Scientists at Brookhaven National LaboratoryBNL found that TPU, a key process at the heart of many TBMs, was poorly represented and that continued inclusion of TPU in terrestrial biosphere models (TBMs) is not supported by current understanding and data. These authors found that inclusion of TPU limitation in TBMs resulted in unrealistic limitation of photosynthesis that in some models could lead to a marked reduction of CO2 uptake and poor representation of the response of photosynthesis to global change. This work brings together several recent lines of evidence and an examination of model representation of TPU that together strongly suggest that TPU should be removed from TBMs (Fig. 3). Current formulations of TPU in TBMs are based on assumptions about the relationship between the capacity for carboxylation and the basal rate of TPU that are not based on measured TPU rates and do not account for the independent temperature response of TPU. A TBM sensitivity analysis demonstrated a limitation of gross primary productivity by TPU at current CO2 concentration but most markedly at high CO2 concentration and at high latitudes. However, a synthesis of measurements clearly demonstrated that TPU did not limit CO2 assimilation at current CO2, even at high latitudes. In addition, it was recently demonstrated that most TBMs that include TPU also include an artifactual limitation on photosynthesis that results in a marked reduction in modeled CO2 assimilation (Walker et al 2021, DOI: 10.1111/gcb.15366).
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