The effect of drought on wood-boring in trees and saplings in tropical rainforests

Abstract

Droughts are predicted to increase in severity in many regions due to climate change and there is strong evidence that such events can lead to increased insect attack and consequent widespread tree mortality in temperate forests. Much less is known about the impact of increased drought on tropical rainforests but in the few large-scale drought manipulation experiments in tropical rainforests, larger trees had higher mortality rates than smaller trees although the cause of death is often uncertain. Previously, we modelled what the impact of drought might be on different types of herbivorous insects and suggested that in rainforests severe drought conditions might lead to increased attack from wood-boring insects. We tested this in a drought manipulation experiment, in Australian tropical rainforest, where we excluded more than 30% of rainfall for two years prior to our study and during it. We compared wood-boring damage of small and large trees between the experiment and a nearby control site. We hypothesized that larger trees would experience greater wood boring whereas smaller trees would present more surface damage. We surveyed 1,778 trees in total across both plots and found that the proportion of trees with termites, hole boring, and surface damage caused by borers (lateral tracks, frass, resin, latex or kino exudation) was significantly higher at the drought plot than at the control plot. There was a significant difference in the proportion of trees with fresh wood-boring damage at the drought site (35%) than at the control site (23%). While all size classes of trees had a higher percentage of fresh wood-boring damage at the drought site compared to control site this was only significant for small trees (dbh < 10 cm). The lack of significant difference for medium sized trees (dbh > 10 cm & <20 cm) and large trees (dbh > 20 cm) may be due to small sample size. Recent termite activity and termite damage was also significantly more frequent in the drought site compared to the control. We conclude that increased severity of drought appears to drive fundamental changes in borer and termite infestation levels with potentially important consequences for long-term tree health and mortality. Increases in tree mortality elevates the risk of forest fires, which are normally rare events in rainforests. Determining which wood boring beetles are responsible for the increased infestation requires further investigation since more than 100 wood-boring beetle species have been collected previously at the study site, including species of Platypodinae, Scolytinae, Anobiidae and Cerambycidae.