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.
Highlights
Drought-induced tree mortality is becoming a major ecological concern as the planet becomes warmer and mortality episodes increase worldwide (Allen et al, 2010)
Many research gaps still exist concerning the patterns of drought-related tree death and the reason why some trees are more prone to die than other coexisting individuals (McDowell et al, 2008). This has fostered the investigation of dieback mechanisms caused by hydraulic failure and carbon starvation (Sevanto et al, 2014)
We focus on such early-warning signals of tree death and we model the mortality probability as a function of recent growth and tree height
Summary
Drought-induced tree mortality is becoming a major ecological concern as the planet becomes warmer and mortality episodes increase worldwide (Allen et al, 2010). Many research gaps still exist concerning the patterns of drought-related tree death and the reason why some trees are more prone to die than other coexisting individuals (McDowell et al, 2008). This has fostered the investigation of dieback mechanisms caused by hydraulic failure (substantial xylem embolism) and carbon starvation (carbon losses greatly exceed carbon gains) (Sevanto et al, 2014). Bennett et al (2015) report in their review that, drought-related mortality increased with tree height in 65% of the cases examined, where few or none involved oak species
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