Éditeur(s) : HAL CCSD Public Library of Science
Résumé : Tree mortality in tropical forests is a complex ecological process for which modelling approaches need to be improved to better understand, and then predict, the evolution of tree mortality in response to global change. The mortality model introduced here computes an individual probability of dying for each tree in a community. The mortality model uses the ontogenetic stage of the tree because youngest and oldest trees are more likely to die. Functional traits are integrated as proxies of the ecological strategies of the trees to permit generalization among all species in the community. Data used to parametrize the model were collected at Paracou study site, a tropical rain forest in French Guiana, where 20,408 trees have been censused for 18 years. A Bayesian framework was used to select useful covariates and to estimate the model parameters. This framework was developed to deal with sources of uncertainty, including the complexity of the mortality process itself and the field data, especially historical data for which taxonomic determinations were uncertain. Uncertainty about the functional traits was also considered, to maximize the information they contain. Four functional traits were strong predictors of tree mortality: wood density, maximum height, laminar toughness and stem and branch orientation, which together distinguished the light-demanding, fast-growing trees from slow-growing trees with lower mortality rates. Our modelling approach formalizes a complex ecological problem and offers a relevant mathematical framework for tropical ecologists to process similar uncertain data at the community level.
ISSN: 1932-6203

hal-01190561
https://hal.archives-ouvertes.fr/hal-01190561
PRODINRA : 208582
PUBMED : 23675500
DOI : 10.1371/journal.pone.0063678

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Variation of dead wood decay rates among tropical trees remains one source of uncertainty in global models of the carbon cycle. Taking advantage of a broad forest plot network surveyed for tree mortality over a 23-year period, we measured the remaining fraction of boles from 367 dead trees from 26 neotropical species widely varying in wood density (0.23–1.24 g cm-3) and tree circumference at death time (31.5–272.0 cm). We modeled decay rates within a Bayesian framework assuming a first order differential equation to model the decomposition process and tested for the effects of forest management (selective logging vs. unexploited), of mode of death (standing vs. downed) and of topographical levels (bottomlands vs. hillsides vs. hilltops) on wood decay rates. The general decay model predicts the observed remaining fraction of dead wood (R2 = 60%) with only two biological predictors: tree circumference at death time and wood specific density. Neither selective logging nor local topography had a differential effect on wood decay rates. Including the mode of death into the model revealed thatstanding dead trees decomposed faster than downed dead trees, but the gain of model accuracy remains rather marginal. Overall, these results suggest that the release of carbon from tropical dead trees to the atmosphere can be simply estimated using tree circumference at death time and wood density.
ISSN: 0029-8549

hal-01094187
https://hal-agroparistech.archives-ouvertes.fr/hal-01094187
DOI : 10.1007/s00442-010-1602-8

Éditeur(s) : HAL CCSD
Résumé : International audience
Radiocarbon dating and "old wood problem" in the Antilles : the state of the artAmong the many effects that may distort the interpretation of radiocarbon dates are those for which the age of the archaeological event is different from the radiocarbon age of the sample or "14C event" age. This type of problem arises in many cases, notably in the "old wood effect”, and occurs in various regions of the world where very long-lived trees species are common. In the Caribbean anthracological studies indicate a very high frequency of some indigenous centuries-old species such as Guaiacum, Guaiacum officinale L. However the absence of tree-ring data in humid tropical zones deprives us of an accurate inventory of long-lived species and especially the assessment of their longevity. Here we present measurements which enable us to estimate the longevity of archaeological Guaiacum wood posts. They highlight a gap of several centuries between wood from the outer edge and the heart of the tree. This example illustrates the additional uncertainty that can result from dating archeological events when dates are realized on unidentified botanical species from which the area that was sampled is unknown. Another source of error could arise from the use of rot-proof dead wood as fuel if we take into consideration that major hurricanes can produce large amounts of dead wood that may be collected for decades, even centuries. To improve chronometric data used in archaeological interpretations, it is necessary to ensure the quality of the dating process by following a protocol similar to those used in other regions of the world based on a better understanding of the material to be dated by preliminary identifications of wood samples: species, anatomical part, wood moisture content (unseasoned or seasoned).
Radiocarbono y "problema madera vieja", que data de las Antillas: estado del arteEntre los muchos efectos que pueden distorsionar la interpretación de fechas de radiocarbono, destacan aquellos para los que la edad de la prueba arqueológica es diferente de la edad de radiocarbono de la muestra, o la edad del "evento de radiocarbono." Este tipo de problema se presenta en muchos casos, especialmente uno llamado "efecto madera vieja." Esto se refleja en diversas regiones del mundo donde las especies de plantas de mucha longevidad son comunes. En el Caribe, los estudios antracológicos disponibles indican una muy alta frecuencia de algunas especies autóctonas como el guayaco, Guaiacum officinale L. Sin embargo, la casi ausencia de datos de anillos de árboles en los trópicos húmedos, nos impide realizar un inventario exacto de especies de mucha longevidad y sobre todo la evaluación de la longevidad. Algunas de las pruebas presentadas aquí permitieron una estimación de la longevidad de de unos postes de guayaco de origen arqueológico. Ponen de manifiesto una brecha de varios siglos entre las partes externas y el corazón del tronco. Este ejemplo ilustra la aparición de una incertidumbre adicional en la datación de los eventos arqueológicos cuando no conocemos la especie botánica y tampoco la zona de la muestra en el árbol. Otra fuente de error puede surgir de la utilización de la madera muerta de calidad duradera como combustible ya que los grandes huracanes pueden producir grandes cantidades de madera muerta, que puede ser requerida durante décadas, incluso siglos. Para mejorar los datos cronométricos utilizados en interpretaciones arqueológicas, es necesario asegurarse de la calidad de las fechas, y por lo tanto seguir un protocolo similar a los utilizados en otras regiones fundado en una mejor comprensión de los elementos: especie, parte anatómica, humedad de la madera usada (verde o seco).
Parmi les nombreux effets susceptibles de fausser l’interprétation des datations par le radiocarbone, figurent ceux pour lesquels l’âge de l’événement archéologique est différent de l’âge radiocarbone de l’échantillon, ou âge de l’« événement radiocarbone ». Ce type de problème se pose dans de nombreux cas, en particulier celui dénommé « effet vieux bois ». Cette question est prise en compte dans diverses régions du monde où des espèces végétales très longévives sont fréquentes. Dans les Antilles les études anthracologiques disponibles indiquent une fréquence très importante de certaines espèces autochtones multiséculaires dont le gaïac, Guaiacum officinale L.. Cependant, la quasi absence de données dendrochronologiques dans la zone tropicale humide, nous privent d’un inventaire précis des espèces longévives et surtout de l’évaluation de leur longévité. Quelques tests présentés ici ont permis une estimation de la longévité de poteaux d’origine archéologique en bois de gaïac. Ils mettent en évidence un décalage de plusieurs siècles entre les parties externes et le cœur du tronc. Cet exemple illustre l’apparition d’une incertitude supplémentaire dans la datation de l’événement archéologique lorsque que l’on ne connaît ni l’espèce botanique ni la zone de prélèvement dans l’arbre. Une autre source d’erreur pourrait provenir de l’utilisation de bois mort de qualité très durable comme combustible, sachant que les grands cyclones peuvent produire d’importantes quantités de bois mort susceptible d’être récolté pendant des décennies, voire des siècles. Afin d’améliorer les données chronologiques utilisées dans les interprétations archéologiques, il apparaît nécessaire de s’assurer de la qualité des datations, et donc de suivre un protocole similaire à ceux utilisés dans d’autres régions basé sur une meilleure connaissance des éléments datés : espèce, partie anatomique, humidité du bois utilisé (vert ou sec).
Puerto Rico 2013 : Actas del 25to Congreso Internacional de Arqueología del Caribe - Proceeding of the 25th International Congress for Caribbean Archeology - Comptes rendus des communications du 25e Congrés International de l'Archéologie de la Caraïbe
San-Juan, Puerto Rico

hal-01198567
https://hal.archives-ouvertes.fr/hal-01198567
https://hal.archives-ouvertes.fr/hal-01198567/document
https://hal.archives-ouvertes.fr/hal-01198567/file/Stouvenot%20et%20al.%202015.%20Effet%20vieux%20bois%20radiocarbone%20Antilles%20HD.pdf

Éditeur(s) : HAL CCSD Wiley
Résumé :  
1. Relationships between tropical rain forest biomass and environmental factors have been determined at regional scales, e.g. the Amazon Basin, but the reasons for the high variability in forest biomass at local scales are poorly understood. Interactions between topography, soil properties, tree growth and mortality rates, and treefalls are a likely reason for this variability. 2. We used repeated measurements of permanent plots in lowland rain forest in French Guiana to evaluate these relationships. The plots sampled topographic gradients from hilltops to slopes to bottomlands, with accompanying variation in soil waterlogging along these gradients. Biomass was calculated for >175 tree species in the plots, along with biomass productivity and recruitment rates. Mortality was determined as standing dead and treefalls. 3. Treefall rates were twice as high in bottomlands as on hilltops, and tree recruitment rates, radial growth rates and the abundance of light-demanding tree species were also higher. 4. In the bottomlands, the mean wood density was 10% lower than on hilltops, the basal area 29% lower and the height:diameter ratio of trees was lower, collectively resulting in a total woody biomass that was 43% lower in bottomlands than on hilltops. 5. Biomass productivity was 9% lower in bottomlands than on hilltops, even though soil Olsen P concentrations were higher in bottomlands. 6. Synthesis. Along a topographic gradient from hilltops to bottomlands there were higher rates of treefall, which decreased the stand basal area and favoured lower allocation to height growth and recruitment of light-demanding species with low wood density. The resultant large variation in tree biomass along the gradient shows the importance of determining site characteristics and including these characteristics when scaling up biomass estimates from stand to local or regional scales.
ISSN: 0022-0477

hal-01032100
https://hal.archives-ouvertes.fr/hal-01032100

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Decreasing growth rate (Feeley et al., 2007) and large die-back due to drought (Phillips et al., 2009) suggest that tropical forests are suffering recent climate changes. Forest vulnerability to external factors (e.g. air pollution, acid rain) is widely studied in northern countries, while only a few attempts have investigated crown integrity in the Tropics. The method needs to be generic enough to account for the large number of species and crown shapes encountered in tropical forests. In the present study, we developed and tested a novel field method that estimates crown fragmentation (main branch mortality (MB) and secondary branch mortality (SB)), liana infestation (LI) and crown position (CP) in the canopy. The relationship between crown fragmentation and annual growth rate (agr) was investigated through multiple regression. Six out of eight canopy tree species showed significant growth decline with increasing crown fragmentation. Higher probability of death was also found in trees with severe crown fragmentation. The capacity of such crown assessment to depict tree vitality in a forest stand is discussed along with potential applications in both forest science and management
ISSN: 0378-1127

halsde-00611465
https://hal.archives-ouvertes.fr/halsde-00611465
DOI : 10.1016/j.foreco.2010.10.025

Résumé : L'importance des forêts tropicales dans le cycle du carbone à l'échelle planétaire est majeure, tant en terme de stock qu’en terme de flux de CO2. Plusieurs études mettent en évidence des changements au sein des forêts tropicales au cours des 20 dernières années, notamment des changements de la dynamique forestière et une augmentation de la biomasse aérienne. Les déterminants de ces variations sont aujourd’hui discutés et nous proposons ici d’apporter une contribution à ce débat. Les données utilisées dans ce travail proviennent du dispositif de suivi forestier à long terme de Paracou, Guyane Française, mis en place en 1984 et qui recouvre plus de 120 ha de forêt tropicale humide. Les données météorologiques proviennent de la tour à flux du dispositif, Guyaflux. Les mesures de diamètre proviennent de la base Guyafor pour les données annuelles et bisannuelles, et des mesures de 260 arbres à proximité de la tour à flux pour les mesures diamétriques intra-annuelles.Cette thèse se divise en deux grandes parties. La première concerne l’analyse de la biomasse du dispositif de suivi forestier de Paracou en Guyane et l’implication des changements de structure de cette forêt sur le bilan de carbone. Cette partie est constituée de deux points. (i) Quelles échelles temporelles et spatiales sont pertinentes pour analyser les composantes de structure (biomasse, aire basale et nombre de tiges) et de dynamique (croissance, recrutement et mortalité) des forêts tropicales afin de minimiser les effets d’échantillonnage ? Nous avons établi une méthodologie permettant de relier les intervalles de temps et la surface de mesure aux coefficients de variation de chacune des variables de structure et de dynamique de la forêt. (ii) Quels processus démographiques sont prépondérants dans l’explication des variations de biomasse et comment se redistribue la biomasse accumulée dans le système ? L’augmentation de la biomasse observée sur le dispositif de Paracou serait liée à la rareté des évènements de mortalité des gros arbres qui portent une part très importante de la biomasse.La deuxième partie de la thèse concerne l’analyse de l’effet du climat à moyen terme, inter et intra-annuel, et les effets directs du climat dans les changements de dynamique de la forêt. Cette partie se divise en 3 points. (i) Comment quantifier le stress hydrique en forêt tropicale humide ? Nous avons réalisé un modèle journalier de réserve en eau du sol pour les arbres en forêt tropicale. (ii) Quelles variables sont explicatives de la croissance des arbres en forêt guyanaise ? Nous avons montré que l’eau dans le sol est le facteur le plus explicatif du déterminisme climatique parmi un panel de variables climatiques. (iii) Quels traits fonctionnels sont prédicteurs de la réponse des arbres aux variations climatique ? Nous avons déterminé que des traits spécifiques, densité du bois et la hauteur maximale, ainsi que le diamètre de l'arbre au moment de la mesure, modulent la croissance des arbres en réponse au climat.
At a global scale tropical forest play a major role in term of carbon stock as well as in term of CO2 fluxes. Several studies have highlighted changes in tropical forest functioning during the last 20 years including a faster turnover and an increase of above ground biomass. The drivers of these changes are discussed and throughout this thesis we propose to contribute to this debate. We use the data from the Paracou experimental site in French Guiana established in 1984 on 120 hectares of moist tropical forest. Meteorological data come from the flux tower of the site, Guyaflux. We use annual and bisannual diameter measurements from the Guyafor database, and intra-annual diameter increments from the measurements of 260 trees near the flux tower.This thesis has two main parts. In the first part we present the biomass analysis of the Paracou permanent plots and the impact of structural changes in this forest on the carbon budget. The first part is constituted by two points. (i) Which temporal and spatial scale used to analyze the structure (biomass, basal area and stem density) and dynamics (tree growth, recruitment and mortality)components of tropical forest in order to minimized sample bias ? We establish a simple method to rely measurement interval between census and surface of measurement to the coefficient of variation of forests structure and dynamic components (ii) Which demographic process are involved in the explanation of biomass variation and how the biomass is distributed in the system ? The observed increase of biomass at Paracou could be link to the rarity of big trees mortality events. These big trees represent the larger part of the biomass. In the second part, we present the analysis of intra and inter-annual climate variation effects on forest dynamic changes. This part is divided in two points. (i) How to model drought stress in moist tropical forest ? We built a daily water balance model for tropical trees. (ii) Which climate variables explain the tree growth in guianian forests? We shown that soil water availability is the determinant factor of tree growth among a panel of climate variables. (iii) Which functional traits are involved in the tropical tree growth responses to climate? In this analysis, we determined that wood specific gravity, maximum tree height and tree diameter modulate the tree growth response to climate variations.

http://www.theses.fr/2011AGUY0481/document

Éditeur(s) : HAL CCSD Cambridge University Press (CUP)
Résumé : We tested the response of seedlings of Dicorynia guianensis, a major timber tree species of French Guiana, to mycorrhizal symbiosis and water limitation in a semi-controlled experiment under natural light conditions. Under well-watered conditions, mycorrhizal colonization resulted in an increase of net photosynthesis, growth and phosphorus uptake. When submitted to water stress, no growth reduction of mycorrhizal seedlings was observed. Mycorrhizal seedlings were more sensitive to drought than non-mycorrhizal ones in terms of carbon assimilation, but not with regard to stomatal closure. In contrast to previous studies on temperate tree seedlings, this result precludes a mycorrhizal effect on the hydraulic properties of this species. Furthermore, our results suggest that below a specific threshold of soil moisture, carbon assimilation of D. guianensis seedlings was decreased by the mycorrhizal symbiosis. This is probably related to the competition between the plant and its host fungus for carbon allocation under low light intensity, even though it did not seem to have a significant effect on mortality in our experiment.
ISSN: 0266-4674

hal-01031901
https://hal.archives-ouvertes.fr/hal-01031901
DOI : 10.1017/S0266467405002348

Éditeur(s) : HAL CCSD Wiley
Résumé : Negative density dependence (NDD) and environmental filtering (EF) shape community assembly, but their relative importance is poorly understood. Recent studies have shown that seedlings mortality risk is positively related to the phylogenetic relatedness of neighbours. However, natural enemies, whose depredations often cause NDD, respond to functional traits of hosts rather than phylogenetic relatedness per se. To understand the roles of NDD and EF in community assembly, we assessed the effects on seedling mortality of functional similarity, phylogenetic relatedness and stem density of neighbouring seedlings and adults in a species-rich tropical forest. Mortality risks increased for common species when their functional traits departed substantially from the neighbourhood mean, and for all species when surrounded by close relatives. This indicates that NDD affects community assembly more broadly than does EF, and leads to the tentative conclusion that natural enemies respond to phylogenetically correlated traits. Our results affirm the prominence of NDD in structuring species-rich communities.
ISSN: 1461-023X

hal-01032429
https://hal.archives-ouvertes.fr/hal-01032429
DOI : 10.1111/j.1461-0248.2011.01705.x

Éditeur(s) : HAL CCSD Springer Verlag/EDP Sciences
Résumé : Key messageWe review the recent findings on the influence of drought on tree mortality, growth or ecosystem functioning in tropical rainforests. Drought plays a major role in shaping tropical rainforests and the response mechanisms are highly diverse and complex. The numerous gaps identified here require the international scientific community to combine efforts in order to conduct comprehensive studies in tropical rainforests on the three continents. These results are essential to simulate the future of these ecosystems under diverse climate scenarios and to predict the future of the global earth carbon balance.[br/]ContextTropical rainforest ecosystems are characterized by high annual rainfall. Nevertheless, rainfall regularly fluctuates during the year and seasonal soil droughts do occur. Over the past decades, a number of extreme droughts have hit tropical rainforests, not only in Amazonia but also in Asia and Africa. The influence of drought events on tree mortality and growth or on ecosystem functioning (carbon and water fluxes) in tropical rainforest ecosystems has been studied intensively, but the response mechanisms are complex.[br/]AimsHerein, we review the recent findings related to the response of tropical forest ecosystems to seasonal and extreme droughts and the current knowledge about the future of these ecosystems.[br/]ResultsThis review emphasizes the progress made over recent years and the importance of the studies conducted under extreme drought conditions or in through-fall exclusion experiments in understanding the response of these ecosystems. It also points to the great diversity and complexity of the response of tropical rainforest ecosystems to drought.[br/]ConclusionThe numerous gaps identified here require the international scientific community to combine efforts in order to conduct comprehensive studies in tropical forest regions. These results are essential to simulate the future of these ecosystems under diverse climate scenarios and to predict the future of the global earth carbon balance.
ISSN: 1286-4560

hal-01269438
https://hal.archives-ouvertes.fr/hal-01269438
DOI : 10.1007/s13595-015-0522-5
PRODINRA : 338568

Éditeur(s) : HAL CCSD
Résumé : International audience
Objective: In order to determine how risk factors of death among HIV-infected patients in French Guiana interact with one another and to define high risk population segments, an alternative statistical method to Cox proportional hazards models was used. Patients and methods: Data from HIV+ patients followed in the three main hospitals of French Guiana were used. Data were extracted from the prospective French Hospital Database for HIV (FHDH). To examine the nature of the relationships between time of death since the first consultation and a set of predictor variables (age at enrolment, gender, nationality, CD4 count at enrolment, presence or absence of HAART at enrolment, a declared addiction, and mode of acquisition of the virus) a survival tree analysis was used. Survival rates at 5 years and 10 years and their 95% confidence interval were also calculated. Results: This analysis permitted to reveal a new specific subgroup of HIV+ patients particularly at risk in French Guiana not already mentioned in the literature: the HIV+ female patients with a declared addiction. Conclusions: Prevention and care in this vulnerable segment of the HIV+ population identified by this methodology should be reinforced.
J AIDS Clinic Res

Droits : info:eu-repo/semantics/OpenAccess
inserm-00914956
http://www.hal.inserm.fr/inserm-00914956
http://www.hal.inserm.fr/inserm-00914956/document
http://www.hal.inserm.fr/inserm-00914956/file/2012_Hanf_-_Hierarchysur Determinants_Underlying_Death_among_HIV-Infected_patients_in_French_Guiana.pdf
DOI : 10.4172/2155-6113.1000172

Éditeur(s) : HAL CCSD
Résumé : Atmospheric carbon dioxide records indicate that the land surface has acted as a strong global carbon sink over recent decades, with a substantial fraction of this sink probably located in the tropics, particularly in the Amazon. Nevertheless, it is unclear how the terrestrial carbon sink will evolve as climate and atmospheric composition continue to change. Here we analyse the historical evolution of the biomass dynamics of the Amazon rainforest over three decades using a distributed network of 321 plots. While this analysis confirms that Amazon forests have acted as a long-term net biomass sink, we find a long-term decreasing trend of carbon accumulation. Rates of net increase in above-ground biomass declined by one-third during the past decade compared to the 1990s. This is a consequence of growth rate increases levelling off recently, while biomass mortality persistently increased throughout, leading to a shortening of carbon residence times. Potential drivers for the mortality increase include greater climate variability, and feedbacks of faster growth on mortality, resulting in shortened tree longevity. The observed decline of the Amazon sink diverges markedly from the recent increase in terrestrial carbon uptake at the global scale, and is contrary to expectations based on models.
ISSN: 1476-4687

hal-01204223
https://hal.archives-ouvertes.fr/hal-01204223
DOI : 10.1038/nature14283
PRODINRA : 294797
PUBMED : 25788097

Éditeur(s) : HAL CCSD Wiley
Résumé : Understanding the mechanisms generating species distributions remains a challenge, especially in hyperdiverse tropical forests. We evaluated the role of rainfall variation, soil gradients and herbivory on seedling mortality, and how variation in seedling performance along these gradients contributes to habitat specialisation. In a 4-year experiment, replicated at the two extremes of the Amazon basin, we reciprocally transplanted 4638 tree seedlings of 41 habitat-specialist species from seven phylogenetic lineages among the three most important forest habitats of lowland Amazonia. Rainfall variation, flooding and soil gradients strongly influenced seedling mortality, whereas herbivory had negligible impact. Seedling mortality varied strongly among habitats, consistent with predictions for habitat specialists in most lineages. This suggests that seedling performance is a primary determinant of the habitat associations of adult trees across Amazonia. It further suggests that tree diversity, currently mostly harboured in terra firme forests, may be strongly impacted by the predicted climate changes in Amazonia.
ISSN: 1461-023X

hal-01531751
https://hal.archives-ouvertes.fr/hal-01531751
DOI : 10.1111/ele.12661
PRODINRA : 387284

Éditeur(s) : HAL CCSD American Society of Hematology
Résumé : International audience
Human immunodeficiency virus (HIV)-associated disseminated Histoplasma capsulatum capsulatum infection often mimics tuberculosis. This disease is well know in the United States but is dramatically underdiagnosed in Central and South America. In the Amazon region, given the available incidence data and the regional HIV prevalence, it is expected that, every year, 1,500 cases of histoplasmosis affect HIV patients in that region alone. Given the mortality in undiagnosed patients, at least 600 patients would be expected to die from an undiagnosed but treatable disease. The lack of a simple diagnostic tool and the lack of awareness by clinicians spiral in a vicious cycle and made a major problem invisible for 30 years. The HIV/acquired immunodeficiency syndrome community should tackle this problem now to prevent numerous avoidable deaths from HIV-associated histoplasmosis in the region and elsewhere.
ISSN: 0006-4971

inserm-00913874
http://www.hal.inserm.fr/inserm-00913874
http://www.hal.inserm.fr/inserm-00913874/document
http://www.hal.inserm.fr/inserm-00913874/file/2013_Nacher_-_How_many_have_died_from_undiagnosed_histoplasmosis.pdf
DOI : 10.4269/ajtmh.13-0226

Éditeur(s) : HAL CCSD Montpellier : CIRAD
Résumé : This article presents the effects of logging on the dynamics of above-ground biomass from the results of the post-logging study within two forests: Cikel in Eastern Para, Brazil and Paracou in French Guiana. The main objective is to compare the impact of commercial logging on the regeneration of the above-ground biomass in these forests whose characteristics differ in terms of structure and growth. In both sites, the intensity of exploitation is a key factor in determining the loss of biomass and the time required for its regeneration. In Paracou, the regeneration of biomass lost during conventional logging of 10 trees per hectare takes 45 years and more than 100 years when operating with higher intensity (21 trees/ha). In Cikei the forest biomass regenerates after 49 years harvesting 6 trees/ha and that takes 87 years after removal of 8 trees/ha. This regeneration needs similar time on both sites but with lower logging intensity at Cikel, in which felled trees are larger with a greater biomass than those of Paracou. This post-logging study has established a direct correlation of the dynamics of the biomass with the initial structure of the forest, as well as with the parameters of forest dynamics: mortality, growth and recruitment. The accumulation of biomass by the tree growth of the two remaining stands is a key parameter for the net carbon storage, while the contribution of recruitment in Paracou becomes significant only after 10 years after felling. Therefore in view to improve the growth of residual trees, it is compulsory to apply adequate silvicultural treatments such as selective thinning or removal of vines. While the two forests are geographically close enough, their regenerative abilities differ and because of the significant difference in size of the trees, the forest could tolerate more intensive harvesting in French Guiana
ISSN: 0006-579X

hal-01032427
https://hal.archives-ouvertes.fr/hal-01032427

Éditeur(s) : HAL CCSD Nature Publishing Group
Résumé : Tropical forests store large amounts of biomass despite they generally grow in nutrient-poor soils, suggesting that the role of soil characteristics in the structure and dynamics of tropical forests is complex. We used data for >34 000 trees from several permanent plots in French Guiana to investigate if soil characteristics could predict the structure (tree diameter, density and aboveground biomass), and dynamics (growth, mortality, aboveground wood productivity) of nutrient-poor tropical forests. Most variables did not covary with site-level changes in soil nutrient content, indicating that nutrient-cycling mechanisms other than the direct absorption from soil (e.g. the nutrient uptake from litter, the resorption, or the storage of nutrients in the biomass), may strongly control forest structure and dynamics. Ecosystem-level adaptations to low soil nutrient availability and long-term low levels of disturbance may help to account for the lower productivity and higher accumulation of biomass in nutrient-poor forests compared to nutrient-richer forests.
ISSN: 2045-2322

hal-01606170
https://hal.archives-ouvertes.fr/hal-01606170
DOI : 10.1038/srep45017
PRODINRA : 389876