophilic, this ecosystem is pauciflorous and paucispecific. The phytoecological data show that this formation, which is a link in the African dense humid forest, presents values far below those known elsewhere; while being faithful to the forests of the Cataractes Plateau. As for the rate of natural regeneration, this index is greater than or equal to 100 for 69.56% of the taxa. Sarcochores and heliophytes are the taxa that best support this natural regeneration. This high rate of regeneration would be correlated with the high level of anthropization, whose large canopy gaps are accompanied by a flow of light conducive to the installation of seedlings.

The synthesis of geological and petroleum research undertaken in the coastal Sedimentary Basin of the Pointe-Noire region enabled the establishment of a lithostratigraphic scale. It has been observed that the order in which the series observed in outcrop and those encountered by deep wells succeed each other has allowed a value to be given to this relative scale. The study area corresponds to a longitudinal tectonic accident, fault or flexure. It belongs to the Cretaceous and Tertiary coastal sedimentary basin covered by the Plio-Pleistocene age formations (series of circuses), formed of highly permeable sands comprising multiple resistant horizons that store large bodies of water whose reserves are considered very important. Hydrographic network is composed of four main basins with a mediocre size. The quality of this groundwater is considered to be satisfactory for household consumption, but the sustainable management of these reserves requires constant checks on their quality as well as on the level of the reserves. All these resources put the agglomeration of Pointe-Noire close to large exploitable water reserves that meet the need for drinking water, even if, on the whole, the agglomeration is still experiencing many difficulties in terms of its drinking water supply.

The study, conducted in the city of Dolisie, provides an overview of plant diversity, ethnobotanical and ecosystem services, people's expectations and perceptions of urban forestry. It is based on the botanical inventory and interviews involving city managers and populations. A total of 34 species belonging to 26 genera and 17 families have been recorded with a fruit tree dominance. The role and importance of trees in urban areas is well perceived by the population. Gender and education level influence urban forestry preferences and expectations. Men with at least a secondary level of education exploit the aesthetic aspects of the environment, while women and individuals with at most a primary level like food, phytotherapeutic and financial aspects. The 10 urban forestry properties identified constitute three categories whose ethnobotanical use values stand out: supply services with 1.96 and regulation services with 1.97. A gender and educational level analysis reveals differences, sometimes significant, in the ethnobotanical use value of a property. As for the level of fidelity, it is carried by the same taxa, especially fruit trees, both in food and phytotherapy. At a time when humanity is suffering from the harmful effects of climate change, the value of urban forestry is undeniable in mitigating the heat islands generated by our cities.

The spatial analysis of the flora of the peri-urban forest of Djoumouna has been conducted in Brazzaville, Congo. Data collection, conducted from April to August 2017, is based on a floristic inventory of 75 plots of each 400 m2 (20 m x 20 m). The study shows 898 trees of dbh ≥ 10 cm corresponding to 106 species and 36 families. The number of individuals per plot averages 11.97 ± 0.42 feet/plot while the number of species per plot averages 7.37 ± 0.30. The land used by projecting the trunk sections is on average 0.74 ± 0.05 m2/plot. The Jaccard similarity coefficient between plots is between 0 and 75% while the average Skewness is 1.26 ± 0.11. The average Shannon index is 1.77 ± 0.05 per plot with an average maximum diversity of 1.93 ± 0.04. The average Pielou index is 0.91 ± 0.01. The absolute diversity index averages 6.54 ± 0.29 while α-Fisher averages 11.44 ± 1.39. The average value of the Green index is -0.04 ± 0.01 and finally 7.55% of taxa have a rarefaction index of species between 40 and 77% compared to 92.45% where this index varies from 81 to 99%. The upward hierarchical classification and the AFC highlight the presence of floristic subfacies resulting from taxa independence. Since most species have a density of less than 1 tree.ha-1, the floristic composition is heterogeneous, resulting in insufficient natural regeneration and low resilience of this ecosystem threatened with extinction in the very short term.

The dynamics of nutrients were compared over three years in a clonal Eucalyptus plantation and in a native savanna in Congo. This paper focuses on the changes in the chemical composition of solutions during their transfer through the soil in both ecosystems. The main characteristics of the soil (Ferralic Arenosol) were similar in the two ecosystems, and the low inter-stand variability allowed reliable comparisons of the influence of afforestation on the soil solution chemistry. Rainfall amounted to about 1400 mm per year during the experimental period. In both ecosystems, an enrichment was observed for most elements during the transfer of solutions through the foliage, but N uptake occurred. Concentrations of H+ and dissolved organic carbon (DOC) in solutions increased through the litter layer in both stands. In the Eucalyptus ecosystem, a quick uptake of nutrients by a dense root mat inside the forest floor likely explained why the concentrations of gravitational waters were not enhanced markedly for ‘base cations’, despite the mineralization of high amounts of nutrients during the litter decay. Soil solutions were collected by zero tension lysimeters (ZTL) at a depth of 15 cm, but these lysimeters were inefficient at collecting gravitational solutions beyond this depth. By contrast, tension lysimeters (TL) maintained at a suction of −60 kPa, collected soil solutions at the depths of 15, 50 cm, 1, 2, 3, 4 m in both ecosystems and 6 m in the plantation. In the topsoil of both stands, the nutrient concentration decreased sharply when the time of residence of solutions increased. This pattern highlighted the crucial role of the inputs by throughfall, stemflow and mineralization of the litter layer for the nutrition of these stands. A combination of high nutrient requirements of the stands and low availability of exchangeable cations in this highly weathered soil might account for the extremely low nutrient concentrations in solutions collected by TL, regardless of the depth.

Since 1978, 42,000 ha of clonal Eucalyptus plantations have been established around Pointe-Noire. These plantations are growing on sandy and acidic soils with small reserves of available nutrients; therefore, risks of nutrient deficiencies exist in the medium or long-term. The effects of slash and litter management practices on soil properties, tree nutrition and growth were quantified to evaluate harvesting methods with respect to sustainable management of the replanted sites. The experimental design was a randomized complete block with four replications and six treatments: R, all aboveground organic residues removed; WTH, whole-trees harvested and removed; SBH, stemwood and bark removed; SH, stemwood-only removed; DS, double slash retained; B, stemwood-only removed and all organic residues burned. After 12 months removal of all slash material (R) had a marked negative effect on total biomass (5.9 t ha-1 vs a mean of 7.2 t ha-1 for the other treatments). During the first 18 months of the study no significant differences among treatments were detected (P < 0.05), but at 24 months, tree growth was significantly reduced where all organic residues were removed (R), compared to DS (12.9 m3 ha-1 per year vs 22.2 m3 ha-1 per year). The best growth was measured in plots with the highest organic residues (treatments DS and SH). Nutrient content in the aboveground biomass of the I-year-old stand was strongly dependent on the type of slash and litter management. Differences in tree nutrient concentrations in various treatments demonstrated the ability of trees to store available nutrients in aboveground biomass. The rate of decomposition of organic residues during the first 8 months after harvesting ranged from 36% (WTH) to 56% (SH). Between 7 and 24 months, the annual production of inorganic nitrogen in the top soil (0-13 cm layer) was 71.6, 66.7, and 85.7 kg ha-1 per year for R, SH, and DS, respectively. The rate of soil nitrification was high, and therefore nitrogen leaching may have occurred in the early development stage when trees are not able to take up all the available nitrogen.

African savannas developed an efficient strategy of nutrient uptake and control of nitrification making it possible to avoid large losses of nutrient by drainage. The conversion of native savanna into commercial forest plantations is an important global change driver that potentially impacts element cycles. Afforestation of native Congolese savannas by Eucalyptus started 30 years ago. Large amounts of nutrients are removed through biomass harvesting every 6-7 years. Losses of nutrients by deep drainage might be a serious threat for the sustainability of these plantations, established on sandy soils with high hydraulic conductivities. We compared the soil N-mineralisation, the nutrient fluxes and deep drainage beneath savanna and Eucalyptus plantation in Congo. Then, we discussed the strategy of nutrient recycling. The water fluxes at a depth of 400 cm were approximately 20% higher in the savanna than in Eucalyptus plantation. Although the nitrification rate and solution chemistry exhibited strong modifications during the first year following both savanna afforestation and the harvesting of the Eucalyptus stand, the losses of nutrients by deep drainage remained unexpectedly low. The largest fluxes of drainage at a depth of 6 m were found for NH4+-N, which reached a maximum of 0.4 g m(-2) yr(-1) in the second year following savanna afforestation and I g m(-2) yr(-1) in the first year after clear cutting. The deep drainage of NO3--N, Mg2+, Ca2+ and K+ did not exceed 0.2 g m(-2) yr(-1) in the savanna and at any stage of plantation development. These results are discussed regarding (i) the roots distribution in the soil of each ecosystem and (ii) the nutrient accumulation in biomass. The limited changes between the nutrient fluxes in both ecosystems were the result of fast root growth in the deep soil layers after planting, combined with an intense uptake of the tree roots to satisfy the large nutrient and water requirements for the development of tree crowns. Intense uptake and cycling of nutrients mitigated the risk due to clear cut and N fertilisation in this forest plantation. Nevertheless, forest managers must carefully fit fertilisation regimes to the nutrient requirements of new clone selected by breeding programmes and reduce as much as possible the delay between harvesting and re-planting to avoid high losses by deep drainage. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.

International audience; The main characteristics of soils under commercial Eucalyptus plantations on the Atlantic coast of the Congo were studied down to a depth of 5 m. The objectives were to invest gate the mineralogical assemblage and distinguish between current versus old pedogenetic processes, and to find evidence of relations between minerals and cation retention in highly weathered sandy soil. These soils, developed from Plio-Pleistocene sandy deposits, were characterized by mineralogical uniformity over depth, with quartz contents ranging from 850 to 930 g kg(-1), a clay fraction dominated by kaolinite and a cation exchange capacity (CEC) < 0.5 cmol(c) kg(-1) whatever the soil layer. Whilst dioctahedral vermiculite amounted to only 3% of the clay fraction, it played a key role in soil cation retention contributing to 49% of the mineral CEC. Vermiculitisation of muscovite was a current process showed by the changes in muscovite/vermiculite ratios with soil depth. The simple mineralogical assemblage of this soil made it possible to estimate CEC values for kaolinite ranging from 6.2 to 6.9 cmol(c) kg(-1) for the 0-2 mu m particle size. Their morphologies and saturation index of soil solutions showed that kaolinite had different possible origins: i) current precipitation from soil solution, ii) pseudomorphic transformation of pre-existing 2:1 phyllosilicates and iii) old particles present in the original deposit. Measured fine earth CEC below estimations summing together the contributions of all the mineralogical fractions showed that particle aggregation greatly influenced the cation retention capacity, even in these poorly structured soils. Pedogenetic differentiations were weak in these soils because the elapsed time since deposition was too short to differentiate 'mineralogical horizons' from weathered materials. Our study showed that beyond the properties that minerals confer to soil, partially controlling the bioavailability of nutrients, their mineralogical and chemical characteristics infer current and former pedogenetic processes in this highly weathered tropical soil. (C) 2011 Elsevier B.V. All rights reserved.

International audience; Tropical plantation forests are meeting an increasing proportion of global wood demand and comprehensive studies assessing the impact of silvicultural practices on tree and soil functioning are required to achieve sustainable yields. The objectives of our study were: (1) to quantify the effects of contrasting organic residue (OR) retention methods on tree growth and soil nutrient pools over a full Eucalyptus rotation and (2) to assess the potential of soil analyses to predict yields of fast-growing plantations established on tropical sandy soils. An experiment was set up in the Congo at the harvesting of the first rotation after afforestation of a native herbaceous savanna. Six treatments were set up in 0.26 ha plots and replicated in 4 blocks, with OR mass at planting ranging from 0 to 46.5 Mg ha(-1). Tree growth over the whole rotation was highly dependent on OR management at planting. Over-bark trunk volume 7 years after planting ranged from 96 m(3) ha-1 in the treatment with forest floor and harvest residue removal at planting to 164 m(3) ha(-1) in the treatment with the largest amount of OR. A comparison of nutrient stocks within the ecosystem at planting and at the end of the rotation suggested that nutrient contents in OR were largely involved in the different response observed between treatments. OR management treatments did not significantly modify most of the nutrient concentrations in the upper layers of the mineral soil. Conventional soil analyses performed before planting and at ages 1 and 3 years were unable to detect differences between treatments despite large differences in tree growth. In contrast, linear regressions between stand aboveground biomass at harvesting and OR mass at planting (independent variable) showed that OR mass was an excellent predictor of stand yield (R(2) = 0.99). A large share of soil fertility comes from organic material above the mineral soil in highly weathered sandy soils and OR mass at planting might be used in conjunction with soil analyses to assess the potential of these soils to support forest plantations.

The dynamics of nutrient accumulation were studied between two annual fires in a herbaceous savanna of the Congolese littoral region. Trees and shrubs were not studied because of their very low density. After fire the aboveground biomass increased for 10 months up to a maximum of 520 g m–2. The underground biomass amounted to roughly 630 g m–2 during the dry season and increased after the fire up to a maximum of 870 g m–2 during the rainy season. In the aerial parts, the accumulation dynamics differed according to the type of nutrient: Ca accumulation was steady until the following dry season in proportion to the total biomass, while the pattern of K accumulation was similar to the living biomass dynamic and reached a maximum four months after the fire. N, P and Mg followed a middle course. For the underground biomass, N accumulation reached a maximum value at the end of the rainy season (10 g m–2) and decreased at the beginning of the dry season. Most of this element was incorporated into the root system. During the rainy season, accumulation in the root system was of the same order of magnitude as in the aerial parts for P and Ca, whereas it was much lower for K and Mg. Transfers of nutrients to the atmosphere during the annual burning amounted, respectively, to 85, 25, 39, 21 and 28% of the amounts of N, P, K, Ca and Mg accumulated in the aerial biomass and litter components. Losses during fire were small for P, K, Ca and Mg compared to the available soil reserves, but not for N. The legume Eriosema erici-rosenii ought to play an important role in N input in this ecosystem.

La matière organique des sols et leur gestion sont des facteurs importants de production. Selon le mode d'exploitation forestière plus ou moins de rémanents (feuilles, branches, écorces, bois) retournent sur le sol. Un essai sur l’impact de la gestion des résidus d'exploitation (rémanents et litière préexistante) sur la croissance d'une replantation d’Eucalyptus a été mis en place en 1998 dans la région de Pointe-Noire (Congo) où les sols sableux sont très pauvres en matière organique et en éléments minéraux. Cet essai compare 6 modes de gestion des résidus d'exploitation sur l’évolution de la croissance des Eucalyptus et sur la fertilité des sols. Ces 6 traitements permettent d'avoir une gradation des quantités de résidus de 0 à 49.4 tMS.ha-1 dont un traitement de référence (la pratique usuelle du gestionnaire des plantations) et un traitement où les résidus ont été brûlés avant plantation. Le suivi de cet essai pendant 7 ans révèle que la croissance des Eucalyptus est immédiatement et fortement influencée par la gestion des résidus d'exploitation. La production en volume à 7 ans est de 84 m3.ha-1 pour le traitement où tous les résidus ont été enlevés, de 146 m3.ha-1 pour le témoin, de 161 m 3.ha-1 pour le traitement où la quantité de résidus a été doublée et de 127 m3.ha-1 pour le traitement où les résidus ont été brûlés. Les résultats d'analyses foliaires, de chutes de litière et de minéralisation montrent que, dans ces sols très pauvres, les Eucalyptus se nourrissent en grande partie à partir du recyclage rapide des matières organiques et grâce à un mat racinaire superficiel. L’évolution des sols indique des modifications notables de la saturation, malgré la fertilité très faible du sol. Ce site appartient à un réseau de 17 sites expérimentaux répartis de la Chine au Brésil en zone intertropicale, où les mêmes traitements ont été appliqués. Ce réseau est coordonné par le Centre International des Recherches Forestières (CIFOR). L’utilisation de la base de données du réseau pour modéliser de l’effet du traitement des rémanents montre que la baisse initiale de production peut ou non disparaître avec le temps, en fonction de la fertilité initiale des sols et à leur capacité de restauration. Ces 24 résultats ont une portée très générale pour discuter de l’effet initial et à long terme du traitement des rémanents en forêt, quel que soit le type de climat.