Which climate indices are relevant for predicting the response of tropical forests to future climate scenarios ?

Climate models predict a range of changes in the amazonian region, including increased frequency of extreme climatic events, increased average temperatures, increased atmospheric CO2 and reduced rainfall intensity. Understanding tree growth response to climate is important because wood production is the main way carbon enters the forest ecosystem. The response of tropical tree growth to changing climate could drive a change in the direction of the flux from terrestrial ecosystems to the atmosphere. The last 20 years has seen a substantial increase in the number of publications with a focus on the effects of climate on tropical tree growth. Citation reports extracted from the Web of Science with the keywords 'climate', 'tropical forest', 'growth' and 'trees' found only 15 articles in 2000 and more than 70 in 2011. These articles are based mostly on modelling approaches combining tree growth and one or more climate indices. We review the different approaches used in order to, (i) identify gaps in current research, (ii) determine which climate index is the most relevant for a range of temporal and spatial scales and (iii) establish which climate indices are the most appropriate to study tropical tree growth in regards to the climate output of the IPCC scenarios. (Résumé d'auteur

Coupling growth and mortality models to detect climate drivers of tropical forest dynamics

Climate models for the coming century predict rainfall reduction in the Amazonian region, including deep changes in water availability for tropical rainforests. Here, we develop an integrated modeling framework in order test the extent to which climate variables related to water regime, temperature and irradiance shape the long-term dynamics of neotropical forests. In a first step, a Bayesian hierarchical model was built to couple tree growth and tree mortality processes into a single modeling framework. Coupling a longitudinal growth model with a punctual mortality model was not an easy task. Past growth, related to the expected growth, was used as an indicator of the individual tree vigor, which is supposed to play a key role in the mortality process. A MCMC approach is used to estimate all the parameters simultaneously. The individual-centered model was explicitly designed to deal with diverse sources of uncertainty, including the complexity of the mortality process itself and the field data, especially historical data for which taxonomic determinations were uncertain. Functional traits are integrated as proxies of the ecological strategies of the trees and permit generalization among all species in the forest community. Data used to parameterize the model were collected at Paracou study site, a tropical rain forest in French Guiana, where 20,408 trees have been yearly censured over 18 years. Climate covariates were finally added as external drivers of the forest dynamics. These drivers are selected in a list of climate variables for which future predictions are available thanks to the IPCC scenario. Amongst climate variables, we highlight the predominant role of water availability in determining interannual variation in the dynamic of neotropical forests. And we stressed the need to include these relationships into forest simulators to test, in silico, the impact of different climate scenarios on the future dynamics of the rainforest. (Texte intégral

Assessing the impact of water availability on growth of neotropical trees

Climate modeling scenarios predict deep changes of the rainfall regime in Amazonia for the next century. The expected increase in dry season length would lead to less water availability for trees. Every month during 3 years, growth measurement was recorded for more than 200 neotropical trees. Tree daily water availability during this period was simulated making use of a soil water balance model. Using a bayesian modeling framework, we analysed (i) the effect of water availability on tree growth and (ii) how functional trait modulated tree responses to drought. For most studied species, tree growth was deeply affected by seasonal water availability. Trees exhibited different water stress sensibilities linked to diffeent functional strategies, from a decrease to a stop in radial stem growth during the drought season. Largest increments were observe at the beginning of the rain season. The remaining part of unexplained variance, may be attributed to ressources competition, indiviual phenology, particular history or genetic. Our results suggest that the intensity of the annual dry season have a large impact on annual tree growth. If rainfall reduction is confirmed in the next decades, tree growth, and hence the whole forest dynamic, will undoubtely be modified. (Résumé d'auteur

What is the carbon balance of tropical managed forests?

Managed forests are a major component of tropical landscapes and almost half of standing primary tropical forests, up to 400 million ha, are designated by national forest services for timber production. However, so far, most of our understanding of the tropical forest carbon cycle yields is from plot networks located in old-growth undisturbed forests while the carbon balance of managed forests at the regional and continental scale remains poorly studied. Here we propose a methodological framework in order to quantify the carbon footprint of selective logging at a regional scale. The yearly balance of a logged forest unit is modeled by aggregating 3 submodels dealing with (i) emissions from extracted wood, (ii) emissions from logging damages and (iii) storage from biomass recovery after logging. Models are parameterized and uncertainties are propagated through a MCMC algorithm. We used the 30-years statistics from the National Forest Service to estimate the carbon balance of managed forests in French Guiana. Over this period, selective logging emitted 0.76 Tg C in the atmosphere. Our results highlight the key role of the local carbon cycle in managed forests for climate regulation at the global scale. (Texte intégral

Modeling soil water availability for neotropical forest trees

The aim of this study was to model the soil water availability to neotropical trees. We developed a discrete-time deterministic water balance model and validated it on a large field data set. Because of some data inaccuracy, the validátion step required us to develop a specific optimization procedure. The model computes daily water fluxes (tree transpiration, understorey evapotranspiration, rainfall interception, and drainage) and soil water content using 3 input variables: daily precipitation, potential evapotranspiration PET and solar radiation. Input data came from a flux tower (Guyaflux) located at the Paracou experimental site (5o 18' N, 52o 55 'W), a lowland tropical rain forest in French Guiana, provided on a half hourly basis a set of meteorological data among which precipitations, temperatures, solar radiations, wind and humidity and PET. The daily output is the relative water available for trees, i.e. the daily available water standardized by the potential available water. Since 2004, soil water moisture was measured monthly with a TDR probe in twenty 3-meters tubes on a representative topographic gradient of Paracou (hilltop, slope, lowland). As the probe was not calibrated with field measurements, we propose an original method to optimize model parameters. This method is based on the variance minimization of the ratio soil water predicted on the soil water probe measurements. Our soil water balance model succeeds in capturing the dynamics of available water for the trees for most of the topographic positions. A sensitivity analysis was performed to determine which parameters were the most influent on the model output. Our results provide a simple method to better use meteorological data in tropical forests and to understand the tropical forest responses to global change. (Résumé d'auteur

Coupling growth and mortality models to detect climate drivers of tropical forest dynamics

Climate models for the coming century predict rainfall reduction in the Amazonian region, including deep changes in water availability for tropical rainforests. Here, we develop an integrated modeling framework in order test the extent to which climate variables related to water regime, temperature and irradiance shape the long-term dynamics of neotropical forests. In a first step, a Bayesian hierarchical model was built to couple tree growth and tree mortality processes into a single modeling framework. Coupling a longitudinal growth model with a punctual mortality model was not an easy task. Past growth, related to the expected growth, was used as an indicator of the individual tree vigor, which is supposed to play a key role in the mortality process. A MCMC approach is used to estimate all the parameters simultaneously. The individual-centered model was explicitly designed to deal with diverse sources of uncertainty, including the complexity of the mortality process itself and the field data, especially historical data for which taxonomic determinations were uncertain. Functional traits are integrated as proxies of the ecological strategies of the trees and permit generalization among all species in the forest community. Data used to parameterize the model were collected at Paracou study site, a tropical rain forest in French Guiana, where 20,408 trees have been yearly censured over 18 years. Climate covariates were finally added as external drivers of the forest dynamics. These drivers are selected in a list of climate variables for which future predictions are available thanks to the IPCC scenario. Amongst climate variables, we highlight the predominant role of water availability in determining interannual variation in the dynamic of neotropical forests. And we stressed the need to include these relationships into forest simulators to test, in silico, the impact of different climate scenarios on the future dynamics of the rainforest. (Texte intégral

Will tropical forests face slow down with ongoing climate changes? [O-2215-04]

In the context of climate changes, identifying and then predicting the impacts of climatic drivers on tropical forest dynamics is becoming a matter of urgency. We used a coupled model of tropical tree growth and mortality, calibrated with forest dynamic data from the 20 year study site of Paracou, French Guiana, in order to introduce and test a set of climatic variables. Three major climatic drivers of the tropical forest dynamics were identified through the variable selection procedure: drought, water saturation and temperature. Drought decreased annual growth and mortality rates, high precipitation increased mortality rates and high temperature decreased growth. Interactions between key functional traits, stature and climatic variables were investigated, showing best resistance to drought for trees with high wood density and for trees with small current diameters. We then used SELVA, an individual-based model to run forest dynamic simulations for the next century using predictions from the IPCC 5AR with 3 different scenarios corresponding to 3 relative concentration pathways. Basal area, above-ground fresh biomass, quadratic diameter, growth and mortality rates exhibited decreasing values as long as the scenario became pessimistic. Temperature is the strongest driver highlighting a drop of 40% in average forest growth for the RCP8.5. Our results highlights the potential slowdown danger that tropical forests will face during the next century. (Texte intégral

Effects of logging on forest stand carbon recovery and tree biomass

We will present recent findings arising from the Tropical managed Forests Observatory. Results from the first regional analysis of above-ground carbon stocks dynamics post-logging in Amazon managed forests will be presented. We found that the percentage of initial carbon lost during logging was the main driver of post-logging dynamic, enabling on his own to accurately predict time of recovery wherever in the Amazon Basin.Moving from forest stand to tree responses, we show how logging, by releasing competition for light, may affect trees morphology. We found a significant reduction of both total and bole heights proportional to logging intensity in a tropical logged forest in French Guiana. This resulted in a 10-13% reduction of tree biomass and timber volume. These results will be discussed in terms of future management and provision of ecosystem services in tropical production forests. (Texte intégral

Confronting satellite and field measurement data to improve the understanding of carbon uptake by tree growth in French Guiana

Climate models predict a range of changes in the amazonian region, including increased frequency of extreme climatic events, increased average temperatures, increased atmospheric CO2 and reduced rainfall intensity. Understanding tree growth response to climate is important because wood production is the main way carbon enters the forest ecosystem. The response of tropical tree growth to changing climate could drive a change in the direction of the flux from terrestrial ecosystems to the atmosphere. Recently, the intra-annual variation of chlorophyll activity in Amazonia and in French Guiana has been assessed with the Enhanced Vegetation Index (EVI) from the MODIS satellite data. As unexpected, peak of biomass increment (early wet season), estimated with diameter increment, were not correlated with peak of chlorophyll activity (early dry season) in French Guiana. In our assumption, this could reflect different timing in the use of photosynthesis products by the plant for primary growth, i.e. shoot growth and leaves production, and secondary growth, i.e. diameter increment. Here we use three datasets covering French Guiana at an intra-annual time scale, MODIS EVI data, modeled intra-annual tree growth data and climate data, to (i) disentangling the timing of carbon use by the trees for primary growth and secondary growth; (ii) analyze the climate determinants of these two components of growth; and (iii) predict the effect of climate change with IPCC scenarios outputs on the carbon entry in the forests of French Guiana. (Résumé d'auteur

Bilan Carbone de l'exploitation forestière sur le domaine forestier permanent de Guyane française

International audienceWe propose a set-valued controller with a signum multifunction nested inside another one. We prove that the controller is well posed and achieves robust ultimate boundedness in the presence of mismatched, non-vanishing disturbances. Even more, the selected output can be made arbitrarily small. Also, by applying an implicit/explicit Euler scheme similar to the one introduced by Acary and Brogliato (2010) for matched disturbances, we derive a selection strategy for the discrete-time implementation of the set-valued control law. Simulations demonstrate that the discrete scheme diminishes chattering substantially, compared with a fully explicit method