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Purpose Members of the plant family Bromeliaceae can uptake nutrients directly from their leaves via leaf absorbing trichomes and their roots have long been reduced to anchorage function,thus overlooked. Recently, evidence has accumulated for a significant role for the roots of some species of tank bromeliads in both water and nutrients absorption. However, to date, little attention has been paid to the importance of the substrate fertility for the structure of the roots and the growth and performance of tank bromeliads. Methods This study investigated the effect of substrate fertility on Aechmea aquilega. Seeds of this tank bromeliad were sowed in a greenhouse in French Guiana in three different substrates: a nutrient-poor, a nutrient-rich and a mixed substrate. The performances of 15-month-old A. aquilega were assessed by measuring leaf and root traits related to nutrient acquisition and resources capture. Results We show that plants growing in nutrient-poor substrate grew twice slower and were smaller than plants grown on the nutrient-rich substrate with fewer leaves and roots, lower total dry mass, and smaller leaves and root length. Overall, 70% of measured traits responded significantly to the experimental treatments indicating that the response of A. aquilega to nutrient availability is a combination of physiological processes, leaf and root structure, and chemistry. Conclusion This study is the first to show that the fertility of the substrate on which the bromeliad A. aquilega grows has a strong and lasting effect on the plant performance and may be a relevant factor for bromeliad ecology.

International audience; Water stress can cause declines in plant function that persist after rehydration. Recent work has defined ‘resilience’ traits characterizing leaf resistance to persistent damage from drought, but whether these traits predict resilience in whole-plant function is unknown. It is also unknown whether the coordination between resilience and ‘resistance’ – the ability to maintain function during drought – observed globally occurs within ecosystems. For eight rainforest species, we dehydrated and subsequently rehydrated leaves, and measured water stress thresholds for declines in rehydration capacity and maximum quantum yield of photosystem II (Fv/Fm). We tested correlations with embolism resistance and dry season water potentials (ΨMD), and calculated safety margins for damage (ΨMD – thresholds) and tested correlations with drought resilience in sap flow and growth. Ψ thresholds for persistent declines in Fv/Fm, indicating resilience, were positively correlated with ΨMD and thresholds for leaf vein embolism. Safety margins for persistent declines in Fv/Fm, but not rehydration capacity, were positively correlated with drought resilience in sap flow.Correlations between resistance and resilience suggest that species' differences in performance during drought are perpetuated after drought, potentially accelerating shifts in forest composition. Resilience to photochemical damage emerged as a promising functional trait to characterize whole-plant drought resilience.

Plant-associated microorganisms have been shown to aid plants in coping with drought. However, the underlying mechanisms are poorly understood and there is uncertainty regarding which microbial taxa and functions are mostly involved. We explored these issues in Neotropical rainforests and identified foliar microorganisms that may play a role in drought tolerance of trees. Our objectives were to (i) test the relationship between drought tolerance traits in Neotropical trees and the diversity and composition of their foliar fungal and bacterial communities and (ii) identify leaf microbial taxa positively or negatively associated with drought tolerance traits. Our results showed that the composition of leaf fungal communities but not bacterial communities was related to drought tolerance. We identified 27 fungal amplicon sequence variants whose relative abundance covaried with drought tolerance traits. Most variants were assigned to fungal clades often described as plant pathogens and increased in abundance with drought susceptibility. This greater relative abundance of leaf pathogens in the most drought-susceptible trees might increase their vulnerability to climate change. Moreover, we identified the Strelitziana and Ochroconis fungal genera as potential candidates for future culture-dependent studies aimed at understanding and improving drought tolerance in Neotropical forests. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Embolism spreading in xylem is an important component of plant drought resistance. Since embolism resistance has been shown to be mechanistically linked to pit membrane characters in stem xylem, we speculate that similar mechanisms account for leaf xylem. We conducted transmission electron microscopy to investigate pit membrane characters in leaf xylem across 18 Neotropical tree species. We also conducted gold perfusion and polar lipid detection experiments on three species covering the full range of leaf embolism resistance. We then related these observations to previously published data on embolism resistance of leaf xylem. We also incorporated previously published data on stem embolism resistance and stem xylem pit membranes to investigate the link between vulnerability segmentation (i.e. difference in embolism resistance) and leaf–stem anatomical variation. Maximum pit membrane thickness (Tpm,max) and the pit membrane thickness-to-diameter ratio (Tpm,max/Dpm) were predictive of leaf embolism resistance, especially when vestured pits were taken into account. Variation in Tpm,max/Dpm was the only trait predictive of vulnerability segmentation between leaves and stems. Gold particles of 5- and 10-nm infiltrated pit membranes in three species, while the entry of 50-nm particles was blocked. Moreover, polar lipids were associated with inner conduit walls and pits. Our results suggest that mechanisms related to embolism spreading are determined by Tpm, pore constrictions (i.e. the narrowest bottlenecks along pore pathways), and lipid surfactants, which are largely similar between leaf and stem xylem and between temperate and tropical trees. However, our mechanistic understanding of embolism propagation and the functional relevance of Tpm,max/Dpm remains elusive.

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

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.

Key message Leaf-stem vulnerability segmentation predicts lower xylem embolism resistance in leaves than stem. However, although it has been intensively investigated these past decades, the extent to which vulnerability segmentation promotes drought resistance is not well understood. Based on a trait-based model, this study theoretically supports that vulnerability segmentation enhances shoot desiccation time across 18 Neotropical tree species. Context Leaf-stem vulnerability segmentation predicts lower xylem embolism resistance in leaves than stems thereby preserving expensive organs such as branches or the trunk. Although vulnerability segmentation has been intensively investigated these past decades to test its consistency across species, the extent to which vulnerability segmentation promotes drought resistance is not well understood. Aims We investigated the theoretical impact of the degree of vulnerability segmentation on shoot desiccation time estimated with a simple trait-based model. Methods We combined data from 18 tropical rainforest canopy tree species on embolism resistance of stem xylem (flow-centrifugation technique) and leaves (optical visualisation method). Measured water loss under minimum leaf and bark conductance, leaf and stem capacitance, and leaf-to-bark area ratio allowed us to calculate a theoretical shoot desiccation time (tcrit). Results Large degrees of vulnerability segmentation strongly enhanced the theoretical shoot desiccation time, suggesting vulnerability segmentation to be an efficient drought resistance mechanism for half of the studied species. The difference between leaf and bark area, rather than the minimum leaf and bark conductance, determined the drastic reduction of total transpiration by segmentation during severe drought. Conclusion Our study strongly suggests that vulnerability segmentation is an important drought resistance mechanism that should be better taken into account when investigating plant drought resistance and modelling vegetation. We discuss future directions for improving model assumptions with empirical measures, such as changes in total shoot transpiration after leaf xylem embolism.

Terrestrial biosphere models typically use the biochemical model of Farquhar, von Caemmerer and Berry (1980) to simulate photosynthesis, which requires accurate values of photosynthetic capacity of different biomes. However, data on tropical forests are sparse and highly variable due to the high species diversity, and it is still highly uncertain how these tropical forests respond to nutrient limitation in terms of C uptake. Tropical forests often grow on phosphorus (P)-poor soils and are, in general, assumed to be P- rather than nitrogen (N)-limited. However, the relevance of P as a control of photosynthetic capacity is still debated. Here, we provide a comprehensive dataset of vertical profiles of photosynthetic capacity and important leaf traits, including leaf N and P concentrations, from two three-year, large-scale nutrient addition experiments conducted in two tropical rainforests in French Guiana. These data present a unique source of information to further improve model representations of the roles of N, P, and other leaf nutrients, in photosynthesis in tropical forests. To further facilitate the use of our data in syntheses and model studies, we provide an elaborate list of ancillary data, including important soil properties and nutrients, along with the leaf data. As environmental drivers are key to improve our understanding of carbon (C)-nutrient cycle interactions, this comprehensive dataset will aid to further enhance our understanding of how nutrient availability interacts with C uptake in tropical forests. The data are available at DOI 10.5281/zenodo.4719242 (Verryckt, 2021).

Climate models predict an increase in the severity and the frequency of droughts. Tropical forests are among the ecosystems that could be highly impacted by these droughts. Here, we explore how hydraulic and photochemical processes respond to drought stress and re-watering. We conducted a pot experiment on saplings of five tree species. Before the onset of drought, we measured a set of hydraulic traits, including minimum leaf conductance, leaf embolism resistance and turgor loss point. During drought stress, we monitored traits linked to leaf hydraulic functioning (leaf water potential (ψmd) and stomatal conductance (gs)) and traits linked to leaf photochemical functioning (maximum quantum yield of photosystem II (Fv/Fm) and maximum electron transport rate (ETRmax)) at different wilting stages. After re-watering, the same traits were measured after 3, 7 and 14 days. Hydraulic trait values decreased faster than photochemical trait values. After re-watering, the values of the four traits recovered at different rates. Fv/Fm recovered very fast close to their initial values only 3 days after re-watering. This was followed by ETRmax, Ψmd and gs. Finally, we show that species with large stomatal and leaf safety margin and low πtlp are not strongly impacted by drought, whereas they have a low recovery on photochemical efficiency. These results demonstrate that πtlp, stomatal and leaf safety margin are a good indicators of plant responses to drought stress and also to recovery for photochemical efficiency.

International audience; Drought-induced xylem embolism is considered to be one of the main factors driving mortality in woody plants worldwide. Although several structure-functional mechanisms have been tested to understand the anatomical determinants of embolism resistance, there is a need to study this topic by integrating anatomical data for many species. We combined optical, laser, and transmission electron microscopy to investigate vessel diameter, vessel grouping, and pit membrane ultrastructure for 26 tropical rainforest tree species across three major clades (magnoliids, rosiids, and asteriids). We then related these anatomical observations to previously published data on drought-induced embolism resistance, with phylogenetic analyses. Vessel diameter, vessel grouping, and pit membrane ultrastructure were all predictive of xylem embolism resistance, but with weak predictive power. While pit membrane thickness was a predictive trait when vestured pits were taken into account, the pit membrane diameter-to-thickness ratio suggests a strong importance of the deflection resistance of the pit membrane. However, phylogenetic analyses weakly support adaptive co-evolution. Our results emphasize the functional significance of pit membranes for air-seeding in tropical rainforest trees, highlighting also the need to study their mechanical properties due to the link between embolism resistance and pit membrane diameter-to-thickness ratio. Finding support for adaptive co-evolution also remains challenging.

Measuring leaf gas exchange from canopy leaves is fundamental for our understanding of photosynthesis and for a realistic representation of carbon uptake in vegetation models. Since canopy leaves are often difficult to reach, especially in tropical forests with emergent trees up to 60 m at remote places, canopy access techniques such as canopy cranes or towers have facilitated photosynthetic measurements. These structures are expensive and therefore not very common. As an alternative, branches are often cut to enable leaf gas exchange measurements. The effect of branch excision on leaf gas exchange rates should be minimized and quantified to evaluate possible bias. We compared light-saturated leaf net photosynthetic rates measured on excised and intact branches. We selected branches positioned at three canopy positions, estimated relative to the top of the canopy: upper sunlit foliage, middle canopy foliage, and lower canopy foliage. We studied the variation of the effects of branch excision and transport among branches at these different heights in the canopy. After excision and transport, light-saturated leaf net photosynthetic rates were close to zero for most leaves due to stomatal closure. However, when the branch had acclimated to its new environmental conditions—which took on average 20 min—light-saturated leaf net photosynthetic rates did not significantly differ between the excised and intact branches. We therefore conclude that branch excision does not affect the measurement of light-saturated leaf net photosynthesis, provided that the branch is recut under water and is allowed sufficient time to acclimate to its new environmental conditions. © 2020 The Association for Tropical Biology and Conservation

The leaf size-stem size spectrum is one of the main dimensions of plant ecological strategies. Yet the anatomical, mechanical, and hydraulic implications of small versus large shoots are still poorly understood. We investigated 42 tropical rainforest tree species in French Guiana, with a wide range of leaf areas at the shoot level. We quantified the scaling of hydraulic and mechanical constraints with shoot size, estimated as the water potential difference (ΔΨ) and the bending angle (ΔΦ), respectively. We investigated how anatomical tissue area, flexural stiffness and xylem vascular architecture affect such scaling by deviating (or not) from theoretical isometry with shoot size variation. Vessel diameter and conductive path length were found to be allometrically related to shoot size, thereby explaining the independence between ΔΨ and shoot size. Leaf mass per area, stem length, and the modulus of elasticity were allometrically related to shoot size, explaining the independence between ΔΦ and shoot size. Our study also shows that the maintenance of both water supply and mechanical stability across the shoot size range are not in conflict.

International audience; Studying the response to drought stress of keystone epiphytes such as tank bromeliads is essential to better understand their resistance capacity to future climate change. The objective was to test whether there is any variation in the carbon, water and nutrient status among different leaf ontogenetic stages in a bromeliad rosette subjected to a gradient of drought stress. We used a semi-controlled experiment consisting in a gradient of water shortage in Aechmea aquilega and Lutheria splendens. For each bromeliad and drought treatment, three leaves were collected based on their position in the rosette and several functional traits related to water and nutrient status, and carbon metabolism were measured. We found that water status traits (relative water content, leaf succulence, osmotic and midday water potentials) and carbon metabolism traits (carbon assimilation, maximum quantum yield of photosystem II, chlorophyll and starch contents) decreased with increasing drought stress, while leaf soluble sugars and carbon, nitrogen and phosphorus contents remained unchanged. The different leaf ontogenetic stages showed only marginal variations when subjected to a gradient of drought. Resources were not reallocated between different leaf ontogenetic stages but we found a reallocation of soluble sugars from leaf starch reserves to the root system. Both species were capable of metabolic and physiological adjustments in response to drought. Overall, this study advances our understanding of the resistance of bromeliads faced with increasing drought stress and paves the way for in-depth reflection on their strategies to cope with water shortage.

International audience; Hydraulic segmentation at the stem-leaf transition predicts higher hydraulic resistance in leaves than in stems. Vulnerability segmentation, however, predicts lower embolism resistance in leaves. Both mechanisms should theoretically favour runaway embolism in leaves to preserve expensive organs such as stems, and should be tested for any potential coordination. We investigated the theoretical leaf-specific conductivity based on an anatomical approach to quantify the degree of hydraulic segmentation across 21 tropical rainforest tree species. Xylem resistance to embolism in stems (flow-centrifugation technique) and leaves (optical visualisation method) was quantified to assess vulnerability segmentation. We found a pervasive hydraulic segmentation across species, but with a strong variability in the degree of segmentation. Despite a clear continuum in the degree of vulnerability segmentation, eight species showed a positive vulnerability segmentation (leaves less resistant to embolism than stems), while the remaining species studied exhibited a negative or no vulnerability segmentation. The degree of vulnerability segmentation was positively related to the degree of hydraulic segmentation, such that segmented species promote both mechanisms to hydraulically decouple leaf xylem from stem xylem. To what extent hydraulic and vulnerability segmentation determine drought resistance requires further integration of the leaf-stem transition at the whole-plant level, including both xylem and outer-xylem tissue.

Trait-environment relationships have been described at the community level across tree species. However, whether interspecific trait-environment relationships are consistent at the intraspecific level is yet unknown. Moreover, we do not know how consistent is the response between organ vs. whole-tree level.We examined phenotypic variability for 16 functional leaf (dimensions, nutrient, chlorophyll) and wood traits (density) across two soil types, Ferralitic Soil (FS) vs. White Sands (WS), on two sites for 70 adult trees of Cecropia obtusa Trécul (Urticaceae) in French Guiana. Cecropia is a widespread pioneer Neotropical genus that generally dominates early successional forest stages. To understand how soil types impact resource-use through the processes of growth and branching, we examined the architectural development with a retrospective analysis of growth trajectories. We expect soil types to affect both, functional traits in relation to resource acquisition strategy as already described at the interspecific level, and growth strategies due to resource limitations with reduced growth on poor soils.Functional traits were not involved in the soil response, as only two traits-leaf residual water content and K content-showed significant differences across soil types. Soil effects were stronger on growth trajectories, with WS trees having the slowest growth trajectories and less numerous branches across their lifespan.The analysis of growth trajectories based on architectural analysis improved our ability to characterise the response of trees with soil types. The intraspecific variability is higher for growth trajectories than functional traits for C. obtusa, revealing the complementarity of the architectural approach with the functional approach to gain insights on the way trees manage their resources over their lifetime. Soil-related responses of Cecropia functional traits are not the same as those at the interspecific level, suggesting that the effects of the acting ecological processes are different between the two levels. Apart from soil differences, much variation was found across sites, which calls for further investigation of the factors shaping growth trajectories in tropical forests.

Key message Abundant Neotropical canopy-tree species are more resistant to drought-induced branch embolism than what is currently admitted. Large hydraulic safety margins protect them from hydraulic failure under actual drought conditions. Context Xylem vulnerability to embolism, which is associated to survival under extreme drought conditions, is being increasingly studied in the tropics, but data on the risk of hydraulic failure for lowland Neotropical rainforest canopy-tree species, thought to be highly vulnerable, are lacking. Aims The purpose of this study was to gain more knowledge on species drought-resistance characteristics in branches and leaves and the risk of hydraulic failure of abundant rainforest canopy-tree species during the dry season. Methods We first assessed the range of branch xylem vulnerability to embolism using the flow-centrifuge technique on 1-m-long sun-exposed branches and evaluated hydraulic safety margins with leaf turgor loss point and midday water potential during normal- and severe-intensity dry seasons for a large set of Amazonian rainforest canopy-tree species. Results Tree species exhibited a broad range of embolism resistance, with the pressure threshold inducing 50% loss of branch hydraulic conductivity varying from − 1.86 to − 7.63 MPa. Conversely, we found low variability in leaf turgor loss point and dry season midday leaf water potential, and mostly large, positive hydraulic safety margins. Conclusions Rainforest canopy-tree species growing under elevated mean annual precipitation can have high resistance to embolism and are more resistant than what was previously thought. Thanks to early leaf turgor loss and high embolism resistance, most species have a low risk of hydraulic failure and are well able to withstand normal and even severe dry seasons.

International audience; Warmer and drier climates over Amazonia have been predicted for the next century with expected changes in regional water and carbon cycles. We examined the impact of interannual and seasonal variations in climate conditions on ecosystem-level evapotranspiration (ET) and water use efficiency (WUE) to determine key climatic drivers and anticipate the response of these ecosystems to climate change. We used daily climate and eddyflux data recorded at the Guyaflux site in French Guiana from 2004 to 2014. ET and WUE exhibited weak interannual variability. The main climatic driver of ET and WUE was global radiation (Rg), but relative extractable water (REW) and soil temperature (Ts) did also contribute. At the seasonal scale, ET and WUE showed a modal pattern driven by Rg, with maximum values for ET in July and August and for WUE at the beginning of the year. By removing radiation effects during water depleted periods, we showed that soil water stress strongly reduced ET. In contrast, drought conditions enhanced radiation-normalized WUE in almost all the years, suggesting that the lack of soil water had a more severe effect on ecosystem evapotranspiration than on photosynthesis. Our results are of major concern for tropical ecosystem modeling because they suggest that under future climate conditions, tropical forest ecosystems will be able to simultaneously adjust CO2 and H2O fluxes. Yet, for tropical forests under future conditions, the direction of change in WUE at the ecosystem scale is hard to predict, since the impact of radiation on WUE is counterbalanced by adjustments to soil water limitations. Developing mechanistic models that fully integrate the processes associated with CO2 and H2O flux control should help researchers understand and simulate future functional adjustments in these ecosystems.

International audience; The water and nutrient uptake mechanisms used by vascular epiphytes have been the subject of a few studies. While leaf absorbing trichomes (LATs) are the main organ involved in resource uptake by bromeliads, little attention has been paid to the absorbing role of epiphytic bromeliad roots. This study investigates the water and nutrient uptake capacity of LATs vs. roots in two epiphytic tank bromeliads Aechmea aquilega and Lutheria splendens. The tank and/or the roots of bromeliads were watered, or not watered at all, in different treatments. We show that LATs and roots have different functions in resource uptake in the two species, which we mainly attributed to dissimilarities in carbon acquisition and growth traits (e.g., photosynthesis, relative growth rate, non-structural carbohydrates, malate), to water relation traits (e.g., water and osmotic potential, relative water content, hydrenchyma thickness) and nutrient uptake (e.g., 15N-labelling). While the roots of A. aquilega did contribute to water and nutrient uptake, the roots of L. splendens were less important than the role played by the LATs in resource uptake. We also provide evidence for a synergistic effect of combined watering of tank and root in the Bromelioideae species. These results call for a more complex interpretation of LATs vs. roots in resource uptake in bromeliads.

Corners rule predicts a positive correlation between leaf dimensions and the cross-sectional area of the primary stem. Although this relationship is usually explained by hydraulic and mechanical requirements, these hypotheses have never been tested empirically. However, Corner’s rule is tricky to investigate since rapid secondary growth of the stem prevents a rigorous link being established between a given leaf and the supporting stem. We chose a twig-like leaf model since petiole anatomy is only linked to the attached lamina. We tested the hypothesis that anatomical adjustments to hydraulic requirements related to vessel size enable reduced investment in tissue in the framework of Corner’s rule. We conducted a functional, mechanistic and intraspecific investigation ofCecropia obtusaTrécul, a Neotropical pioneer tree, by integrating morphological, anatomical and theoretical hydraulic traits around the lamina-petiole size relationship. The twig-like structure of the leaf and the strong lamina-petiole correlation of this model tree species made it possible to use the leaf-level model for a rigorous investigation of the functional implications of Corner’s rule. We found a positive correlation between petiole size, lamina size, the ratio of mean vessel area to vessel frequency in the petiole xylem and theoretical specific conductivity in the petiole xylem. Hydraulic function supports Corner’s rule to a lesser extent than previously thought. Variations in vessel dimensions mainly drive xylem hydraulic performances and avoid disproportionate petiole cross-sections to answer to hydraulic requirements associated with lamina size.

1. Water availability is a key determinant of forest ecosystem function and tree species distributions. While droughts are increasing in frequency in many ecosystems, including in the tropics, plant responses to water supply vary with species and drought intensity and are therefore difficult to model. Based on physiological first principles, we hypothesized that trees with a lower turgor loss point (pi(tlp)), that is, a more negative leaf water potential at wilting, would maintain water transport for longer into a dry season. 2. We measured sapflux density of 22 mature trees of 10 species during a dry season in an Amazonian rainforest, quantified sapflux decline as soil water content decreased and tested its relationship to tree pi(tlp), size and leaf predawn and midday water potentials measured after the onset of the dry season. 3. The measured trees varied strongly in the response of water use to the seasonal drought, with sapflux at the end of the dry season ranging from 37 to 117% (on average 83 +/- 5 %) of that at the beginning of the dry season. The decline of water transport as soil dried was correlated with tree pi(tlp) (Spearman's rho >= 0.63), but not with tree size or predawn and midday water potentials. Thus, trees with more drought-tolerant leaves better maintained water transport during the seasonal drought. 4. Our study provides an explicit correlation between a trait, measurable at the leaf level, and whole-plant performance under drying conditions. Physiological traits such as pi(tlp) can be used to assess and model higher scale processes in response to drying conditions.

Light is of primary importance in structuring tropical tree communities. Light exposure at seedling and adult stages has been used to characterize the ecological profile of tropical trees, with many implications in forest management and restoration ecology. Most shade-tolerance classification systems have been proposed based on empirical observations in a specific area and thus result in contradictions among categories assigned to a given species. In this study, we aimed to quantify the light requirements for seedling growth of a Central African timber tree, Lophira alata (Ochnaceae), taking into account effects of population origin. In two controlled experiments: a light response experiment and a comparative population experiment, conducted in southwestern Cameroon, using seeds collected from four populations (three from Cameroon and one from Gabon), we examined the quantitative responses to irradiance of seedlings. After 2 years, mortality was very low (

The maintenance in the long run of a positive carbon balance under very low irradiance is a prerequisite for survival of tree seedlings below the canopy or in small gaps in a tropical rainforest. To provide a quantitative basis for this assumption, experiments were carried out to determine whether construction cost (CC) and payback time for leaves and support structures, as well as leaf life span (i) differ among species and (ii) display an irradiance-elicited plasticity. Experiments were also conducted to determine whether leaf life span correlates to CC and payback time and is close to the optimal longevity derived from an optimization model. Saplings from 13 tropical tree species were grown under three levels of irradiance. Specific-CC was computed, as well as CC scaled to leaf area at the metamer level. Photosynthesis was recorded over the leaf life span. Payback time was derived from CC and a simple photosynthesis model. Specific-CC displayed only little interspecific variability and irradiance-elicited plasticity, in contrast to CC scaled to leaf area. Leaf life span ranged from 4 months to >26 months among species, and was longest in seedlings grown under lowest irradiance. It was always much longer than payback time, even under the lowest irradiance. Leaves were shed when their photosynthesis had reached very low values, in contrast to what was predicted by an optimality model. The species ranking for the different traits was stable across irradiance treatments. The two pioneer species always displayed the smallest CC, leaf life span, and payback time. All species displayed a similar large irradiance-elicited plasticity.

Leaf traits of tropical tree species display an important inter-specific diversity, as detected for instance in the large range of values of leaf mass : area ratio (LMA). They also demonstrate a large irradiance-elicited plasticity, but there is still debate whether this plasticity differs among species. To address this question, leaf traits were recorded on saplings from 12 rainforest tree species in French Guiana, grown under approximately 5, 10 and 20% relative irradiance. Fifteen structural and physiological leaf traits related to photosynthesis were measured. The irradiance-elicited plasticity was quantified using a relative distance plasticity index. A large inter-specific diversity was detected for all leaf traits. A principal component analysis opposed species with a large mass-based photosynthesis, respiration, N content and photosynthetic nitrogen use efficiency, to species with a large leaf mass : area ratio, LMA. The two pioneer species used in this study displayed the largest photosynthetic capacity (and lowest LMA) and ranked at one end of the species continuum. Relative irradiance affected almost all traits with the exception of mass-based photosynthesis. A weak interaction was found between species and relative irradiance and the species ranking was maintained among relative irradiance treatments for the majority of the traits. A principal component analysis of the values of relative-distance plasticity index failed to reveal any consistent patterns of traits or species. We concluded that irradiance-elicited plasticity of leaf traits was similar among species irrespective of LMA and successional status, despite the occurrence of a large inter-specific diversity for the investigated traits.

International audience; Chlorophyll meters such as the SPAD-502 offer a simple, inexpensive and rapid method to estimate foliar chlorophyll content. However, values provided by SPAD-502 are unitless and require empirical calibrations between SPAD units and extracted chlorophyll values. * Leaves of 13 tree species from the tropical rain forest in French Guiana were sampled to select the most appropriate calibration model among the often-used linear, polynomial and exponential models, in addition to a novel homographic model that has a natural asymptote.* The homographic model best accurately predicted total chlorophyll content (μg cm−2) from SPAD units (R2 = 0.89). Interspecific differences in the homographic model parameters explain less than 7% of the variation in chlorophyll content in our data set. * The utility of the general homographic model for a variety of research and management applications clearly outweighs the slight loss of model accuracy due to the abandon of the species' effect.

Irradiance elicits a large plasticity in leaf traits, but little is known about the modulation of this plasticity by ontogeny. Interactive effects of relative irradiance and ontogeny were assessed on leaf traits for two tropical rainforest tree species: Dicorynia guianensis Amshoff and Tachigali melinonii (Harms) Barneby (Fabaceae, Caesalpinioideae). Eleven morphological and physiological leaf traits, relative to photosynthetic performance, were measured on saplings at three different architectural development stages (ASD 1, 2 and 3) and used to derive composite traits like photosynthetic N-use efficiency. Measurements were made along a natural irradiance gradient. The effect of ASD was very visible and differed between the two species. For Dicorynia guianensis, only leaf mass-per-area (LMA) significantly increased with ASDs whereas for Tachigali melinonii, almost all traits were affected by ASD: LMA, leaf N content and photosynthetic capacity increased from ASD 1 to ASD 3. Photosynthetic N-use-efficiency was not affected by ASD in any species. Leaf traits were severely modulated by irradiance, whereas the degree of plasticity was very similar among ASDs. Only few interactions were detected between irradiance and ASD, for leaf thickness, carbon content, and the ratio Chl/N in T. melinonii and for photosynthetic capacity in D. guianensis. We conclude that ontogenic development and irradiance-elicited plasticity modulated leaf traits, with almost no interaction, i.e., the degree of irradiance-elicited plasticity was stable across development stages and independent of ontogeny in these two species, at least in the early stages of development assessed here., Les traits foliaires varient fortement avec l'éclairement mais également avec l'ontogenèse. Cependant, l'impact de l'ontogenèse sur la plasticité induite par l'éclairement reste mal connu. Les interactions entre ontogenèse et éclairement relatif ont ainsi été étudiées pour des traits fonctionnels foliaires de deux espèces de forêt tropicale humide de Guyane française : Dicorynia guianensis Amshoff et Tachigali melinonii (Harms) Barneby (Fabaceae, Caesalpinioideae). • Onze traits morphologiques et physiologiques foliaires, relatifs aux capacités photosynthétiques, ont été mesurés sur de jeunes arbres ayant atteint trois stades architecturaux de développement successifs (ASD 1, 2 et 3) et utilisés pour en déduire des traits composites. Les mesures ont été réalisées le long d'un gradient naturel d'éclairement en forêt. • L'impact de stades successifs de développement différait entre espèces. Pour D. guianensis, seule la masse surfacique (LMA) a augmenté significativement avec les stades de développement tandis que pour T. melinonii, presque tous les traits foliaires étaient modulés. Pour cette espèce, LMA, teneur en N et capacité photosynthétique ont augmenté de ASD 1 à ASD 3. Par contre, l'efficience d'utilisation de N par la photosynthèse est restée stable entre ASDs. • Les traits foliaires ont été fortement modifiés par l'éclairement mais le degré de plasticité était similaire entre les ASDs. Des interactions entre ASD et éclairement n'ont été observées que pour l'épaisseur, la teneur en C et le rapport Chl/N de T. melinonii ainsi que pour la capacité photosynthétique de D. guianensis. • En dépit de l'effet significatif de l'ontogénèse sur les traits foliaires, seules de très faibles interactions ont pu être détectées avec l'éclairement pour les deux espèces. On peut en conclure que les stades successifs de développement ne modifient pas le degré de plasticité foliaire en réponse à l'éclairement tout au moins dans le cas des stades jeunes pris en compte ici.

We characterised the among species variability in leaf gas exchange and morphological traits under controlled conditions of seedlings of 22 tropical rainforest canopy species to understand the origin of the variability in leaf carbon isotope discrimination (Delta) among species with different growth and dynamic characteristics (successional gradient). Our results first suggest that these species pursue a consistent strategy in terms of (Delta) throughout their ontogeny (juveniles grown here versus canopy adult trees from the natural forest). Second, leaf (Delta) was negatively correlated with WUE and N, and positively correlated with gs, but among species differences in (Delta) were mainly explained by differences in WUE. Finally, species belonging to different successional groups display distinct leaf functional and morphological traits. We confirmed that fast growing early successional species maximise carbon assimilation with high stomatal conductance. In contrast, fast and slow growing late successional species are both characterised by low carbon assimilation values, but by distinct stomatal conductance and leaf morphological features. Along the successional gradient, these differences result in much lower (Delta) for the intermediate species (i.e. fast growing late successional) as compared to the two other groups., Nous avons caractérisé la variabilité interspécifique des échanges gazeux et des traits morphologiques foliaires en conditions environnementales contrôlées de jeunes plants de 22 espèces d'arbres de la canopée en forêt tropicale humide afin de comprendre l'origine de la variabilité de la discrimination isotopique du carbone foliaire (Delta) observée entre ces espèces présentant des caractéristiques de croissance et de dynamique distinctes (groupes successionnels). Nous montrons premièrement que les espèces tropicales possèdent une stratégie très conservée de (Delta) au cours de leur ontogénie (juvéniles élevés ici versus arbres adultes de la canopée en forêt naturelle). Deuxièmement,(Delta) était négativement corrélée à WUE et N, et positivement à gs_, mais les différences de (Delta) entre espèces sont principalement expliquées par des différences de WUE. Enfin, nous montrons que les espèces appartenant à des groupes successionnels distincts présentent des traits fonctionnels et morphologiques foliaires distincts. Nous confirmons que les espèces à croissance rapide qui s'installent en premier au cours de la succession écologique (FE) maximisent A avec de fortes conductances stomatiques. Les espèces climax (qui s'installent en second dans la succession écologique), à croissance rapide (FL) ou à croissance faible (SL), présentent des valeurs de A identiques, mais des valeurs de gs, ainsi que des caractéristiques morphologiques foliaires distinctes. Dans la succession écologique, ces différences se traduisent par des valeurs de(Delta)nettement plus faibles pour les espèces intermédiaires (c'est-à-dire les espèces climax à croissance rapide) par rapport aux deux autres groupes.

International audience

International audience; In beech (Fagus sylvatica L.), the number of leaf primordia preformed in the buds determines the length and the type (long versus short) of annual growth units, and thus, branch growth and architecture. We analyzed the correlation between the number of leaf primordia and the hydraulic conductance of the vascular system connected to the buds. Terminal buds of short growth units and axillary buds of long growth units on lower branches of mature trees were examined. Buds with less than four and more than five leaf primordia formed short and long growth units, respectively. Irrespective of the type of growth unit the bud was formed on, the occurrence of a large number of leaf primordia was associated with high xylem hydraulic conductance. Xylem conductance was correlated to the area of the outermost annual ring. These results suggest that organogenesis and primary growth in buds correlates with secondary growth of the growth units and thus with their hydraulic architecture. Possible causal relationships between the variables are discussed.

Variability of leaf traits related to photosynthesis was assessed in seedlings from 14 tree species growing in the tropical rain forest of French Guiana. Leaf photosynthetic capacity (maximum rate of carboxylation and maximum rate of electron transport) was estimated by fitting a biochemical model of photosynthesis to response curves of net CO2 assimilation rate versus intercellular CO2 mole fraction. Leaf morphology described by leaf mass per unit leaf area (LMA), density and thickness, as well as area- and mass-based nitrogen (N) and carbon (C) concentrations, were recorded on the same leaves. Large interspecific variability was detected in photosynthetic capacity as well as in leaf structure and leaf N and C concentrations. No correlation was found between leaf thickness and density. The correlations between area- and mass-based leaf N concentration and photosynthetic capacity were poor. Conversely, the species differed greatly in relative N allocation to carboxylation and bioenergetics. Principal component analysis (PCA) revealed that, of the recorded traits, only the computed fraction of total leaf N invested in photosynthesis was tightly correlated to photosynthetic capacity. We also used PCA to test to what extent species with similar shade tolerances displayed converging leaf traits related to photosynthesis. No clear-cut ranking could be detected among the shade-tolerant groups, as confirmed by a one-way ANOVA. We conclude that the large interspecific diversity in photosynthetic capacity was mostly explained by differences in the relative allocation of N to photosynthesis and not by leaf N concentration, and that leaf traits related to photosynthetic capacity did not discriminate shade-tolerance ranking of these tropical tree species.

Tropical rainforests play a central role in biogeochemical cycles at the global scale. Seasonal droughts alter their functioning, but severe droughts can severely affect tree growth and mortality. Increasingly frequent severe droughts in the past decades over the entire Amazon Basin have triggered changes in tree community composition, but our capacity to predict the fate of these communities is partly hampered by our lack of knowledge on the physiological mechanisms involved in tree drought-resistance. This thesis explores the interspecific diversity of drought resistance mechanisms of tropical rainforest tree species in French Guiana and aims at enhancing our present knowledge on their functioning. This thesis reveals that tropical rainforest canopy tree species can be very resistant to drought-induced stem embolism and that a majority of species achieve high hydraulic safety during the dry season, with strong interspecific differences. However, for most trees, a decrease in soil water availability leads to a decrease in their sap flux density during the dry season, with part of their sensitivity being related to physiological mechanisms involved in drought-resistance strategies. The underlying physiological mechanisms to their strategies vary strongly between species for young tree saplings. Some benefit from an early stomatal closure and low minimum leaf conductance, while others are more resistant to stem embolism and rely on a positive hydraulic vulnerability segmentation. During a severe imposed drought, there was a strong reduction in carbohydrate contents, which highlights the interdependence between the water and carbon balance of trees. However, the main physiological mechanism causing tree mortality seems to be hydraulic failure. The strong interspecific variability within these communities could lead to contrasting responses of tree populations to an increase in the frequency and severity of drought events, which could impact tree community composition in French Guiana.; Les forêts tropicales humides jouent un rôle clé dans les cycles biogéochimiques à l’échelle globale. Les évènements de sécheresse saisonniers entrainent des modifications dans le fonctionnement de ces forêts. L’augmentation de la fréquence et de l’intensité des évènements de sécheresse de forte intensité à l’échelle de l’Amazonie entraine déjà un changement dans la composition des communautés d’arbres, mais notre capacité à prédire leur réponse future dépend en partie de nos connaissances des mécanismes physiologiques leur permettant de résister à la sécheresse. Cette thèse explore la diversité interspécifique des mécanismes de résistance à la sécheresse des arbres en forêt tropicale humide de Guyane et vise à améliorer les connaissances actuelles. Elle montre que les espèces d’arbres de canopée en forêt naturelle peuvent avoir une grande résistance à l’embolie des tiges et qu’une majorité des espèces ont un grand niveau de sureté hydraulique en saison sèche, avec une grande variabilité interspécifique. Cependant, pour la majorité d’entre eux, une diminution de la disponibilité en eau du sol entraine une diminution de leur densité de flux de sève en saison sèche, et une partie de cette sensibilité peut être expliquée par des mécanismes physiologiques liés à des stratégies de résistance à la sécheresse. Les mécanismes physiologiques sous-jacents à ces stratégies varient fortement entre espèces pour de jeunes arbres. Certaines bénéficient d’une fermeture stomatique précoce et d’une faible conductance minimum, alors que d’autres ont une plus grande résistance à l’embolie des tiges et un plus fort degré de segmentation de vulnérabilité, parfois associés à une grande tolérance à l’embolie des tiges. Lors d’une sécheresse de forte intensité, nous avons observé une forte réduction des teneurs en carbohydrates, ce qui souligne l’interdépendance entre le fonctionnement hydraulique et carboné des espèces. Cependant, le principal processus physiologique causant la mort semble être la défaillance hydraulique. Cette forte variabilité interspécifique amène à postuler que les populations d’arbres de forêt tropicale humide pourraient répondre de manière contrastée à une intensification des épisodes de sécheresse, ce qui pourrait avoir des conséquences sur la composition des communautés d’arbres en Guyane.