OOS 38-2
From source- to sink limited vegetation modelling
Predictions of future terrestrial carbon and water cycles rely on numerical tools that simulate the dynamics of vegetation from assimilation of carbon through stomata to long-term forest development at the global scale. Current vegetation models use the photosynthesized carbon to drive plant growth. However, experimental evidence suggests that plant tissue growth is very sensitive to direct controls of temperature and plant water status, independent of the amount of assimilated carbon. Here, we examine the hierarchy of plant growth control and suggest a paradigm change from source (photosynthesis) to sink (tissue expansion) modeling of vegetation dynamics. We illustrate this with an attempt to improve the LPJ DGVM (Lund-Potsdam-Jena dynamic global vegetation model) by replacing the source-limited (SoL) by a sink-limited (SiL) tree growth algorithm. This is achieved by limiting annual Net Primary Production of the LPJ DGVM by an algorithm based on the annual sum of growing degree-days (GDD), assuming that maximum plant growth is reached asymptotically with increasing GDD.
Results/Conclusions
The reversal in our understanding of plant growth drivers from a source- to a sink-limited control has serious implications for carbon fluxes and storage in the long-term. In our case study for temperature-limited forests at the treeline, we show that the location of both the northern and the alpine treeline is estimated more accurately when using a SiL algorithm than when using the commonly employed SoL algorithm. For example, In the range of 60°-80° N over Europe and Asia, carbon stored in vegetation is estimated to be c. 50% higher in the LPJ standard version (LPJ-SoL) compared to LPJ-SiL, resulting in a global difference in estimated biomass of 25 Pg (c. 5% of the global terrestrial standing biomass). In conclusion, we argue that there is enough observational evidence to tackle the challenge of revising the core structure of current DGVMs not only in temperature, but also in water and nutrient limited systems, which will lead from source- to sink-driven modelling of plant growth.