Competing ecosystem model hypotheses for the CO2 response of a nutrient- and water-limited mature Eucalypt woodland (EucFACE)

Abstract:

Model projections of the response of the terrestrial biosphere to increasing atmospheric CO₂ concentrations, are strongly limited by uncertainties relating to the role of nutrients. A number of long-term (~10 years) Free-Air CO₂ Enrichment (FACE) experiments have now been carried out, providing significant new understanding of system responses to increased CO₂ concentrations. Recently, these data have been successfully used to examine the underlying hypotheses in a series of state-of-the-art terrestrial biosphere models (TBMs). However, these comparisons are limited in a number of ways: (i) experiments on forests, have predominantly been focussed in the temperate zone; (ii) stands are often limited to young or plantation forests; and (iii) model-data comparisons have occurred after the conclusions of these experiments. The Eucalyptus Free-Air CO₂ Enrichment (EucFACE) experiment, which has recently been established in a nutrient-limited woodland in Sydney, Australia, thus presents a unique opportunity to address all of these issues. We applied 7 TBMs that incorporate contrasting hypotheses about nutrient (nitrogen and phosphorous) cycling, to make predictions before the experiment commenced, with particular focus on the role of potential drought. The aim was both to examine the likely model responses from the experiment and to identify key areas of model uncertainty where additional measurements could constrain models.