The process of photosynthesis determines the amount of carbon available for plants to grow. However, experimental work has highlighted that the flux of photosynthesis and timing of growth in plants are not always correlated, which presents a problem for current carbon cycle models. These models currently assume a tight connection between photosynthesis (source) and growth (sink), whereas new insights point to a more direct environmental (e.g. temperature, water-stress, lack of nutrients) control on growth, termed “sink-limitation”. To accurately predict plant growth responses to environmental change, our models need to account for both source and sink limitations.

To get an insight into this lack of correlation between photosynthesis and growth, colleagues at the Hawkesbury Institute for the Environment, Colgate University and a CLEX researcher, applied a novel data assimilation approach (the statistical combination of observations with a model) to a recently concluded experiment. This experiment was designed to probe the processes that are affected by sink limitations by manipulating the rooting volume of Eucalyptus tereticornis seedlings. By restricting rooting volume, the experiment imposes a limitation on growth.

The analysis was able to infer that the imposed limitation not only led to a reduction in photosynthesis but also a reduction in the rate at which stored carbohydrates were used. It also showed that leaf turnover and respiration were increased and the allocation of carbon for growth was modified. Each of these effects had a significant effect upon the plant biomass and the original experiment was unable to identify these casual processes as it could not accurately quantify all components of the plant carbon balance.

The researchers’ application of a simple carbon balance model, combined with a data assimilation approach to a manipulation experiment, demonstrates the potential of this approach to unlock new insights from other similar experiments. In doing so, approaches like this one will facilitate improvements in the process understanding embedded in models used to predict responses of the carbon cycle to climate change.

  • Paper: Mahmud, K., Medlyn, B. E., Duursma, R. A., Campany, C., and De Kauwe, M. G.: Inferring the effects of sink strength on plant carbon balance processes from experimental measurements, Biogeosciences, 15, 4003-4018,, 2018.