International audienceSustainable wood production requires appropriate management of commercial forest plantations. Establishment of industrial eucalypt plantations on poor sandy soils leads to a high loss of nutrients including nitrogen (N) after wood harvesting. An ecological intensification of eucalypt plantations was tested with the replacement of half of the Eucalyptus urophylla x E. grandis by Acacia mangium in the eucalypt monoculture to sustain soil fertility through enhancement of the N biological cycle. A randomised block design was set up on ferralitic arenosol in the Congolese coastal plains to assess differences in soil N mineralisation, N fluxes in litterfall, and N stocks in forest floor litter and soil between pure acacia (100A), pure eucalypt (100E) and mixed-species treatments (50A50E). Soil N mineralisation was enhanced under acacia, reaching on average 0.17 and 0.15 mg kg(-1) soil d(-1) in 100A and 50A50E, respectively, compared with 0.09 mg kg(-1) soil d(-1) in 100E. Higher amounts of N returning to the soil through harvest residues and litterfall were observed under acacia than under eucalypt. However, N stock in mineral soil was not increased in 100A and exhibited a limited increase only in the top soil layer of 50A50E. Our results suggest a much faster N turnover under acacia than under eucalypt. Although A. mangium is an exotic N-2-fixing tree in central Africa, it appears to be well adapted to the climatic and edaphic conditions of the Congo, showing an efficient growth strategy. Eucalypt trees could benefit from the increase in soil N availability in mixed-species stands
-• Sustainability of Eucalyptus plantations is often questioned in resource-limited environments, especially in areas characterized by soils with poor nutrient and water holding capacities. Yet, field-based observations of fine root dynamics in relation with the seasonality of rainfall are lacking.• This study was undertaken on two Eucalyptus stands planted in the Kouilou Region (south-western Congo), which is characterized by a four-monthlong dry season. Fine root (less than 2 mm in diameter) dynamics were studied using rhizotron observations of root elongation in the field.• Fine root elongation rates displayed a seasonal variation in the two stands, with higher elongation rates during the rainy season than during the dry season. Positive and significant correlations were found between fine root elongation rates and soil water content at all depths, but a better correlation was found with soil water content in the deep soil horizon than in the superficial horizons.• These results suggest that the temporal variations in fine root elongation were related to the seasonality of rainfall, and they were probably associated with seasonal changes in tree water status, carbon assimilation and belowground allocation. fine roots / growth / rhizotron / Eucalyptus / soil water content Résumé -Élongation des racines dans les plantations tropicales d'Eucalyptus : effet du contenu en eau du sol.• La question de la durabilité des plantations d'Eucalyptus est souvent posée dans les environnements où les ressources sont limitées, en particulier les zones où les sols ont une faible capacité à retenir l'eau et les nutriments et où la saison sèche est longue. Pourtant, les observations in situ de la dynamique racinaire en relation avec la saisonnalité des pluies sont inexistantes.• Cette étude a été réalisée dans deux plantations d'Eucalyptus de la région du Kouilou dans le sud ouest du Congo, qui est caractérisée par quatre mois de saison sèche. La dynamique des racines fines (moins de 2 mm de diamètre) a été étudiée à l'aide de rhizotrons permettant d'observer l'élongation racinaire au champ.• La vitesse d'élongation des racines fines montrait une variation saisonnière dans les deux plantations, avec des vitesses plus élevées en saison des pluies qu'en saison sèche. Des corrélations positives et significatives ont été trouvées entre la vitesse d'élongation des racines fines et la teneur en eau du sol à toutes les profondeurs, mais les meilleures corrélations ont été observées avec la teneur en eau des horizons profonds.• Cela suggère que les variations temporelles de l'élongation des racines fines sont reliées à la saisonnalité des précipitations, et qu'elles sont associées aux changements saisonniers d'état hydrique des arbres, d'assimilation carbonée et d'allocation vers les parties souterraines. racines fines / croissance / rhizotron / Eucalyptus / teneur en eau du sol
The forest in the south-west of the Central African Republic has enormous potential for non-timber forest products (NTFPs) of plant origin. The objective of this study is to identify the medicinal value of these NTFPs of plant origin with high health potential and the effects of human activities on their sustainability. The study was carried out in 09 villages in the intervention zone of the Kadéi Forestry Company. From various methodological approaches (ethno botanical survey, participant observation, inventory technique), data were collected, processed and analysed. From the results obtained 91 non-wood medicinal plant species belonging to 29 families and to one large systematic group, the Magnoliophytes were identified. These species represent a means of therapeutic remedies used by 91% of surveyed populations to treat many diseases, symptoms and infections. Tree barks (43.26%) and leaves (39.21%) are the most commonly harvested organs. The variables collected related to the types of uses allowed to identify 19 priority NTFPs medicinal and underline the process of degradation of 7 important NTFPs. This degradation is mainly due to the way in which the natural resource is harvested, causing a large number of non- woody medicinal plant species to become rare. The definition of strategies based on ecological and socioeconomic harvesting techniques of NTFPs are solutions to ensure sustainable management of these products for the benefit of future generations.
1984, The periodic induction of dormancy during the rhythmic growth of Gnetum africanum -Physiol, Plant, 61: 309-313, ^ ", ^^ ,The rhythmic growth of Gnetum africatmm Welw, has an endogenous origin. Correlations between the apical bud and young leaves located at the top of the epicotyl can explain this kind of growth. Young Icaves inhibit the apical bud and a prolonged inhibition leads to bud dormancy. The period of inhibition coincides with the growth of these leaves. When growth stops, dormancy is induced, Motcover a removal of young leaves suppresses inhibition which prevents dormancy induction; here again a continuous growth results, A prolonged exogenous treattiient of the apical bud with abscisic acid aitd auxin shows that these two growth substances can play a part in the induction of dormancy after removal of the young Icaves,
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