A model for simulating structure-function relationships in walnut tree growth processes.

An ecophysiological growth process model, called INCA, for simulating the growth and development of a young walnut tree (Juglans regia L.) during three or four years, is presented. This tool, currently under development, aims at integrating architectural and physiological knowledge of the processes involved, in order to give a more rational understanding of the pruning operation. The model describes a simple three-dimensional representation of tree crown, solar radiation interception, photosynthesis, respiration, growth and partitioning of assimilates to leaves, stems, branches and roots. It supports the hypothesis that the tree grows as a collection of semiautonomous, interacting organs that compete for resources, based on daily sink strengths and proximity to sources. The actual growth rate of organs is not predetermined by empirical data, but reflects the pattern of available resources. The major driving variables are solar radiation, temperature, topological, geometrical and physiological factors. Outputs are hourly and daily photosynthate production and respiration, daily dimensional growth, starch storage, biomass production and total number of different types of organ. The user can interact or override any or all of the input variables to examine the effects of such changes on photosynthate production and growth. Within INCA, the tree entities and the surrounding environment are structured in a frame-based representation whereas the processes are coded in a rule-based language. The simulation mechanism is primarily based on the rule chaining capabilities of an inference engine

Intercomparison of model predictions of 14C concentrations in agricultural plants following acute exposures to airborne 14C

International audienceCarbon-14 (14C) is one of the main radionuclides released during normal operation by nuclear power plants, nuclear defense facilities and nuclear fuel reprocessing plants. It is mainly released in the form of carbon dioxide gas denoted 14CO2, which has the specificity of being incorporated into food webs via photosynthesis by primary producing organisms. In order to better assess the environmental and human impacts of 14CO2 under normal operating conditions - or after potential accidental releases - from nuclear facilities, it is necessary to improve our understanding and our predictions of the behaviour of this radionuclide along the human food chain. To achieve this goal, the International Atomic Energy Agency (IAEA) Environmental Modelling for Radiation Safety (EMRAS) model evaluation programme included the Tritium and 14C Working Group (TCWG) which dealt with the intercomparison exercises between several models of environmental transfer in the case of routine and accidental releases of these radionuclides into the environment, and their performance testing. The TOCATTA-χ model developed at IRSN is a dynamic compartment model with high temporal resolution, which simulates the transfer of 14C (and tritium) in grassland ecosystems exposed to gaseous 14CO2 (and HTO) from nuclear facilities under normal or accidental operating conditions. Following this work, IRSN proposed a related project to extend the application of the TOCATTA-χ model to 14C estimates in leafy vegetables, fruits and roots. This article deals with the application of the TOCATTA-χ model to a specific real-case scenario identified within the framework of the TCWG. The scenario provides experimental data and predicted results from models developed at the international level. Model-model and model-data intercomparison exercises were thus carried out to validate the evaluations of the TOCATTA-χ model. In addition, this paper discusses the parameterization of the TOCATTA-χ model for this scenario and the development of modules for 14C concentrations in potato tubers, based on the assumption that photosynthetic transfer occurs directly from leaves to tubers and depends mainly on the growth stage of the tubers. It is observed that the predictions of the TOCATTA-χ model for the concentrations of 14C in leaves and tubers are slightly better than the other models due to the modelling approaches adopted by TOCATTA-χ for the calculation of key ecophysiological processes that govern plant functioning. Overall, the TOCATTA-χ model reduces the Root Mean Square Error (RMSE) by a factor of less than 8 compared to other models. In addition, most of the predicted results of the TOCATTA-χ model better match the measurements and are within the measurement uncertainty limit, while a few are overestimated. This could be due to the high uncertainty associated with the experimentally measured 14C activities, which reflects the field variability in plant growth rate

Carbon-based models of individual tree growth: A critical appraisal

Twenty-seven individual tree growth models are reviewed. The models take into account the same main physiological processes involved in carbon metabolism (photosynthate production, respiration, reserve dynamics, allocation of assimilates and growth) and share common rationales that are discussed. It is shown that the spatial resolution and representation of tree architecture used mainly depend on model objectives. Beyond common rationales, the models reviewed exhibit very different treatments of each process involved in carbon metabolism. The treatments of all these processes are presented and discussed in terms of formulation simplicity, ability to account for response to environment, and explanatory or predictive capacities. Representation of photosynthetic carbon gain ranges from merely empirical relationships that provide annual photosynthate production, to mechanistic models of instantaneous leaf photosynthesis that explicitly account for the effects of the major environmental variables. Respiration is often described empirically as the sum of two functional components (maintenance and growth). Maintenance demand is described by using temperature-dependent coefficients, while growth efficiency is described by using temperature-independent conversion coefficients. Carbohydrate reserve pools are generally represented as black boxes and their dynamics is rarely addressed. Storage and reserve mobilisation are often treated as passive phenomena, and reserve pools are assumed to behave like buffers that absorb the residual, excessive carbohydrate on a daily or seasonal basis. Various approaches to modelling carbon allocation have been applied, such as the use of empirical partitioning coefficients, balanced growth considerations and optimality principles, resistance mass-flow models, or the source-sink approach. The outputs of carbon-based models of individual tree growth are reviewed, and their implications for forestry and ecology are discussed. Three critical issues for these models to date are identified: (i) the representation of carbon allocation and of the effects of architecture on tree growth is Achilles' heel of most of tree growth models; (ii) reserve dynamics is always poorly accounted for; (iii) the representation of below ground processes and tree nutrient economy is lacking in most of the models reviewed. Addressing these critical issues could greatly enhance the reliability and predictive capacity of individual tree growth models in the near future.Les modèles de croissance d'individus arbres basés sur le fonctionnement carboné : une évaluation critique. Vingt-sept modèles simulant la croissance d'arbres à l'échelle individuelle sont évalués. Ces modèles prennent en compte les principaux processus impliqués dans le métabolisme carboné (assimilation photosynthétique, respiration, dynamique des réserves, allocation des assimilats et croissance). Les concepts communs à tous ces modèles sont discutés. Il est montré que l'échelle d'espace et la représentation de l'architecture utilisées dépendent principalement des objectifs du modèle. Au-delà de concepts communs, les modèles évalués utilisent des représentations très différentes pour chacun des processus impliqués dans le métabolisme carboné. Les différentes représentations de ces processus sont présentées et discutées en termes de simplicité de formulation, de capacité à prendre en compte la réponse aux variables environnementales, et de capacités prédictives. La représentation des gains de carbone va de relations purement empirique calculant la production annuelle de photosynthétats jusqu'à des modèles de photosynthèse foliaire à bases mécanistes prenant explicitement en compte les effets des principales variables environnementales. La respiration est souvent décrite de façon empirique comme la somme de deux composantes (maintenance et croissance). La demande de maintenance est calculée à partir de coefficients dépendant de la température, alors que l'efficience de croissance est calculée à partir de coefficients de conversion indépendant de la température. Les réserves carbonées sont généralement représentées comme des boîtes noires, et leur dynamique est rarement prise en compte. La mise en réserve et l'utilisation des réserves sont souvent traitées comme des processus passifs, les réserves servant souvent de compartiment tampon absorbant les assimilats produits en excès sur une base journalière ou saisonnière. De nombreuses approches ont été utilisées pour modéliser l'allocation de carbone, telles que l'utilisation de coefficients d'allocation empiriques, l'application des principes de l'équilibre fonctionnel et d'optimisation, l'utilisation de schémas flux-résistance, ou des approches sources-puits. Les sorties des modèles simulant le bilan carboné et la croissance de plantes ligneuses à l'échelle individuelle sont présentées, et leurs implications en foresterie et en écologie sont discutées. Trois points particulièrement critiques actuellement pour ces modèles sont identifiés : (i) la représentation de l'allocation du carbone et des effets de l'architecture sur la croissance de l'arbre est le talon d'Achille de la majorité de ces modèles ; (ii) la dynamique des réserves est toujours faiblement représentée ; (iii) la représentation du fonctionnement racinaire et de la gestion des nutriments dans l'arbre est absente dans presque tous les modèles évalués. Une meilleure prise en compte de ces points critiques devrait fortement améliorer la fiabilité et les capacités prédictives des modèles de croissance d'arbres à l'échelle individuelle dans le futur

The TOCATTA-chi model for assessing C-14 transfers to grass: an evaluation for atmospheric operational releases from nuclear facilities

International audienceRadioactive C-14 is formed as a by-product of nuclear power generation and from the operation of nuclear fuel reprocessing plants like AREVA-NC La Hague (North France), which releases about 15 TBq per year of C-14 into the atmosphere. This article evaluates a recently improved radioecology model (TOCATTA-chi) to assess C-14 transfers to grassland ecosystems under normal operating conditions. The new version of the TOCATTA model (TOCATTA-chi) includes developments that were derived from PaSiM, a pasture model for simulating grassland carbon and radiocarbon cycling. The TOCATTA-chi model has been tested against observations of C-14 activity concentrations in grass samples collected monthly from six plots which are located around the periphery of the reprocessing plant. Simulated C-14 activities are consistent with observations on both intensively managed and poorly managed grasslands, but an adaptation of the mean turn-over time for C-14 within the plant is necessary in the model to account for different management practices. When atmospheric C-14 activity concentrations are directly inferred from observations, TOCATTA-chi performs better than TOCATTA (the root mean square error is decreased by 45%), but when atmospheric C-14 activity concentrations are not known and must be calculated, the uncertainty associated with the TOCATTA-chi model outcomes is estimated to be larger than the standard deviation of the observations. (C) 2013 Elsevier Ltd. All rights reserved