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Abstract:
Despite extensive research on the role of plant species richness for the regulation of ecosystem functions,
the mechanistic understanding of the underlying processes, especially in species-rich communities, is still
limited. Biogeochemical models of vegetation dynamics could potentially be used to complement empirical
studies, but it is unclear how the particular process description within these models affects simulations of
species performance and resulting ecosystem functions.
We evaluate the models’ process descriptions to simulate the response of different species, their inter-
actions and their joint performance to drought and mowing. Therefore, we compare simulations of two
grassland models of different complexity for monocultures and two-species mixtures in a grassland experi-
ment in Jena, Germany.
Models’ process representations are crucial for species’ performance and interaction. We provide an
in-depth analysis of the processes responsible for model behavior to identify potential fields of model devel-
opment and discuss our findings in the context of other modeling approaches.
Both models simulated similar average above-ground biomass (AGB) but showed different intra-annual
variability. Generally, the models had difficulties representing a balanced species composition in multiple
species mixtures and competition for space was the main driver of community composition in both models.
The resulting communities were dominated by the more competitive species, while the weak competitor was
only marginally present in most mixtures independent of drought and mowing. The competitive strength
which we derived from the calibrated parameter sets of the species differed between the models and the
agreement on which species dominate specific mixtures was mixed. While both models simulated reduced
soil water content and above-ground biomass in response to drought, the strength and duration of these
responses differed. Despite these differences, simulated species interactions were barely affected, and strong
competitors remained dominant. In both models, the representation of competition for below-ground re-
sources (water and nutrients) is less complex than that for above-ground resources (space and light). We
found that in both models the transpiration reduction from water stress is too strong when soil water con-
tent is close to field capacity, which weakened the drought effects. Mowing had opposing effects on the
competition for space in the models, which could be attributed to the different representations of plants in
the two models.
Here, we demonstrated that process-based vegetation models in general allow for a detailed comparison
of the modelled processes and their links to both – emerging vegetation responses and underlying plant
parameters/traits. Such a model intercomparison supports the understanding of how much model complexity
is necessary to simulate specific research questions and objectives. Concerning our example of drought
effects on plant species competition, we conclude that a better representation of multi-species mixtures and
their response to disturbances in grassland models is needed to allow for more robust future projections of grassland dynamics under future management and climate change.
