Canopy gaps and associated losses of biomass – combining UAV imagery and field data in a central Amazon forest
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Published:2023-09-13
Issue:17
Volume:20
Page:3651-3666
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ISSN:1726-4189
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Container-title:Biogeosciences
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language:en
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Short-container-title:Biogeosciences
Author:
Simonetti Adriana, Araujo Raquel Fernandes, Celes Carlos Henrique Souza, da Silva e Silva Flávia Ranara, dos Santos Joaquim, Higuchi Niro, Trumbore SusanORCID, Magnabosco Marra DanielORCID
Abstract
Abstract. Understanding mechanisms of tree mortality and the
dynamics of associated canopy gaps is relevant for robust estimates of
carbon balance in forests. We combined monthly RGB images acquired from an
unoccupied aerial vehicle with field surveys to identify gaps in an 18 ha plot
installed in an old-growth central Amazon forest. We measured the size and
shape of gaps and analyzed their temporal variation and correlation with
rainfall over a period of 28 months. We further described associated modes
of tree mortality (i.e., snapping, uprooting and standing dead) and branch
fall and quantified associated losses of biomass. In total, we detected 32
gaps either in the images or field ranging in area from 9 to 835 m2. Relatively small gaps (< 39 m2) opened by branch fall
were the most frequent (11 gaps). Out of 18 gaps for which both field and
image data were available, three could not be detected remotely. Gaps
observed in the field but not captured on the imagery were relatively small
and mainly formed by the fall of branches from live and standing dead trees.
Our data show that ∼ 17 % of the tree-mortality and
branch-fall events only affected the lower canopy and the understory of the
forest and are likely neglected by top-of-the-canopy assessments.
Regardless of the detection method, the size distribution was best described
by a lognormal function for gaps starting from the smallest detected size (9 and 10 m2 for field and imagery data, respectively), and the
Weibull and Power functions for gaps larger than 25 m2. Properly
assessing associated confidence intervals requires larger sample sizes.
Repeated field measurements reveal that gap area does not differ
significantly among modes of tree mortality or branch fall in central Amazon
forests, with the last contributing the least to biomass loss. Predicting
mechanisms of gap formation based on associated area and biomass loss
remains challenging, which highlights the need for larger datasets. The rate
of gap area formation was positively correlated with the frequency of
extreme rainfall events, which may be related to a higher frequency of
storms propagating extreme rain and wind gusts. While remote sensing has proven to be an accurate and precise method for mapping gaps compared to field data (i.e., ground truth), it is important to note that our sample size was relatively small. Therefore, the extrapolation of
these results beyond our study region and landscape shall be made
cautiously. Apart from improving landscape assessments of carbon balance,
regional information on gap dynamics and associated mechanisms of formation
are fundamental to address forest responses to altered disturbance regimes
resulting from climate change.
Funder
Ministério da Ciência, Tecnologia e Inovação Bundesministerium für Bildung und Forschung
Publisher
Copernicus GmbH
Subject
Earth-Surface Processes,Ecology, Evolution, Behavior and Systematics
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