Exploring environmental and physiological drivers of the annual carbon budget of biocrusts from various climatic zones with a mechanistic data-driven model
Authors: Ma Yunyao , Weber Bettina , Kratz Alexandra , Raggio José , Colesie Claudia , Veste Maik , Bader Maaike Y. , Porada Philipp 
Published: July 2023

Abstract. Biocrusts are a worldwide phenomenon, contributing
substantially to ecosystem functioning. Their growth and survival depend on
multiple environmental factors, including climatic ones, and the relations
of these factors to physiological processes. Responses of biocrusts to
individual environmental factors have been examined in a large number of
field and laboratory experiments. These observational data, however, have rarely been assembled into a comprehensive, consistent framework that allows
quantitative exploration of the roles of multiple environmental factors and
physiological properties for the performance of biocrusts, in particular
across climatic regions. Here we used a data-driven mechanistic modelling
framework to simulate the carbon balance of biocrusts, a key measure of
their growth and survival. We thereby assessed the relative importance of
physiological and environmental factors for the carbon balance at six study
sites that differ in climatic conditions. Moreover, we examined the role of
seasonal acclimation of physiological properties using our framework, since
the effects of this process on the carbon balance of biocrusts are poorly
constrained so far. We found substantial effects of air temperature,
CO2 concentration, and physiological parameters that are related to
respiration on biocrust carbon balance, which differ, however, in their
patterns across regions. The ambient CO2 concentration is the most
important factor for biocrusts from drylands, while air temperature has the
strongest impact at alpine and temperate sites. Metabolic respiration cost
plays a more important role than optimum temperature for gross
photosynthesis at the alpine site; this is not the case, however, in
drylands and temperate regions. Moreover, we estimated a small annual carbon
gain of 1.5 gm-2yr-1 by lichen-dominated biocrust and 1.9 gm-2yr-1 by moss-dominated biocrust at a dryland site, while the
biocrusts lost a large amount of carbon at some of the temperate sites
(e.g. −92.1 for lichen-dominated and −74.7 gm-2yr-1 for moss-dominated
biocrust). These strongly negative values contradict the observed survival
of the organisms at the sites and may be caused by the uncertainty in
environmental conditions and physiological parameters, which we assessed in
a sensitivity analysis. Another potential explanation for this result may be
the lack of acclimation in the modelling approach, since the carbon balance
can increase substantially when testing for seasonally varying parameters in
the sensitivity analysis. We conclude that the uncertainties in air
temperature, CO2 concentration, respiration-related physiological
parameters, and the absence of seasonal acclimation in the model for humid
temperate and alpine regions may be a relevant source of error and should be
taken into account in future approaches that aim at estimating the long-term
biocrust carbon balance based on ecophysiological data.