Early-Warning Signals of Individual Tree Mortality Based on Annual Radial Growth

Abstract

Tree mortality is a key driver of forest dynamics and its occurrence is projected to increase in the future due to climate change. Despite recent advances in our understanding of the physiological mechanisms leading to death, we still lack robust indicators of mortality risk that could be applied at the individual tree scale. Here, we build on a previous contribution exploring the differences in growth level between trees that died and survived a given mortality event to assess whether changes in temporal autocorrelation, variance, and synchrony in time-series of annual radial growth data can be used as early warning signals of mortality risk. Taking advantage of a unique global ring-width database of 3065 dead trees and 4389 living trees growing together at 198 sites (belonging to 36 gymnosperm and angiosperm species), we analyzed temporal changes in autocorrelation, variance, and synchrony before tree death (diachronic analysis), and also compared these metrics between trees that died and trees that survived a given mortality event (synchronic analysis). Changes in autocorrelation were a poor indicator of mortality risk. However, we found a gradual increase in inter-annual growth variability and a decrease in growth synchrony in the last ∼20 years before mortality of gymnosperms, irrespective of the cause of mortality. These changes could be associated with drought-induced alterations in carbon economy and allocation patterns. In angiosperms, we did not find any consistent changes in any metric. Such lack of any signal might be explained by the relatively high capacity of angiosperms to recover after a stress-induced growth decline. Our analysis provides a robust method for estimating early-warning signals of tree mortality based on annual growth data. In addition to the frequently reported decrease in growth rates, an increase in inter-annual growth variability and a decrease in growth synchrony may be powerful predictors of gymnosperm mortality risk, but not necessarily so for angiosperms.

Ghosts of the past: how drought legacy effects shape forest functioning and carbon cycling

“…Despite their ubiquity, legacy effects are highly variable across taxa (Camarero et al 2018) and can even vary within the same species due to differences in age (Gao et al 2018), site conditions such as stand density and diversity (Gazol & Camarero 2016; Bottero et al 2017), and drought characteristics such as severity and seasonal timing (Merlin et al 2015; Huang et al 2018; Kannenberg et al 2019a). Finally, legacy effects can alter demographic processes such as mortality (Berdanier & Clark 2016; Vanoni et al 2016; Cailleret et al 2017; Cailleret et al 2019), which in turn have cascading consequences for ecosystem fluxes (Anderegg et al 2016).…”

Forecasting Forest Vulnerability to Drought in Pyrenean Silver Fir Forests Showing Dieback

“…Recent evidence suggests that forest dieback may be manifested as a loss of growth resilience capacity to extreme events and the presence of negative growth trends (Camarero et al, 2015(Camarero et al, , 2018Cailleret et al, 2017). In fact, stand level negative growth trends can indicate a gradual loss in tree vigor and can be used as an early warning signal of tree vulnerability to drought (e.g., Camarero et al, 2015;Cailleret et al, 2019). When growth decline becomes widespread within a stand it can be indicative of increased forest vulnerability (e.g., Camarero et al, 2018).…”

Radial Growth Patterns Associated with Tree Mortality in Nothofagus pumilio Forest

“…This incentivizes efforts to develop and employ common methodological strategies across many sites globally to assess relationships between tree-ring-based records of growth and mortality. The representativeness of angiosperms in tree mortality studies using global databases, in particular the South American tree broadleaf species, is very poor [6,[27][28][29].…”