Anthromes, CO2 and Terrestrial Carbon – Session 2 (part 1): CO2 and land use change as drivers of terrestrial carbon dynamics
Authors: Arneth Almut , Norby Richard 
Published: April 2023

11.20 - 11.50: Carbon emissions and uptake from land use and land cover changes – uncertainties and implications for the land carbon sink, Almut Arneth* (presenting remotely) 11.50 – 12.10: Carbon dynamics in mature forests under elevated CO2: Evidence from three contrasting forests, Rich NorbyDue to technical difficulties, Almut's talk is not currently available.Carbon emissions and uptake from land use and land cover changes – uncertainties and implications for the land carbon sinkDue to technical difficulties, the recording of Almut's talk is not currently available.Historical emissions of CO2 from land conversions and management intensity remain the largest source of uncertainty in the global carbon budget. Within- and between model uncertainty, different land-cover and land-use change reconstructions and lack of process-representations all contribute to the lack of clarity regarding past emissions. In the presentation I will highlight some examples, focussing on three aspects:1. The net carbon emission estimates from human-induced land cover changes need to capture a complex interplay between gross emissions and uptake (e.g., emissions from deforestation vs uptake from reforestation); 2. Likewise, the land carbon sink in response to climate change is a complex interplay between gross sinks and sources (e.g., CO 2 fertilisation vs mortality due to weather extremes); 3. Management intensity can be as important as land cover change and is typically not included when modelling carbon emissions or uptake. All of these vary greatly between regions and differ in time. I will present some illustrative examples and discuss -given the carbon budget’s ‘budget approach’- the implications the lack of understanding of the land-use change related net sources has for our estimates of the net carbon sink on land.Carbon dynamics in mature forests under elevated CO2: Evidence from three contrasting forestsFree-air CO2 enrichment experiments in young tree plantations have demonstrated the capacity for increased forest growth in elevated CO2, which could dampen the rate of increase in atmospheric CO2 concentration. But will older, more established forests that often are limited by nutrient availability respond similarly? Ongoing and developing FACE experiments provide tests of that question. In the EucFACE experiment in a native Eucalyptus tereticornis forest in New South Wales, Australia, the trees are taking up additional CO2 in elevated CO2, but a phosphorus limitation apparently is precluding carbon accumulation in long-lived biomass. In the BIFoR FACE experiment in a 180-year-old Quercus robur woodland in central England, basal area increment calculated from tree ring analysis is 25-43% greater in elevated CO2. However, detection of a response in net primary productivity has been difficult because of pretreatment differences and uncertainty in allometric models. The AmazonFACE experiment, under construction in a wet tropical forest in Brazil, will test whether elevated CO2 confers mechanisms to alleviate phosphorus deficiency to forest productivity. A particular challenge will be detecting response in this highly diverse forest. In all three experiments, substantial carbon is allocated belowground, and belowground responses will be key to predicting long-term response.