Large tropical trees and a few dominant species were recently identified as the main structuring elements of tropical forests. However, such result did not translate yet into quantitative approaches ...which are essential to understand, predict and monitor forest functions and composition over large, often poorly accessible territories. Here we show that the above-ground biomass (AGB) of the whole forest can be predicted from a few large trees and that the relationship is proved strikingly stable in 175 1-ha plots investigated across 8 sites spanning Central Africa. We designed a generic model predicting AGB with an error of 14% when based on only 5% of the stems, which points to universality in forest structural properties. For the first time in Africa, we identified some dominant species that disproportionally contribute to forest AGB with 1.5% of recorded species accounting for over 50% of the stock of AGB. Consequently, focusing on large trees and dominant species provides precise information on the whole forest stand. This offers new perspectives for understanding the functioning of tropical forests and opens new doors for the development of innovative monitoring strategies.
► Plant diversity patterns of urban domestic gardens correspond to a socio-economic gradient. ► Most plants in the domestic gardens in Bujumbura are alien species. ► Gardens are a source of ...potentially invasive species.
Domestic gardens can have both positive and negative effects on urban biodiversity conservation. We analyzed the flora of 1045 gardens in Bujumbura, Burundi. Among the 567 species identified in the gardens, 85% were not native. They originate mainly from tropical America (43%), tropical Asia (30%) or Africa (17%). Three plant assemblages were defined from a cluster analysis, corresponding to ornamental gardens, utilitarian gardens (plants used for fencing and food production), and mixed gardens (intermediate between the two former types). A Non-metric Multidimensional Scaling analysis showed that the main floristic gradients in the garden vegetation were correlated to the socio-economical status of the neighborhood. High standing neighborhoods have higher proportion of ornamental gardens while most gardens in the popular and semi-rural neighborhoods are utilitarian. The species richness per garden was highest in high standing neighborhoods, mainly due to an area effect. When controlling for garden area, species richness is highest in the middle class neighborhoods. City gardens play an important role in the functioning of the urban ecosystem and provide shelter to native plant species and to the urban fauna. However, gardens also are sources of introduction of alien species. In Bujumbura, 9% of the alien garden species have already naturalized in the city, among which 14 invasive species. This number is expected to increase in the coming decennia as there are about 121 other garden species that present a high invasion risk: they are invasive in other parts of the world in climates comparable to that of the Bujumbura region.
Nous avons évalué l'impact anthropique sur la richesse, la diversité, l'abondance et la densité relative des Rongeurs dans quatre habitats de la Réserve de Masako au nord-est de la République ...Démocratique du Congo. Une forêt primaire à Gilbertiodendron dewevrei et trois habitats d'origine anthropique (forêt secondaire, jachère et zone de contact jachère/forêt secondaire (lisière)) ont été investigués de mai 2009 à avril 2010. 1275 spécimens répartis entre 24 espèces et 5 familles inventoriées ont permis de quantifier des différences de composition et d'abondances entre un habitat non perturbé et des milieux anthropisés, ainsi qu'entre une lisière et des habitats adjacents. Nos résultats montrent une distribution non uniforme des Rongeurs dans les quatre milieux. La richesse et la diversité sont élevées en lisière et faibles en forêt primaire. Chaque milieu héberge une densité relative différente. Les plus fortes densités sont observées en forêt secondaire et jachère; la plus faible est observée en forêt primaire. Les abondances diffèrent significativement entre les habitats, à l'exception de la paire forêt secondaire - jachère. Les paramètres dans la lisière diffèrent de ceux des habitats adjacents, traduisant l'existence d'un effet de lisière. Les équitabilités faibles obtenues dans tous les habitats traduisent l'instabilité relative de cet écosystème forestier. Le test de χ2 atteste l'existence de l'effet « saison » sur les abondances des Rongeurs. De même, ce test confirme l'existence de l'effet « activités anthropiques » sur les abondances dans les milieux anthropisés, mais pas en forêt primaire.
La zone tropicale connaît actuellement une réduction catastrophique de la superficie des écosystèmes forestiers qui jouent pourtant un rôle essentiel dans la régulation climatique et qui constituent ...un réservoir inestimable de la biodiversité. Les causes de cette déforestation sont multiples et complexes. Même si l’agriculture constitue l’une des causes majeures de la déforestation en zones tropicales, l’Afrique sub-saharienne a connu des influences exceptionnelles liées aux conflits sociopolitiques. Au Burundi, l’instabilité sociopolitique survenue en 1993 a entrainé un déplacement massif de la population constituée essentiellement d’agriculteurs. L’objectif de cette étude est de caractériser la dynamique spatio-temporelle de l’occupation du sol en mettant un accent particulier sur l’influence de cette migration forcée de la population. Ainsi, l’analyse de la dynamique de l’occupation du sol autour des camps a été réalisée en combinant une analyse d’images satellitaires et des observations sur le terrain. L’impact négatif de cette instabilité socio-politique sur la végétation naturelle a été mis en évidence par la diminution de l’anthropisation au fur et à mesure que la distance camps-végétation augmentait. Il a été confirmé par le fait que l’anthropisation de la zone située autour des camps est plus importante après leur implantation. Ainsi, la réhabilitation de ces zones déboisées ou dégradées s’avère indispensable.
The Contribution of Internally Displaced Persons Camps to the Landscape Dynamics in Southern and Southeastern of Burundi.Nowadays, tropical zones are characterized by a catastrophic decline of forest ecosystems areas which play however an important role in climate regulation and biodiversity conservation. There are numerous and complex causes of deforestation. Even if agriculture is one of the main causes of deforestation in most tropical regions, sub-Saharan Africa had exceptional influences related to social conflicts. In Burundi, socio-political instability which occurred in 1993 also led to massive waves of displaced people, essentially farmers. The present study aims to analyze the land cover spatiotemporal dynamics and focuses particularly on the influence of this population forced to migrate. The study combines satellite images analysis with field observations to analyze land cover dynamic in the camp’s surrounding zone. The negative impact of the socio-political instability has been demonstrated by vegetation anthropization decrease when the distance from the camps increases. The greatest anthropogenic pressure which is observed during the period that followed the outbreak of the socio-political instability also confirms this negative impact. The rehabilitation of those deforested or degraded zones is therefore essential.
Key message
Wood density profiles revealed significant differences in wood formation along a precipitation gradient in the Congo Basin. The response of trees to climate change varies depending on ...leaf phenology properties.
Tropical forests face increasing pressures due to climate change and yet, the response of trees to varying climate conditions remains poorly understood. In the present study, we aim to fill some gaps by comparing the leaf phenology and the pith-to-bark wood anatomical variability of 13 common tree species of the Democratic Republic of Congo among three sites presenting contrasted rainfall regimes. We measured pith-to-bark density profiles on which we applied wavelet analyses to extract three descriptors, which we further used as proxies to describe and compare wood anatomical variability. They describe the growth periodicity, regularity and the amplitude of variations of the anatomical patterns. Our results show that evergreen species tend to have significantly higher anatomical variability where rainfall seasonality is more pronounced. Deciduous species, in spite of shedding leaves for longer periods in drier sites, did not show significant differences in their anatomical variability. The analyses of density profiles and phenology records suggest that the seasonality of precipitation influences both leaf phenology and cambial activity. The high intra-site variability in phenology and anatomy suggests that site-related micro-climate conditions also influence cambial activity.
La forêt étudiée est une aire protégée située au sud du Burundi à proximité de la ville de Bururi et de villages agricoles. La protection effective de cette réserve forestière a débuté en 1980 mais ...l’aire protégée a été délimitée en 2000. L’objectif de cette étude est d’analyser la dynamique spatiale de cette forêt afin de vérifier si son statut de protection a limité les pressions anthropiques auxquelles elle était soumise avant sa protection. Les changements d’occupation du sol au sein de la réserve ont été analysés à l’aide de six images satellitaires multispectrales de type Landsat et des observations sur le terrain. L’analyse des changements d’aire et du périmètre de la forêt de Bururi montre que la dynamique de cette forêt comprend deux phases : la première est principalement caractérisée par la déforestation et la savanisation en bordure des zones agricoles mais également par la cicatrisation de son intérieur entre 1986 et 2001. La deuxième phase est caractérisée par l’augmentation de la superficie et du périmètre forestier suite à la transformation des zones antérieurement savanisées. La limitation de la pression anthropique liée notamment à la délimitation de l’aire protégée, la perturbation des activités agricoles suite à l’instabilité sociopolitique ainsi que l’augmentation du nombre de garde-forestiers, seraient à l’origine de l’augmentation de la superficie entre 2001 et 2011. L’implication des populations locales dans la protection de cette forêt pourrait limiter davantage la pression anthropique et favoriser ainsi la régénération des zones dégradées. Dans ce contexte, l’innovation des pratiques agropastorales dans les villages riverains de l’aire protégée pourrait également contribuer à limiter les perturbations anthropiques.
The Forest Spatial Dynamic in the Bururi Forest Nature Reserve, Burundi.The studied forest is a protected area located in Southern Burundi, close to Bururi city and to agricultural villages. The effective protection of this forest started in 1980 but the protected area delimitation occurred in 2000. The forest spatial dynamic is analyzed to assess the effectiveness of this protection status. The study combines six Landsat multispectral satellite images analysis with field observations. Forest area and perimeter analysis highlights its dynamic in two phases: the first one is mainly characterised by deforestation and savannah development around agricultural lands but also by forest regrowth processes between 1986 and 2001. The second phase (2001-2011) of the forest dynamic is characterized by the increase of its surface and perimeter following the transformation of savannah zones. The anthropogenic effect limitation linked to the protected area delimitation, agricultural activities disturbance during socio-political instability period and to the increasing number of forest-rangers, would have influenced the surface and perimeter gain between 2001 and 2011. Local population implication in forest protection could further limit human pressure and promote degraded zones regeneration. Thus, agropastoral practices innovation in neighboring villages of the protected zone could contribute to limit these anthropogenic disturbances.
Background and aims — A synchronic study has been achieved in fallows distributed in three age classes (0—2 years, 3—5 years and more than five years) in the natural forest reserve of Kigwena ...(Burundi). The aim was to show that differences in the proportion of plant functional groups could be used as indicators in characterizing the post-cultivation succession stages. Methods — The retained functional groups concerned biological and phytogeographic types, leaf size and diaspores dispersion. Key results — The data analysis gave five floristic associations. The functional group analysis evidences that the young successional stages are often characterized by sclerochorous herbaceous (therophyts) with often anemochorous dispersion, while the aged stages are essentially dominated by sarcochorous phanerophyts with essentially autochorous dispersion. The proportion of species with large distribution distribution with the age of the fallow contrary to species with regional distribution. A progressive increase with the age of species number with large leaves and a reduction of species with small leaves has been observed. Conclusion — The follow-up of the temporal variability of biological traits of species permits to characterize stages of post-cultivation succession. Objectifs — Une étude synchronique a été réalisée dans les jachères réparties en trois classes d'âges (0—2 ans, 3—5 ans et plus de cinq ans) dans la réserve naturelle forestière de Kigwena (Burundi). L'objectif était de montrer que les différences dans les spectres des groupes fonctionnels de plantes pouvaient être utilisées comme indicateurs pour caractériser les stades de la succession post-culturale. Méthodes—Les groupes fonctionnels retenus concernaient la forme biologique, l'origine phytogéographique, la morphologie foliaire et le mode de dispersion des diaspores. Résultats — Après traitement des données phytosociologiques, cinq groupements végétaux ont été individualisés sur base de la composition floristique. L'analyse des groupes fonctionnels met en évidence que les stades jeunes sont caractérisés par les thérophytes à diaspores sclérochores souvent anémochores, tandis que les stades âgés sont dominés par des phanérophytes à diaspores sarcochores essentiellement zoochores. La proportion des espèces à large distribution diminue avec l'âge de la jachère contrairement aux espèces à distribution régionale. Une augmentation progressive avec l'âge de la jachère d'espèces mésophylles et macrophylles et une diminution d'espèces nanophylles ont été observées. Conclusion — Le suivi de la variabilité temporelle des spectres des traits biologiques des espèces permet de caractériser les stades de la succession post-culturale.
► Alien plant species dominate the vegetation of the city of Bujumbura (Sub-Saharan Africa). Most of these species originate from tropical America. ► The urbanization process leads to the ...homogenisation of the vegetation in favour of ruderal plant assemblages and introduced species. ► Urban planners in African cities should take measures to preserve the last patches of semi-natural vegetation and to reduce the expansion of invasive species.
Although many African cities are confronted to an important population growth, there are almost no studies describing their vegetation and how it is affected by the urbanization process. We present here the description of the spontaneous vegetation of the capital city of Burundi, Bujumbura, through the analysis of 437 vegetation plots. Ruderal plant assemblages dominate in the most urbanized areas while in the outskirts of the city, the vegetation shows similarities to natural plant assemblages in the region. The variability of the plant assemblages is further influenced by the degree of shade, the presence of a permanent source of humidity and trampling by humans and cattle. Over the 404 species recorded in the plots, 57% are native and 43% are introduced. Sixty percent of the 173 introduced species originate from tropical America. It is very likely that in the future, if no biodiversity conservation measures are taken, the urbanization process will lead to the homogenisation of the vegetation in favour of ruderal plant assemblages and introduced species. Similarly to what is done in temperate countries, urban planners in African cities should take measures to preserve the last patches of semi-natural vegetation, not only for their conservation value, but also for the diversity of services they offer to the population.
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