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Zusammenfassung:
The response of terrestrial ecosystems to rising atmospheric CO2 concentration (Ca), particularly under nutrient-limited
conditions, is a major uncertainty in Earth System models. The Eucalyptus Free-Air CO2 Enrichment (EucFACE)
experiment, recently established in a nutrient- and water-limited woodland presents a unique opportunity to address
this uncertainty, but can best do so if key model uncertainties have been identified in advance. We applied seven vegetation
models, which have previously been comprehensively assessed against earlier forest FACE experiments, to
simulate a priori possible outcomes from EucFACE. Our goals were to provide quantitative projections against which
to evaluate data as they are collected, and to identify key measurements that should be made in the experiment to
allow discrimination among alternative model assumptions in a postexperiment model intercomparison. Simulated
responses of annual net primary productivity (NPP) to elevated Ca ranged from 0.5 to 25% across models. The simulated
reduction of NPP during a low-rainfall year also varied widely, from 24 to 70%. Key processes where assumptions
caused disagreement among models included nutrient limitations to growth; feedbacks to nutrient uptake;
autotrophic respiration; and the impact of low soil moisture availability on plant processes. Knowledge of the causes
of variation among models is now guiding data collection in the experiment, with the expectation that the experimental
data can optimally inform future model improvements.
