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Traissac, S., & Pascal, J. - P. (2014). Birth and life of tree aggregates in tropical forest: Hypotheses on population dynamics of an aggregated shade-tolerant species. J. Veg. Sci., 25(2), 491–502.
Abstract: Questions: Several studies have described aggregated spatial patterns in tropical tree species. This study investigates aggregate dynamics of Vouacapoua americana (Aublet), a climax species whose spatial pattern is not simply related to light and soil conditions or to its short seed dispersal range. Location: Two rain forest sites: Nouragues and Paracou, in the Guiana Shield. Methods: We described the spatial pattern of tree locations and spatial autocorrelation of tree diameters, using statistics derived from Ripley's K. We particularly used methods to define analysis subplots according to local density or local mean diameter. We investigated relationships between spatial distributions of adults and saplings. Results: At both sites, populations of Vouacapoua demonstrated several nested levels of aggregation. Tree diameters were spatially autocorrelated, revealing the existence of clusters with similar diameters. In the largest aggregates, tree diameters declined from the centre to the edge. Regeneration was aggregated and occurred mainly at cluster edges and around rare isolated trees, and sapling densities and basal area of adults were negatively correlated. We show that long-distance dispersal events are rare. Conclusions: Environmental factors and seed dispersal only explain part of the observed spatial patterns. We provide two main hypotheses about Vouacapoua population dynamics. First, the lack of regeneration in aggregate centres results in the ageing of existing aggregates. We suggest that this lack of recruitment close to mature trees is due to a Janzen-Connell effect. However, aggregates can continue to grow along colonization fronts. Second, long-distance dispersal events allow the formation of new clusters and play a crucial role in the colonization process. We investigate aggregate dynamics of Vouacapoua americana (Aublet) whose spatial pattern is not simply related to environmental conditions or to its seed dispersal. Regeneration does not occur in centers of aggregate of adults. We suggest that rare long-distance dispersal events and density-dependence predation of seeds and seedlings play a crucial role in formation of new clusters and structuration of larger aggregates. © 2013 International Association for Vegetation Science.
Keywords: Clusters; Colonization strategy; Janzen-Connell; Spatial analysis; Spatial pattern; Vouacapoua americana
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Malé, P. - J. G., Ferdy, J. - B., Leroy, C., Roux, O., Lauth, J., Avilez, A., et al. (2014). Retaliation in Response to Castration Promotes a Low Level of Virulence in an Ant-Plant Mutualism. Evol. Biol., 41(1), 22–28.
Abstract: The diversion of a host's energy by a symbiont for its own benefit is a major source of instability in horizontally-transmitted mutualisms. This instability can be counter-balanced by the host's retaliation against exploiters. Such responses are crucial to the maintenance of the relationship. We focus on this issue in an obligate ant-plant mutualism in which the ants are known to partially castrate their host plant. We studied plant responses to various levels of castration in terms of (1) global vegetative investment and (2) investment in myrmecophytic traits. Castration led to a higher plant growth rate, signalling a novel case of gigantism induced by parasitic castration. On the other hand, completely castrated plants produced smaller nesting and food resources (i.e. leaf pouches and extra floral nectaries). Since the number of worker larvae is correlated to the volume of the leaf pouches, such a decrease in the investment in myrmecophytic traits demonstrates for the first time the existence of inducible retaliation mechanisms against too virulent castrating ants. Over time, this mechanism promotes an intermediate level of castration and enhances the stability of the mutualistic relationship by providing the ants with more living space while allowing the plant to reproduce. © 2013 Springer Science+Business Media New York.
Keywords: Allomerus decemarticulatus; Cheater; Evolutionary conflict; Hirtella physophora; Mutualism breakdown; Overexploitation
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Rowland, L., Hill, T. C., Stahl, C., Siebicke, L., Burban, B., Zaragoza-Castells, J., et al. (2014). Evidence for strong seasonality in the carbon storage and carbon use efficiency of an Amazonian forest. Global Change Biol., 20(3), 979–991.
Abstract: The relative contribution of gross primary production and ecosystem respiration to seasonal changes in the net carbon flux of tropical forests remains poorly quantified by both modelling and field studies. We use data assimilation to combine nine ecological time series from an eastern Amazonian forest, with mass balance constraints from an ecosystem carbon cycle model. The resulting analysis quantifies, with uncertainty estimates, the seasonal changes in the net carbon flux of a tropical rainforest which experiences a pronounced dry season. We show that the carbon accumulation in this forest was four times greater in the dry season than in the wet season and that this was accompanied by a 5% increase in the carbon use efficiency. This seasonal response was caused by a dry season increase in gross primary productivity, in response to radiation and a similar magnitude decrease in heterotrophic respiration, in response to drying soils. The analysis also predicts increased carbon allocation to leaves and wood in the wet season, and greater allocation to fine roots in the dry season. This study demonstrates implementation of seasonal variations in parameters better enables models to simulate observed patterns in data. In particular, we highlight the necessity to simulate the seasonal patterns of heterotrophic respiration to accurately simulate the net carbon flux seasonal tropical forest. © 2013 The Authors Global Change Biology Published by John Wiley & Sons Ltd.
Keywords: Carbon use efficiency; Dalec; Data assimilation; Ecosystem respiration; French Guiana; Seasonal carbon fluxes; Tropical forest
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Lamarre, G. P. A., Mendoza, I., Fine, P. V. A., & Baraloto, C. (2014). Leaf synchrony and insect herbivory among tropical tree habitat specialists. Plant Ecol., 215(2), 209–220.
Abstract: Growth defense tradeoff theory predicts that plants in low-resource habitats invest more energy in defense mechanisms against natural enemies than growth, whereas plants in high-resource habitats can afford higher leaf loss rates. A less-studied defense against herbivores involves the synchrony of leaf production, which can be an effective defense strategy if leaf biomass production exceeds the capacity of consumption by insects. The aim of this study was to determine whether leaf synchrony varied across habitats with different available resources and whether insects were able to track young leaf production among tree habitat specialists in a tropical forest of French Guiana. We predicted that high-resource habitats would exhibit more synchrony in leaf production due to the low cost and investment to replace leaf tissue. We also expected closer patterns of leaf synchrony and herbivory within related species, assuming that they shared herbivores. We simultaneously monitored leaf production and herbivory rates of five pairs of tree species, each composed of a specialist of terra firme or white-sand forests within the same lineage. Our prediction was not supported by the strong interaction of habitat and lineage for leaf synchrony within individuals of the same species; although habitat specialists differed in leaf synchrony within four of five lineages, the direction of the effect was variable. All species showed short time lags for the correlation between leaf production and herbivory, suggesting that insects are tightly tracking leaf production, especially for the most synchronous species. Leaf synchrony may provide an important escape defense against herbivores, and its expression appears to be constrained by both evolutionary history and environmental factors. © 2014 Springer Science+Business Media Dordrecht.
Keywords: Escape; French Guiana; Herbivorous insects; Phenology; Resource availability; Time lag
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Rockwell, C. A., Kainer, K. A., d'Oliveira, M. V. N., Staudhammer, C. L., & Baraloto, C. (2014). Logging in bamboo-dominated forests in southwestern Amazonia: Caveats and opportunities for smallholder forest management. For. Ecol. Manage., 315, 202–210.
Abstract: Guadua sarcocarpa and Guadua weberbaueri (Poaceae: Bambuseae) have a negative influence on tree regeneration and recruitment in bamboo-dominated forests of southwestern Amazonia. The lack of advanced regeneration and sparse canopy in this forest type present a considerable challenge for developing sustainable timber management plans. We conducted field studies in the Porto Dias Agroextractive Settlement Project in Acre, Brazil to assess influences of logging in bamboo-dominated forest sites. Taxonomic composition, stand structure, aboveground biomass, commercial timber volume, and commercial tree seedling and bamboo culm density were compared between five logged vs. unlogged sites in different landholdings, using modified 0.5. ha Gentry plots. No differences in taxonomic composition, aboveground biomass, adult and juvenile stem density, or woody seedling and bamboo culm density were detected between paired logged and unlogged sites. Commercial timber volume, however, was reduced by almost two-thirds in logged plots, suggesting that long-term timber management goals in this forest type are compromised since so few future crop trees remained onsite. Our findings indicate that in order to maximize local management objectives, community forest managers must approach logging in bamboo-dominated forests with caution. We suggest an integration of non-timber forest product extraction with low harvest intensity and low-impact logging, tending of natural regeneration, and diversification of commercial species. © 2014 Elsevier B.V.
Keywords: Bamboo; Community forest management; Guadua; Logging; Timber management; Tropical forest
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Woolfit, M., Iturbe-Ormaetxe, I., Brownlie, J. C., Walker, T., Riegler, M., Seleznev, A., et al. (2013). Genomic evolution of the pathogenic Wolbachia strain, wMelPop. Genome Biolog. Evol., 5(11), 2189–2204.
Abstract: Most strains of the widespread endosymbiotic bacterium Wolbachia pipientis are benign or behave as reproductive parasites. The pathogenic strain wMelPop is a striking exception, however: it overreplicates in its insect hosts and causes severe life shortening. The mechanism of this pathogenesis is currently unknown. We have sequenced the genomes of three variants of wMelPop and of the closely related nonpathogenic strain wMelCS. We show that the genomes of wMelCS and wMelPop appear to be identical in the nonrepeat regions of the genome and differ detectably only by the triplication of a 19-kb region that is unlikely to be associated with life shortening, demonstrating that dramatic differences in the host phenotype caused by this endosymbiont may be the result of only minor genetic changes. We also compare the genomes of the original wMelPop strain from Drosophila melanogaster and two sequentialderivatives, wMelPop-CLA and wMelPop-PGYP. To develop wMelPop as a novel biocontrol agent, it was first transinfected into and passaged in mosquito cell lines for approximately 3.5 years, generating wMelPop-CLA. This cell line-passaged strain was then transinfected into Aedesaegypti mosquitoes, creating wMelPop-PGYP,which wassequenced after 4yearsin the insecthost. We observe a rapid burst of genomic changes during cell line passaging, but no further mutations were detected after transinfection into mosquitoes, indicating either that host preadaptation had occurred in cell lines, that cell lines are a more selectively permissive environment than animal hosts, or both. Our results provide valuable data on the rates of genomic and phenotypic change in Wolbachia associated with host shifts over short time scales. © The Author(s) 2013. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution.
Keywords: Endosymbiont; Evolution; Genomics; Wolbachia
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Houel, E., Rodrigues, A. M. S., Jahn-Oyac, A., Bessière, J. - M., Eparvier, V., Deharo, E., et al. (2014). In vitro antidermatophytic activity of Otacanthus azureus (Linden) Ronse essential oil alone and in combination with azoles. J. Appl. Microbiol., 116(2), 288–294.
Abstract: Aims: We determined the chemical composition and investigated the antifungal activity of Otacanthus azureus (Linden) Ronse essential oil (EO) against a range of dermatophytes alone or in combination with azole antifungals. Methods and Results: Aerial parts of the plant were steam-distilled and the obtained oil was analysed by gas chromatography/mass spectrometry and 1H-NMR. It was shown to be largely composed of sesquiterpenes, with the main component being β-copaen-4-α-ol. Using broth microdilution techniques, this oil was found to have remarkable in vitro antifungal activities. Minimum inhibitory concentrations as low as 4 μg ml-1 were recorded. The analysis of the combined effect of the O. azureus EO with azoles using chequerboard assays revealed a synergism between the EO and ketoconazole, fluconazole or itraconazole against Trichophyton mentagrophytes. Notably, the O. azureus essential oil showed low cytotoxicity to VERO cells. Conclusions: The O. azureus essential oil alone or in combination with azoles is a promising antifungal agent in the treatment for human dermatomycoses caused by filamentous fungi. Significance and Impact of the Study: There is much interest in the study of essential oils for the discovery of new antimicrobial drugs. This study has highlighted the antidermatophytic activity of the O. azureus EO. © 2013 The Society for Applied Microbiology.
Keywords: Antifungal activity; Azoles; Dermatophytes; Essential oil; Otacanthus azureus; Synergy
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Lalague, H., Csilléry, K., Oddou-Muratorio, S., Safrana, J., de Quattro, C., Fady, B., et al. (2014). Nucleotide diversity and linkage disequilibrium at 58 stress response and phenology candidate genes in a European beech (Fagus sylvatica L.) population from southeastern France. Tree Genetics and Genomes, 10(1), 15–26.
Abstract: European beech (Fagus sylvatica L.) is one of the most economically and ecologically important deciduous trees in Europe, yet little is known about its genomic diversity and its adaptive potential. Here, we detail the discovery and analysis of 573 single nucleotide polymorphisms (SNPs) from 58 candidate gene fragments that are potentially involved in abiotic stress response and budburst phenology using a panel of 96 individuals from southeastern France. The mean nucleotide diversity was low (θ π = 2.2 × 10-3) but extremely variable among gene fragments (range from 0.02 to 10), with genes carrying insertion/deletion mutations exhibiting significantly higher diversity. The decay of linkage disequilibrium (LD) measured at gene fragments >800 base pairs was moderate (the half distance of r 2 was 154 bp), consistent with the low average population-scaled recombination rate (ρ = 5.4 × 10-3). Overall, the population-scaled recombination rate estimated in F. sylvatica was lower than for other angiosperm tree genera (such as Quercus or Populus) and similar to conifers. As a methodological perspective, we explored the effect of minimum allele frequency (MAF) on LD and showed that higher MAF resulted in slower decay of LD. It is thus essential that the same MAF is used when comparing the decay of LD among different studies and species. Our results suggest that genome-wide association mapping can be a potentially efficient approach in F. sylvatica, which has a relatively small genome size. © 2013 Springer-Verlag Berlin Heidelberg.
Keywords: Climate adaptation; Effective population size; Forest tree; Genomic diversity; Minor allele frequency (MAF); Recombination rate; Single nucleotide polymorphism (SNP)
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Perrin, A. - S., Fujisaki, K., Petitjean, C., Sarrazin, M., Godet, M., Garric, B., et al. (2014). Conversion of forest to agriculture in Amazonia with the chop-and-mulch method: Does it improve the soil carbon stock? Agric. Ecosyst. Environ., 184, 101–114.
Abstract: Fire-free forest conversion with organic inputs as an alternative to slash-and-burn could improve agro-ecosystem sustainability. We assessed soil carbon mass changes in a sandy-clayey and well-drained soil in French Guiana after forest clearing by the chop-and-mulch method and crop establishment. At the experimental site of Combi, native forest was cut down in October 2008; woody biomass was chopped and incorporated into the top 20cm of soil. After about one year of legume and grass cover, three forms of land management were compared: grassland (Urochloa ruziziensis), maize/soybean crop rotation with disk tillage and in direct seeding without tillage. There were four replicates. We measured 14.16kgm-2 of carbon in 2mm-sieved soil down to 2m depth for the initial forest. Forest clearing did not induce significant soil compaction; neither did any specific agricultural practice. In converted soils, C stocks were measured in the 0-30cm layer after each crop for three years. Carbon mass changes for soil fractions <2mm (soil C stock) and >2mm (soil C pool) in the 0-5, 5-10, 10-20 and 20-30cm soil layers were assessed on an equivalent soil mass basis. One year and 1.5 years after deforestation, higher C stocks (+0.64 to 1.16kgCm-2yr-1) and C pools (+0.52 to 0.90kgCm-2yr-1) were measured in converted soils, compared to those of the forest into the top 30cm of soil. However, the masses of carbon in these converted soils declined later. The highest rates of carbon decrease were measured between 1.5 and 2 years after forest conversion in the <2mm soil fraction, from 0.46kgCm-2yr-1 (in grassland soils) to 0.71kgCm-2yr-1 (in cropland under no tillage). The carbon pool declined during the third year at rates of 0.41kgCm-2yr-1 (cropland under disk tillage) to 0.76kgCm-2yr-1 (grassland soils). Three years after forest conversion, C masses in the top 30cm of soils for grassland showed similar values than for forest. In comparison, the carbon stock in cropped soils managed under no tillage in direct seeding (without mulch) was significantly 17% and 16% lower than in forest and grassland soils, respectively. None of the studied agricultural practices succeeded in accumulating carbon from the chopped forest biomass. © 2013 Elsevier B.V.
Keywords: Annual crops; Brachiaria; Deforestation; Fire-free; French Guiana; No-tillage
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Fortunel, C., Paine, C. E. T., Fine, P. V. A., Kraft, N. J. B., & Baraloto, C. (2014). Environmental factors predict community functional composition in Amazonian forests. J. Ecol., 102(1), 145–155.
Abstract: The consequences of biodiversity loss for ecosystem services largely depend on the functional identities of extirpated species. However, poor descriptions of spatial patterns of community functional composition across landscapes hamper accurate predictions, particularly in highly diverse tropical regions. Therefore, understanding how community functional composition varies across environmental gradients remains an important challenge. We sampled 15 functional traits in 800 Neotropical tree species across 13 forest plots representative of the broad climatic and soil gradients encompassed by three widespread lowland forest habitats (terra firme forests on clay-rich soils, seasonally flooded forests and white-sand forests) at opposite ends of Amazonia (Peru and French Guiana). We combined univariate and multivariate approaches to test the magnitude and predictability of environmental filtering on community leaf and wood functional composition. Directional shifts in community functional composition correlated with environmental changes across the 13 plots, with denser leaves, stems and roots in forests occurring in environments with limited water and soil-nutrient availability. Critically, these relationships allowed us to accurately predict the functional composition of 61 additional forest plots from environmental data alone. Synthesis. Environmental filtering consistently shapes the functional composition of highly diverse tropical forests at large scales across the terra firme, seasonally flooded and white-sand forests of lowland Amazonia. Environmental factors drive and allow the prediction of variation in community functional composition among habitat types in Amazonian forests. © 2013 British Ecological Society.
Keywords: Amazonian landscape; Climatic and soil gradients; Determinants of plant community diversity and structure; Environmental filtering; Functional traits; Tree communities; Tropical forests
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