Bois et forêts des tropiques

Abstracts & articles: n° 327
(1st quarter 2016)


All abstracts
(In French, English and Spanish)

New issue

Issue 327


 

WINDTHROW PATTERN AND REGENERATION OF TREES IN THE IPASSA BIOSPHERE RESERVE IN GABON

J.- P. Obame Engone, L. Bélanger, S. Assame

This study was conducted to characterise the pattern of disturbance caused by windthrow in the forest of Ipassa (Gabon) and to analyze the links between types of windthrow and the regeneration of different tree species. The pattern of windthrow disturbance was studied in a permanent 38 hectares plot. Windthrow was monitored annually for five years from 2005 to 2009. We also assessed the natural regeneration of trees according to different types of windthrow. The study results show that 72% of windthrown trees in the forest of Ipassa are partially fallen or single trees. Each year, these occupied 1.3% of the surface area studied. The scale of windthrow varied on average from 52 ± 13 m2 to 625 ± 220 m2, with a half-life of about 53 years. Multiple Correspondence Analysis showed that in 47 species (25.7%), regeneration is associated with the different types of windthrow. Twelve species of sun-loving trees regenerate exclusively in multiple or complex windthrow situations, while 35 shade-tolerant species tend to regenerate in partial and single-tree windthrow situations. In several tree species, there is no obvious link between regeneration and windthrow patterns. This suggests that other factors besides windthrow, such as limited dispersal, influence the regeneration of tree species.

Keywords: sciaphilous species, natural regeneration, pattern of windthrow, tropical forest, Gabon


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DYNAMICS OF FOREST COVER AND CARBON STOCK CHANGE IN THE DEMOCRATIC REPUBLIC OF CONGO: CASE OF WOOD-FUEL SUPPLY BASIN FOR KINSHASA

V. Gond, E. Dubiez, M. Boulogne, M. Gigaud, A. Peroches, A. Pennec, N. Fauvet, R. Peltier

To contribute to the development of methods for the sustainable management of forest ecosystems in Central Africa, the following research question was addressed: can analyses of land cover change help to understand and document the spatial organization and mechanisms of forest degradation? To answer this question in the Democratic Republic of Congo, the Makala project mapped the tree and forest resources of Kinshasa’s wood-fuel supply basin and attempted to predict how they would evolve in the future. Maps were made for four periods (1984, 2001, 2006 and 2012) using a mosaic of four LANDSAT images. The above-ground biomass was estimated in 2012 using forest inventories in 317 plots distributed across the four types of plant cover found on the Bateke plateau (4,337 trees and 44 species were recorded). This inventory data combined with the satellite data allowed us to make the first comprehensive assessment of above-ground biomass in the study area. Between 2000 and 2012, the average volume of wood-fuel fell by more than 50%. Over the entire period studied (1984 to 2012), carbon stocks fell by 75%. In the wood-fuel supply area for Kinshasa, the drastic loss of forest cover, shortened fallow periods, savannah encroachment and the decline of biomass and carbon stocks are clear signals of degradation. However, these initial estimations were derived from a small sample that was extrapolated to the entire supply area. It would be very useful to increase sampling in order to obtain more accurate and realistic figures. The experience of the Makala project clearly shows that the analysis of land cover change helps to understand and document the spatial organization and mechanisms of forest degradation. However, only with a sound wood-fuel resource policy and sustainable community land management, combined with very dynamic tree reintroduction on agricultural land, will it be possible to initiate a sustainable process of restoration.

Keywords: wood-fuel, carbon stocks, forest cover, forest degradation, forest inventory, forest restoration, remote sensing, Kinshasa, Democratic Republic of Congo.


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VEGETATIVE PROPAGATION OF VITEX DONIANA SWEET FROM ROOT SEGMENT CUTTINGS

P. M. Mapongmetsem, G. Fawa, J. B. Noubissie-Tchiagam, B. A. Nkongmeneck, S. S. H. Biaou, R. Bellefontaine

Vitex doniana Sweet is a common multipurpose species in tropical Africa, which has considerable socio-economic importance. Unfortunately, it is extracted from the wild and there has been little or no focused effort to domesticate and cultivate the species. Mastering its propagation through root segment cuttings (RSC) is a real alternative to the difficult process of regeneration from seeds. The aim of our study was to assess two of the key factors (rooting medium and root diameter) that influence its rooting ability. The root system of 23 trees was partially excavated to a depth of 20 cm. Root segment cuttings 15 cm in length were carefully cut and arranged horizontally in a polypropagator, on five different substrates. The experimental setup involved a split-plot design with three replications. The main treatment was applied to all five substrates: black soil (Bs), fine sand (S), sawdust (Sd), 50% Bs/50% Sc (Bs-Sc) and 50% Bs:50% S (Bs-S). The secondary treatment covered two diameter size classes (0.5-1cm; 1.1-2.5 cm). The latency time before emergence was 8 weeks for the aerial shoots and 12 weeks for the roots. After 28 weeks, the percentage of leafy shoots formed varied from 28.3% in the black soil substrate (Bs) to 55.0% in the sand (S). Aerial shoots developed mainly (82%) on the distal pole. The RSC diameter had a significant influence (P < 0.01) on the development of leafy shoots. The budding rate of the RSC was 21 ± 1.8% for 0.5-1 cm RSC and 86 ± 7.8% for 1.1-2.5 cm RSC. The diameter also had a significant impact (P < 0.001) on RSC rooting. The rooting rate was 12 ± 2.3% for 0.5-1 cm RSC and 59.3 ± 4.7% for 1.1-2.5 cm RSC. These results show that vegetative propagation by RSC could improve the economic value of V. doniana in Cameroon’s savannah uplands.

Keywords: Vitex doniana, vegetative propagation, domestication, rooting ability, root segment cuttings, Cameroon.


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LOCAL PERCEPTIONS AND KNOWLEDGE OF LOCAL OIL-SEED PLANTS IN BURKINA FASO’S KÉNÉDOUGOU PROVINCE

F. R. S. Tiétiambou, A. M. Lykke, G. Korbéogo, A. Thiombiano, A. Ouédraogo

Local oil-producing plants have many different uses and are a source of income for rural communities. The aim of our study is to understand local perceptions and knowledge of these species and the oil they produce in Burkina Faso’s Kenedougou Province. The methodology used combines a participatory social approach with ethnobotanical surveys. We interviewed a total of 336 people from 4 community groups (Toussian, Siamou, Bolon and Fulani) across 14 villages. The results show that 11 oil-seed plants are well known, four of which account for 94% of all citations. Significant variations in knowledge on the rarely cited species are age-related (Χ2 = 8.11 ; p = 0.0173). Adults and elderly people are more knowledgeable.Women have the most knowledge of oil extraction and processing techniques (Χ2 = 19.79; p < 0.001). These techniques are known for seven species, but are only applied to extract oil from four of them, Vitellaria paradoxa, Elaeis guineensis, Carapa procera and Lophira lanceolata. The oils from all the 11 cited species are mainly used to prepare medicines (47%), food (37%) and cosmetics (16%). Preferences for different oils among the communities are influenced by their uses but also by knowledge of extraction techniques, their ease of use and the oil yield that can be extracted from the seeds. The figure for the informant consensus factor (68%) shows that the species that communities are willing to preserve are those that are most useful to them. Promoting the oils produced from these plants could be a key to their preservation.

Keywords: local knowledge, local plants, oil-seed plants, community preferences, use value, Burkina Faso.


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NATURE AND PERIODICITY OF GROWTH RINGS IN AFRICAN TIMBER: CAN THEY BE USED TO DETERMINE THE AGE OF TREES?

A. Mariaux I. Bossanyi (Trans.)

To draw up logging plans for African forests, it is essential to know at what age the trees can be felled, whether for timber from dense humid forests or for fuelwood from savannah areas. However, attempts are rarely made to monitor tree growth over a number of years. Could growth rings be used to provide at least an approximation of the age of trees marked for felling? This methodological article published nearly 40 years ago, and now translated into English, first gives an overview of the nature and anatomy of growth rings in African timber, explaining how, even when they are hard to discern with a magnifying glass, they can be counted individually by applying a mechanical treatment to the surface, followed by X-rays. The second part of the article discusses ways of determining the periodicity of growth ring formation, with a close look at the main difficulty that arises for tropical species, which is that growth rates in one and the same species do not necessarily match the seasonal cycle. Various methods have been used since the 1920s to analyse the dynamics of wood formation in tropical tree species. Taking periodical samples of the cambium around the circumference of the trunk produces good results but the method is destructive. Incisions made into the bark leave scars from which different points in the wood can be dated. Another method involves making an incision each year and placing dendrometric tape around the tree. Regular monthly readings then show the exact periods when wood is being formed. Two years of observation are enough to produce good results.

Keywords: growth rings, growth, felling diameter, X-ray, tropical timber, Africa.


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