Leaf Mottling: Relation to Growth Form and Leaf Phenology and Possible Role as Camouflage

Abstract

The arguments of Smith (1986) bearing on the adaptive significance of leaf mottling are criticized and an alternative hypothesis is presented. I propose that mottling may serve to camouflage the foliage of certain groups of shortstatured forest herbs, by disrupting their outline as perceived by colour-blind vertebrate herbivores in sun-dappled understoreys. Certain phenological groups are likely to be particularly vulnerable to herbivores, based on their high leaf nitrogen content (spring ephemerals, spring leaves of summeractive species), leaf activity when few other species possess foliage (evergreen species, wintergreen species, winter leaves of dimorphic species) and/or relative cost of replacing consumed foliage (evergreen species on sterile soils). These groups are also exposed to relatively high irradiances and so are less likely to suffer photosynthetic losses as a result of the reduced leaf absorptance that accompanies mottling. A survey of the incidence of leaf mottling in the native flora of the north-eastern USA supports these ideas: mottled leaves occur almost exclusively among forest herbs and are substantially over-represented among evergreen, wintergreen, and spring ephemeral species, and among the winter leaves of dimorphic species and the spring leaves of summer-active species. Mottled leaves are especially common among evergreen forest herbs. Testable extensions of the camouflage hypothesis to account for the relative abundance of mottled leaves among tropical forest herbs are discussed and parallels with other recently proposed mechanisms of vegetative crypsis in angiosperms based on leaf size, shape, texture, colour, and/or movement (i.e. Mimosa pudica) are outlined. Key-words: Anti-herbivore defence, camouflage, forest herbs, leaf mottling, leaf phenology