Modelling the carbon balance in bryophytes and lichens: Presentation of PoiCarb 1.0, a new model for explaining distribution patterns and predicting climate-change effects

Premise  Bryophytes and lichens have important functional roles
in many ecosystems. Insight into how their CO2 exchange responds to
climatic conditions is essential for understanding current and predicting
future productivity and biomass patterns, but responses are hard to
quantify at time-scales beyond instantaneous measurements. We present
PoiCarb 1.0, a model to study how CO2 exchange rates of these
poikilohydric organisms change through time as a function of weather
conditions. Methods PoiCarb simulates diel fluctuations of CO2 exchange
and estimates long-term carbon balances, identifying optimal and limiting
climatic patterns. Modelled processes are net photosynthesis, dark
respiration, evaporation and water uptake. Measured CO2-exchange responses
to light, temperature, atmospheric CO2 concentration, and thallus water
content (calculated in a separate module) are used to parameterise the
model's carbon module. We validated the model by comparing modelled
diel courses of net CO2 exchange to such courses from field measurements
on the tropical lichen Crocodia aurata. To demonstrate the model's
usefulness, we simulated potential climate-change effects. Results Diel
patterns were reproduced well and modelled and observed diel carbon
balances were strongly positively correlated. Simulated warming effects
via changes in metabolic rates were consistently negative, while effects
via faster drying were variable, depending on the timing of hydration.
Conclusions Being able to reproduce the weather-dependent variation in
diel carbon balances is a clear improvement compared to simple
extrapolations of short-term measurements or potential photosynthetic
rates. Apart from predicting climate-change effects, future uses of
PoiCarb include testing hypotheses about distribution patterns of
poikilohydric organisms and guiding species' conservation.