Implications of carbon source-sink feedbacks for plant growth responses to environmental factors

Global models of vegetation dynamics are largely carbon (C) source-driven, with behaviour primarily determined by the environmental responses of photosynthesis. However, real plants operate as integrated wholes, with feedbacks between sources and sinks resulting in homeostatic concentrations of metabolites such as sugars. An approach to implementing such behaviour in a plant growth model is presented and its implications for responses to generic environmental factors assessed. The approach is based on Hill functions representing inhibition of C sources (net photosynthesis) and activation of sinks (structural growth) based on sugar concentrations. The model is parameterised for a mature tropical rainforest site and its qualitative behaviour is found to be consistent with experimental observations. Key findings are that sinks and sources strongly regulate each other. For example, doubling potential net photosynthesis (i.e. the rate that would occur without feedback) results in growth increasing by only 1/3 at equilibrium, with increased sugar concentration causing feedback-inhibition of photosynthesis. A source-only driven response, as in current global models, would result in close to a doubling of growth. Hence environmental factors that affect potential net photosynthesis, such as atmospheric CO ₂ , have greatly reduced effects on growth when homeostatic behaviour of sugars is considered. The implications of considering source-sink coupling for the behaviour of models of vegetation dynamics are likely to be profound.