Abstract
Abstract. 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.
Highlights
Lichens and bryophytes are different from vascular plants: Lichens are not real plants, but a symbiosis of a fungus and at least one green alga or cyanobacterium, whereas bryophytes, such as mosses or liverworts, are plants which have no specialised tissue such as roots or stems
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
They use a large amount of data from field experiments or lab measurements to estimate characteristic mean values of net carbon uptake and nitrogen fixation for each of the world’s biomes. By multiplying these mean values with the area of the respective biome, they arrive at global numbers for uptake of carbon and nitrogen. While their estimate for global net carbon uptake amounts to 7 % of terrestrial net primary productivity (NPP), the derived value of nitrogen fixation corresponds to around 50 % of the total terrestrial biological nitrogen fixation (BNF), representing a large impact on the global nitrogen cycle
Summary
Lichens and bryophytes are different from vascular plants: Lichens are not real plants, but a symbiosis of a fungus and at least one green alga or cyanobacterium, whereas bryophytes, such as mosses or liverworts, are plants which have no specialised tissue such as roots or stems. We present the first process-based modelling approach to estimate net carbon uptake of lichens and bryophytes at the global scale In this way, we are able to assess the role of these organisms regarding global biogeochemical cycles. The equations use environmental factors such as radiation or water supply as input values, which are either prescribed or derived from climate forcing data In spite of their simplicity, global vegetation models are capable of predicting NPP to a reasonable accuracy (Randerson et al, 2009). Similar to these models, our model describes lichens and bryophytes as reservoirs of biomass located either on the soil or in the canopy and it is based on equations to represent photosynthesis and other physiological processes. Poradisa eetiathl.:eEr sttihmeatcinagngolpoybalocrartbhoen ugprotauknedb,y alischewneslal nadsbrtyhoephsyutrersounding vegetation, which is 6991 135 described by a biome classification
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