Tree mode of death and mortality risk factors across Amazon forests

Abstract

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.

Linking plant hydraulics and the fast–slow continuum to understand resilience to drought in tropical ecosystems

“…We postulate faster growth has a direct negative effect on hydraulic risk, independent of the environmental factors affecting growth rate variation. Inherent growth rates (maximum growth potential) should therefore be regarded as a good proxy for tree mortality risk during drought as evidenced by a recent study in the Amazon basin (Esquivel‐Muelbert et al ., 2020). The HSM‐growth axis of trait variation provides a mechanistically roadmap for understanding the widespread growth‐mortality trade‐off in tropical trees and explains the high variability in tree mortality risk and productivity across the tropics.…”

“…In fact, nutrient‐rich habitats (e.g. western Amazonian forests near the Andes) are more productive, have higher mortality rates (Esquivel‐Muelbert et al ., 2020) and likely lower HSMs, while communities in old landscapes and nutrient‐poor soils, such as lowland evergreen Amazon forests in the Guyana shield should be less productive, have higher safety margins (Ziegler et al ., 2019) and lower mortality rates during droughts (from left to right in Fig. 6).…”

“…Mortality risk differs across sites and species in the tropics suggesting that intrinsic factors such as tree ecological traits and strategies are also important (Fig. 1a,b; Esquivel‐Muelbert et al ., 2020). Within communities, inherent fast‐growing species tend to have higher mortality rates (Fig.…”

Large-scale variations in the dynamics of Amazon forest canopy gaps from airborne lidar data and opportunities for tree mortality estimates

“…As a result, the estimates of mortality from lidar canopy gaps are likely biased to some extent on observing tree mortality associated with broken and uprooted mode of death. In the Amazon, this mode of death represents approximately 39-55% of the total mortality but, on average, it is not significantly distinguished from the standing dead mode of death [22][23][24] .…”

Review of Hiltner et al.

“…Tree height is the strongest predictor of tree mortality out of which basket of indicators? At the individual tree level other predictors can be very important (see e.g Esquivel-Muelbert et al, 2020),. so it needs to be clear what is being compared to what.…”