Growth and survival of termite-hollowed trees decrease with degree of piping, contrary to commonly-held belief that termites benefit host trees

Background/Question/Methods

Trees in tropical and sub-tropical savannas develop hollow cores, or pipes, caused by termite activity.  It is commonly accepted that such tree hollows an adaptive trait whereby trees benefit by the release of nutrients, yet remarkably little has been documented about their effect on tree growth or survival. In the natural mesic savanna of Kakadu National Park in northern Australia, 541 trees with known growth and survival histories were cored to determine pipe diameters. Generalized linear modelling and multi-model inference was used to analyze frequency and degree of piping relative to initial tree diameter at breast height (dbh). Growth (dbh increment) and survival (4 years) were analyzed relative to initial tree size, pipe ratio (pipe diameter:dbh) and eco-taxonomic group. Separate analyses were also conducted on 267 trees of  the two most canopy species, Eucalyptus tetrodonta and E. miniata, whose hollowed stems are harvested to make didgeridoos, the traditional musical instrument of Aboriginal peoples now experiencing high demand from international markets.

Results/Conclusions

Growth and survival of eucalyptus trees increased with tree diameter and decreased with pipe ratio. For example, from modelled data, 10-cm-diameter trees without pipes grew 0.14 cm y^-1 with 85% survival but those with pipe ratios of 0.60 had near-zero growth and only 46% survival. Comparing 40-cm-diameter trees without pipes to those having pipe ratios of 0.80, growth was 0.22 vs. 0.05 cm y^-1, with little difference in survival, 97–99%, respectively. Contrary to the accepted belief that termite piping benefits trees, in this study, the net effect on individual tree growth and survival was strongly negative. Results also shed light on potential long-term effects of current large-scale commercial harvesting of trees to make didgeridoos which take trees with higher growth and survival rates–those trees most likely to contribute to sustainable tree populations, whereas traditional Aboriginal methods of tree harvesting incidentally remove only those smaller hollow trees that are already suffering low growth rates and are likely to die before reaching maturity.