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.

Abstract: 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.

Abstract: Plant leaf litter generally decomposes faster as a group of different species than when individual species decompose alone, but underlying mechanisms of these diversity effects remain poorly understood. Because resource C : N : P stoichiometry (i.e. the ratios of these key elements) exhibits strong control on consumers, we supposed that stoichiometric dissimilarity of litter mixtures (i.e. the divergence in C : N : P ratios among species) improves resource complementarity to decomposers leading to faster mixture decomposition. We tested this hypothesis with: (i) a wide range of leaf litter mixtures of neotropical tree species varying in C : N : P dissimilarity, and (ii) a nutrient addition experiment (C, N and P) to create stoichiometric similarity. Litter mixtures decomposed in the field using two different types of litterbags allowing or preventing access to soil fauna. Litter mixture mass loss was higher than expected from species decomposing singly, especially in presence of soil fauna. With fauna, synergistic litter mixture effects increased with increasing stoichiometric dissimilarity of litter mixtures and this positive relationship disappeared with fertilizer addition. Our results indicate that litter stoichiometric dissimilarity drives mixture effects via the nutritional requirements of soil fauna. Incorporating ecological stoichiometry in biodiversity research allows refinement of the underlying mechanisms of how changing biodiversity affects ecosystem functioning. © 2014 The Author(s) Published by the Royal Society. All rights reserved.

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.

Abstract: Black pod rot, caused by Stramenopiles of the genus Phytophthora, leads to serious production losses in all cocoa growing zones. In order to reduce the impact of these pests, preference is given to genetic control using resistant varieties, and sources of resistance are actively being sought, particularly in wild cacao trees. Surveys were undertaken in the natural cacao tree populations of south-eastern French Guiana between 1985 and 1995 and an abundant amount of plant material belonging to a particular genetic group, the “Guiana” group, was collected. A great deal of work has shown the merits of this genetic group as a source of resistance to Phytophthora palmivora and megakarya. We describe here the results of a global study to assess the resistance of the 186 clones in the “Guiana” group “core collection” to a Guianese strain of Phytophthora capsici (strain Reg 2-6). This study, which used an efficient methodology (fifteen series of tests on leaf discs and a statistical test adapted to the ordinal nature of the basic data), showed that the “Guiana” genetic group is a major source of resistance to P.capsici. Strain Reg 2-6 proves to be particularly virulent, as the Scavina 6 control, an international reference for resistance to Phytophthora, is not resistant to it. However, 24 clones of the “Guiana” group are, and 92 have proved to be more resistant than Scavina 6, thereby showing the interest of the group in genetically controlling P.capsici.Thus, of the clones in the Guiana group that are more resistant to P.capsici than Scavina 6, some, which are also resistant to P.palmivora and/or Phytophthora megakarya, and also displaying some other notable qualities, could be incorporated into cocoa genetic improvement programmes in countries where P.capsici is rife on cacao trees.