Relationships between leaf growth and holes caused by herbivores

Abstract

There are two approaches to the estimation of herbivory in a canopy. One is to measure the absolute amount consumed by herbivores in terms of bites of leaf tissue removed and express defoliation as metres squared of leaf surface area (or kilogram of photosynthetic tissue). This approach is useful for examining the population dynamics and feeding behaviour of the insects or, in a sense, to approach herbivory from a herbivore's perspective. The second is to measure the proportion of leaf area missing, usually expressed as a percentage of total leaf surface area. The latter approach was used for long-term studies of herbivory in Australian rainforests (Lowman 1982, 1984, 1985). and is currently in use for similar studies on eucalypt canopies (Lowman & Heatwole 1987, unpubl. data). One of the most frequently asked questions regarding defoliation measurements expressed as percentage leaf area losses is: 'Does the proportion of area of defoliation stay the same as a leaf expands?' Many herbivores exhibit marked preference for young leaf tissue (Lowman & Box 1983; Selman & Lowman 1983; Wint 1983). A direct measurement (cm2) of holes in mature leaves, therefore. may be an overestimation of the actual amount ingested by herbivores that fed upon young, partially expanded leaves. A proportional estimate of defoliation will remain accurate throughout the life of a leaf, however, if leaf holes expand at the same rate as leaf area. This was tested on young leaves of one species of a northern temperate forest. Liriodendron tullpifera L. (albeit with deciduous, and unusual tulip-shaped leaves), and its holes expanded at a rate proportional to leaf growth (Reichle et al. 1973). In this study, a similar test was performed for a range of Australian rainforest tree species exhibiting different leaf shapes, textures, and growth habits.