Preprints
https://doi.org/10.5194/bg-2021-218
https://doi.org/10.5194/bg-2021-218

  23 Aug 2021

23 Aug 2021

Review status: this preprint is currently under review for the journal BG.

Thirty-eight years of CO2 fertilization have outpaced growing aridity to drive greening of Australian woody ecosystems

Sami W. Rifai1, Martin G. De Kauwe1,2,3,4, Anna M. Ukkola1,5, Lucas A. Cernusak6, Patrick Meir7,8, Belinda E. Medlyn9, and Andy J. Pitman1,2 Sami W. Rifai et al.
  • 1ARC Centre of Excellence for Climate Extremes, University of New South Wales, Sydney, NSW 2052, Australia
  • 2Climate Change Research Centre, University of New South Wales, Sydney, NSW 2052, Australia
  • 3Evolution & Ecology Research Centre, University of New South Wales, Sydney, NSW 2052, Australia
  • 4School of Biological Sciences, University of Bristol, Bristol, BS8 1TQ, United Kingdom
  • 5Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
  • 6College of Science and Engineering, James Cook University, Cairns, QLD 4188, Australia
  • 7Research School of Biology, The Australian National University, Acton, ACT 2601, Australia
  • 8School of Geosciences, University of Edinburgh, Edinburgh EH89XP, UK
  • 9Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW 2753, Australia

Abstract. Climate change is projected to increase the imbalance between the supply (precipitation) and atmospheric demand for water (i.e. increased potential evapotranspiration), stressing plants in water-limited environments. Plants may be able to offset increasing aridity because rising CO2 increases water-use-efficiency. CO2 fertilization has also been cited as one of the drivers of the widespread ‘greening’ phenomenon. However, attributing the size of this CO2 fertilization effect is complicated, due in part to a lack of long-term vegetation monitoring and interannual to decadal-scale climate variability. In this study we asked the question, how much has CO2 contributed towards greening? We focused our analysis on a broad aridity gradient spanning eastern Australia’s woody ecosystems. Next we analysed 38-years of satellite remote sensing estimates of vegetation greenness (normalized difference vegetation index, NDVI) to examine the role of CO2 in ameliorating climate change impacts. Multiple statistical techniques were applied to separate the CO2-attributable effects on greening from the changes in water supply and atmospheric aridity. Widespread vegetation greening occurred despite a warming climate, increases in vapor pressure deficit, and repeated record-breaking droughts and heatwaves. Between 1982–2019 we found that NDVI increased (median 11.3 %) across 90.5 % of the woody regions. After masking disturbance effects (e.g. fire), we statistically estimated an 11.7 % increase in NDVI attributable to CO2, broadly consistent with a hypothesized theoretical expectation of an 8.6 % increase in water-use-efficiency due to rising CO2. In contrast to reports of a weakening CO2 fertilization effect, we found no consistent temporal change in the CO2 effect. We conclude rising CO2 has mitigated the effects of increasing aridity, repeated record-breaking droughts, and record-breaking heat waves in eastern Australia. However, we were unable to determine whether trees or grasses were the primary beneficiary of the CO2 induced change in water-use-efficiency, which has implications for projecting future ecosystem resilience. A more complete understanding of how CO2 induced changes in water-use-efficiency affect trees and non-tree vegetation is needed.

Sami W. Rifai et al.

Status: open (until 04 Oct 2021)

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Sami W. Rifai et al.

Sami W. Rifai et al.

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Short summary
Australia's woody ecosystems have experienced widespread greening despite a warming climate and repeated record-breaking droughts and heatwaves. Increasing atmospheric CO2 increases plant water-use-efficiency, yet quantifying the CO2 effect is complicated due to co-occurring effects of global change. Here we harmonized a 38-year satellite record to separate the effects of climate change, land use change, and disturbance to quantify the CO2 fertilization effect on the greening phenomenon.
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