Size Matters a Lot: Drought-Affected Italian Oaks Are Smaller and Show Lower Growth Prior to Tree Death

Abstract

Hydraulic theory suggests that tall trees are at greater risk of drought-triggered death caused by hydraulic failure than small trees. In addition the drop in growth, observed in several tree species prior to death, is often interpreted as an early-warning signal of impending death. We test these hypotheses by comparing size, growth, and wood-anatomy patterns of living and now-dead trees in two Italian oak forests showing recent mortality episodes. The mortality probability of trees is modeled as a function of recent growth and tree size. Drift-diffusion-jump (DDJ) metrics are used to detect early-warning signals. We found that the tallest trees of the anisohydric Italian oak better survived drought contrary to what was predicted by the theory. Dead trees were characterized by a lower height and radial-growth trend than living trees in both study sites. The growth reduction of now-dead trees started about 10 years prior to their death and after two severe spring droughts during the early 2000s. This critical transition in growth was detected by DDJ metrics in the most affected site. Dead trees were also more sensitive to drought stress in this site indicating different susceptibility to water shortage between trees. Dead trees did not form earlywood vessels with smaller lumen diameter than surviving trees but tended to form wider latewood vessels with a higher percentage of vessel area. Since living and dead trees showed similar competition we did not expect that moderate thinning and a reduction in tree density would increase the short-term survival probability of trees.

Drought Decreases Growth and Increases Mortality of Coexisting Native and Introduced Tree Species in a Temperate Floodplain Forest

“…We found that dead oaks were characterized as smaller than surviving oaks, as previously reported [17,50]. This suggests that a smaller size predisposes to drought-induced dieback and death through a lower capacity to buffer the negative effects of soil water deficit by increasing hydraulic capacitance [79].…”

“…In previous studies, we have shown that taller oaks were more prone to survive after drought than smaller oaks [50]. We applied Firth's bias-Reduced penalized-likelihood logistic regression [51,52].…”

Site factors and stand conditions associated with Persian oak decline in Zagros mountain forests

“…Forest structure mediates the effect of drought on oak forest dynamics at several scales (Clark et al, 2016;Colangelo et al, 2017b). Indeed, recruitment patterns and mortality in several oak species seem to be related to canopy closure (Gómez-Aparicio et al, 2008).…”

“…Notwithstanding, such suppressed and presumably older stems, growing as coppices, might be particularly vulnerable to decline (Hamzehpour et al, 2011). Specifically, these trees subjected to higher competition show lower radial growth and they are more prone to die following extreme drought events (Franklin et al, 1987;Das et al, 2008Das et al, , 2011Colangelo et al, 2017b). Greater mortality in coppice oaks could be related with the abandonment of stems harvest at earlier age, or with the undone reduction of the number of stems of each coppice, both management practices that could reduce stand-level decline and mortality (Di Filippo et al, 2010).…”

“…Episodic tree mortality is usually related to dry site conditions and high stand density (Chapman et al, 2006;Gea-Izquierdo et al, 2009). Specifically, trees subjected to higher competition show lower radial growth and they are more prone to die following extreme drought events (Das et al, 2008;Galiano et al, 2010;Linares et al, 2010;Colangelo et al, 2017b). As a result, contrasting stand structure may act modulating the individual tree responses to drought (Lechuga et al, 2017).…”