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Abstract
One factor that determines a tree’s resilience capacity to drought is its level of interaction with aggravating biotic factors, such as bark beetles, which in turn depends on whether insect populations are at an endemic or epidemic stage. In a Mediterranean natural forest ecosystem, we investigated the ecophysiological characteristics of trees which (i) predispose Pinus halepensis to attacks of endemic populations of bark beetles (Tomicus destruens) and (ii) that arise as a result of these attacks. The annual life cycle of Tomicus destruens features an initial dispersal phase of sexually mature adults towards pine trunks for the purposes of brood production, and a second dispersal phase of callow adults from trunks to pine shoots for sexual maturation. During a three-year survey of endemic bark beetle attacks in a stand of 178 trees, we found that trees targeted for trunk attacks showed a more aggregated distribution pattern than those targeted for shoot attacks. Moreover, shoot-attacked trees were clearly larger and taller, had lower wood density, and were less exposed to competition than non-attacked trees. No differences were found between the characteristics of trunk-attacked trees and non-attacked ones. We found crown defoliation, lower primary growth and alteration of trunk non-structural carbon in phloem in the year following trunk attacks, although this did not affect vulnerability to cavitation of the vascular system, secondary growth, wood density, and xylem starch. Conversely, the health status and physiological variables of the shoot-attacked trees were not altered, which may be explained by their initial vigor. We conclude that the distribution of Tomicus destruens attacks at the plot level was independent of the ecophysiological traits of the host trees. It was mainly determined by the dispersal strategy of the endemic bark beetle population especially during brood production as only trunk attacks significantly weakened the trees.
Highlights
Global changes are likely to have a significant impact on the distribution of many tree forest species, and their associated animal and plant communities
These results suggest that the distribution of T. destruens attacks in an endemic state were not a consequence of the distribution of the health status of the trees but rather the dispersal strategies of the bark beetles, which change during different phases in their life cycle
We show that a population of T. destruens at an endemic stage may not necessarily attack weakened host trees, and that notably they targeted the shoots of the most vigorous trees during the sexual maturation phase of their life cycle
Summary
Global changes are likely to have a significant impact on the distribution of many tree forest species, and their associated animal and plant communities. Increases in the frequency of dieback and mortality in some tree species are expected in this context (Allen et al., 2010). Such declines in tree populations are usually triggered by repeated droughts and heatwaves, individual trees can display resilience which, in some cases, enable them to avoid mortality and benefit from the return of favorable conditions. Biotic aggravating factors include numerous phytophagous insect species e.g. bark beetles that commonly feed on the cambial tissues of dead, recently damaged or even heavily-defended healthy trees (Paine et al, 1997). Bark beetles are considered as significant disturbance agents in forested areas worldwide (Raffa et al, 2008) affecting critical processes in forest dynamics and forest services (Kurz et al, 2008; Hicke et al, 2012; Weed et al, 2013) with important subsequent societal issues (Abrams et al, 2017; Morris et al, 2018)
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