Vegetation traits and herbivory distribution in an Australian subtropical forest.

Abstract

We tested the hypothesis that leaves in the canopy should have higher sclerophylly indices compared with understorey leaves, which should, along with other physical foliage traits, allow greater gall-forming insect survivorship in the canopy and result in higher leafchewing damage in the understorey. The study was conducted in the subtropical rainforest of Lamington National Park, Queensland, Australia. Along an altitudinal gradient, four independent canopy pin-transects and one equivalent, horizontal understorey pintransect (20 metres long) were conducted at each of four altitudes, 300, 700, 900 and 1100 m above sea level (a.s.l.). Each discrete layer (stratum) of foliage within the 1 m diameter pin-transects was considered a sample. From each sample, various leaf and meristem measurements were taken, and the number of damaged leaves was counted. Healthy leaves were also collected for analysis of specific leaf mass, an indirect measure of sclerophylly. All vegetation resources were more abundant in the canopy than in the understorey, and also increased from lower to higher altitudes. In the canopy, leaf density increased steadily from 300 to 1100 m sites, but in the understorey 300 and 900 m had denser foliage than 700 and 1100 m. Young leaves were more available in the canopy than the understorey. However, the sites at 900 and 1100 m had as many young leaves in the understorey as 300 m sites had in the canopy. The ratio of young/ mature leaves increased with altitude, with no difference between understorey and canopy. Leaf area did not vary between canopy and understorey. Active meristems were found in greater numbers in the canopy. A significant increase in sclerophylly with increasing canopy stratum height was found. Comparing altitudes, 300 and 1100 m sites had more active meristems than those at 700 and 900 m. Out of 72 plant species, 29 presented galls of which the greatest densities were concentrated on seven host species. Herbivory was more intense at lower altitudes, suggesting that micro-climate and host specificity may drive the insect distribution patterns  canopy-understorey vegetation gradient, gall density, leaf herbivory, gall super hosts, Lamington National Park, sclerophyllous habitats.