Results from the months of January, April, and July are analyzed to demonstrate the model behavior in different environmental conditions. The site experiences a drought season during the summer months in which the vegetation becomes water stressed. Flux simulations are compared with Eddy Covariance field measurements collected from 2004 to 2007. The Advanced Canopy–Atmosphere–Soil Algorithm (ACASA) model is used to predict energy, water and carbon fluxes over a Mediterranean maquis site located in North-Western Sardinia (Italy) and the model performance is evaluated. Addressing these critical issues could greatly enhance the reliability and predictive capacity of individual tree growth models in the near future 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. The outputs of carbon-based models of individual tree growth are reviewed, and their implications for forestry and ecology are discussed. 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. 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.
Carbohydrate reserve pools are generally represented as black boxes and their dynamics is rarely addressed. Maintenance demand is described by using temperature-dependent coefficients, while growth efficiency is described by using temperature-independent conversion coefficients. Respiration is often described empirically as the sum of two functional components (maintenance and growth). 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. 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. Beyond common rationales, the models reviewed exhibit very different treatments of each process involved in carbon metabolism. It is shown that the spatial resolution and representation of tree architecture used mainly depend on model objectives.
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. Twenty-seven individual tree growth models are reviewed.
The main results of the simulations were: (i) spatial variations of stomatal conductance were mainly responsible for the important spatial variation of the leaf-to-air decoupling factor () within the tree crown (ii) high coupling was observed at tree level, so that whole tree transpiration was primarily controlled by air VPD and secondarily by radiation (iii) wind velocity and direction influenced only weakly the local transpiration, and had no effect on photosynthetic rates. Simulations by the RATP model, i.e., a 3D numerical model of radiation transfer, leaf transpiration and photosynthesis (Sinoquet et al., 1999), were used (i) to study the spatial variations of transpiration, photosynthesis and leaf-to-atmosphere coupling within the tree crown and (ii) to analyse coupling at tree level, in order to evaluate the sensitivity of total transpiration to climate variations. Wind speed attenuation within the tree crown was strongly correlated to the cumulative leaf area along the wind path into the crown, as deduced from the 3D distribution of the tree foliage, obtained by the combined use of digitizing and allometric relationships. In situ measurements and model simulations were used to analyse the spatial variations of wind speed (U) and leaf boundary layer conductance (g H b), and their effects on leaf transpiration and photosynthesis within the crown of a 20-year-old walnut tree (Juglans regia L.).
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