Bernard Riera

Estimation of biomass in a neotropical forest of French Guiana : spatial and temporal variability

Biomass content and turnover rate were estimated for a lowland wet rain forest in French Guiana. A regression model relating the biomass of a tree to its dbh (diameter at breast height) was deduced from previously published data. A power-law allometric relationship of the form AGTB = aD b was used to estimate the tree biomass, AGTB (Mg ha -1 ), from its dbh D (cm). Using direct measurements of tree biomass in the literature, the best-fit allometric exponent b = 2.42 (SD = 0.02) was found. The logarithm of the coefficient a was normally distributed with an aver- age of -2.00 (SD = 0.27). This method was applied to two permanent research sta- tions of the lowland tropical rain forest of French Guiana: the Nouragues and Piste de Saint-Elie. At the Nouragues, the biomass was estimated from trees 10 cm in dia- meter on two plots covering a total surface area of 22 ha and yielded an average bio- mass of 309 Mg ha -1 (± 32 Mg ha -1 , 95% confidence interval). Spatial variability was also addressed at the Nouragues by estimating the biomass of trees M 30 cm dbh over a total surface area of 82 ha. For the wet tropical forest vegetation type, an average of 284 Mg ha -1 was obtained (spatial variability ± 55 Mg ha -1 ). Biomass turnover was evaluated at Piste de Saint-Elie from two transects (0.78 and 1 ha) on which all trees M 5 cm in diameter were recorded and mapped twice in 10 y. Transect 1 showed a slight increase in biomass, from 245 to 260 Mg ha -1 (338 to 345 Mg ha -1 for transect 2), corresponding to a net increase of 1.9 Mg ha -1 y -1 (0.7 Mg ha -1 y -1 ), and the bio- mass ingrowth was 3.2 Mg ha -1 y -1 (2.8 Mg ha -1 y -1 ). These figures are discussed in the light of the natural recruitment dynamics of tropical forests.

Identification of Amazonian Trees with DNA Barcodes

Background Large-scale plant diversity inventories are critical to develop informed conservation strategies. However, the workload required for classic taxonomic surveys remains high and is particularly problematic for megadiverse tropical forests. Methodology/Principal Findings Based on a comprehensive census of all trees in two hectares of a tropical forest in French Guiana, we examined whether plant DNA barcoding could contribute to increasing the quality and the pace of tropical plant biodiversity surveys. Of the eight plant DNA markers we tested (rbcLa, rpoC1, rpoB, matK, ycf5, trnL, psbA-trnH, ITS), matK and ITS had a low rate of sequencing success. More critically, none of the plastid markers achieved a rate of correct plant identification greater than 70%, either alone or combined. The performance of all barcoding markers was noticeably low in few species-rich clades, such as the Laureae, and the Sapotaceae. A field test of the approach enabled us to detect 130 molecular operational taxonomic units in a sample of 252 juvenile trees. Including molecular markers increased the identification rate of juveniles from 72% (morphology alone) to 96% (morphology and molecular) of the individuals assigned to a known tree taxon. Conclusion/Significance We conclude that while DNA barcoding is an invaluable tool for detecting errors in identifications and for identifying plants at juvenile stages, its limited ability to identify collections will constrain the practical implementation of DNA-based tropical plant biodiversity programs.

Above-ground biomass and productivity in a rain forest of eastern South America

Abstract: The dynamics of tropical forest woody plants was studied at the Nouragues Field Station, central French Guiana. Stem density, basal area, above-ground biomass and above-ground net primary productivity, including the contribution of litterfall, were estimated from two large permanent census plots of 12 and 10 ha, established on contrasting soil types, and censused twice, first in 1992?1994, then again in 2000?2002. Mean stem density was 512 stems ha?1 and basal area, 30m2 ha?1. Stem mortality rate ranged between 1.51% and 2.06% y?1. In both plots, stem density decreased over the study period. Using a correlation between wood density and wood hardness directly measured by a Pilodyn wood tester,we found that the mean wood densitywas 0.63 g cm?3, 12% smaller than the mean of wood density estimated from the literature values for the species occurring in our plot. Above-ground biomass ranged from 356 to 398Mgha?1 (oven-dry mass), and it increased over the census period. Leaf biomass was 6.47Mg ha?1. Our total estimate of aboveground net primary productivity was 8.81 MgC ha?1 y?1 (in carbon units), not accounting for loss to herbivory, branchfalls, or biogenic volatile organic compounds, whichmay altogether account for an additional 1MgC ha?1 y?1. Coarse wood productivity (stem growth plus recruitment) contributed to 4.16 MgC ha?1 y?1. Litterfall contributed to 4.65MgC ha?1 y?1 with 3.16 MgC ha?1 y?1 due to leaves, 1.10 MgC ha?1 y?1 to twigs, and 0.39MgC ha?1 y?1 to fruits and flowers. The increase in above-ground biomass for both trees and lianas is consistentwith the hypothesis of a shift in the functioning of Amazonian rain forests driven by environmental changes, although alternative hypotheses such as a recovery from past disturbances cannot be ruled out at our site, as suggested by the observed decrease in stem density. Key Words: above-ground biomass, carbon, French Guiana, net primary productivity, tropical forest

Simulating the Long-term Response of Tropical Wet Forests to Fragmentation

AbstractIn the coming decades, a large fraction of the worlds tropical forests will be fragmented into remnants surrounded by secondary vegetation, land-use areas, or roads. It is important to develop integrative tools to monitor the evolution of these fragmented ecosystems. We used the individual-oriented and process-based forest growth simulator FORMIND2.0 to investigate the spatial and temporal effects on standing biomass and functional diversity of various intensities and patterns of fragmentation within a forest landscape. The simulator was calibrated for an old-growth wet forest in French Guiana, South America. We found that the standing biomass of forest remnants was reduced significantly compared to a similar area of nonfragmented forest. When fewer but larger remnants were created rather than many small ones, the total loss in biomass and the increase in the abundance of early-successional species were significantly reduced, confirming that edge effects dominate the functioning of forest remnants. We also performed simulations of secondary succession after the landscape had been abandoned. The simulated recovery time in those secondary forests depends on both the size of cleared area and the spatial pattern of the remnant forests.

Agreement between floristic and soil organic carbon isotope (13C/12C,14C) indicators of forest invasion of savannas during the last century in Cameroon

In an area of savanna-forest (S-F) mosaic of Cameroon, at Kandara near Bertoua, an enclosed savanna bordered by young semi-deciduous forests was selected for detailed studies of vegetation and soil carbon isotope compositions with a view to estimating the rate of forest advance into savannas. Forest floristic composition and structure were analysed in small plots along two S-F transects and within two large stands. Tree species counts and basal area (BA) measurements gave convergent results defining (1) an edge forest with low BA values that forms an irregular strip parallel to the S-F border, (2) a large colonization zone zone composed of pioneer species ( Albizia species, with individuals of very large diameter) and (3) a mature forest composed of abundant Rinorea individuals and large individuals of Triplochiton scleroxylon and Piptadeniastrum africanum . Carbon stable isotopes were deter mined from organic matter of soil profiles sampled at various depths in savanna, colonization-zone and mature forest. In the deep soil horizons (40-50 cm) of the colonization-zone profiles, δ 13 C values similar to those of the present savanna reveal the past existence of a large-tree savanna. In subsurface horizons (15-20 cm), δ 13 C values intermediate between those of savanna and mature forest prove the encroachment of the forest ecosystem on savanna. Using 14 C measurements, the mean residence time (MRT) of soil organic matter of these last horizons was determined with precision owing to the atmospheric 14 C pulse from nuclear bomb tests prior to 1964. By assuming an exponential age distribution of organic compounds and by taking account of MRT and remaining carbon from the savanna, the coloniza tion zone was found to be 60-80 y old. The age of the colonization zone being the same near the present savanna and near the mature forest, it seems that the forest advance was probably not a linear process but would result from the coalescence of Albizia thickets born in savanna.

Pollen-rain-vegetation relationships along a forest-savanna transect in southeastern Cameroon.

Modern soil and litter samples from southeastern Cameroon, collected along a continuous forest-savanna transect were analysed for pollen content to define modern pollen-vegetation relationships. The pollen results, completed and compared with botanical inventories, leaf area index and basal area measurements performed in the same area, clearly registered the physiognomy, the main floristic composition and floral richness of the two sampled ecosystems. Distortions were observed between sampled vegetations and their pollen rain, related to important differences in pollen production and dispersal of plant species: this is a general feature in many tropical regions. The pollen data in the area studied reflected well the recent transgression of forest versus savanna. This permitted us to define inside the forest ecosystem more successional vegetation communities than the botanical surveys allowed.

Forest perturbations and biodiversity during the last ten thousand years in French Guiana

Tropical forests can be described as a mosaic of juxtaposed eco-units corresponding to different stages of regeneration after treefals. However, these small-scale regeneration mechanisms alone cannot account for the different patterns of species distribution, plant communities and population structures found in this habitat. The presence of charcoal layers in the soil and the study of sediments along streams suggest that large-scale forest fires deeply affected the tropical forest vegetation, even in high rainfall areas such as French Guiana. Many atypical plant distribution and population structure patterns, in relation to what would be expected from present-time regeneration processes, can be explained by these large-scale events which happened during the last few thousand years.

Fast determination of light availability and leaf area index in tropical forests

An important property of plant communities is the Leaf Area Index (LAI),which is the vertically integrated surface of leaves per unit of ground area.Leavesaretheprimarysitesofphotosynthesisandtranspiration,thustheLAI,which conditions the light interception by the canopy, is directly related tocarbon and water exchange with the atmosphere at the stand scale(McNaughton&Jarvis1983).LAIalsohasanimpactontreegrowththroughtheinterceptionoflight.Lightavailabilitybelowcanopiesistheprincipallimit-ing factor of tree recruitment and growth in forests (Denslow

The use of diachronic spatial approaches and predictive modelling to study the vegetation dynamics of a managed heathland

According to the EU Habitats Directive, heathlands are a semi-natural habitat type of community interest. This status aims at conserving these habitats, especially where and when they are threatened by various changes, including natural vegetation succession. We present results of a study of the dynamics of a typical dry heathland plot located in the Fontainebleau massif (France). An exhaustive observation of vegetation changes were made on this area of four hectares between 2000 and 2008, employing a spatial approach. We recorded the expansion of Molinia caerulea (L.) Moench at the expense of Ericaceae. The potential future vegetation of the site was modelled using Markov chains coupled to a GIS programme. This model predicted a gradual change in the floristic composition of heathland in favour of M. caerulea at the expense of Calluna vulgaris (L.) Hull and Erica tetralix L., and the expansion of Pinus sylvestris L. The study demonstrates how spatial methods can contribute to the design of reliable management methods of habitats such as the heathlands.

Influence of the spatial variability of soil type and tree colonization on the dynamics of Molinia caerulea (L.) Moench in managed heathland

European heathland communities on acid, nutrient-poor soils have a high ecological value due to their special environmental conditions. Natural succession (tree colonization and the emergence of grasses) poses a threat to this type of habitat and different types of management strategy must be considered if it is to be maintained. A previous study on a dry heathland area located in the Fontainebleau forest (France) showed a gradual shift from a pure ericaceous stand to a mosaic of grasses and Ericaceae, despite the application of measures such as removal of woodland species to sustain the habitat. Habitat change was due to local expansion of a grass, Molinia caerulea (L.) Moench. The present paper aimed to identify factors responsible for the expansion of M. caerulea and the subsequent decrease in ericaceous heath. We focused our study on spatial variability of soil properties (soil horizons, pH, water content) and reforestation (density of birch individuals and proximity to woodland) as a suite of possible factors promoting the expansion of M. caerulea. We show that the development of grasses was correlated with thin soil E horizon and spatial distribution of old shoots of birch, Betula pendula Roth, which are regularly cut and then resprout. These results suggest that new methods to avoid tree colonization must be introduced if typical heathland is to be maintained.