Abstract
Tropical ecosystems adapted to high water availability may be highly impacted by climatic changes that increase soil and atmospheric moisture deficits. Many tropical regions are experiencing ...significant changes in climatic conditions, which may induce strong shifts in taxonomic, functional and phylogenetic diversity of forest communities. However, it remains unclear if and to what extent tropical forests are shifting in these facets of diversity along climatic gradients in response to climate change. Here, we show that changes in climate affected all three facets of diversity in West Africa in recent decades. Taxonomic and functional diversity increased in wetter forests but tended to decrease in forests with drier climate. Phylogenetic diversity showed a large decrease along a wet-dry climatic gradient. Notably, we find that all three facets of diversity tended to be higher in wetter forests. Drier forests showed functional, taxonomic and phylogenetic homogenization. Understanding how different facets of diversity respond to a changing environment across climatic gradients is essential for effective long-term conservation of tropical forest ecosystems.
Climatic changes have profound effects on the distribution of biodiversity, but untangling the links between climatic change and ecosystem functioning is challenging, particularly in high diversity ...systems such as tropical forests. Tropical forests may also show different responses to a changing climate, with baseline climatic conditions potentially inducing differences in the strength and timing of responses to droughts. Trait‐based approaches provide an opportunity to link functional composition, ecosystem function and environmental changes. We demonstrate the power of such approaches by presenting a novel analysis of long‐term responses of different tropical forest to climatic changes along a rainfall gradient. We explore how key ecosystem's biogeochemical properties have shifted over time as a consequence of multi‐decadal drying. Notably, we find that drier tropical forests have increased their deciduous species abundance and generally changed more functionally than forests growing in wetter conditions, suggesting an enhanced ability to adapt ecologically to a drying environment.
Abstract Background Taxonomic identification of wood specimens provides vital information for a wide variety of academic (e.g. paleoecology, cultural heritage studies) and commercial (e.g. wood ...trade) purposes. It is generally accomplished through the observation of key anatomical features. Classic methodologies mostly require destructive sub-sampling, which is not always acceptable. X-ray computed micro-tomography (µCT) is a promising non-destructive alternative since it allows a detailed non-invasive visualization of the internal wood structure. There is, however, no standardized approach that determines the required resolution for proper wood identification using X-ray µCT. Here we compared X-ray µCT scans of 17 African wood species at four resolutions (1 µm, 3 µm, 8 µm and 15 µm). The species were selected from the Xylarium of the Royal Museum for Central Africa, Belgium, and represent a wide variety of wood-anatomical features. Results For each resolution, we determined which standardized anatomical features can be distinguished or measured, using the anatomical descriptions and microscopic photographs on the Inside Wood Online Database as a reference. We show that small-scale features (e.g. pits and fibres) can be best distinguished at high resolution (especially 1 µm voxel size). In contrast, large-scale features (e.g. vessel porosity or arrangement) can be best observed at low resolution due to a larger field of view. Intermediate resolutions are optimal (especially 3 µm voxel size), allowing recognition of most small- and large-scale features. While the potential for wood identification is thus highest at 3 µm, the scans at 1 µm and 8 µm were successful in more than half of the studied cases, and even the 15 µm resolution showed a high potential for 40% of the samples. Conclusions The results show the potential of X-ray µCT for non-destructive wood identification. Each of the four studied resolutions proved to contain information on the anatomical features and has the potential to lead to an identification. The dataset of 17 scanned species is made available online and serves as the first step towards a reference database of scanned wood species, facilitating and encouraging more systematic use of X-ray µCT for the identification of wood species.
Tropical forests play a key role in absorbing carbon from the atmosphere into the land surface. Recent analyses of long‐term (1985–2014) forest inventory plots across the tropics show that ...structurally intact tropical forest are a large carbon sink, but that this sink has saturated and is projected to be in long‐term decline. Here we compare these results with estimates from the two latest generations of Earth System Models, Climate Modelling Intercomparison Project 5 (CMIP5) (19 models) and CMIP6 (17 models). While CMIP5 and CMIP6 are of similar skill, they do not reproduce the observed 1985–2014 carbon dynamics. The “natural” pan‐tropical carbon sink from inventory data is 0.99 Pg C yr−1 (95% CI 0.7–1.3, n = 614) between 2000 and 2010, the best sampled decade, double the CMIP6 multimodel‐mean of 0.45 Pg C yr−1 (95% CI 0.35–0.55). The observed saturating and declining sink is not captured by the models, which show modest increases in sink strength. The future (2015–2040) “natural” pan‐tropical sink from a statistical model driven by extrapolating past trends of its putative environmental drivers decreases by 0.23 Pg C per decade (95% CI 0.09–0.39) until the 2030s, while the CMIP6 multimodel‐mean under the climate change scenario closest to the statistical model project an increasing carbon sink (0.54 Pg C per decade; 95% CI 0.25–0.67). CMIP multimodel‐means reproduce the response of carbon gains from tree growth to environmental drivers, but the modeling of carbon losses from tree mortality does not correspond well to the inventory data. The model‐observation differences primarily result from the treatment of mortality in models.
Plain Language Summary
The land surface is absorbing carbon from the atmosphere. Tropical forests play a key role in this carbon uptake. Recent long‐term analyses of forest inventory plots across Africa and Amazonia show that structurally intact tropical forest are a large carbon sink, but that this sink has recently saturated and is projected to be in long‐term decline. Earth System Models featuring in the Climate Modelling Intercomparison Project (CMIP) are used to project the influence of future climate change on this critical part of the global carbon cycle. As the current CMIP6 and its predecessor, CMIP5, directly inform future policy decisions through the IPCC Assessment Report and other channels, assessing the models' perfomance is crucial to better inform future climate policy. We show that the observed saturating and declining carbon sink is not captured by the CMIP models, which show modest increases in sink strength. The future pan‐tropical net sink from the statistical model decreases by 0.23 Pg C per decade until the 2030s, while CMIP6 multimodel‐means project an increasing carbon sink under all scenarios (0.01–0.03 Pg C per decade) bar one (−0.02 Pg C per decade). Reason for this is the treatment of vegetation mortality in the models.
Key Points
Climate Modelling Intercomparison Project (CMIP) models do not reproduce observed saturation of carbon sink in intact tropical forests
In modeling carbon gains and losses CMIP5 and CMIP6 are of similar skill
CMIP6 models project an increase in tropical forest carbon sink under most future scenarios, the statistical model a decline in the sink
•Multispecies H-D allometric equations underestimate the total height of the emergent tree species Pericopsis elata.•The H-D allometric equation specific to P. elata varies significantly between ...sites.•This site-specific variation showed to be associated with differences in the stand-level basal area at the regional-scale and tree-level light-availability at the local-scale.•The stand-level maximum asymptotic height increased with tree-level light-availability.
Reliable tree height-diameter (H-D) allometric equations are a key tool for the estimation of forest productivity and Above Ground Biomass (AGB). Most existing H-D allometric equations developed for the tropical region are based on large-scale multi-species datasets, and their use to derive information on productivity and AGB at the species level is prone to uncertainties. The single-species H-D allometric equations available are mainly focused on monocultures or stands with simple tree species mixtures and did not account for the site effects. Here we measured the height and diameter of 2,288 trees of the emergent tree species Pericopsis elata (Harms) Meeuwen in the Democratic Republic of the Congo (DRC) and in Cameroon. We first examined how accurate multispecies H-D allometric equations are in predicting the total height of P. elata. We then tested whether single-species H-D allometric equations vary between sites. We developed the first H-D allometric equation of P. elata and tested whether and how stand-level and environmental variables induce changes in H-D allometric relationship of P. elata at the regional level. We additionally evaluated whether tree-level variables are important at the local level where climate and stand development stage are expected to be less variable. We found that pantropical, regional and local H-D allometric equations significantly underestimate the total height of P. elata. The local multi-species H-D allometric equation developed for Yangambi showed the highest underestimation in all the studied sites. This result supports the need for an H-D allometric equation specific for P. elata. The species-level H-D allometric equation developed showed significant underestimations for trees from the disturbed and undisturbed forests in DRC, while overestimations were observed for similar sites in Cameroon. Using a mixed-effect H-D allometric equation, we showed that even within a single species, a substantial variation exists between sites. This variation showed to be driven by the differences in the maximum asymptotic height (Hmax) between sites. We found that P. elata trees are taller and attain higher Hmax in DRC than in Cameroon. The basal area showed to be a significant covariate accounting for the site effects at the regional-scale where climate variables showed minor effects. However, at the local-scale, none of climate or stand variables showed to be significant. Local-scale variation showed to be associated with differences in light availability, highlighting the potential of management options that shape the local environment in driving species productivity.
This article reports on new archaeological data obtained in 2018 from the coastal region of the Kongo Central province of the Democratic Republic of Congo (DRC). The area’s ancient history is ...virtually unknown but is potentially of paramount importance in the context of the development of early village communities in Central Africa. The article focuses on the Muanda 6 site, dated to ⁓ 1400 BP, and offers a multidisciplinary analysis of the finds uncovered there. The site’s pottery is associated with shell and stone beads, ironworking remains, a stone quern, and biological remains testifying to a mixed subsistence system that took advantage of the region’s ecological diversity. The Early Iron Age occupants of Muanda 6 practiced ocean and mangrove fishing, gathered beach and mangrove gastropods and bivalves, hunted, and exploited oil palms. The article also discusses the Muanda 13 site, which is more recent (⁓ 1100 BP), and the results of the profile sampling at the Katala village. Both Muanda 6 and Muanda 13 yielded different ceramic types. Other pottery styles, surface collected between the Congo River and the Angolan province of Cabinda and probably of more recent date, are briefly described. The results provide new perspectives about the Iron Age in the DRC, providing further evidence of the cultural diversity in the Lower Congo region and an outline of the cultural sequence along the Atlantic Ocean coast.
Résumé
Cet article rend compte de nouvelles données archéologiques obtenues en 2018 dans la région côtière de la Province du Kongo-Central en République démocratique du Congo. L’histoire ancienne de cette région est pratiquement inconnue, mais elle est potentiellement d'une importance capitale dans le contexte de l'installation des premiers villages en Afrique centrale. Les travaux se concentrent sur le site de Muanda 6, daté vers 1400 BP, dont les trouvailles sont étudiées dans une perspective multidisciplinaire. La poterie de ce site est associée à des perles de coquillages et de pierre, à des traces de travail du fer, et à une meule en pierre ainsi qu'à des restes biologiques témoignant d'un système de subsistance mixte reposant sur divers écotones. Les occupants de l’Age du Fer Ancien de Muanda 6 pratiquaient la pêche en mer et dans la mangrove, la collecte de gastéropodes et de bivalves sur la plage et dans la mangrove, la chasse et l'exploitation des palmiers à huile. Le site plus récent de Muanda 13 (vers 1100 BP), ainsi qu’une collecte stratigraphique dans la berge du village de Katala, ont permis de découvrir un autre type de céramique. L’article décrit un dernier style de poterie probablement plus récent et découvert en surface entre le fleuve Congo et la frontière de la province angolaise de Cabinda. Les résultats présentés ici offrent de nouvelles perspectives à l’Age du Fer sur la diversité culturelle dans la région du Bas-Congo et propose l’esquisse d’une séquence culturelle au long de la côte de l’Océan Atlantique.
Most Central African rainforests are characterized by a remarkable abundance of light‐demanding canopy species: long‐lived pioneers (LLP) and non‐pioneer light demanders (NPLD). A popular explanation ...is that these forests are still recovering from intense slash‐and‐burn farming activities, which abruptly ended in the 19th century. This “human disturbance” hypothesis has never been tested against spatial distribution patterns of these light demanders. Here, we focus on the 28 most abundant LLP and NPLD from 250 one‐ha plots distributed along eight parallel transects (~50 km) in the Yangambi forest. Four species of short‐lived pioneers (SLP) and a single abundant shade‐tolerant species (Gilbertiodendron dewevrei) were used as reference because they are known to be strongly aggregated in recently disturbed patches (SLP) or along watercourses (G. dewevrei). Results show that SLP species are strongly aggregated with clear spatial autocorrelation of their diameter. This confirms that they colonized the patch following a one‐time disturbance event. In contrast, LLP and NPLD species have random or weakly aggregated distribution, mostly without spatial autocorrelation of their diameter. This does not unambiguously confirm the “human disturbance” hypothesis. Alternatively, their abundance might be explained by their deciduousness, which gave them a competitive advantage during long‐term drying of the late Holocene. Additionally, a canonical correspondence analysis showed that the observed LLP and NPLD distributions are not explained by environmental variables, strongly contrasting with the results for the reference species G. dewevrei, which is clearly aggregated along watercourses. We conclude that the abundance of LLP and NPLD species in Yangambi cannot be unambiguously attributed to past human disturbances or environmental variables. An alternative explanation is that present‐day forest composition is a result of adaptation to late‐Holocene drying. However, results are inconclusive and additional data are needed to confirm this alternative hypothesis.
The objective of this paper was to contribute to the elucidation of the light‐demanding species persistence puzzle in the Congo Basin rainforest canopy. Three hypotheses have been tested, which suggest: (H1) legacy of past disturbances, (H2) legacy of adaptation to a drier climate, and (H3) consequence of heterogeneity of the environment. Our results clearly support H1 for the species of short‐lived pioneer guild and do not unambiguously support a single hypothesis for the long‐lived pioneer and non‐pioneer light‐demanding guilds, although they seem to favor H2.
Wood density profiles reveal a tree’s life strategy and growth. Density profiles are, however, rarely defined in terms of tissue fractions for wood of tropical angiosperm trees. Here, we aim at ...linking these fractions to corresponding density profiles of tropical trees from the Congo Basin. Cores of 8 tree species were scanned with X-ray Computed Tomography to calculate density profiles. Then, cores were sanded and the outermost 3 cm were used to semi-automatically measure vessel lumen, parenchyma and fibre fractions using the Weka segmentation tool in ImageJ. Fibre wall and lumen widths were measured using a newly developed semi-automated method. An assessment of density variation in function of growth ring boundary detection is done. A mixed regression model estimated the relative contribution of each trait to the density, with a species effect on slope and intercept of the regression. Position-dependent correlations were made between the fractions and the corresponding wood density profile. On average, density profile variation mostly reflects variations in fibre lumen and wall fractions, but these are species- and position-dependent: on some positions, parenchyma and vessels have a more pronounced effect on density. The model linking density to traits explains 92% of the variation, with 65% of the density profile variation attributed to the three measured traits. The remaining 27% is explained by species as a random effect. There is a clear variation between trees and within trees that have implications for interpreting density profiles in angiosperm trees: the exact driving anatomical fraction behind every density value will depend on the position within the core. The underlying function of density will thus vary accordingly.
In the tropics, more precisely in equatorial dense rainforest, xylogenesis is driven by a little distinct climatological seasonality, and many tropical trees do not show clear growth rings. This ...makes retrospective analyses and modeling of future tree performance difficult. This research investigates the presence, the distinctness, and the periodicity of growth ring for dominant tree species in two semi‐deciduous rainforests, which contrast in terms of precipitation dynamics. Eighteen tree species common to both forests were investigated. We used the cambial marking technique and then verified the presence and periodicity of growth‐ring boundaries in the wood produced between pinning and collection by microscopic and macroscopic observation. The study showed that all eighteen species can form visible growth rings in both sites. However, the periodicity of ring formation varied significantly within and between species, and within sites. Trees from the site with clearly defined dry season had a higher likelihood to form periodical growth rings compared to those from the site where rainfall seasonality is less pronounced. The distinctness of the formed rings however did not show a site dependency. Periodical growth‐ring formation was more likely in fast‐growing trees. Furthermore, improvements can be made by a detailed study of the cambial activity through microcores taken at high temporal resolution, to get insight on the phenology of the lateral meristem.
In the tropical Congo Basin, our study revealed varying tree‐ring formation among species and sites. While all monitored trees exhibited growth rings, their periodicity differed significantly. Notably, the distinctness of the rings was consistent across sites, and faster‐growing trees exhibited a higher likelihood of periodic ring formation.
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