Estimating global carbon uptake by lichens and bryophytes with a process-based model

Lichens and bryophytes are abundant globally and they may even form the dominant autotrophs in (sub) polar ecosystems, in deserts and at high altitudes. Moreover, they can be found in large amounts as epiphytes in old-growth forests. Here, we present the first process-based model which estimates the net carbon uptake by these organisms at the global scale, thus assessing their significance for biogeochemical cycles. The model uses gridded climate data and key properties of the habitat (e.g. disturbance intervals) to predict processes which control net carbon uptake, namely photosynthesis, respiration, water uptake and evaporation. It relies on equations used in many dynamical vegetation models, which are combined with concepts specific to lichens and bryophytes, such as poikilohydry or the effect of water content on CO2 diffusivity. To incorporate the great functional variation of lichens and bryophytes at the global scale, the model parameters are characterised by broad ranges of possible values instead of a single, globally uniform value. The predicted terrestrial net uptake of 0.34 to 3.3 Gt yr(-1) of carbon and global patterns of productivity are in accordance with empirically-derived estimates. Considering that the assimilated carbon can be invested in processes such as weathering or nitrogen fixation, lichens and bryophytes may play a significant role in biogeochemical cycles.