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Dejean, A., Azémar, F., Libert, M., Compin, A., Herault, B., Orivel, J., et al. (2017). Ant-lepidopteran associations along African forest edges. The Science of Nature, 104(1), 7.
Abstract: Working along forest edges, we aimed to determine how some caterpillars can co-exist with territorially dominant arboreal ants (TDAAs) in tropical Africa. We recorded caterpillars from 22 lepidopteran species living in the presence of five TDAA species. Among the defoliator and/or nectarivorous caterpillars that live on tree foliage, the Pyralidae and Nymphalidae use their silk to protect themselves from ant attacks. The Notodontidae and lycaenid Polyommatinae and Theclinae live in direct contact with ants; the Theclinae even reward ants with abundant secretions from their Newcomer gland. Lichen feeders (lycaenid; Poritiinae), protected by long bristles, also live among ants. Some lycaenid Miletinae caterpillars feed on ant-attended membracids, including in the shelters where the ants attend them; Lachnocnema caterpillars use their forelegs to obtain trophallaxis from their host ants. Caterpillars from other species live inside weaver ant nests. Those of the genus Euliphyra (Miletinae) feed on ant prey and brood and can obtain trophallaxis, while those from an Eberidae species only prey on host ant eggs. Eublemma albifascia (Erebidae) caterpillars use their thoracic legs to obtain trophallaxis and trophic eggs from ants. Through transfer bioassays of last instars, we noted that herbivorous caterpillars living in contact with ants were always accepted by alien conspecific ants; this is likely due to an intrinsic appeasing odor. Yet, caterpillars living in ant shelters or ant nests probably acquire cues from their host colonies because they were considered aliens and killed. We conclude that co-evolution with ants occurred similarly in the Heterocera and Rhopalocera.
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Ghislain, B., Nicolini, E. - A., Romain, R., Ruelle, J., Yoshinaga, A., Alford, M. H., et al. (2016). Multilayered structure of tension wood cell walls in Salicaceae sensu lato and its taxonomic significance. Bot. J. Linn. Soc., 182(4), 744–756.
Abstract: Salicaceae have been enlarged to include a majority of the species formerly placed in the polyphyletic tropical Flacourtiaceae. Several studies have reported a peculiar and infrequently formed multilayered structure of tension wood in four of the tropical genera. Tension wood is a tissue produced by trees to restore their vertical orientation and most studies have focused on trees developing tension wood by means of cellulose-rich, gelatinous fibres, as in Populus and Salix (Salicaceae s.s.). This study aims to determine if the multilayered structure of tension wood is an anatomical characteristic common in other Salicaceae and, if so, how its distribution correlates to phylogenetic relationships. Therefore, we studied the tension wood of 14 genera of Salicaceae and two genera of Achariaceae, one genus of Goupiaceae and one genus of Lacistemataceae, families closely related to Salicaceae or formerly placed in Flacourtiaceae. Opposite wood and tension wood were compared with light microscopy and three-dimensional laser scanning confocal microscopy. The results indicate that a multilayered structure of tension wood is common in the family except in Salix, Populus and one of their closest relatives, Idesia polycarpa. We suggest that tension wood may be a useful anatomical character in understanding phylogenetic relationships in Salicaceae. Further investigation is still needed on the tension wood of several other putatively close relatives of Salix and Populus, in particular Bennettiodendron, Macrohasseltia and Itoa.
Keywords: Flacourtiaceae; G-layer; multilayered tension wood; reaction wood
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Alméras, T., & Clair, B. (2016). Critical review on the mechanisms of maturation stress generation in trees. J R Soc Interface, 13(122).
Abstract: Trees control their posture by generating asymmetric mechanical stress around the periphery of the trunk or branches. This stress is produced in wood during the maturation of the cell wall. When the need for reaction is high, it is accompanied by strong changes in cell organization and composition called reaction wood, namely compression wood in gymnosperms and tension wood in angiosperms. The process by which stress is generated in the cell wall during its formation is not yet known, and various hypothetical mechanisms have been proposed in the literature. Here we aim at discriminating between these models. First, we summarize current knowledge about reaction wood structure, state and behaviour relevant to the understanding of maturation stress generation. Then, the mechanisms proposed in the literature are listed and discussed in order to identify which can be rejected based on their inconsistency with current knowledge at the frontier between plant science and mechanical engineering.
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Hein, P. R. G., Chaix, G., Clair, B., Brancheriau, L., & Gril, J. (2016). Spatial variation of wood density, stiffness and microfibril angle along Eucalyptus trunks grown under contrasting growth conditions. Trees – Structure and Function, 30(3), 871–882.
Abstract: Sloped terrains tend to produce trees with higher radial variation in wood density while at high wind speeds, the wood tends to present higher stiffness and lower MFA in Eucalyptus.
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Stahl, C., Freycon, V., Fontaine, S., Dezecache, C., Ponchant, L., Picon-Cochard, C., et al. (2016). Soil carbon stocks after conversion of Amazonian tropical forest to grazed pasture: importance of deep soil layers. Regional Environmental Change, 16(7), 2059–2069.
Abstract: Recent studies suggest that carbon (C) is stored in the topsoil of pastures established after deforestation. However, little is known about the long-term capacity of tropical pastures to sequester C in different soil layers after deforestation. Deep soil layers are generally not taken into consideration or are underestimated when C storage is calculated. Here we show that in French Guiana, the C stored in the deep soil layers contributes significantly to C stocks down to a depth of 100 cm and that C is sequestered in recalcitrant soil organic matter in the soil below a depth of 20 cm. The contribution of the 50–100 cm soil layer increased from 22 to 31 % with the age of the pasture. We show that long-term C sequestration in C4 tropical pastures is linked to the development of C3 species (legumes and shrubs), which increase both inputs of N into the ecosystem and the C:N ratio of soil organic matter. The deep soil under old pastures contained more C3 carbon than the native forest. If C sequestration in the deep soil is taken into account, our results suggest that the soil C stock in pastures in Amazonia would be higher with sustainable pasture management, in particular by promoting the development of legumes already in place and by introducing new species.
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Morel-Journel, T., Piponiot, C., Vercken, E., & Mailleret, L. (2016). Evidence for an optimal level of connectivity for establishment and colonization. Biol Lett, 12(11), 20160704.
Abstract: Dispersal is usually associated with the spread of invasive species, but it also has two opposing effects, one decreasing and the other increasing the probability of establishment. Indeed, dispersal both slows population growth at the site of introduction and increases the likelihood of surrounding habitat being colonized. The connectivity of the introduction site is likely to affect dispersal, and, thus, establishment, according to the dispersal behaviour of individuals. Using individual-based models and microcosm experiments on minute wasps, we demonstrated the existence of a hump-shaped relationship between connectivity and establishment in situations in which individual dispersal resembled a diffusion process. These results suggest that there is an optimal level of connectivity for the establishment of introduced populations locally at the site of introduction, and regionally over the whole landscape.
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Rutishauser, E., Herault, B., Petronelli, P., & Sist, P. (2016). Tree Height Reduction After Selective Logging in a Tropical Forest. Biotropica, 48(3), 285–289.
Abstract: By harvesting scattered large trees, selective logging increases light availability and thereby stimulates growth and crown expansion at early-life stage among remnant trees. We assessed the effects of logging on total and merchantable bole (i.e., lowest branch at crown base) heights on 952 tropical canopy trees in French Guiana. We observed reductions in both total (mean, −2.3 m) and bole (mean, −2.0 m) heights more than a decade after selective logging. Depending on local logging intensity, height reductions resulted in 2–13 percent decreases in aboveground tree biomass and 3–17 percent decreases in bole volume. These results highlight the adverse effects of logging at both tree and stand levels. This decrease in height is a further threat to future provision of key environmental services, such as timber production and carbon sequestration.
Keywords: carbon sequestration; forest management; logging; tropical forests; wood production
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Goulamoussene, Y., Bedeau, C., Descroix, L., Deblauwe, V., Linguet, L., & Herault, B. (2016). Weak Environmental Controls of Tropical Forest Canopy Height in the Guiana Shield Remote Sensing. Remote Sens, 8(9), 747.
Abstract: Canopy height is a key variable in tropical forest functioning and for regional carbon inventories. We investigate the spatial structure of the canopy height of a tropical forest, its relationship with environmental physical covariates, and the implication for tropical forest height variation mapping. Making use of high-resolution maps of LiDAR-derived Digital Canopy Model (DCM) and environmental covariates from a Digital Elevation Model (DEM) acquired over 30,000 ha of tropical forest in French Guiana, we first show that forest canopy height is spatially correlated up to 2500 m. Forest canopy height is significantly associated with environmental variables, but the degree of correlation varies strongly with pixel resolution. On the whole, bottomland forests generally have lower canopy heights than hillslope or hilltop forests. However, this global picture is very noisy at local scale likely because of the endogenous gap-phase forest dynamic processes. Forest canopy height has been predictively mapped across a pixel resolution going from 6 m to 384 m mimicking a low resolution case of 3 points·km − 2 . Results of canopy height mapping indicated that the error for spatial model with environment effects decrease from 8.7 m to 0.91 m, depending of the pixel resolution. Results suggest that, outside the calibration plots, the contribution of environment in shaping the global canopy height distribution is quite limited. This prevents accurate canopy height mapping based only on environmental information, and suggests that precise canopy height maps, for local management purposes, can only be obtained with direct LiDAR monitoring.
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Sullivan, M. J. P., Talbot, J., Lewis, S. L., Phillips, O. L., Qie, L., Begne, S. K., et al. (2017). Diversity and carbon storage across the tropical forest biome. Sci. Rep., 7, 39102.
Abstract: Tropical forests are global centres of biodiversity and carbon storage. Many tropical countries aspire to protect forest to fulfil biodiversity and climate mitigation policy targets, but the conservation strategies needed to achieve these two functions depend critically on the tropical forest tree diversity-carbon storage relationship. Assessing this relationship is challenging due to the scarcity of inventories where carbon stocks in aboveground biomass and species identifications have been simultaneously and robustly quantified. Here, we compile a unique pan-Tropical dataset of 360 plots located in structurally intact old-growth closed-canopy forest, surveyed using standardised methods, allowing a multi-scale evaluation of diversity-carbon relationships in tropical forests. Diversity-carbon relationships among all plots at 1 ha scale across the tropics are absent, and within continents are either weak (Asia) or absent (Amazonia, Africa). A weak positive relationship is detectable within 1 ha plots, indicating that diversity effects in tropical forests may be scale dependent. The absence of clear diversity-carbon relationships at scales relevant to conservation planning means that carbon-centred conservation strategies will inevitably miss many high diversity ecosystems. As tropical forests can have any combination of tree diversity and carbon stocks both require explicit consideration when optimising policies to manage tropical carbon and biodiversity. © The Author(s) 2017.
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Bompy, F., Lequeue, G., Imbert, D., & Dulormne, M. (2014). Increasing fluctuations of soil salinity affect seedling growth performances and physiology in three Neotropical mangrove species. Plant and Soil, 380(1), 399–413.
Abstract: Background: Micro-tidal wetlands are subject to strong seasonal variations of soil salinity that are likely to increase in amplitude according to climate model predictions for the Caribbean. Whereas the effects of constant salinity levels on the physiology of mangrove species have been widely tested, little is known about acclimation to fluctuations in salinity. Aims and methods: The aim of this experiment was to characterize the consequences of the rate of increase in salinity (slow versus fast) and salinity fluctuations over time versus constant salt level. Seedling mortality, growth, and leaf gas exchange of three mangrove species, Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle were investigated in semicontrolled conditions at different salt levels (0, 685, 1025, and 1370 mM NaCl). Results: Slow salinity increase up to 685 mM induced acclimation, improving the salt tolerance of A. germinans and L. racemosa, but had no effect on R. mangle. During fluctuations between 0 and 685 mM, A. germinans and R. mangle were not affected by a salinity drop to zero, whereas L. racemosa took advantage of the brief freshwater episode as shown by the durable improvement of photosynthesis and biomass production. Conclusions: This study provides new insights into physiological resistance and acclimation to salt stress. We show that seasonal variations of salinity may affect mangrove seedlings' morphology and physiology as much as annual mean salinity. Moreover, more severe dry seasons due to climate change may impact tree stature and species composition in mangroves through higher mortality rates and physiological disturbance at the seedling stage. © 2014 Springer International Publishing Switzerland.
Keywords: Acclimation; Avicennia germinans; Hypersalinity; Laguncularia racemosa; Leaf gas exchange; Rhizophora mangle; Salt stress
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