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

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

Ex vitro survival, rooting and initial development on in vitro rooted vs unrooted microshoots from juvenile and mature Tectona grandis genotypes

L'influence de l'appareil racinaire de type adventif formé in vitro sur l'acclimatation et les premiers stades de développement ex vitro de microboutures de génotypes juvéniles et mature de teck (Tectona grandis) a été étudiée. Globalement, 7 semaines après leur transfert, les microboutures enracinées in vitro se distinguent par des taux de survie et d'enracinement ex vitro supérieurs à leurs homologues non enracinées in vitro. L'influence de l'âge se ressent au niveau des taux de mortalité plus élevés 7 et 15 semaines après le transfert pour les microboutures provenant du clone mature. Le nombre de racines formées in vitro influe sur le nombre de racines développées en conditions ex vitro 7 semaines après le transfert, ainsi que sur la hauteur des microboutures à la date du transfert, après 7 semaines, et dans une moindre mesure après 15 semaines. Les différences de hauteur mises en évidence entre les deux origines à la date du transfert tendent à s'estomper durant l'acclimatation. Plus de 80% de l'ensemble des microboutures transférées ex vitro ont pu être acclimatés avec succès. Ces résultats sont discutés en considérant principalement l'impact du phénomène de maturation sur la rhizogénèse ex vitro de microboutures issues d'in vitro, ainsi que l'influence de certains facteurs exogènes prépondérants. (Résumé d'auteur

Pan-Tropical Analysis of Climate Effects on Seasonal Tree Growth

Climate models predict a range of changes in tropical forest regions, including increased average temperatures, decreased total precipitation, reduced soil moisture and alterations in seasonal climate variations. These changes are directly related to the increase in anthropogenic greenhouse gas concentrations, primarily CO2. Assessing seasonal forest growth responses to climate is of utmost importance because woody tissues, produced by photosynthesis from atmospheric CO2, water and light, constitute the main component of carbon sequestration in the forest ecosystem. In this paper, we combine intra-annual tree growth measurements from published tree growth data and the corresponding monthly climate data for 25 pan-tropical forest sites. This meta-analysis is designed to find the shared climate drivers of tree growth and their relative importance across pan-tropical forests in order to improve carbon uptake models in a global change context. Tree growth reveals significant intra-annual seasonality at seasonally dry sites or in wet tropical forests. Of the overall variation in tree growth, 28.7% was explained by the site effect, i.e. the tree growth average per site. The best predictive model included four climate variables: precipitation, solar radiation (estimated with extrasolar radiation reaching the atmosphere), temperature amplitude and relative soil water content. This model explained more than 50% of the tree growth variations across tropical forests. Precipitation and solar radiation are the main seasonal drivers of tree growth, causing 19.8% and 16.3% of the tree growth variations. Both have a significant positive association with tree growth. These findings suggest that forest productivity due to tropical tree growth will be reduced in the future if climate extremes, such as droughts, become more frequent

The water relations of two tropical rainforest species (Virola surinamensis and Eperua falcata): Is Virola unusual as previously reported?

HYDROInternational audienceThe objective of this study was to examine the water relations and hydraulic architecture and vulnerability to cavitation in Virola surinamensis and V. michelii and to compare to similar measurements in Eperua falcata. In several previous reports Virola was seen to have a rather narrow range of xylem pressure potentials (Ψx) near zero in the course of a wet-season day while having water fluxes quite close to Eperua. We tested the hypothesis that the narrow range of Ψx might be consistent with very high hydraulic conductivities of stems, roots and shoots and high vulnerability to cavitation in Virola compared to Eperua. When this hypothesis proved false we concluded that the previous determinations of Ψx might be wrong in Virola due to latex. We re-examined the determination of Ψx in Virola by the pressure chamber technique and compared results to determination of Ψleaf by the thermocouple psychrometer technique and found that the likely range of Ψx are more negative than previously reported. Problems concerning the determination of Ψx in species with latex are discussed

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

International audienc

Performance Trade‐Offs Among Tropical Tree Seedlings In Contrasting Microhabitats

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/117031/1/ecy20058692461.pd

Tree mortality and Extreme Rainfall in the Amazon

Mesoscale Convective Systems (MCS) are responsible for severe rainfall in the Amazon and can produce strong descending winds that can uprooting or break trees the most dominant mode of tree mortality in the Amazon. Our results show that severe rainfall help to explain the observed residence time of woody biomass and tree mortality in the Amazon

Global parameterization and validation of a two-leaf light use efficiency model for predicting gross primary production across FLUXNET sites:TL-LUE Parameterization and Validation

Light use efficiency (LUE) models are widely used to simulate gross primary production (GPP). However, the treatment of the plant canopy as a big leaf by these models can introduce large uncertainties in simulated GPP. Recently, a two-leaf light use efficiency (TL-LUE) model was developed to simulate GPP separately for sunlit and shaded leaves and has been shown to outperform the big-leaf MOD17 model at six FLUX sites in China. In this study we investigated the performance of the TL-LUE model for a wider range of biomes. For this we optimized the parameters and tested the TL-LUE model using data from 98 FLUXNET sites which are distributed across the globe. The results showed that the TL-LUE model performed in general better than the MOD17 model in simulating 8 day GPP. Optimized maximum light use efficiency of shaded leaves (εmsh) was 2.63 to 4.59 times that of sunlit leaves (εmsu). Generally, the relationships of εmsh and εmsu with εmax were well described by linear equations, indicating the existence of general patterns across biomes. GPP simulated by the TL-LUE model was much less sensitive to biases in the photosynthetically active radiation (PAR) input than the MOD17 model. The results of this study suggest that the proposed TL-LUE model has the potential for simulating regional and global GPP of terrestrial ecosystems, and it is more robust with regard to usual biases in input data than existing approaches which neglect the bimodal within-canopy distribution of PAR