Abstract: Quercus robur L. is a mid-European broadleaved tree species that grows readily on temporary waterlogged soils. An experiment aiming to identify potential markers of tolerance to waterlogging in this species and to assess the degree of genetic control over the corresponding traits was conducted. Quantitative trait loci (QTL) were assessed in an F-1 progeny for responses to waterlogging, and the relevance of the observed traits as markers of tolerance was investigated using a precise description of the time course of their expression. Five significant QTL involved in the response to waterlogging were identified. In particular, QTL were detected for the development of hypertrophied lenticels and for the degree of leaf epinasty, but not for the formation of adventitious roots. A multi-environment QTL model allowed a detailed description of the time course (7 weeks) of the allelic substitution effect of some of these QTL. Correlation clustering identified significant clusters of QTL, at inter-trait as well as at intra-trait level. These clusters suggest the occurrence of a genetically controlled response cascade to waterlogging.

Abstract: Leaf-level net photosynthesis (An) estimates and associated photosynthetic parameters are crucial for accurately parameterizing photosynthesis models. For tropical forests, such data are poorly available and collected at variable light conditions. To avoid over- or underestimation of modeled photosynthesis, it is critical to know at which photosynthetic photon flux density (PPFD) photosynthesis becomes light-saturated. We studied the dependence of An on PPFD in two tropical forests in French Guiana. We estimated the light saturation range, including the lowest PPFD level at which Asat (An at light saturation) is reached, as well as the PPFD range at which Asat remained unaltered. The light saturation range was derived from photosynthetic light-response curves, and within-canopy and interspecific differences were studied. We observed wide light saturation ranges of An. Light saturation ranges differed among canopy heights, but a PPFD level of 1,000 µmol m−2 s−1 was common across all heights, except for pioneer trees species that did not reach light saturation below 2,000 µmol m−2 s−1. A light intensity of 1,000 µmol m−2 s−1 sufficed for measuring Asat of climax species at our study sites, independent of the species or the canopy height. Because of the wide light saturation ranges, results from studies measuring Asat at higher PPFD levels (for upper canopy leaves up to 1,600 µmol m−2 s−1) are comparable with studies measuring at 1,000 µmol m−2 s−1. © 2020 The Association for Tropical Biology and Conservation

Abstract: Although the leaf economic spectrum has deepened our understanding of leaf trait variability, little is known about how leaf traits scale with leaf area. This uncertainty has resulted in the assumption that leaf traits should vary by keeping the same pace of variation with increases in leaf area across the leaf size range. We evaluated the scaling of morphological, tissue-surface and vascular traits with overall leaf area, and the functional significance of such scaling. We examined 1,271 leaves for morphological traits, and 124 leaves for anatomical and hydraulic traits, from 38 trees of Cecropia obtusa Trécul (Urticaceae) in French Guiana. Cecropia is a Neotropical genus of pioneer trees that can exhibit large laminas (0.4 m2 for C. obtusa), with leaf size ranging by two orders of magnitude. We measured (i) tissue fractions within petioles and their second moment of area, (ii) theoretical xylem hydraulic efficiency of petioles and (iii) the extent of leaf vessel widening within the hydraulic path. We found that different scaling of morphological trait variability allows for optimisation of lamina display among larger leaves, especially the positive allometric relationship between lamina area and petiole cross-sectional area. Increasing the fraction of pith is a key factor that increases the geometrical effect of supportive tissues on mechanical rigidity and thereby increases carbon-use efficiency. We found that increasing xylem hydraulic efficiency with vessel size results in lower leaf lamina area: xylem ratios, which also results in potential carbon savings for large leaves. We found that the vessel widening is consistent with hydraulic optimisation models. Leaf size variability modifies scaling of leaf traits in this large-leaved species. © The Author(s) 2020. Published by Oxford University Press. All rights reserved. For permissions, please email: journals.permission@oup.com.

Abstract: Niche differentiation is a key issue in the current debate on community assembly mechanisms. In highly diverse moist tropical forests, tree species sensitivity to canopy openness is thought to be a major axis in niche differentiation. In the past, the syndrome of traits driving the demographic trade-off involved in the niche-based theory of coexistence has always been established among species situated at the two extremities of the shade-tolerance gradient, even though most tropical tree species have intermediate light requirements. In addition, trait plasticity has seldom been linked to tropical tree species distribution along environmental gradients. This article examines covariations between leaf traits, whole-plant traits and niche parameters among 14 tree species with intermediate light requirements in French Guiana and across a range of canopy openness. Each functional trait measured under field conditions was characterized by a median value and a degree of plasticity expressed under contrasting light regimes. Niche differentiation was characterized in terms of spatial light gradient. We first examined covariations between functional traits then explored to what degree the median value and plasticity in functional traits could predict light niche characteristics at the sapling stage and the ontogenetic change in light availability estimated by adult stature. Leaf mass per area (LMA) was positively correlated with leaf life span (LLS); species with higher LMA and higher LLS displayed lower diameter growth rates (GRs) and lower responsiveness to canopy gap at both whole-plant and population levels. This proved that the relationships previously established over a broader range of species held true within the narrow range of the light requirements covered. Height GR plasticity accounted for 49% of the variation in light niche optimum. LMA plasticity, unlike LLS plasticity, was significantly correlated with light niche breadth and adult stature. Synthesis. This study demonstrates the relevance of considering the phenotypic plasticity in functional traits in community ecology, particularly for quantifying breadth of species distribution over environmental gradients. Our findings did not support Hubbell's hypothesis of functional equivalence and suggest that even a rather subtle variation in forest canopy disturbance promotes the coexistence of tropical tree species. © 2012 The Authors. Journal of Ecology © 2012 British Ecological Society.

Abstract: While spiders constitute the most abundant and diverse arthropods in many habitats, they remained under-studied, especially in tropical rainforests. The goal of this study is to assess the spatial distribution of the spider family Ctenidae by assessing associations of species diversity and population traits among different habitat conditions. Fieldwork was carried out during 2013 in habitats varying in flooding frequency (plateau vs. flooded forest) and elevation (inselberg vs. lowland) in the Nouragues National Natural Reserve, French Guiana. Assemblage composition, population structure, and trait measurements of one dominant species were assessed using hand collection in replicated quadrats. We found strong effects on ctenid assemblages attributable to both elevation and flooding, with changes in relative abundance of species among habitats, but few correlated densities between species. At the population level, main differences in species distribution between and within habitats were detected only when juveniles were taken into account. No effect of elevation was found on the measurements of traits of the dominant species, but legs were proportionally shorter in flooded habitats, suggesting reduced active dispersal in these habitats. Our study highlights the value of complementary of measures of diversity and traits at different biological scales in Ctenidae.