Search

Wood density (WD) relates to important tree functions such as stem mechanics and resistance against pathogens. This functional trait can exhibit high intraindividual variability both radially and vertically. With the rise of LiDAR-based methodologies allowing nondestructive tree volume estimations, failing to account for WD variations related to tree function and biomass investment strategies may lead to large systematic bias in AGB estimations. Here, we use a unique destructive dataset from 822 trees belonging to 51 phylogenetically dispersed tree species harvested across forest types in Central Africa to determine vertical gradients in WD from the stump to the branch tips, how these gradients relate to regeneration guilds and their implications for AGB estimations. We find that decreasing WD from the tree base to the branch tips is characteristic of shade-tolerant species, while light-demanding and pioneer species exhibit stationary or increasing vertical trends. Across all species, the WD range is narrower in tree crowns than at the tree base, reflecting more similar physiological and mechanical constraints in the canopy. Vertical gradients in WD induce significant bias (10%) in AGB estimates when using database-derived species-average WD data. However, the correlation between the vertical gradients and basal WD allows the derivation of general correction models. With the ongoing development of remote sensing products providing 3D information for entire trees and forest stands, our findings indicate promising ways to improve greenhouse gas accounting in tropical countries and advance our understanding of adaptive strategies allowing trees to grow and survive in dense rainforests.

Climbing plants need to reach supports and position their leaves for light capture. Vines and lianas develop a large diversity of self-supporting shoots among diverse species and different kinds of attachment. A searcher’s reach is a crucial trait for colonising supports in complex three-dimensional spaces. We explore the reach capacity and diversity of searcher shoots among representative temperate and tropical climbing plants. We investigate the overall range of variation between short- and long-reach searchers; the mechanical and anatomical organisations underlying reach capacities; how searcher architectures are linked to different climbing strategies such as stem twining, tendril climbing, root climbing, and branch-angle-hook climbing. We investigated reach and mechanical and anatomical organisations (stem rigidity and stiffness, stem and tissue geometry) in 29 climbing plant species from temperate and tropical habitats. Searchers show a wide range of maximal reach per species from 0.1 to 2.5 m. Flexural rigidity (EI) at the base of searchers increased with reach length; overall this increase was proportional although some longest-reaching shoots develop proportionally thinner searcher bases with higher stiffness [structural Young’s modulus (Estr)] than shorter-reach shoots. Bases of short-reach searchers rely more on primary tissues compared to long-reach shoots, which rely more on wood production. We identified different mechanical architectures for a given reach capacity across all species. These are linked to different kinds of attachment mechanisms, support foraging, and possibly leaf display. Plants attaching by twining of the main stem showed a wide range of reach capacity. They also developed lighter, more slender, less rigid, but generally relatively stiff (higher Estr) shoots compared with tendril climbers and branch-angle-hook climbers. Differences in the mechanical architecture of searcher shoots in climbing plants are informative for understanding how diverse climbing plant species explore and colonise different kinds of three-dimensional spaces. This is a key feature that distinguishes different habitat preferences. We discuss how such knowledge is not only important for understanding functional biology and ecology of climbing plants but is also of interest for developing new technologies in soft robotics that mimic climbing plants that can navigate through unstructured environments.

Aim Tree crowns determine light interception, carbon and water exchange. Thus, understanding the factors causing tree crown allometry to vary at the tree and stand level matters greatly for the development of future vegetation modelling and for the calibration of remote sensing products. Nevertheless, we know little about large-scale variation and determinants in tropical tree crown allometry. In this study, we explored the continental variation in scaling exponents of site-specific crown allometry and assessed their relationships with environmental and stand-level variables in the tropics. Location Global tropics. Time period Early 21st century. Major taxa studied Woody plants. Methods Using a dataset of 87,737 trees distributed among 245 forest and savanna sites across the tropics, we fitted site-specific allometric relationships between crown dimensions (crown depth, diameter and volume) and stem diameter using power-law models. Stand-level and environmental drivers of crown allometric relationships were assessed at pantropical and continental scales. Results The scaling exponents of allometric relationships between stem diameter and crown dimensions were higher in savannas than in forests. We identified that continental crown models were better than pantropical crown models and that continental differences in crown allometric relationships were driven by both stand-level (wood density) and environmental (precipitation, cation exchange capacity and soil texture) variables for both tropical biomes. For a given diameter, forest trees from Asia and savanna trees from Australia had smaller crown dimensions than trees in Africa and America, with crown volumes for some Asian forest trees being smaller than those of trees in African forests. Main conclusions Our results provide new insight into geographical variability, with large continental differences in tropical tree crown allometry that were driven by stand-level and environmental variables. They have implications for the assessment of ecosystem function and for the monitoring of woody biomass by remote sensing techniques in the global tropics.

Les savanes de la République du Congo sont exploitées pour des fins agricoles et pastorales. Dans la Cuvette congolaise (nord du Congo), les savanes sont encore peu étudiées. La gestion durable de ces savanes nécessite leur connaissance sur le plan floristique. C’est ainsi qu’une étude floristique a été menée dans les savanes de la zone. Elle avait pour objectif d’évaluer l’intérêt pastoral des savanes de cette cuvette. A cet effet, des relevés floristiques ont été réalisés par la méthode des points quadrats alignés, afin d’inventorier la diversité floristique et les principales espèces fourragères. Ainsi, 37 espèces réparties en 33 genres et 10 familles ont été recensées dans la savane à Hyparrhenia diplandra (Hack.) Stapf , et 27 espèces réparties en 22 genres et 8 familles dans la savane à Loudetia simplex (Nees) C.E. Hubbard. Dans l’ensemble, les familles les plus riches en espèces étaient les Poaceae, les Fabaceae et les Cyperaceae. L’indice de diversité de Shannon et l’équitabilité de Piélou avaient des valeurs moyennes de 2,23 bits et 0,57 pour la savane à H. diplandra, et entre 2,1bits et 0,61 pour la savane à L. simplex. Il y a donc une diversité floristique non négligeable, dans ces savanes. Elle varie en fonction des stations d’étude et des phytocénoses. Une gestion planifiée de ces formations permettrait d’assurer une meilleure planification pour des fins pastorales.Mots-clés : savanes, phytodiversité, herbacée, pastoralisme, cuvette congolaise.Evaluation of herbaceous floristic diversity of Congolese basin savannahs (Republic of Congo)The savannas of the Republic of Congo are exploited for agricultural and pastoral. In the Congolese Basin (northern Congo), savannas are still poorly studied. Sustainable management of these savannas requires knowledge in terms of flora. Thus, a floristic study was conducted in the savannah area. It aimed to assess the value of pastoral savannas of the basin. To this end, the floristic surveys were conducted by the method of quadrat points aligned to inventory plant diversity and major forage species. Thus, 37 species, 33 kinds and 10 families were identified in the savannah of Hyparrhenia diplandra (Hack.) Stapf and 27 species, 22 kinds and 8 families in the savannah of Loudetia simplex (Nees) CE Hubbard. Overall, the richest families in species were Poaceae, Fabaceae and the Cyperaceae. The Shannon diversity index and equitability of Pielou had average values of 2.23 bits and 0.57 for the savannah of H. diplandra, and between 2.1 bits and 0.61 for the savannah of L. simplex. So there is substantial plant diversity in these savannas. It varies from study sites and phytocenoses. Planned management of such training would ensure better planning for pastoral purposes.Keywords : savannas, plant diversity, herbaceous, pastoralism, congolese basin.

La savane à Hyparrhenia diplandra Stapfet celle à Loudetia simplex C.E. Hubbard ont été étudiées dans la Cuvette congolaise (Nord du Congo-Brazzaville), sur le plan de la production herbacée. Les sites retenus pour l’étude sont Ollombo, Owando et Makoua. L’étude a démarré à partir des feux de la saison « sèche ». L’objectif de cette étude était d’évaluer la phytomasse aérienne herbacée et sa productivité, en vue d’aider à la prise de décision sur le choix des espaces à occuper pour des activités agricoles et pastorales. Laphytomasse aérienne herbacée a été mesurée par la méthode de la récolte et sa productivité primaire nette réelle par la méthode de la différence entre le moment où la phytomasse est maximale et le moment où elle est minimale dans un cycle annuel. Les échantillons de sols ont été prélevés sous ces deux types de savanes. Leurs analyses granulométrique et chimiques révèlent que ces sols sont dans l’ensemble très sableux, acides et pauvres en matière organique. Le taux en argiles est légèrement supérieur dans le sol sous savane à H. diplandra (jusqu’à 14%) que dans celui sous savane à L. simplex (1% au maximum). La phytomasse aérienne totale maximale est enregistré à dix mois après les feux, en mai : 9,48 – 10,44 t MS ha-1 pour la savane à H. diplandra et 3,43 – 4,52 t MS ha-1 pour la savane à L. simplex. La productivité de la phytomasse varie de 9,29 – 10,04 t MS ha-1an-1 pour la savane à H. diplandra et de 3,09 – 4,64 t MS ha-1an-1 pour la savane à L. simplex. La savane à H. diplandra serait donc à recommander pour des activitésagricoles et pastorales que celle à L. simplex.Mots-clés : savanes, sols, phytomasse, strate aérienne herbacée, productivité, cuvette congolaise.Productivity of Congolese Basin savannas (Congo-Brazzaville)The Hyparrhenia diplandra Stapf and the Loudetia simplex C.E. Hubbard savannas have been studied in the Cuvette congolaise (the North of Congo-Brazzaville) on the herbaceous production level. The selected sites for this research are: Ollombo, Owando and Makoua. The study started with the fires of the dry season. The objective of this survey was to evaluate the aerial herbaceous phytomass and its productivity, in order to help the decision taking concerning the choice of the areas for agricultural and pastoral activities. Theherbaceous aerial phytomass has been examined within the harvesting framework and the net primary productivity of the phytomass is determined by the method of difference between the time when the phytomass is maximal and when it is minimal in an annual cycle. The soil samples have been taken from these two types of savannas. The granulometric and chemical analyses of soils reveal that the soils of the studied savannas are generally very sandy (79, 94-98, 38 %), acidic and poor in organic materials. The clay rate is slightly superior in the soil of the H. diplandra savanna (up to 14%) than in that of L. simplex (1% maxi). The maximal total aerial phytomass is recorded ten months after the fires, in may: 9, 48-10, 44 t MS ha-1 for H. diplandra savanna and 3, 43-4, 52 t MS ha-1 for L. simplex savanna (Figures 1 and 2). The phytomass productivity varies from 9, 29-10, 04 t MS ha-1an-1 for H. diplandra and from 3, 09-4, 64 t MS ha-1 an-1 for L. simplex. The H. diplandra savanna is more productive than the L. simplex savanna. The former should be recommended for agricultural and pastoral activities than the latter.Keywords : savannas, soils, phytomass, herbaceous aerial stratum, productivity, congolese cuvette.

The floristic dynamics of the woody facies of the Aucoumea klaineana forest of the Congolese littoral has been done according to a gradient evolving of the pioneer front towards the interior of the ecosystem. This settlement integrates a dynamics going from the youthful stages towards the most stable of this formation. The objective of this study was focused on the dynamics of the woody formation, the monodominant installation of the Aucoumea klaineana facies with d1.30 . 10 cm and highlights the evolution and the correlation between the density, the basal area and the floristic diversity. The phytoecological parameters count 1186 trees of d1.30 . 10 cm (an average 395 arbres.ha-1) for 71 species. The trees number by subplot oscillated between 70 and 159 trees (6 to 13% of listed trees). The species number by subplot varies from 8 to 28 (11 to 39% of inventoried species). Basal area by plot oscillates of 22 and 35 m2.ha-1 (27 m2.ha-1 an average). It varies from 4 to 10 m2 per subplot (7 m2 by . ha an average). The trees number by subplot and diameter classes vary from 3 to 636 trees (1 to 212 trees.ha-1 an average). The trees of 10 < d1.30 . 20 cm are dominating with 54% and this prevalence is regular in all the subplots. As for the indices of diversity, the Green index (GI) decrease from 0.9 to 0.1 as the ecosystem increases in age. This observation is also true for the skewness coefficient which regresses from pioneer front towards the interior of the forest. The values are respectively 2.10-2 in the first plot, 7.10-4 in the second and 4.10-3 for the third. In this inventory where Aucoumea klaineana is dominating, its density by subplot varies from 42 to 384 trees.ha-1 (183 trees.ha-1 an average). Just like the density, basal area is decreasing to pioneer front towards the interior of the forest. The values oscillate from 87 to 32% of basal area of the subplot (19 m2.ha-1 an average). The report basal area by the density of Aucoumea klaineana is lower than 1 for an average of 0.1. The ecological parameters analysis which allowed seizing the floral dynamics of this forest is stressed by a numerical multivariate analysis (grouping and ordination). It highlights the weak resemblance degree between the twelve plots of the inventory and clearly marks the existence of variation characterizing the Congolese coastal forest, notably during the first stages of installation.Keywords: Congo, Aucoumea klaineana, biological diversity indices, Coastal forest, floristic diversity, phytoecological parameters.