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

Twenty-seven individual tree growth models are reviewed. The models take in to account the same main physiological processes involved in carbon metabol ism (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 architectu re used mainly depend on model objectives. Beyond common rationales, the mo dels 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 respo nse to environment, and explanatory or predictive capacities. Representatio n of photosynthetic carbon gain ranges from merely empirical relationships that provide annual photosynthate production, to mechanistic models of inst antaneous leaf photosynthesis that explicitly account for the effects of th e major environmental variables. Respiration is often described empirically as the sum of two functional components (maintenance and growth). Maintena nce 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 mobilisa tion 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 allocatio n 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 mode ls of individual tree growth are reviewed, and their implications for fores try and ecology are discussed. Three critical issues for these models to da te are identified: (i) the representation of carbon allocation and of the e ffects of architecture on tree growth is Achilles' heel of most of tree gro wth models; (ii) reserve dynamics is always poorly accounted for; (iii) the representation of below ground processes and tree nutrient economy is lack ing 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.