Tree mode of death and mortality risk factors across Amazon forests

The carbon sink capacity of tropical forests is substantially affected by tree mortality. However, the main drivers of tropical tree death remain largely unknown. Here we present a pan-Amazonian assessment of how and why trees die, analysing over 120,000 trees representing > 3800 species from 189 long-term RAINFOR forest plots. While tree mortality rates vary greatly Amazon-wide, on average trees are as likely to die standing as they are broken or uprooted—modes of death with different ecological consequences. Species-level growth rate is the single most important predictor of tree death in Amazonia, with faster-growing species being at higher risk. Within species, however, the slowest-growing trees are at greatest risk while the effect of tree size varies across the basin. In the driest Amazonian region species-level bioclimatic distributional patterns also predict the risk of death, suggesting that these forests are experiencing climatic conditions beyond their adaptative limits. These results provide not only a holistic pan-Amazonian picture of tree death but large-scale evidence for the overarching importance of the growth–survival trade-off in driving tropical tree mortality.


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Software and code
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Data analysis
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Data
Policy information about availability of data All manuscripts must include a data availability statement. This statement should provide the following information, where applicable: -Accession codes, unique identifiers, or web links for publicly available datasets -A list of figures that have associated raw data -A description of any restrictions on data availability Adriane Esquivel Muelbert Jul 13, 2020 No software was used for data collection.
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Study description
Research sample Sampling strategy

Data collection
We provide a large-scale spatial assessment of mode of tree death (Figure 1 and 2) and the risk factors associated with tree death (Figure 3) across structurally intact forests in Amazonia. To do so, we collected, compiled and analysed data from 124,571 trees (of which 23,683 died during the monitoring period), from 189 forest plots monitored for over 30 years.
Using a Cox proportional hazard approach, we distinguished between the influence of individual condition (size and growth prior to death) and species traits (species mean growth rate, maximum stem diameter size, wood density and drought tolerance, estimated as water deficit affiliation) as risk factors of tree death.
We analyzed 124,571 trees from 189 forest plots monitored for over 30 years. All plots analysed were located in lowland (<1,000 m.a.s.l.), terra firme, intact forest and were monitored regularly. The plot average size is 1.23 hectares (95% CI = 1.1, 1.37) with a total area of 331.05 hectares.
No sample size calculation was performed. We selected all available plots meeting the criteria described above. All Amazon regions are adequately represented.
Forest plots data were collected by teams led by at least one of researchers co-authoring this manuscript across the Amazon basin as part of the RAINFOR network (Malhi et al. 2002), accessed via the ForestPlots.net repository (Lopez-Gonzalez et al. 2009) and following a standardized protocol (Lopez-Gonzalez et al. 2011).
Forest plot data: All trees and palms that have stem diameter at 1.3 m (or above buttresses) of "10 cm are measured, tagged, and identified, when possible, to the species level. In every census, when the plot is revisited, the living trees are measured, the new recruits that attain stem diameter "10 cm are tagged and measured, and notes are taken about the dead trees. Lianas and nonwoody arborescent individuals from the families Strelitziaceae and Cyatheaceae were excluded from these analyses.
Mode of death: Dead trees were diagnosed as having died standing or non-standing (broken or uprooted) following a standardized protocol for assessing the mode of death based on an analysis of the tree when it is found dead (Phillips et al. 2016, Chao et al. 2009).
Species traits (wood density, maximum size, mean growth and climate affiliation) were obtained from previous studies. Wood density data (in g cm-3) were obtained using previous studies from measurements in different areas of the Amazon (Zanne et al. 2009). Water-deficit affiliation (WDA, in mm) was derived in a previous study using relative abundances across 513 inventory plots distributed along a large water-deficit gradient across the Western Neotropics (Esquivel-Muelbert et al. 2017). Mean growth (in mm y-1) and maximum stem diameter size (in mm) were estimated by previous studies based on a large number of inventory plots distributed across Amazonia (Coelho de Souza 2016, Esquivel-Muelbert 2019). The maximum size represents the 95th quantile of the distribution of size and growth rates across all individuals of a given species (Coelho de Souza 2016, Esquivel-Muelbert 2019).