Preprints
https://doi.org/10.5194/bg-2021-30
https://doi.org/10.5194/bg-2021-30

  22 Feb 2021

22 Feb 2021

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

Drought effects on leaf fall, leaf flushing and stem growth in Neotropical forest; reconciling remote sensing data and field observations

Thomas Janssen1, Ype van der Velde1, Florian Hofhansl2, Sebastiaan Luyssaert3, Kim Naudts1, Bart Driessen4, Katrin Fleischer5, and Han Dolman1 Thomas Janssen et al.
  • 1Department of Earth Sciences, Vrije Universiteit Amsterdam, Amsterdam, the Netherlands
  • 2International Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria
  • 3Department of Ecological Sciences, Vrije Universiteit Amsterdam, Amsterdam, the Netherlands
  • 4Department of Computer Science, Universidad de Alcala de Henares, Madrid, Spain
  • 5Department of Biogeochemical Signals, Max Planck Institute for Biogeochemistry, Jena, Germany

Abstract. Large amounts of carbon flow through tropical ecosystems every year, from which a part is sequestered in biomass through tree growth. However, the effects of ongoing warming and drying on tree growth and carbon sequestration in tropical forest is still highly uncertain. Field observations are sparse and limited to a few sites while remote sensing analysis shows diverging growth responses to past droughts that cannot be interpreted with confidence. To reconcile data from field observations and remote sensing, we collated in situ measurements of stem growth and leaf litterfall from inventory plots across the Neotropics. This data was used to train two machine learning models and to evaluate model performance on reproducing stem growth and litterfall rates. The models utilized multiple climatological variables and other geospatial datasets as explanatory variables. The output consisted of monthly estimates of leaf litterfall (R2 = 0.67, NRMSE = 9.5 %) and stem growth (R2 = 0.51, NRMSE = 11.2 %) across the neotropics from 1982 to 2019 at a high spatial resolution (0.1°). Modelled time series allowed to assess the impacts of the 2005 and 2015 droughts in the Amazon basin on regional scales. Both droughts were estimated to have caused widespread declines in stem growth (−0.6σ ~ −1.8σ), coinciding with enhanced leaf fall (+0.7σ ~ +0.9σ). Regions in the Amazon basin that flushed leaves at the onset of both droughts (+1.1σ ~ +1.9σ), showed positive anomalies in remotely sensed enhanced vegetation index, while sun-induced fluorescence and vegetation optical depth were reduced. The previously observed counterintuitive response of canopy green-up during drought in the Amazon basin detected by many remote sensing analyses can therefore be explained by enhanced leaf flushing at the onset of a drought. The long-term estimates of leaf litterfall and stem growth point to a decline of stem growth and a simultaneous but weaker increase in leaf litterfall in the Amazon basin since 1982 that is not observed in long-term inventory plots. These trends are associated with increased warming and drying of the Amazonian climate.

Thomas Janssen et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-30', Anonymous Referee #1, 18 Mar 2021
    • AC1: 'Reply on RC1', Thomas Janssen, 06 May 2021
  • RC2: 'Comment on bg-2021-30', Anonymous Referee #2, 02 Apr 2021
    • AC2: 'Reply on RC2', Thomas Janssen, 06 May 2021

Thomas Janssen et al.

Thomas Janssen et al.

Viewed

Total article views: 428 (including HTML, PDF, and XML)
HTML PDF XML Total Supplement BibTeX EndNote
281 134 13 428 31 0 5
  • HTML: 281
  • PDF: 134
  • XML: 13
  • Total: 428
  • Supplement: 31
  • BibTeX: 0
  • EndNote: 5
Views and downloads (calculated since 22 Feb 2021)
Cumulative views and downloads (calculated since 22 Feb 2021)

Viewed (geographical distribution)

Total article views: 354 (including HTML, PDF, and XML) Thereof 354 with geography defined and 0 with unknown origin.
Country # Views %
  • 1
1
 
 
 
 
Latest update: 16 May 2021
Download
Short summary
Satellite images show that the Amazon forest has greened up during past droughts. Measurements of tree stem growth and leaf litterfall upscaled using machine learning algorithms show that leaf flushing at the onset of a drought results in canopy rejuvenation and green-up during drought while simultaneously trees excessively shed older leaves and tree stem growth declines. Canopy green-up during drought therefore does not necessarily point to enhanced tree growth and improved forest health.
Altmetrics