The successional status of tropical rainforest tree species is associated with differences in leaf carbon isotope discrimination and functional traits

International audienc

A cost–benefit analysis of acclimation to low irradiance in tropical rainforest tree seedlings: leaf life span and payback time for leaf deployment

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

Nitrogen-fixing legumes and silvigenesis in a rain forest in French Guiana: a taxonomic and ecological approach

International audienc

Peut-on gérer la flore adventice sans recours aux herbicides de synthèse dans les systèmes de cultures amazoniens ? Sensibilisation aux apports et intérêts des principes agroécologiques

Comme partout dans le monde, les agriculteurs doivent garantir leur revenu et produire des aliments ou des services dans le respect de l’environnement et de la santé des consommateurs, tout en recherchant les conditions optimales pour la culture. De manière singulière, le contrôle des adventices dans les régions tropicales constitue une contrainte majeure pour les productions végétales et mobilise toute l’attention des exploitants. Les conditions climatiques humides et chaudes permettent un développement des adventices tout au long de l’année, alors que le nombre de jours disponibles à leur éradication est limitant (pluies, vent, sol gorgé d’eau…). La nature et l’environnement diversifié des bordures de parcelles, souvent de petite taille et jouxtant la forêt, contribuent également au maintien d’un stock semencier et au retour rapide des adventices. Dans un contexte de réduction, voire de suppression des herbicides chimiques de synthèse (Loi Ecophyto), des alternatives agroécologiques visant au contrôle des adventices fondées sur :[br/] - Le contrôle lumineux (paillage vivant ou mort),[br/] - Le contrôle biologique (pâturage), (allélopathie), même chimique[br/] font désormais partie du panel de solutions techniques mobilisables en cultures tropicales. A l’échelle des systèmes de culture, le choix de succession des cultures de rente, combiné à la maîtrise des jachères offrent par ailleurs des possibilités de contrôle de la pression des adventices. Il existe, pour ce faire, une grande variété de plantes de service qui peuvent être cultivées dans les jachères ou en association avec les cultures de rente. L’objectif de cet article est de proposer aux agriculteurs un ensemble de principes agroécologiques à raisonner et à combiner pour une mise en application optimale dans leurs systèmes de culture, en lien avec leur contexte environnemental et sociotechnique. Il n’existe pas en effet une gestion de l’enherbement, mais des gestions d’enherbements que nous essayerons d’examiner à partir de différents exemples de Guyane et de Guadeloupe.As all around the world, the farmers have to guarantee their income and produce foods, or services, that respect the environment and health of consumers, while protecting or improving the fertility of soils. More than anywhere else, the control of weeds in tropical regions is a major constraint for crop production and mobilizes the attention of the farmers! In tropical areas weeds benefit from favorable growing conditions throughout the year, while the number of days available for their eradication is limiting (rains, wind, soils full of water…). The origin and the environment of the agricultural parcel, often small-sized and in heterogeneous and diversified context (border…), also facilitate the weeds return. The agroecological solutions based on:[br/] - Light control (green or dead mulch)[br/] - Limitation of development with a biological control (pasture), mechanical (tillage, rollers like FACA), thermal, or burndown (allelopathy properties).[br/] At the scale of cropping systems, a strategy based on cash crops rotation combined with controlled fallow (e.g. weed suppressive cover species such as nitrogen-fixing legume) also offers opportunities to control weed pressure. Additionally this creates an opportunity for coverspecies to deliver additional ecosystem services (e.g. fertility building, conservation of nutrients in the soil …) The objective of this article is to offers farmers a set of methods against weed pressure based on agroecological principles. The choice of the method to be used must be reasoned in each cropping system, according to its environmental and socio-technical context.[br/] There is no one but many methods of weed management, that we will examine from different examples in French Guyana and Guadeloupe

Nitrogen cycling in the tropical rain forest of French Guiana: comparison of two sites with contrasting soil types using delta 15N

The natural 15N abundance method for estimating symbiotic biological N2-fixation was tested on legume trees from two rain forests on contrasting soils (oxisols and spodosols) in French Guiana. When possible, the significance of N2-fixing species in the plant community was evaluated in terms of density, biomass and contribution of N2-fixation to the building up of the total nitrogen mass in the leaves. Of the two sites, the rain forest on spodosols was the less favourable for application of the [delta]15N method: the available soil nitrogen was isotopically similar to fixed-N2. Hence, the results showed that a reliable estimate of N2-fixation could not be obtained. A substantial contribution of fixed-N2 to the nitrogen nutrition of legumes was found on oxisols, with an average value of 54 % Ndfa (Nitrogen derived from the atmosphere). The contribution of the N2-fixing legumes to the biomass of the stand was estimated to be 2 t ha[minus sign]1 for the leaf biomass and 136 t ha[minus sign]1 for the total above-ground plant biomass. With 7.5 % of trees in the stand able to fix N2 (462 out of 6156), N2-fixation was estimated to be 7 kg ha[minus sign]1 y[minus sign]1. These results are the first use of the [delta]15N method to estimate nitrogen input by N2-fixing legumes to a natural rain forest. The inter-site variability observed in the [delta]15N of the non-fixing plants suggested different nitrogen-cycling processes in the two soils. The [delta]15N of the non-N2-fixing plants could be related to the soil nitrogen availability and be used as an indicator of efficient or non-efficient nitrogen-cycling rain forests. The spatial variability of the [delta]15N in the plant-available soil nitrogen pool and the nitrogen balance in tropical rain forests are discussed

Gestion durable de la fertilité des sols par l'utilisation de matières organiques : retours d'expérience en Guyane française

Les sols sont au cœur des grands enjeux planétaires tels que la sécurité alimentaire et forment une ressource naturelle qu’il faut protéger et utiliser durablement. Afin de maintenir, voire d’améliorer, la fertilité des sols de Guyane déjà naturellement pauvres, des pratiques agro-écologiques, par valorisation d’apport en matières organiques (MO) permettant de stimuler la vie des sols, sont développées dans les systèmes de cultures actuels. Par cette approche de gestion durable, le sol n’est plus considéré comme un simple support mais comme un milieu vivant qui produit et fournit les éléments nutritifs aux cultures. Dans le cadre du projet Guyafer du RITA, différents types de matières organiques ont été testés (bois raméal fragmenté - BRF, charbon de bois, compost, plantes de couverture) pour en déterminer leur capacité à améliorer la qualité du sol. Cette qualité est appréhendée par l’utilisation de bio-indicateurs d’activités microbiennes du sol liées aux deux grands cycles biochimiques que sont ceux du carbone et de l’azote. Si les différents types de matières organiques amènent à des améliorations de la qualité des sols - la biomasse microbienne active du sol est souvent largement stimulée par l’apport de MO. Cette amélioration peut être très différente en intensité et en durée suivant le type de MO utilisée. Certains de ces apports n’entrainent pas de changement profond de la qualité, changement évalué par les indicateurs de variations de diversité.Soils are at the heart of major global challenges: food security, quality of groundwater and surface water, air quality, climate change and biodiversity. Soil is a natural resource that must be protected and sustainably used. To maintain or even improve the soil fertility of French Guiana, already naturally poor, agro-ecological processes focusing organic matter in order to stimulate the soil life are developed in the current cropping systems. Through this sustainable management approach, soil is no longer considered as an inert support but also as a living component that produces and provides nutrients to crops. Various types of organic matter (Rameal Chipped Wood - RCW, charcoal, compost, mulch of cover plants) were tested to determine their impact on the soil, particularly through the use of bio-indicators of soil microbial activities related to the carbon and nitrogen cycles. Every tested organic matters increase soil quality (high stimulation of active microbial biomass). However, different types of organic matters affect differently the soil microbial diversity

Estimating N transfers between N2-fixing actinorhizal species and the non-N2-fixing Prunus avium under partially controlled conditions

International audienceTwo methods of N transfer between plants—by litter decomposition and root-to-root exchange—were examined in mixed plantations of N-fixing and non-fixing trees. Nitrogen transfers from decaying litters were measured by placing 15N-labelled litters from four actinorhizal tree species around shoots of containerized Prunus avium. Nitrogen transfers by root-to-root exchanges were measured after foliar NO3-15N fertilization of Alnus subcordata and Elaeagnus angustifolia growing in containers in association with P. avium. During the first 2 years of litter decomposition, from 5–20% of the N, depending on the litter identity, was released and taken up by P. avium. N availability in the different litters was strongly correlated with the amount of water-soluble N, which was highest in leaves of E. angustifolia. In the association between fixing and non-fixing plants, 7.5% of the A. subcordata N and 25% of E. angustifolia N was transferred to P. avium by root exchange. These results showed that the magnitude of N transfers by root exchange depended on the associated N2-fixing species. Among the species investigated, E. angustifolia displayed the highest capacity for exudating N from roots as well as for releasing N from litters. These qualities make this tree a promising species for enhancing wood yields in mixed stands

Gestion durable de la fertilité des sols par l'utilisation de matières organiques : retours d'expérience en Guyane française

Soils are at the heart of major global challenges: food security, quality of groundwater and surfacewater, air quality, climate change and biodiversity. Soil is a natural resource that must be protected andsustainably used. To maintain or even improve the soil fertility of French Guiana, already naturally poor,agro-ecological processes focusing organic matter in order to stimulate the soil life are developed in thecurrent cropping systems. Through this sustainable management approach, soil is no longer consideredas an inert support but also as a living component that produces and provides nutrients to crops.Various types of organic matter (Rameal Chipped Wood - RCW, charcoal, compost, mulch of coverplants) were tested to determine their impact on the soil, particularly through the use of bio-indicators ofsoil microbial activities related to the carbon and nitrogen cycles. Every tested organic matters increasesoil quality (high stimulation of active microbial biomass). However, different types of organic mattersaffect differently the soil microbial diversity.Les sols sont au cœur des grands enjeux planétaires tels que la sécurité alimentaire et forment uneressource naturelle qu’il faut protéger et utiliser durablement. Afin de maintenir, voire d’améliorer, lafertilité des sols de Guyane déjà naturellement pauvres, des pratiques agro-écologiques, parvalorisation d’apport en matières organiques (MO) permettant de stimuler la vie des sols, sontdéveloppées dans les systèmes de cultures actuels. Par cette approche de gestion durable, le sol n’estplus considéré comme un simple support mais comme un milieu vivant qui produit et fournit leséléments nutritifs aux cultures.Dans le cadre du projet Guyafer du RITA, différents types de matières organiques ont été testés (boisraméal fragmenté - BRF, charbon de bois, compost, plantes de couverture) pour en déterminer leurcapacité à améliorer la qualité du sol. Cette qualité est appréhendée par l’utilisation de bio-indicateursd’activités microbiennes du sol liées aux deux grands cycles biochimiques que sont ceux du carbone etde l’azote. Si les différents types de matières organiques amènent à des améliorations de la qualité dessols - la biomasse microbienne active du sol est souvent largement stimulée par l’apport de MO. Cetteamélioration peut être très différente en intensité et en durée suivant le type de MO utilisée. Certains deces apports n’entrainent pas de changement profond de la qualité, changement évalué par lesindicateurs de variations de diversité