50 research outputs found
VariabilitĂ© de la croissance en circonfĂ©rence des arbres dans les forĂȘts semi-dĂ©cidues de Lamto (CĂŽte d'Ivoire)
Girth increments of trees were studied using band-dendrometers in some semi-deciduous forests in Lamto (Ivory Coast). Every tree over 20 cm gbh was monitored monthly, during 5 to 9 years, in seven stands. Girth increments' distributions were log-normal in the forest stands studied, a few individuals contributing to the largest part of the forest production. Girth increments vary widely; even between trees of similar size; several types of girth increment were described (regular, intermittent, increasing or decreasing etc.). A yearly periodicity of growth, in conformity with the local climatic cycle, is the pattern most commonly observed, mainly in fast growing trees, but yearly bimodal patterns and dry season increments were also observed. The variability of growth patterns is mainly due to intra-specific variability. At the community level the mortality rate of trees ranges, between stands, from 1 to 4%
Couvert angulaire et architecture forestiĂšre: Etudes en CĂŽte d'ivoire
A method for studying the forest cover is described. The possible uses of the method are developed on two examples, one consisting of the analysis of some plots, the other with a set of stands characterized by their architecture
Variations de la phytomasse herbacée le long d'une toposéquence en zone soudano-guinéenne, Ouango-Fitini (CÎte-d'Ivoire)
Des mesures de la phytomasse maximale aĂ©rienne (qui englobe la matiĂšre vivante et la matiĂšre morte) ont Ă©tĂ© faites par la mĂ©thode de la rĂ©colte le long d'une toposĂ©quence de la rĂ©gion de Ouango-Fitini (4°01'W, 9°35âN) dans le nord-est de la CĂŽte-d'Ivoire en zone phytogĂ©ographique soudano-guinĂ©enne. La phytomasse herbacĂ©e maximale varie entre 4 et 12 t/ha. Une influence de la position topographique, des caractĂ©ristiques Ă©daphiques et de l'importance du couvert ligneux est manifeste
Substratum géologique et partage des terres dans le sud du Tchad Région des lacs de Léré et de Tréné et réserve de faune de Binder Nayri
National audienceĂ travers une analyse du substratum gĂ©ologique et de l'occupation du sol dans unerĂ©gion sahĂ©lo-soudanienne au sud du Tchad : le massif du Mayo-KĂ©bbi (carte 1), nousproposons de montrer :â quels sont les liens entre la nature du substratum gĂ©ologique (socle et formationssuperficielles) et les paysages ;â comment se rĂ©partissent les ressources naturelles entre les paysages et comment serĂ©alise le partage entre les zones fortement anthropisĂ©es et les zones naturelles dĂ©diĂ©esĂ une rĂ©serve de faune.BasĂ©e sur une analyse de terrain spatialisĂ©e (cartographie gĂ©ologique et pĂ©dologique,enquĂȘte villageoise...), complĂ©tĂ©e par des Ă©lĂ©ments de bibliographie, la mĂ©thode faitĂ©galement appel aux outils de tĂ©lĂ©dĂ©tection pour la mise Ă jour de la carte d'occupationdu sol. La compilation globale aboutit Ă une meilleure comprĂ©hension de lâorganisationdes paysages
Phenology of Woody Species Along the Climatic Gradient in West Tropical Africa
International audienc
The condition of pastoral resources of a land of reception and transit for transhumant herders: the Kotchari territory (Southeastern Burkina Faso)
Kotchari pasturelands, which include a portion
of the W National Park, were characterized by three complementary methods: an analysis of a colour
composition of an Aster image, a floristical analysis and a pastoral survey. Nine pastoral landscape units
with diverse tree cover were described, they include ten agrostological groups. The landscape units located
outside the protected area, especially on thin to fairly deep soils, show the highest density in ligneous species
and the highest herbaceous richness as well as the least dominance between species. Conversely, herbaceous
phytomass and ligneous cover are higher in the protected area, particularly on the deep hydromorphic soils
units. In addition, these units are more or less heterogeneous, some containing several agrostological groups.
In general, the pasture lands located outside the protected area tend to be the most degraded: in some of
them there is a regeneration of trees and a decrease of perennial herbaceous species. In all, although the
phytomass is rather abundant and the pastoral values are acceptable in most units, those located outside the
W park appear not very interesting in terms of pastoral use. Their ability to produce herbaceous regrowth
after vegetation fires is low, the landscape is highly fragmented and the access to some units is not possible
all year long. The pastoral status of the pastoral units inside the protected area, which have the highest phytomasses
and the best pastoral values, is clearly much better. The strong interest of transhumant herders in
this region seems guided on the one hand by the quality and availability of forage that is better at the end of
the dry season than in their home grazing territory, and on the other hand by the opportunity of illegal use of
pastoral resources in protected areasLes pùturages du terroir de Kotchari, qui incluent une portion du Parc national du W, ont été caractérisés par trois méthodes complémentaires: une analyse en composition colorée d'une image satellitaire Aster, une analyse
floristique et une enquĂȘte pastorale. Neuf unitĂ©s paysagĂšres pastorales plus ou moins densĂ©ment boisĂ©es qui comprennent dix groupements agrostologiques ont Ă©tĂ© dĂ©crites. Celles situĂ©es Ă l'extĂ©rieur de l'aire protĂ©gĂ©e prĂ©sentent la densitĂ© en ligneux la plus Ă©levĂ©e et la strate herbacĂ©e la
plus riche en espÚces avec la dominance entre espÚces la moins marquée, surtout sur les sols minces à moyennement profonds. La phytomasse herbacée sur pied et le recouvrement ligneux apparaissent en revanche plus élevés dans l'aire protégée, notamment dans les unités sur sols profonds
hydromorphes. Par ailleurs, ces unités sont plus ou moins hétérogÚnes, certaines renfermant plusieurs groupements agrostologiques. De maniÚre générale, il y a une tendance à la dégradation des pùturages dans les unités situées en dehors de l'aire protégée: certaines d'entre elles présentent
un début d'embuissonnement et les herbacées pérennes y sont en diminution. Au total, malgré une phytomasse globalement satisfaisante et des valeurs pastorales acceptables, la plupart des unités situées hors du parc W apparaissent peu intéressantes au plan pastoral. Leur potentiel en
repousses herbacées aprÚs les feux est faible, les milieux sont fortement fragmentés et il est impossible d'accéder à certaines d'entre elles toute l'année. Le statut pastoral des unités pastorales à l'intérieur de l'aire protégée, qui ont les plus fortes phytomasses et les meilleures
valeurs pastorales, est clairement bien meilleur. Le fort attrait des transhumants pour ce terroir semble s'expliquer par une production fourragĂšre meilleure et plus disponible en fin de saison sĂšche que dans les territoires d'attache d'une part, par l'usage possible des ressources
interdites des aires protégées d'autre part
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Climate seasonality limits leaf carbon assimilation and wood productivity in tropical forests
The seasonal climate drivers of the carbon cycle in tropical forests remain poorly known, although these forests account for more carbon assimilation and storage than any other terrestrial ecosystem. Based on a unique combination of seasonal pan-tropical data sets from 89 experimental sites (68 include aboveground wood productivity measurements and 35 litter productivity measurements), their associated canopy photosynthetic capacity (enhanced vegetation index, EVI) and climate, we ask how carbon assimilation and aboveground allocation are related to climate seasonality in tropical forests and how they interact in the seasonal carbon cycle. We found that canopy photosynthetic capacity seasonality responds positively to precipitation when rainfall is â<â2000âŻmmâŻyrâ»Âč (water-limited forests) and to radiation otherwise (light-limited forests). On the other hand, independent of climate limitations, wood productivity and litterfall are driven by seasonal variation in precipitation and evapotranspiration, respectively. Consequently, light-limited forests present an asynchronism between canopy photosynthetic capacity and wood productivity. First-order control by precipitation likely indicates a decrease in tropical forest productivity in a drier climate in water-limited forest, and in current light-limited forest with future rainfall â<â2000âŻmmâŻyrâ»Âč
Utilisation de l'indice de rougeur de Madeira pour la reconnaissance des sols de la région de Bondoukuy (ouest burkinabé) à partir d'images satellitaires SPOT
Ce travail prĂ©sente quelques rĂ©sultats prĂ©liminaires obtenus pour Ă©valuer l'intĂ©rĂȘt de l'indice de rougeur de Madeira (XS2(2)/XS1(3)) pour la reconnaissance des sols ferrallitiques et ferrugineux tropicaux dans un site soudanien du Burkina Faso. Une premiĂšre analyse d'une image SPOT a montrĂ© que l'indice de rougeur donne une meilleure reprĂ©sentation de la distribution toposĂ©quentielle des sols rouges que l'indice de couleur (XS2-XS1)(XS2+XS1). L'analyse des propriĂ©tĂ©s radiomĂ©triques et des principales caractĂ©ristiques de la surface des sols confirme que l'indice de rougeur permet une diffĂ©rentiation plus prĂ©cise des sols les plus rouges que l'indice de couleur. (RĂ©sumĂ© d'auteur
To what extent does land-use affect relationships between the distribution of woody species and climatic change ? A case study along an aridity gradient in western Burkina Faso
The study aims at evaluating how land-use modifies the link between the observed distribution of species and the climatic variability and at detecting species involved in that modification. The area studied covers the phytogeographical transition between the South-Sudanian sector and the North-Sudanian sector in western Burkina Faso. It lies along an aridity gradient, of which the aridity index (UNESCO-MAB) ranges from 0.363 to 0.533. The data studied were derived from observations performed on 192 woody species in 603 vegetation survey plots inside and outside protected areas. The species turn-over (beta diversity) and the mutual information were assessed along the aridity gradient inside protected areas on one hand and outside protected areas on the other. Our study shows that the statistical links between the geographical distributions of species and the aridity gradient differ according to whether the observations are performed inside or outside the protected areas. Anthropogenic disturbances, mainly agricultural in the region of study, result in a decrease in the beta diversity and in the average mutual information between the distribution of all the woody species and the climatic gradient. Moreover, the variation in mutual information differs according to the species: it diversely decreases with agricultural land-use for most of them, but increases for some. Thus, land-use leads to species-specific changes in the realised climatic niches