The AmazonFACE Programme – the first Free-Air CO2 Enrichment experiment in a tropical forest

Tropical rainforests play an important role in the global carbon cycle. They store massive amounts of biomass in their trees and soils, and contribute to climate mitigation by removing carbon from the atmosphere through photosynthesis. It is assumed that plant responses to rising atmospheric CO2 concentrations may have induced an increase in biomass and thus, increased the carbon sink in forests worldwide. Rising CO2 directly stimulates photosynthesis (the so-called CO2-fertilization effect) and tends to reduce stomatal conductance, leading to enhanced water-use efficiency, which may provide an important buffering effect for plants during adverse climate conditions and also have implications for water resources by reducing the loss of soil moisture through transpiration. For these reasons, current global climate simulations consistently predict that undisturbed tropical forests will continue to sequester more carbon in aboveground biomass. However, several lines of evidence point towards a decreasing carbon sink strength of the Amazon rainforest in the coming decades, potentially driven by higher temperatures, droughts or other factors. Mechanistically modelling the effects of rising CO2 in the Amazon rainforest are hindered by a lack of direct observations from ecosystem scale CO2 experiments. To address these critical issues, we are currently building a Free-Air CO2 Enrichment (FACE) experiment in an old-growth, highly diverse, tropical forest in the Brazilian Amazon and here we present our main hypotheses that underpin the AmazonFACE Programme. AmazonFACE is focused on six research areas: carbon, nutrients, water, biodiversity, socioenvironmental impacts and modelling. We give an update on the state of the experimental design, current status of construction and main research questions and challenges.