Kleidon,A. and Heimann,M.(1998): A method of determining rooting depth from a terrestrial biosphere model and its impacts on the global water and carbon cycle. Global Change Biology, 4, 275-286.


 We outline a method of inferring rooting depth from a Terrestrial Biosphere Model by maximizing the benefit of the vegetation within the model. This corresponds to the evolutionary principle that vegetation has adapted to make best use of its local environment. We demonstrate this method with a simple coupled biosphere/soil hydrology model and find that deep rooted vegetation is predicted in most parts of the tropics. Even with a simple model like the one we use, it is possible to reproduce biome averages of observations fairly well. By using the optimized rooting depths global Annual Net Primary Production (and transpiration) increases substantially compared to a standard rooting depth of one meter, especially in tropical regions that have a dry season. The decreased river charge due to the enhanced evaporation complies better with observations. We also found that the optimization process is primarily driven by the water deficit/surplus during the dry/wet season for humid and arid regions, respectively. Climate variability further enhances rooting depth estimates. In a sensitivity analysis where we simulate changes in the water use efficiency of the vegetation we find that vegetation with an optimized rooting depth is less vulnerable to variations in the forcing. We see the main application of this method in the modelling communities of land surface schemes of General Circulation Models and of global Terrestrial Biosphere Models. We conclude that in these models, the increased soil water storage is likely to have a significant impact on the simulated climate and the carbon budget, respectively. Also, effects of land use change like tropical deforestation are likely to be larger than previously thought.

Keywords: land use change; net primary production; optimization; rooting depth; terrestrial biosphere model; water cycle』


 Model description
 Forcing of the model
 Determination of rooting depth
 Sensitivity to increased water use efficiency

 Optimum rooting depth
 Impacts on NPP

Fig. 1 Global distribution of rooting depth obtained from maximization of NPP. The scattered values in desert ecosystems is due to the stochastic precipitation generated by a weather generator.

図1 NPPの最大化から得られた根の深さの世界的分布。砂漠生態系での値のばらつきは、気象発生プログラムにより引き起こされた確率的な降水量のせいである。

Kleidon,A. and Heimann,M.(1998): A method of determining rooting depth from a terrestrial biosphere model and its impacts on the global water and carbon cycle. Global Change Biology, 4, 275-286.

 Impacts on watershed hydrology
 Mechanism behind the optimization process
 Sensitivity to increased water use efficiency
 Limitations of the model
  Net Primary Production
  Vertical heterogeneity of soil water distribution/groundwater
  Bare soil evaporation
  Bedrock/impermeable soil layers
  Nutrient uptake
  Frozen soil
 Limitations of the method
  Equilibrium of vegetation/potential vegetation
  Drought-avoidance strategy
  Optimal behaviour
Summary and conclusion