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See detailPast human disturbances and soil fertility both influence the distribution of light-demanding tree species in a Central African tropical forest
Vleminckx, Jason; Bauman, David; Demanet, Marine et al

in Journal of Vegetation Science (2020), 31

Questions: In vast areas of Central African forests, the upper canopy is presently dominated by light-demanding tree species. Here, we confront three hypotheses to explain this dominance: (a) these ... [more ▼]

Questions: In vast areas of Central African forests, the upper canopy is presently dominated by light-demanding tree species. Here, we confront three hypotheses to explain this dominance: (a) these species have expanded their distribution because of widespread past slash-and-burn activities, as suggested by important charcoal amounts recorded in the soils of the region; (b) their abundance is rather explained by soil properties, as this guild establishes preferentially on favourable physico-chemical conditions for rapid growth; (c) soil properties have been substantially influenced by past human disturbances and those two effects cannot be disentangled. Location: Pallisco-CIFM logging concession, southeastern Cameroon (300,000 ha). Methods: We quantified soil charcoal abundance and measured ten soil variables at the basis of 60 target trees that belonged to a list of three long-living pioneer lightdemanding (LLP) and four shade-bearer (SB) species. We identified all stems with a diameter at breast height (DBH) ≥ 20 cm within a distance of 15 m around each target tree. Species were characterised by their wood-specific gravity (WSG), which reflected their light requirement. Multiple regression models were used to quantify and test the relative effects of charcoal abundance and soil variables on the mean WSG of the 60 tree communities, as well as the abundance of three guilds: LLP, SB, and non-pioneer light demanders (NPLD). Results: The mean WSG was the only response variable significantly explained by soil variables and charcoal abundance combined. It was significantly negatively associated with soil calcium and Mg content and with charcoal abundance, with soil and charcoal influencing the mean WSG independently. Conclusion: Our study provides evidence that past human disturbances and soil fertility have independently promoted the establishment of light-demanding species in western Central African forests, thereby shedding light on tree community assembly rules in these ecosystems which remain considerably understudied compared to the tropical forests of other continents. [less ▲]

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See detailCarte blanche : "Quels arbres pour la forêt de demain?"
Fanal, Aurore ULiege; Monty, Arnaud ULiege; Claessens, Hugues ULiege et al

Article for general public (2019)

17 scientifiques et académiques de la Fédération Wallonie – Bruxelles cosignent cette carte blanche pour alerter sur les risques de destruction de l’écosystème des forêts wallonnes par l’introduction ... [more ▼]

17 scientifiques et académiques de la Fédération Wallonie – Bruxelles cosignent cette carte blanche pour alerter sur les risques de destruction de l’écosystème des forêts wallonnes par l’introduction massive d’espèces végétales exotiques. [less ▲]

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See detailNest composition, stable isotope ratios and microbiota unravel the feeding behaviour of an inquiline termite
Hellemans, Simon; Marynowska, Martyna; Drouet, Thomas et al

in Oecologia (2019)

Termites are eusocial insects having evolved several feeding, nesting and reproductive strategies. Among them, inquiline termites live in a nest built by other termite species: some of them do not forage ... [more ▼]

Termites are eusocial insects having evolved several feeding, nesting and reproductive strategies. Among them, inquiline termites live in a nest built by other termite species: some of them do not forage outside the nest, but feed on food stored by the host or on the nest material itself. In this study, we characterized some dimensions of the ecological niche of Cavitermes tuberosus (Termitidae: Termitinae), a broad-spectrum inquiline termite with a large neotropical distribution, to explain its ecological success. We used an integrative framework combining ecological measures (physico-chemical parameters, stable isotopic ratios of N and C) and Illumina MiSeq sequencing of 16S rRNA gene to identify bacterial communities and to analyse termites as well as the material from nests constructed by different termite hosts (the builders). Our results show that (1) nests inhabited by C. tuberosus display a different physico-chemical composition when compared to nests inhabited by its builder alone; (2) stable isotopic ratios suggest that C. tuberosus feeds on already processed, more humified, nest organic matter; and (3) the gut microbiomes cluster by termite species, with the one of C. tuberosus being much more diverse and highly similar to the one of its main host, Labiotermes labralis. These results support the hypothesis that C. tuberosus is a generalist nest feeder adapted to colonize nests built by various builders, and explain its ecological success. [less ▲]

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See detailGrowth determinants of timber species Triplochiton scleroxylon and implications for forest management in central Africa
Ligot, Gauthier ULiege; Fayolle, Adeline ULiege; Gourlet-Fleury, Sylvie et al

in Forest Ecology and Management (2019), 437

The sustainability of the polycyclic logging system in tropical forests has been increasingly questioned for a variety of reasons, and particularly in central Africa as commercial species, mostly light ... [more ▼]

The sustainability of the polycyclic logging system in tropical forests has been increasingly questioned for a variety of reasons, and particularly in central Africa as commercial species, mostly light-demanding long-lived pioneer species, usually fail to recover a stable number of large trees after exploitation. Several factors are known to affect tropical tree demographic processes, like tree growth, survival and recruitment. Tree growth has particularly been showed to depend on ecological conditions, tree genetics, and competition with surrounding vegetation, as well as tree size or ontogeny. Yet, due to the paucity of available data, the importance of such factors is unclear and usually ignored when estimating future timber yields. To fill this gap, we chose to evaluate the variability in growth of one African long-lived pioneer and commercially very important species: Triplochiton scleroxylon K. Schum, gathering a broad dataset composed of tree ring data recorded in one site in Cameroon and periodic field inventory data recorded in seven sites across central Africa. In total, we analyzed 13,225 records of annual tree diameter increments recorded over 920 trees from seven sites in Cameroon, Republic of the Congo and Central African Republic. We evaluated (i) to what extent the average growth of trees that reach harvestable dimensions differs from population average and (ii) to what extent past perturbations influence the growth of remaining trees. We found the diameter growth of T. scleroxylon to be remarkably variable and this study provided an unprecedented quantification of the magnitude of some key growth determinants. In unlogged forests, the diameter increment of T. scleroxylon ranged between 0.40 cm year-1 in Southern Cameroon and 0.83 cm year-1 in South-Eastern Cameroon. The diameter increment was weakly related to tree size but increased twofold from unlogged to logged forests. Perturbation caused by logging stimulates growth of T. scleroxylon for at least 10-15 years. Finally, harvestable timber stock of large-sized T. scleroxylon was found to be constituted by trees that grew in average twice faster than trees of the entire extant population. As more and more inventory data become available, quantifying these effects could be replicated for other timber species and in other sites, to improve the accuracy of future timber resource estimates and improve forest management guidelines. [less ▲]

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See detailLe Hêtre face au changement climatique : le cas de la Belgique
Latte, Nicolas ULiege; Lebourgeois, François; Kint, Vincent et al

in Revue Forestière Française (2018), 69(3), 205-218

This article explores the relationship between beech (Fagus sylvatica L.) and its environment on the scale of Belgium. Radial increment is used to assess the effects of global change, particularly climate ... [more ▼]

This article explores the relationship between beech (Fagus sylvatica L.) and its environment on the scale of Belgium. Radial increment is used to assess the effects of global change, particularly climate change, on the development and vitality of beech since the beginning of the 20th century. The average radial increment of beech on the scale of Belgium has decreased while its variability has increased. Climate has had a growing influence on growth and has become the predominant factor since the beginning of the 1990’s. The increments of all trees have become synchronised mainly due to the increase in frequency and intensity of summer heat waves and spring droughts. The results are interpreted on the basis of knowledge about the ecophysiology of beech and the consequences of current and future climate change are discussed. [less ▲]

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See detailThe influence of spatially structured soil properties on tree community assemblages at a landscape scale in the tropical forests of southern Cameroon
Vleminckx, Jason; Doucet, Jean-Louis ULiege; Morin, Julie ULiege et al

in Journal of Ecology (2016)

Species distribution within plant communities results from both the influence of deterministic processes, related to environmental conditions, and neutral processes related to dispersal limitation and ... [more ▼]

Species distribution within plant communities results from both the influence of deterministic processes, related to environmental conditions, and neutral processes related to dispersal limitation and stochastic events, the relative importance of each factor depending on the observation scale. Assessing the relative contribution of environment necessitates controlling for spatial dependences among data points. Recent methods, combining multiple regression and Moran's eigenvectors maps (MEM), have been proved successful in disentangling the influence of pure spatial processes related to dispersal limitation, pure environmental variables (not spatially structured) and spatially structured environmental properties. However, the latter influence is usually not testable when using advanced spatial models like MEM. To overcome this issue, we propose an original approach, based on torus-translations and Moran spectral randomizations, to test the fraction of species abundance variation that is jointly explained by space and seven soil variables, using three environmental and tree species abundance data sets (consisting of 120, 52 and 34 plots of 0·2 ha each, located along 101-, 66- and 35-km-long transect-like inventories, respectively) collected in tropical moist forests in southern Cameroon. The overall abundance of species represented by ≥30 individuals, and 27% of these species taken individually, were significantly explained by fine-scale (<5 km) and/or broad-scale (5–100 km) spatially structured variations in soil nutrient concentrations (essentially the concentration of available Mn, Mg and Ca) along the 120-plots area. The number of significant tests considerably decreased when investigating the two smaller data sets, which mostly resulted from low statistical power rather than weaker floristic and/or edaphic variation captured among plots. Synthesis. Our results provide evidence that tree species turnovers are partly controlled by spatially structured concentrations in soil nutrients at scales ranging from few hundreds of metres to c. 100 km, a poorly documented subject in Central African forests. We also highlight the usefulness of our testing procedure to correctly interpret the space-soil fraction of variation partitioning analyses (which always accounted here for the most important part of the soil contribution), as this fraction was sometimes relatively high (R2 values up to c. 0·3) but nearly or not significant. [less ▲]

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See detailTaller trees, denser stands and greater biomass in semi-deciduous than in evergreen lowland central African forests
Fayolle, Adeline ULiege; Loubota Panzou, Grâce Jopaul ULiege; Drouet, Thomas et al

in Forest Ecology and Management (2016), 374

Accurate height-diameter allometry is crucial for the estimation of forest biomass and carbon stocks. Tree height measurements over a large range of diameters and species are urgently needed in the ... [more ▼]

Accurate height-diameter allometry is crucial for the estimation of forest biomass and carbon stocks. Tree height measurements over a large range of diameters and species are urgently needed in the tropics, specifically in central Africa, for the development of locally derived height-diameter allometric equations and the conversion of forest inventory data into biomass estimates, and for the validation of remotely sensed canopy height that mostly rely on a few specific field sites. In this study, we aimed to identify the variation in height-diameter allometry of tropical trees between forest types and among species in central Africa, and we examined the consequences for biomass estimation. Height and diameter were measured for a total of 521 trees over a large range of diameters in two forest types in southern Cameroon, 10–240 cm in the evergreen forest and 11–182 cm in the semi-deciduous forest. A total of ten allometric models including asymptotic and non-asymptotic models were fitted to the heightdiameter data. Measured tree diameters, grouped into 10 cm wide diameter classes up to 150, from commercial forest inventory data (0.5 ha plots, n = 2101 and n = 5152, respectively in the evergreen and in the semi-deciduous forests) were converted into biomass estimates using general allometric models with and without including our site-specific height-diameter allometry. Though debated in the literature, our results supported a saturation of tree height with tree diameter both at site and species level, with asymptotic models better depicting the height-diameter allometry. Height-diameter allometry significantly differed between forest types and these local height-diameter equations also differed from published equations. For a given diameter, trees tended to be taller in the semi-deciduous forest than in the evergreen forest, as already reported between moist and wet forests in pantropical studies. Similar trends were reported within species for the three species shared by both forest types, suggesting an environmental control of tree allometry. Because of the low performance of the bioclimatic stress variable to predict tree height and of the slight soil differences between the two forest types, the environmental determinants of height-diameter allometry remain to be explored. In addition to tree allometry variation, structural differences (basal area and density) were also identified between the two forest types using commercial forest inventory data at genus level, and both allometry and forest structure (taller trees and denser stands) contributed to the greater biomass per hectare of the semi-deciduous forest. [less ▲]

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See detailDendroécologie du hêtre en forêt de Soignes : Les cernes des arbres nous renseignent sur les changements récents et futurs
Latte, Nicolas ULiege; Kint, Vincent; Drouet, Thomas et al

in Forêt.Nature (2015), 137(Octobre-Novembre-Décembre), 24-37

La dendroécologie combine la dendrochronologie et l’écologie forestière. Elle a permis d’analyser les cernes des hêtres de la forêt de Soignes et de comparer les tendances dans le contexte plus large des ... [more ▼]

La dendroécologie combine la dendrochronologie et l’écologie forestière. Elle a permis d’analyser les cernes des hêtres de la forêt de Soignes et de comparer les tendances dans le contexte plus large des hêtraies de Belgique. Depuis la fin des années ‘70, le hêtre devient de plus en plus sensible au climat, surtout en Soignes. L’augmentation de la fréquence et de l’intensité des sécheresses printanières et des canicules ont affecté négativement la croissance annuelle des hêtres. Les prévisions climatiques pour le 21e siècle présage une rapide aggravation. Les risques encourus par l’essence aboutissent à de nouvelles réflexions sur la gestion de la hêtraie de Soignes. Pour les peuplements existants : une sylviculture dynamique avec des éclaircies fortes et fréquentes afin de limiter la concurrence pour les ressources et favoriser la croissance individuelle des arbres. Pour la régénération des vieilles hêtraies : les remplacer par d’autres essences plus tolérantes ou favoriser le mélange et confiner le hêtre aux surfaces limitées des stations les plus fraîches. Un risque mesuré pourrait être pris en regard du rôle patrimonial des hêtraies cathédrales en Soignes. Toutefois, ce faciès peut être obtenu avec d’autres essences. [less ▲]

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See detailSoil charcoal to assess the impacts of past human disturbances on tropical forests
Vleminckx, Jason; Morin-Rivat, Julie ULiege; Biwolé, Achille et al

in PLoS ONE (2014), 9(11),

The canopy of many central African forests is dominated by light-demanding tree species that do not regenerate well under themselves. The prevalence of these species might result from ancient slash-and ... [more ▼]

The canopy of many central African forests is dominated by light-demanding tree species that do not regenerate well under themselves. The prevalence of these species might result from ancient slash-and-burn agricultural activities that created large openings, while a decline of these activities since the colonial period could explain their deficit of regeneration. To verify this hypothesis, we compared soil charcoal abundance, used as a proxy for past slash-and-burn agriculture, and tree species composition assessed on 208 rainforest 0.2 ha plots located in three areas from Southern Cameroon. Species were classified in regeneration guilds (pioneer, non-pioneer light-demanding, shade-bearer) and characterized by their woodspecific gravity, assumed to reflect light requirement. We tested the correlation between soil charcoal abundance and: (i) the relative abundance of each guild, (ii) each species and family abundance and (iii) mean wood-specific gravity. Charcoal was found in 83% of the plots, indicating frequent past forest fires. Radiocarbon dating revealed two periods of fires: ‘‘recent’’ charcoal were on average 300 years old (up to 860 BP, n = 16) and occurred in the uppermost 20 cm soil layer, while ‘‘ancient’’ charcoal were on average 1900 years old (range: 1500 to 2800 BP, n = 43, excluding one sample dated 9400 BP), and found in all soil layers. While we expected a positive correlation between the relative abundance of light demanding species and charcoal abundance in the upper soil layer, overall there was no evidence that the current heterogeneity in tree species composition can be explained by charcoal abundance in any soil layer. The absence of signal supporting our hypothesis might result from (i) a relatively uniform impact of past slash-and-burn activities, (ii) pedoturbation processes bringing ancient charcoal to the upper soil layer, blurring the signal of centuries-old Human disturbances, or (iii) the prevalence of other environmental factors on species composition. [less ▲]

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See detailSoil charcoal to assess the impacts of past Human disturbances on tropical forests
Vleminckx, Jason; Morin, Julie ULiege; Biwolé, Achille et al

Conference (2014, July 24)

The canopy of many central African forests is dominated by light-demanding tree species that do not regenerate well under themselves. The prevalence of these species might result from ancient slash-and ... [more ▼]

The canopy of many central African forests is dominated by light-demanding tree species that do not regenerate well under themselves. The prevalence of these species might result from ancient slash-and-burn agricultural activities that created large openings, while a decline of these activities since the colonial period could explain their deficit of regeneration. To verify this hypothesis, we compared soil charcoal abundance, used as a proxy for past slash-and-burn agriculture, and tree species composition assessed on 208 rainforest 0.2 ha plots located in three areas from Southern Cameroon. Species were classified in regeneration guilds (pioneer, non-pioneer light-demanding, shade-bearer) and characterized by their wood-specific gravity, assumed to reflect light requirement. We tested the correlation between soil charcoal abundance and: (i) the relative abundance of each guild, (ii) each species and family abundance and (iii) mean wood-specific gravity. Charcoal was found in 83% of the plots, indicating frequent past forest fires. Radiocarbon dating revealed two periods of fires: “recent” charcoal were on average 300 years old (up to 860 BP, n = 16) and occurred in the uppermost 20 cm soil layer, while “ancient” charcoal were on average 1900 years old (range: 1500 to 2800 BP, n = 43, excluding one sample dated 9400 BP), and found in all soil layers. While we expected a positive correlation between the relative abundance of light-demanding species and charcoal abundance in the upper soil layer, overall there was no evidence that the current heterogeneity in tree species composition can be explained by charcoal abundance in any soil layer. The absence of signal supporting our hypothesis might result from (i) a relatively uniform impact of past slash-and-burn activities, (ii) pedoturbation processes bringing ancient charcoal to the upper soil layer, blurring the signal of centuries-old Human disturbances, or (iii) the prevalence of other environmental factors on species composition. [less ▲]

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See detailImpacts of past Human disturbances on present-day tree species assembly in a tropical forest of South-East Cameroon
Vleminckx, Jason; Doucet, Jean-Louis ULiege; Daïnou, Kasso ULiege et al

Poster (2013, November 06)

Many evidence have been found for intensive past Human presence in the forests of Central Africa, notably widespread charcoal occurrence in the soil. Forest clearing for slash-and-burn agriculture may ... [more ▼]

Many evidence have been found for intensive past Human presence in the forests of Central Africa, notably widespread charcoal occurrence in the soil. Forest clearing for slash-and-burn agriculture may have favored the competitiveness of light-demanding species (LD) to the detriment of shade-bearer species (SB). Hypothesis: Positive correlation between abundance of charcoal in the soil (proxy for past Human clearing) and abundance of LD.Mostly “young” charcoals were thought to reflect past Human disturbances that would have shaped present-day species assembly. However, CAI 0-20cm and CAI 20-100cm were highly correlated with each other (r-Pearson = 0.55; P<0.001) and both displayed positive correlations with Non-Pioneer LD abundance (significant with a classic test) and negative correlations with SB abundance. Although this observation is coherent with our hypothesis, significance disappeared when correcting for spatial autocorrelation [4], even after removing small-diameter trees potentially too young to be linked with last Human disturbances (not shown). Correlation of CAI between the two soil layers => Humans found appropriate conditions for settlement in the same area at different periods? Absence of significant correlation in ❸ (i) Last Human disturbances are too old to detect any signal on present-day tree species assembly. (ii) Human impact is weak compared to other factors (soil properties, dispersal limitation,…) (iii) Local scale heterogeneity of LD abundance is weak compared to landscape scale. Parallel large scale gradients in the abundance of Non-Pioneer LD and charcoal abundance (proxy for past slash-and-burn activities) were observed, but a causal link cannot be established so far. [less ▲]

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See detailImpacts of past Human disturbances on present day tree species assembly in the tropical forests of South-East Cameroon
Vleminckx, Jason; Doucet, Jean-Louis ULiege; Daïnou, Kasso ULiege et al

Poster (2013, June 25)

Non-random spatial distribution of trees is the result of both neutral and deterministic factors. Neutral models suggest that species within a community are equally competitive, with spatial structures ... [more ▼]

Non-random spatial distribution of trees is the result of both neutral and deterministic factors. Neutral models suggest that species within a community are equally competitive, with spatial structures mainly due to dispersal limitation. Deterministic (or non-neutral) models consider species assemblages as the result of what we name “induced spatial dependence”, where forcing (explanatory) variables shape diversity organization. However, deterministic models have often included habitat variables only, without considering human disturbance which we know enhances the competitive advantage of heliophytic (light-demanding) species and therefore the floristic composition of phytocenoses. Based on charcoal abundance in the soil (used as an indicator of anthropogenic perturbation), species abundance, and environmental data from a forest of south-east Cameroon, we applied modern variation partitioning methods to assess the relative impact of human disturbance on floristic patterns, controlling for purely spatial and habitat effects. Significant signals of human influence have been found so far, and a new collection of data should establish with a better precision the importance of the anthropogenic impact on tree species assemblages. [less ▲]

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See detailEffets de la fragmentation des forêts par l'agriculture itinérante sur la dégradation de quelques propriétés physiques d'un ferralso échantillonné à Yangambi, R.D. Congo
Alongo, Sylvain; Visser, Marjolein; Drouet, Thomas et al

in Tropicultura (2013), 31(1), 36-43

The knowledge of changes occurring in the soil cover at the forest edge is needed to better understand the impact of forest fragmentation on soil parameters. The objective of this study is to evaluate the ... [more ▼]

The knowledge of changes occurring in the soil cover at the forest edge is needed to better understand the impact of forest fragmentation on soil parameters. The objective of this study is to evaluate the effects of forest fragmentation caused by shifting agriculture on texture, bulk density, total porosity and soil hydraulic conductivity in the layers at 0-10, 10-20 and 20-30 cm depth by the transect method in a perspective of sustainable land management in the region of Yangambi. The results obtained show that the edge between the fallow grassland and dense forest has a width of 70 m. Degradation of all the physical properties of soils relative to the dense forest is very significant (p<0.01) for the studied layers. Fragmentation of forests by shifting cultivation increases the phenomenon of selective loss of clays of the surface soil layers and consequently increases its bulk density and decreases the permeability and porosity with depth under the fallow grass and edge. The texture data associated with textural classes in the FAO textural triangle indicate a change in texture of the soil under different land uses for the tree layers studied. [less ▲]

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See detailQuel avenir pour la "hêtraie cathédrale" de Soignes ? Bases de réflexion pour une prise de décision
Claessens, Hugues ULiege; Langohr, Roger; Drouet, Thomas et al

in Forêt.Nature (2012), (120), 3-21

A l'heure où le plan de gestion de la forêt de Soignes bruxelloise doit être révisé, il est apparu nécessaire de synthétiser les éléments objectifs sur lesquels la gestion doit s'appuyer. Rassemblant ... [more ▼]

A l'heure où le plan de gestion de la forêt de Soignes bruxelloise doit être révisé, il est apparu nécessaire de synthétiser les éléments objectifs sur lesquels la gestion doit s'appuyer. Rassemblant leurs expertises respectives pour une analyse commune des nombreuses études existantes, pédologues, écologues, sylviculteurs et gestionnaires tirent la sonnette d'alarme : la forêt de Soignes n'est pas bien armée pour affronter l'avenir. Forts de ce constat, ils proposent et argumentent divers scénarios de gestion alternatifs. [less ▲]

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