Assessing Evidence for a Pervasive Alteration in Tropical Tree Communities

In Amazonian tropical forests, recent studies have reported increases in aboveground biomass and in primary productivity, as well as shifts in plant species composition favouring fast-growing species over slow-growing ones. This pervasive alteration of mature tropical forests was attributed to global environmental change, such as an increase in atmospheric CO2 concentration, nutrient deposition, temperature, drought frequency, and/or irradiance. We used standardized, repeated measurements of over 2 million trees in ten large (16–52 ha each) forest plots on three continents to evaluate the generality of these findings across tropical forests. Aboveground biomass increased at seven of our ten plots, significantly so at four plots, and showed a large decrease at a single plot. Carbon accumulation pooled across sites was significant (+0.24 MgC ha−1 y−1, 95% confidence intervals [0.07, 0.39] MgC ha−1 y−1), but lower than reported previously for Amazonia. At three sites for which we had data for multiple census intervals, we found no concerted increase in biomass gain, in conflict with the increased productivity hypothesis. Over all ten plots, the fastest-growing quartile of species gained biomass (+0.33 [0.09, 0.55] % y−1) compared with the tree community as a whole (+0.15 % y−1); however, this significant trend was due to a single plot. Biomass of slow-growing species increased significantly when calculated over all plots (+0.21 [0.02, 0.37] % y−1), and in half of our plots when calculated individually. Our results do not support the hypothesis that fast-growing species are consistently increasing in dominance in tropical tree communities. Instead, they suggest that our plots may be simultaneously recovering from past disturbances and affected by changes in resource availability. More long-term studies are necessary to clarify the contribution of global change to the functioning of tropical forests

Structures spatiales de la richesse spécifique dans quelques blocs forestiers du nord-est du Bassin congolais: Implication pour la diversité régionale et la conservation

The assessment of species diversity remains an indispensable tool in the development of biodiversity conservation strategies and ecosystem management. In tropical rainforest, richness and species diversity are almost always high but they vary spatially. Several studies have been conducted at the local level to assess the richness and floristic composition of the forests of the Congolese central basin. With a hundred of inventory plots we performed in four forest sites in northeastern Congo Basin, we have shown that tree stands have significant variations in species diversity and species composition. The northern sites in Ituri and Rubi-Tele are less diverse than those of Yoko and Uma around Kisangani.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Plantations monospécifiques d’espèces exotiques, déserts biologiques ou îlots de diversité floristique ?Exemple de situations biotiques et abiotiques contrastées au Nord-Kivu en République démocratique du Congo

While forest plantations are a topic attracting growing interest in Central Africa, their undergrowth has been little studied. In order to assess the potential contribution of plantations of exotic species to the restoration of plant cover on anthropised land in Kivu (Democratic Republic of the Congo), the plant diversity of their understorey was characterised and compared in a selection of single-species plantations of Eucalyptus saligna and Grevillea robusta on two sites (Sake and Kirumba) with different altitudinal and pedoclimatic conditions. The variations in plant composition found by the surveys was then analysed according to certain ecological parameters. Finally, indicator species of the altitudinal and pedoclimatic conditions at each site were identified. The data collected in 2018 and 2020 in 12 plantations, using the minimum area method, showed a high degree of heterogeneity in the plant composition of the understorey. Although herbaceous species predominated, some woody and liana species were also found in these understoreys. The species richness did not differ significantly between sites or between the tree species planted. Species richness varied from one survey to another depending on latitude, soil pH and bioavailable aluminium and magnesium concentrations. Thirteen indicator species for the altitudinal and pedoclimatic conditions of the sites were identified altogether, nine for Sake and four for Kirumba. Repetition of the surveys increased the sampling effort even though the minimum area did not change significantly over two years. This study highlighted the contribution of exotic species plantations to the recovery of plant cover on potentially degraded sites in Kivu and suggested management measures suited to the physico-chemical properties of the soils. The use of indicator species of pedoclimatic conditions as predictors of site productivity deserves to be tested in the plantations studied, in order to verify whether their presence could be used as a tool for selecting sites for reforestation.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Local spatial structure of forest biomass and its consequences for remote sensing of carbon stocks

Advances in forest carbon mapping have the potential to greatly reduce uncertainties in the global carbon budget and to facilitate effective emissions mitigation strategies such as REDD+ (Reducing Emissions from Deforestation and Forest Degradation). Though broad-scale mapping is based primarily on remote sensing data, the accuracy of resulting forest carbon stock estimates depends critically on the quality of field measurements and calibration procedures. The mismatch in spatial scales between field inventory plots and larger pixels of current and planned remote sensing products for forest biomass mapping is of particular concern, as it has the potential to introduce errors, especially if forest biomass shows strong local spatial variation. Here, we used 30 large (8-50 ha) globally distributed permanent forest plots to quantify the spatial variability in aboveground biomass density (AGBD in Mg ha-1) at spatial scales ranging from 5 to 250 m (0.025-6.25 ha), and to evaluate the implications of this variability for calibrating remote sensing products using simulated remote sensing footprints. We found that local spatial variability in AGBD is large for standard plot sizes, averaging 46.3% for replicate 0.1 ha subplots within a single large plot, and 16.6% for 1 ha subplots. AGBD showed weak spatial autocorrelation at distances of 20-400 m, with autocorrelation higher in sites with higher topographic variability and statistically significant in half of the sites. We further show that when field calibration plots are smaller than the remote sensing pixels, the high local spatial variability in AGBD leads to a substantial "dilution" bias in calibration parameters, a bias that cannot be removed with standard statistical methods. Our results suggest that topography should be explicitly accounted for in future sampling strategies and that much care must be taken in designing calibration schemes if remote sensing of forest carbon is to achieve its promise. © Author(s) 2014