Nutrients are central in the limitation of forest productivity. Due to frequent biomass exports and poor soils, tropical eucalypt plantations are an example of strongly nutrient-limited ecosystems. The plantations cover a wide surface ~7.5 Mha) in Brazil. Large-scale nutrient (N, P, K) omission experiments have been conducted over multiple rotations, and an important coherent data set of the response of this system to nutrient availability is available. In many of these experiments, Potassium (K) was identified as the major limiter of wood productivity.Mechanistic forest models have proven to be relevant tools to explore the response of forest ecosystems to nutrient limitations by Nitrogen (N) and recently, Phosphorus (P). In this thesis we created the first coupled C-H2O-K process-based model in an attempt to quantify and identify processes responsible for the limitation of wood productivity by K. We used the model as a tool to explore two main questions regarding the K-limitation of wood productivity in tropical eucalypt plantations: 1) What is the impact of K plant availability on photosynthesis (C-source)? 2) Is the limitation of the C source enough to explain the limitation of wood productivity by K deficiency in tropical eucalypt plantations? To this end, the processes impacted by plant K availability were identified in a literature review (Chapter II). In the modelling work, a specific focus was thus put on leaves (Chapter III), the recycling of K in the plant and allocation of K to the different organs (chapter IV). The assimilation of C was greatly reduced under K deficiency. This was mainly the result of a decrease in leaf area combined to a decline in photosynthetic capacity per leaf area. The importance of processes governing K internal plant recycling was also underlined (chapter IV). Simulations also showed that this reduction in C assimilation was enough to explain the observed reduction of wood productivity in the K omission stand and that no stoichi