We investigated variation in the nitrogen content of young leaves and the rate of leaf expansion in tropical shade tolerant species. Both these factors can influence rates of herbivory: low nitrogen content makes leaves less attractive to herbivores, while rapid expansion reduces the time that young leaves are vulnerable to herbivory. Among the 13 study species, nitrogen in young leaves (at 5% of full expansion) ranged from 20 to 71 mg/g DW. Most species had higher nitrogen in young leaves compared to mature leaves, but one species showed no change with age. The ratio of nitrogen import relative to dry weight import was low (1-4%) and dropped during expansion. The amount of nitrogen imported daily into the leaf peaked at 70-75 percent of-full expansion, though species varied in the percent of final nitrogen that was imported prior to full expansion (67%-93%). There was a significant positive correlation between nitrogen content of young leaves and the rate of leaf expansion. We suggest that the age-specific patterns of nitrogen can best be understood as a trade-off between high palatability to herbivores and escape through rapid expansion. IN MOST SPECIES, young expanding leaves are more heavily attacked by both chewing and sucking insects than are mature leaves (e.g., Feeny 1976, Raupp & Denno 1983, Lowman 1985, Crawley 1986, Lightfoot & Whitford 1987, Slansky & Rodriguez 1987). This pattern has been documented in communities as diverse as grasslands, tundra, and temperate forests, but may be most pronounced in tropical rain forests (Coley & Aide 1991). For 24 shade tolerant species studied in Panama, rates of herbivory were 25 times faster on young leaves compared to mature leaves (Coley 1983). This means that although young leaves are expanding for only 3 percent of their lifetime, almost 50 percent of the lifetime damage occurs during this period. Therefore, to fully understand the factors that regulate losses to herbivores, we must examine the characteristics of young leaves. Young leaves have a variety of characteristics that are likely to function as anti-herbivore defenses. In many species, young leaves are protected by ants associated with extrafloral nectaries (Bentley & Elias 1983, McKey 1989). Young leaves tend to be more pubescent and to contain higher concentrations of tannins and other secondary metabolites (Coley 1983, Coley & Aide 1991). Synchronous production of young leaves during times of low herbivore density may permit plants to avoid most herbivores and satiate those that are present (Feeny 1976; McKey 1979, 1989; Lieberman & Lieberman 1984; l Received 24 December 1989, revision accepted 13 November 1990. Crawley 1986; Aide 1988). Rapid expansion of individual leaves may also reduce the period of vulnerability before leaves have expanded, toughened, and become better defended against herbivores (Orians & Janzen 1974, McKey 1979, Hay et al. 1988, Aide & Londono 1989). Despite these defenses, young leaves generally have higher rates of herbivory than mature leaves. One reason may be because young leaves are less tough and easier to chew and digest. For mature leaves of tropical species, leaf toughness is quite negatively correlated with herbivory and appears to be the most effective defense (Coley 1983). However, toughness is not an option for expanding leaves because the formation of a lignified secondary cell wall is not compatible with cell division and cell expansion. Once young leaves reach full size and begin to toughen, rates of herbivory can drop precipitously (Coley 1983). Another factor which may contribute to high herbivory on young leaves is their greater nutritional value. Young leaves typically have two to four times the nitrogen content of mature leaves (Ho et al. 1984, Mattson & Scriber 1987, Coley & Aide 1991). Diets with higher nitrogen can increase herbivore fitness (Mattson & Scriber 1987). It is therefore not surprising that nitrogen content is positively correlated with food choice for both chewing and sucking insects. Any changes in the developmental patterns of young leaves that reduce the concentration of nitrogen might therefore reduce the rates of herbivory on young leaves (Moran & Hamilton 1980, Neuvonen & Haukioja 1984).
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Kursar et al. (1991) studied this question.