Effects of sclerophylly and host choice on gall densities andherbivory distribution in an Australian subtropical forest.

Abstract

We hypothesize that upper canopies contrast with the understorey vegetation in gall density and survival be-cause of greater mortality in the latter. We expect that high sclerophylly rates in the upper canopy leaves are a main cause ofsuch pattern, and more important than other environmental traits, for instance related to altitude. The study was con-ducted in Lamington National Park, Queensland, Australia. Four independent vertical cylindrical transects through theforest canopy and one equivalent, horizontal understorey transect (20 m long and 1 m in diameter) were sampled at dif-ferent altitude (300, 700, 900 and 1100 m above sea level) during two seasons. Total and damaged leaves were counted,and galls were collected and opened to determine if they were alive or what may have been the cause of death. Sclerophyllywas estimated as specific foliar mass. Out of 72 sampled plant species, 29 presented galls, of which the greatest densitieswere concentrated on seven host species. A significant increase in sclerophylly with increasing canopy stratum height wasobserved, but had no direct effect on gall distribution. Total and live galls were more abundant in the canopy than in theunderstor ey for all altitud es but 300 m, where a specific infestation on saplings of the canopy tree Arytera divaricata oc-curred. We found a positive gall establishment and survivorship in the upper canopy, along with a decrease in chewing her-bivory, which resulted in decreasing risks of gall death by herbivory. An overall high sclerophylly rate in both canopy andunderstorey and the total number of galled host species suggest that the plant community studied is prone to gall estab-lishment and evolution. However, a few hosts species with extreme infestation, such as A.divaricata and Ficus watkinsiana,override the community-wide effect of sclerophylly. Our results emphasize how scarce and patchy distributed galls are.