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Abstract:
In many current dynamic global vegetation models (DGVMs), including those incorporated into Earth system
models (ESMs), terrestrial vegetation is represented by
a small number of plant functional types (PFTs), each with
fixed properties irrespective of their predicted occurrence.
This contrasts with natural vegetation, in which many plant
traits vary systematically along geographic and environmental
gradients. In the JSBACH DGVM, which is part of the
MPI-ESM, we allowed three traits (specific leaf area (SLA),
maximum carboxylation rate at 25 C (Vcmax25) and maximum
electron transport rate at 25 C (Jmax25)) to vary within
PFTs via trait–climate relationships based on a large trait
database. The R2
adjusted of these relationships were up to
0.83 and 0.71 for Vcmax25 and Jmax25, respectively. For
SLA, more variance remained unexplained, with a maximum
R2
adjusted of 0.40. Compared to the default simulation, allowing
trait variation within PFTs resulted in gross primary productivity
differences of up to 50% in the tropics, in >35%
different dominant vegetation cover, and a closer match with
a natural vegetation map. The discrepancy between default
trait values and natural trait variation, combined with the
substantial changes in simulated vegetation properties, together
emphasize that incorporating climate-driven trait variation,
calibrated on observational data and based on ecological
concepts, allows more variation in vegetation responses in DGVMs and as such is likely to enable more reliable projections
in unknown climates.
