•Antithetic and randomized branch are unbiased and cost effective sampling techniques.•Model using basal area and 3-knot cubic spline provides good prediction of AGB.•Model predicted by 3-knot cubic ...spline accounts for heteroscedasticity.•General models overestimate amounts of carbon for Gilbertiodendron dewevrei.•For more precise estimates of biomass site-and-species specific model are important.
Estimates of global carbon stocks in standing forests are subject to uncertainty because of regional and tree species differences that are usually ignored in global allometric equations. The absence of appropriate site-specific and individual tree allometric equations has led to broad use of pan moist tropical equations, which use has raised questions on the accuracy of the resulting predictions of standing biomass. Here we develop site-specific individual tree allometric equations for estimating biomass of Gilbertiodendron dewevrei, a canopy tree that dominates extensive areas of forest in the Congo basin region. We applied both antithetic and randomized branch sampling to sample for total aboveground biomass components. We evaluated a series of regression models (linear and non-linear) for predicting total aboveground biomass asa function of commonly measured variables including diameter, basal area and total height of 43 sample trees for Ituri and Yoko forests in the Democratic Republic of Congo. We found the best model for total aboveground biomass to be a linear, 3-knot cubic spline model that used only basal area asa predictor with the lowest AIC and BIC of 602 and 610 respectively. The incorporation of height in biomass equations did not significantly improve model performance, while models with diameter alone or in combination with height perform poorly. Our results show that models using only basal area are sufficient to accurately estimate biomass for Gilbertiodendron dewevrei and carbon stock, which is an important outcome given that height measurements are usually difficult to acquire. We also demonstrate that general models used can significantly overestimate amounts of carbon for Gilbertiodendron dewevrei as compared to the site and species-specific regression model that we have developed.
Though substantial research has been conducted on possible historical, physiological, and symbiotic mechanisms that permit monodominance to occur within tropical lowland rainforests, less is known ...about the successional rates at which monodominance exerts itself on surrounding forest structures. Here we extend efforts to evaluate the longitudinal dynamics of Gilbertiodendron dewevrei-dominated forest in Central Africa by considering this species' spatial dynamics. Using three 10-ha censused field plots measured across three time periods, we present the first quantitative estimates of the spatial propagation of Gilbertiodendron into adjacent mixed species forest. Using three analytical strategies, we demonstrate that Gilbertiodendron is increasing in dominance and that monodominant forest patches are expanding into the surrounding forest at a statistically significant rate. The rates of successional advance vary by patch and direction, but average 0.31 m year-1, with speeds greatest in the direction of the prevailing winds. We show that the advancement of Gilbertiodendron is significantly slower than documented rates from other forest ecotones across Central Africa. When paired with stress tolerance traits and ectomycorrhizal associations, these findings help to clarify the means by which Gilbertiodendron dewevrei gains dominance in otherwise species-diverse regions.
The phytochemical investigation of the methanol extract of the stem bark of Gilbertiodendron dewevrei led to the isolation of two new secondary metabolites, ...5,7-dihydroxy-4′-methoxyisoflavan-2,4-dione (1) and 23-hydroxy-2-tricosanone (2) along with 19 known compounds (3-21). The structure of these compounds were established by interpretation of their spectral data, mainly HR-TOFESIMS, 1 D NMR (
1
H,
13
C and DEPT) and 2 D NMR (
1
H-
1
H COSY, HSQC, HMBC, and NOESY), and by comparison with those reported in the literature. The methanol extract and some isolates were screened for their antiradical, antibacterial, and inhibitory properties against acetylcholinesterase.
Tropical forests are hyperdiverse, yet extensive areas of monodominant forest occur in the tropics worldwide. Most long‐lived and persistent monodominant tree species form ectomycorrhizal fungi ...symbioses, allowing them to obtain nutrients directly from soil organic matter. This might promote monodominance by reducing nutrient availability to co‐occurring species, the majority of which form associations with arbuscular mycorrhizal fungi.
Gilbertiodendron dewevrei forest is the most widespread monodominant forest in tropical Africa. Its distribution appears determined in part by moisture availability, but its monodominance is not thought to be driven by its fungal partner or soil fertility.
Here, we compare soil fertility of 20 G. dewevrei stands to mixed forest from three sites across an 8,400 km2 region of the Central African Republic and the Republic of Congo. In contrast to previous studies, we find monodominant G. dewevrei stands associated with infertile soils, as base cations (calcium, magnesium, total exchangeable bases) and extractable manganese are extremely low, and significantly lower in soils under G. dewevrei forest compared to mixed forest. Further, and consistent with ectomycorrhizal forests globally, soil carbon to nitrogen and carbon to phosphorus ratios are significantly higher in G. dewevrei stands than in mixed forest stands, providing evidence in support of direct acquisition of nitrogen and phosphorus from soil organic matter by ectomycorrhizal fungi.
Gilbertiodendron dewevrei recruits from the seedling bank, with its large seedlings surviving in high densities for over a decade. We tested whether light plasticity could facilitate monodominance by growing seedlings of G. dewevrei under controlled light conditions. We found that its seedlings grow well under a wide range of irradiance levels and conclude that this plasticity affords a competitive advantage.
Synthesis. We reframe the discussion of factors contributing to monodominance of Gilbertiodendron dewevrei into one of resource acquisition and use efficiency. In particular, G. dewevrei is associated with moist and infertile soils and competes well under a variety of light conditions. Our data are consistent with a model where root associations with ectomycorrhizal fungi drive monodominance through the direct acquisition of nutrients from soil organic matter, promoting nutrient limitation of co‐occurring species.
Foreign Language Résumé
Les forêts tropicales sont hyperdiverses, mais de vastes zones de forêts monodominantes existent dans les tropiques du monde entier. La plupart des espèces à vie longue et à monodominances persistantes forme une association symbiotique avec de champignons ectomycorhiziens, leur permettant d'obtenir des nutriments directement à partir de la matière organique du sol. Cela pourrait favoriser la monodominance en réduisant la disponibilité des éléments nutritifs pour les espèces concurrentes, dont la plupart forment des associations avec les champignons mycorhiziens arbusculaires.
Gilbertiodendron dewevrei est la forêt monodominante la plus répandue en Afrique tropicale. Sa distribution semble déterminée en partie par la disponibilité en humidité, mais sa monodominance ne semble pas être liée par son association avec les mycorhizes ni par la fertilité du sol.
Ici, nous comparons la fertilité du sol de vingt peuplements de G. dewevrei à celle de trois sites indépendants des forêts mixtes répartis sur une superficie de 8 400 km2 en République centrafricaine et en République du Congo. Contrairement aux études précédentes, nous trouvons des peuplements monodominants de G. dewevrei associés à des sols infertiles, car les cations de base (calcium, magnésium, bases échangeables totales) et le manganèse extractible sont extrêmement faibles et significativement plus faibles dans les sols des forêts à G. dewevrei par rapport aux sols des forêts mixtes. De plus, et conformément aux forêts ectomycorhiziennes du monde entier, les rapports carbone/azote et carbone/phosphore du sol sont nettement plus élevés dans les peuplements de G. dewevrei que dans les peuplements des forêts mixtes; ce qui confirme l'hypothèse d'une acquisition directe d'azote et de phosphore dans la matière organique du sol par les champignons ectomycorhiziens.
Gilbertiodendron dewevrei recrute dans la banque de semis, ses gros semis survivant à des densités élevées pendant une décennie. Nous avons vérifié si la plasticité de la lumière pouvait faciliter la monodominance en semant les plants de G. dewevrei dans des conditions de lumière contrôlée. Nous avons constaté que les plants de G. dewevrei poussent bien sous une large gamme de niveaux d'irradiance et avons conclu que cette plasticité conférait un avantage compétitif.
Synthèse. Nous recentrons la discussion sur les facteurs contribuant à la monodominance de Gilbertiodendron dewevrei en un problème d'acquisition de ressources et d'efficacité d'utilisation. G. dewevrei est, en particulier, associé à des sols humides et infertiles et est bien compétitif dans diverses conditions de luminosité. Nos données sont cohérentes avec un modèle où les associations de racines avec les champignons ectomycorhiziens conduisent à la monodominance via l'acquisition directe d'éléments nutritifs à partir de la matière organique du sol, favorisant ainsi la limitation des éléments nutritifs d'espèces concurrentes.
For a tree species to dominate a site over generations, it must have evolved strategies to be competitively superior at acquiring and maintaining the necessary resources for survival and growth. In the case of Gilbertiodendron dewevrei forest in Central Africa, we reframe thinking related to the ability of tree species to form monodominant stands and forests into a resource acquisition, use, and cycling context.
Key message
Under similar site conditions, leaf litter decomposition beneath Central African rainforests was largely driven by average leaf litter quality. Although significant, the additional ...variability related to litter mixing and to the decomposition environment was limited.
Context
Under given site conditions, litter decomposition is expected to mainly depend on its average quality. However, the additional impacts of litter diversity as well as of the local decomposition environment remain rather inconclusive.
Aims
This study investigates the litter mixture effects on decomposition and home-field advantage for two emblematic old growth forest types of the Congo Basin: the
Scorodophloeus zenkeri
Harms mixed forests and the evergreen
Gilbertiodendron dewevrei
(De Wild.) J. Léonard monodominant forests.
Methods
Based on a litterbag experiment, variations in leaf litter mass loss were measured from the eight most important tree species under mixed and monodominant forests and for all possible two-species combinations.
Results
Remaining mass was largely explained (90%) by a multivariate measure of initial litter quality including 11 functional traits, which performed better than any single leaf litter trait. For the litter mixtures, the average deviation from expectation based on simple additive effects ranged from slightly synergistic (+ 2.56%) to slightly antagonistic (− 0.86%) after 1 and 6 months, respectively. Mixture identity and chemical dissimilarity contributed to explaining the mixing effects, yet the effect of chemical dissimilarity at the whole mixture level was only detected through an interaction with incubation time. In addition, the initial decomposition rates of
S. zenkeri
and
G. dewevrei
were accelerated under their own forest type.
Conclusion
In agreement with the “home-field advantage” theory, our results highlighted that the functional composition of the host forest did have an indirect impact on decomposition. In addition, leaf litter decomposition was mainly controlled by average litter quality, which in turn was closely related to a multivariate measure of green leaf quality. This suggests that increased knowledge of tree species leaf traits in tropical forests would greatly help in better understanding the litter decomposition dynamics.
Aims
In tropical rainforests, soil respiration accounts for the major part of ecosystem respiration, yet a deep understanding of the influence of forest type and species composition is still lacking. ...We therefore selected patches of the rainforest in the Central Congo basin differing in their species composition, some patches under the
Scorodophloeus zenkeri
Harms mixed forests (MIF) and others in the
Gilbertiodendron dewevrei
(De Wild.) J.Léonard monodominant forests (MOF). We measured daily soil respiration over a one-year period.
Methods
By fitting a simple conceptual model of soil respiration, including fine root biomass, soil organic C stocks and ground climate measurements (soil moisture and temperature), we attempted to distinguish autotrophic and heterotrophic soil respiration, and to better understand the drivers behind total soil respiration.
Results
On an annual basis, soil respiration was 10% higher under MOF (22.10 Mg C ha
−1
y
−1
) compared to MIF (20.01 Mg C ha
−1
y
−1
) (
p
< 10
−3
). While the estimated autotrophic and heterotrophic soil respiration contributed about equally to soil respiration in MOF, autotrophic soil respiration slightly dominated (59%) in MIF. In both forests, the combined contribution of litterfall inputs and fine roots productivity was lower than the heterotrophic flux, with the largest difference observed under MOF (−6.16 Mg C ha
−1
year
−1
) compared to MIF (−2.62 Mg C ha
−1
year
−1
). The sensitivity analysis of the model showed that the higher heterotrophic soil respiration under MOF was driven by the twofold C accumulation in MOF topsoil compared to MIF. Soil moisture was a major driver of temporal changes in soil respiration, but hardly impacted the differences in annual soil respiration between forests.
Conclusion
While the difference in SOC accumulation between forests was driven by the low nutrient to C ratios of
Gilbertiodendron dewevrei
tissues, additional research is needed to identify the causes behind the unbalanced C budget
.
Aims Gilbertiodendron dewevrei (De Wild.) J. Léonard monodominant forests (MOF) and Scorodophloeus zenkeri Harms mixed forests (MIF) frequently co-occur on similar soil conditions in the Central ...Congo basin. Although tree species composition is known to impact C storage, the patterns of C sequestration between those contrasted forest types as well as the associated drivers remain unknown. Methods Annual litterfall, as well as soil (forest floor and mineral soil down to 220 cm depth) organic C (SOC) and aboveground C (AGC) stocks were investigated in MIF and MOF located on highly weathered sandy soils in the Yoko Reserve (DRC). Results The annual leaf litterfall was similar under both forests but litterfall quality in MOF strongly differed by a set of traits related to organic matter recalcitrance. The SOC stock down to 220 cm was 55% higher under MOF compared to MIF, and the differences between forests remained significant down to 100 cm. While the combined SOC and AGC stocks were similar in both forests, the SOC stocks accounted for ca. 19 and 33% of the total C stocks in MIF and MOF, respectively. Conclusions Because of similar litterfall C inputs, we conclude that the greater SOC accumulation under G. dewevrei results from a limitation of the decomposition rate, in agreement with the traits of the corresponding leaf litter.
Background and aims – The forests of the Congo Basin contain high levels of biodiversity, and are globally important for carbon storage. In order to design effective conservation strategies, and to ...accurately model carbon stocks, a fine-scale understanding of the different forest types that make up this forest block is needed. Monodominant Gilbertiodendron dewevrei forest covers large areas of the Congo Basin, but it is currently unclear whether it is sufficiently distinct from adjacent mixed terre firme forest to warrant separate treatment for conservation planning and carbon calculations. This study aimed to compare the structure and diversity of monodominant and mixed forest, and ask whether there is a unique vascular plant community associated with G. dewevrei forest. Material and methods – We utilised a combination of plot data and herbarium specimens collected in the Sangha Trinational (a network of protect areas in Cameroon, Central African Republic, and the Republic of Congo). Plot inventories were used to compare G. dewevrei forest and mixed forest for stem density, basal area, above ground biomass, stem size distribution, species diversity, and species composition. In addition, a database of 3,557 herbarium specimens was used to identify species of vascular plant that are associated with G. dewevrei forest. Key results – Gilbertiodendron dewevrei forest is distinct in both structure and species composition from mixed forest. Gilbertiodendron dewevrei forest has a lower stem number (of trees ≥ 10 cm), but a greater proportion of larger trees (> 70 cm), suggesting higher carbon stocks. The species composition is distinct from mixed forest, with 56 species of vascular plant significantly associated with G. dewevrei forest. Conclusion – Monodominant G. dewevrei forest in the Sangha Trinational is both compositionally and structurally distinct from mixed forest. We therefore recommend this forest type be considered separately from mixed forest for conservation planning and carbon stock calculations.
Small-scale comparisons help us to understand how habitat features and food availability impact primate abundance. This is particularly useful at sites without human impacts, as it allows for the ...investigation of the natural factors influencing nesting patterns and great ape abundance. We provide a small-scale study of sympatric great ape nests in an unlogged old-growth forest without poaching activities. We conducted a line transect survey (52 km total effort) around Mbeli Bai, a forest clearing in Nouabalé–Ndoki National Park, Congo, applying on-site nest decay rates and assigning nest builder using logistic regression. We found a high occurrence of monodominant
Gilbertiodendron dewevrei
at Mbeli Bai (34%) that correlates with low great ape densities at Mbeli Bai. Chimpanzees (
Pan troglodytes troglodytes
) showed a preference for nesting in trees in closed canopy monodominant forest. We found a low percentage of gorilla (
Gorilla gorilla gorilla
) nests in mixed species forest (35%) and a higher percentage in trees (64%) compared to other study sites. However, generalized additive models found higher gorilla nest encounter rates in mixed species forest with dense understory than in monodominant forest and open understory. We found no indication of higher gorilla densities close to Mbeli Bai than elsewhere, and line transect estimates were lower than the number of gorillas revealed from direct observations. There were substantial differences between our findings and those for nearby sites, demonstrating the utility of small-scale comparisons to further understand the factors determining chimpanzee and gorilla densities within and between sites and the limitations of nest surveys.
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