Herbivory, Foliar Enzyme Inhibitors, Nitrogen and Leaf Structure of Young and Mature Leaves in a Tropical Forest

Abstract

In Bako National Park in Sarawak we investigated leaf damage, enzyme inhibition, condensed tannins, total nitrogen and leaf structure in young and mature leaves of Eugenia ochneocarpa and three species of Shorea. The amount of leaf removed by herbivores (measured as the total percentage area, not as a rate) was 3.8 percent in young leaves and 6.2 percent in mature leaves for E. ochneocarpa, and was 12.1 percent in young leaves and 13.4 percent in mature leaves for Shorea spp. The relative importance of the three guilds of herbivores examined was edge-grazing, mining, then holing, in both taxa. E. ochneocarpa mature leaves were 425 ,um thick with a specific leaf area (SLA) of 93 cm2 g-'; Shorea spp. mature leaves were 180 ,um thick with an SLA of 98 cm2 g. Young leaves of both taxa had higher enzyme inhibition than mature leaves, and young leaves of E. ochneocarpa were over five times more inhibitory than those of Shorea spp. Leaf nitrogen (per area) was lower in young leaves than in mature leaves for both taxa. The concentrations of condensed tannins were similar in young and mature leaves for both species, but higher in E. ochneocarpa than Shorea spp. In both taxa, when young and mature leaves were compared, increased damage was associated with decreased SLA, but with decreased leaf nitrogen and increased enzyme inhibition. Comparing the two taxa, we found that for young leaves, though not for mature leaves, increased damage was associated with increased leaf nitrogen, and decreased enzyme inhibition and condensed tannin. A FLUSH OF RED YOUNG LEAVES iS an eye catching sight in the green of a tropical forest canopy. Nearly unfolded leaves are often partially or totally lacking in chlorophyll but brightly coloured due to anthocyanin pigmentation. Before expansion some young leaves appear wilted due to a lack of mechanical tissue; Richards (1952) describes the development of young leaves. The low fibre content and fleshy nature of young leaves might make them more palatable to herbivores than mature leaves, and their redness may alter their attractiveness. In a seasonal environment young leaves can avoid damage by developing before the increase in herbivore numbers (Feeny 1970). In the less seasonal environment of Sarawak some counter-balancing chemical protection could be expected. Alternatively substantial loss of young leaves would be anticipated during development. We wished to test the former hypothesis: that young leaves are no more susceptible to damage than mature leaves, because they are protected chemically. To this end, two major questions were asked: Is there a difference in the proportion of the leaf area removed by herbivores (the damage) between young and mature leaves? Could the damage level be related to leaf palatability: the enzyme inhibition by leaf extracts, condensed tannin, hydrolysable tannin, and total nitrogen levels, or the structure of leaves (measured as specific leaf area)? Three further questions elaborated this work: What are the major guilds of herbivores to which the leaf samples had been exposed and how did the intensity of their attack vary between young and mature leaves? How was damage distributed across the population of young leaves, and how did the damage affect the subsequent population of mature leaves? What role did condensed tannins play in enzyme inhibition by leaf extracts? A more general hypothesis was also tested, that total nitrogen, leaf toxins and leaf structure are related to damage when different species are compared. We therefore applied the questions above in a comparison between members of two genera of trees. It is thought that tannins reduce the nutritional qual1 Received 16 May 1983, revised 3 January 1984, accepted 13 January 1984. 2 All authors contributed equally. I Present address: Department of Applied Biology, New Museum Site, Pembroke Street, Cambridge, England. 4Present address: Department of Psychology, Downing Site, Cambridge, England. I Present address and address for correspondence: Department of Pharmaceutical Chemistry, University of Strathclyde, Royal College Building, 204 George Street, Glasgow GI IXW, Scotland. BIOTROPICA 16(4): 257-263 1984 257 This content downloaded from 157.55.39.161 on Mon, 23 May 2016 06:09:20 UTC All use subject to http://about.jstor.org/terms ities of plant material by complexing with plant proteins and so rendering them indigestible (Feeny 1969). Tannins may also attack the digestive enzymes in a herbivore's gut reducing their catalytic action. Tannins are not the only protein attacking agents; many plant phenolics and their oxidation products can have similar effects (Swain 1979). Chemical methods were used to assay condensed tannins and total nitrogen, and an enzyme inhibition reaction to detect a broad spectrum of protein attacking agents. As we were interested in chemical defenses that operate by attacking proteins, families where the chemical defense takes other forms were avoided.