Individual tree damage dominates mortality risk factors across six tropical forests

Abstract

Summary The relative importance of tree mortality risk factors remains unknown, especially in diverse tropical forests where species may vary widely in their responses to particular conditions. We present a new framework for quantifying the importance of mortality risk factors and apply it to compare 19 risks on 31 203 trees (1977 species) in 14 one‐year periods in six tropical forests. We defined a condition as a risk factor for a species if it was associated with at least a doubling of mortality rate in univariate analyses. For each risk, we estimated prevalence (frequency), lethality (difference in mortality between trees with and without the risk) and impact (‘excess mortality’ associated with the risk, relative to stand‐level mortality). The most impactful risk factors were light limitation and crown/trunk loss; the most prevalent were light limitation and small size; the most lethal were leaf damage and wounds. Modes of death (standing, broken and uprooted) had limited links with previous conditions and mortality risk factors. We provide the first ranking of importance of tree‐level mortality risk factors in tropical forests. Future research should focus on the links between these risks, their climatic drivers and the physiological processes to enable mechanistic predictions of future tree mortality.

Climate and crown damage drive tree mortality in southern Amazonian edge forests

“…These trees have crowns most exposed to light, making them more susceptible to water stress caused by intense droughts or lengthening of dry seasons (Aleixo et al, 2019; Costa et al, 2010). Thus, the mortality of these trees may be caused by a combination of physiological and mechanical factors (Zuleta et al, 2021). Our results suggest that the stress generated by drought may be increasing the predisposition of trees to break just as much as it kills directly via hydraulic failure.…”

“…The likelihood and cause of tree death vary across different canopy layers (Camac et al, 2018). Competition for light is likely to impact especially trees growing in the understorey that are shaded by neighbours and are potentially close to their carbon compensation point (Camac et al, 2018; McDowell et al, 2018; Wright et al, 2010; Zuleta et al, 2021). Sub‐canopy trees may be more susceptible to death from mechanical damage generated by canopy trees' death or breakage and extreme weather events (Toledo et al, 2012; Yang et al, 2003).…”

Tree crown damage and its effects on forest carbon cycling in a tropical forest

“…We used both the living length and b l to estimate the biomass of a living tree based on an allometric function that accounts for the vertical distribution of volume in the trunk vs. crown (Ver Planck & MacFarlane, 2014). Specifically, we estimated the proportion of crown volume below a given height (within the living length) and multiplied it by the relative biomass of the crown, which was set to 1/3 of the total biomass of the tree based on empirical data from 611 harvested tropical trees (Chambers et al, 2001; Duque et al, 2017), see (Zuleta et al, 2021, 2022), and Methods S3 for a full explanation. Based on estimated damage we grouped trees into five damage classes (corresponding to the damage classes used in FATES) and calculated mortality, M , for each class aswhere d is damage class, N 1 is the number of individuals alive in census 1, N 2 is the number of individuals alive in census 2 (regardless of damage class in census 2), and t is time in years between censuses.…”

“…Crown damage is an important predictor of individual tree mortality (Arellano et al, 2019; Reis et al, 2022). After light limitation, crown loss was found to have the largest impact on forest wide mortality out of 19 risk factors assessed across six tropical forests (Zuleta et al, 2021). A number of mechanisms could drive the damage‐mortality relationship.…”

Analyses of three‐dimensional species associations reveal departures from neutrality in a tropical forest

“…Tree stature relates to different tradeoffs between species such as the growth versus survival tradeoff (Wright et al, 2010) and the early reproduction versus annual fecundity tradeoff (Wright, 2005). Species of different size have access to different levels of light and show different tolerances to shade (Poorter et al, 2005); they have different life spans (Lieberman et al, 1985), reproductive strategies (Gilbert et al, 2006), dispersal potential (Thompson et al, 2011); and are exposed to different mortality risks (Zuleta et al, 2022). Vertical stratification in tropical forests has been studied for decades (Horn, 1971; Terborgh, 1985) and clearly has a niche or life‐strategy component, with some species being reported as understory species and other species as typical canopy species.…”