Implications of foliar monoterpenoid variation among ontogenetic stages of the California bay tree (Umbellularia californica) for deer herbivory

As part of a larger detailed investigation of foliar monoterpenoid variation in Umbellularia californica, monoterpenoid content was measured from trees at several sites in three ontogenetic stages: seedlings (<30 cm), saplings (between 1 and 2 m) and adult trees (>4 m). Mature leaves were used in this study to standardize against the changes that occur during leaf development. Furthermore, as shown in other studies new growth of all three ontogenetic stages had extremely low terpenoid total yields (mg all monoterpenoids/g leaf dry weight). Variation due to collecting site, selected across a variety of community types and environments, was not significant for any of the variables studied. However, monoterpenoid total yields varied significantly (P < 0.001) among ontogenetic stages: seedlings had less than half the yields found in the sapling and adult tree foliage, whereas total yields between sapling and adult tree foliage did not differ statistically. No significant difference occurred among the ontogenetic stages in the ratios of hydrocarbons to oxygenated monoterpenoids. Little variation in monoterpenoid composition (% of the total yield by individual monoterpenoids) occurred among the ontogenetic stages. A year-long field study of Blacktailed Deer browse intensity revealed no strong preference among tagged shoots of mature leaves among ontogenetic stages with differing total yields. These results are discussed in relation to suggestions regarding plant chemical defense against mammalian herbivory.