12 Jan 2023
 | 12 Jan 2023
Status: this preprint is currently under review for the journal BG.

Gap geometry, seasonality and associated losses of biomass – combining UAV imagery and field data from a Central Amazon forest

Adriana Simonetti, Raquel Fernandes Araujo, Carlos Henrique Souza Celes, Flávia Ranara da Silva e Silva, Joaquim dos Santos, Niro Higuchi, Susan Trumbore, and Daniel Magnabosco Marra

Abstract. Understanding mechanisms of tree mortality and geometric patterns of canopy gaps is relevant for robust estimates of carbon stocks and balance in tropical forests, and for assessing how they are responding to climate change. We combined monthly RGB images acquired from an unmanned aerial vehicle with field surveys to identify gaps in an 18-ha permanent plot in an old-growth Central Amazon forest over a period of 28 months. In addition to detecting, we measured the size and shape of gaps, and analyzed their temporal variation and correlation with rainfall. We further described associated modes of tree mortality or branch fall and quantified associated losses of biomass. Overall, the sensitivity of gap detection differed between field surveys and imagery data. In total, we detected 32 gaps either in the images and field, ranging in area from 9 m2 to 835 m2. Relatively small gaps (< 39 m2) associated with branch fall were the most frequent (11 gaps). Out of 18 gaps for which both field and imagery data were available, three could not be detected remotely. This result shows that a considerable fraction of tree mortality and branch-fall events (~ 17 %) affect only the lower canopy and the understory of the forest and thus, are likely neglected by assessments of top of the canopy. Regardless the detection method, the size distribution of gaps in our study region was better captured by a Weibull function. As confirmed by our detailed field surveys, we believe that this pattern was not biased by gaps possibly undetected from image data. Although not related to differences in gap size, the main modes of tree mortality partially explained associated losses of biomass. The rate of gap area formation expressed as the percent per month was positively correlated with the frequency of extreme rainfall events, which may be related to a higher frequency of storms propagating destructive wind gusts. Our results demonstrate the importance of combining field observations with remote sensing methods for monitoring gap dynamics in dense forests. The correlation between modes of tree mortality and gap geometry with associated losses of biomass provide evidence on the importance of small-scale events of tree mortality and branch fall as processes that contribute to landscape patterns of carbon balance and species diversity in Amazon forests. Regional assessments of the dynamics and geometry of canopy gaps formed from branch fall and individual tree-mortality (e.g., from few to hundreds of m2) up to catastrophic blowdowns associated with extreme rain and wind (e.g., from hundreds of m2 to thousands of ha) can reduce the uncertainty of landscape assessments of carbon balance, especially as the frequency and intensity of storms causing these events is likely to change with future Amazon climate.

Adriana Simonetti 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-2022-251', Anonymous Referee #1, 30 Jan 2023
    • AC1: 'Reply on RC1', Adriana Simonetti Lopes Peixoto, 26 Apr 2023
  • RC2: 'Comment on bg-2022-251', Anonymous Referee #2, 09 Mar 2023
    • AC2: 'Reply on RC2', Adriana Simonetti Lopes Peixoto, 26 Apr 2023

Adriana Simonetti et al.

Adriana Simonetti et al.


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Short summary
Our study confirms the reliability of drone images for monitoring canopy dynamics of dense tropical forests. We combined two years of monthly drone-acquired RGB (red-green-blue) imagery with field surveys in a Central Amazon forest. Our results indicate that small gaps associated with branch fall were the most frequent. Although not related to variations in gap area, the studied modes of tree mortality and branch fall were associated with biomass losses.