Abstract
The response of terrestrial ecosystems to rising atmospheric CO2 concentration (Ca), particularly under nutrient-lim-
ited 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 woodlands, 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 experi-
ments, 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 intercompari-
son. 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 nutri-
ent 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.
