Leaf Mottling: Relation to Growth Form and Leaf Phenology and Possible Role As Camouflage Author(S): T
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Leaf Mottling: Relation to Growth Form and Leaf Phenology and Possible Role as Camouflage Author(s): T. J. Givnish Source: Functional Ecology, Vol. 4, No. 4 (1990), pp. 463-474 Published by: British Ecological Society Stable URL: http://www.jstor.org/stable/2389314 Accessed: 27-04-2016 13:23 UTC Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at http://about.jstor.org/terms JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. British Ecological Society, Wiley are collaborating with JSTOR to digitize, preserve and extend access to Functional Ecology This content downloaded from 130.223.51.59 on Wed, 27 Apr 2016 13:23:21 UTC All use subject to http://about.jstor.org/terms Functional ESSAY REVIEW Ecology 1990, 4, 463-'474 Leaf mottling: relation to growth form and leaf phenology and possible role as camouflage T. J. GIVNISH tion or masking of chlorophyll over portions of the Department of Botany, University of Wisconsin, upper leaf surface, often in a more or less mottled Madison, Wisconsin, USA pattern. Leaves mottled in various shades of green, or marked by splashes or stripes of white (or sometimes purple, red or pink) are characteristic Abstract. The arguments of Smith (1986) bearing of many species of angiosperms, particularly on the adaptive significance of leaf mottling are among understorey herbs in temperate and tropi- criticized and an alternative hypothesis is pre- cal forests (Neger, 1913; Richards, 1952; Morgan, sented. I propose that mottling may serve to 1971; Smith, 1986; Givnish, 1988a; Evenari, 1989). camouflage the foliage of certain groups of short- Smith (1986) discusses some of the selection statured forest herbs, by disrupting their outline as pressures that may favour mottled leaves, in terms perceived by colour-blind vertebrate herbivores in of processes that could help maintain a population sun-dappled understoreys. Certain phenological polymorphic for mottled and unmottled leaves. He groups are likely to be particularly vulnerable to hypothesizes that mottled leaves might be advan- herbivores, based on their high leaf nitrogen con- tageous by: (i) serving to mimic leaf miner damage tent (spring ephemerals, spring leaves of summer- and thereby deterring actual oviposition by active species), leaf activity when few other miners, which are posited to avoid the reduced species possess foliage (evergreen species, resource levels or induced defences (Schultz & wintergreen species, winter leaves of dimorphic Baldwin, 1982) that may be associated with mined species) and/or relative cost of replacing con- leaves; (ii) reducing leaf temperature and transpir- sumed foliage (evergreen species on sterile soils). ation in sunny, hot or dry microsites; (iii) visually These groups are also exposed to relatively high diverging from other foliage sought by visually irradiances and so are less likely to suffer photo- orienting herbivores (see Gilbert, 1975 and synthetic losses as a result of the reduced leaf Rausher, 1981 for similar suggestions based on absorptance that accompanies mottling. A survey differences in leaf shape); and/or (iv) serving as of the incidence of leaf mottling in the native flora aposomatic coloration, advertising a chemical, of the north-eastern USA supports these ideas: mechanical or biological defence. Set against these mottled leaves occur almost exclusively among potential advantages is (v) the reduction of light forest herbs and are substantially over-represented interception likely to result from the masking or among evergreen, wintergreen, and spring ephem- absence of chlorophyll over the mottled areas of eral species, and among the winter leaves of the leaf. dimorphic species and the spring leaves of Smith (1986) presents data that bear on these summer-active species. Mottled leaves are es- hypotheses for Byttneria aculeata Jacq. (Stercu- pecially common among evergreen forest herbs. liaceae), a weedy tropical subcanopy vine growing Testable extensions of the camouflage hypothesis in Panama. Juvenile plants of Byttneria have either to account for the relative abundance of mottled mottled or unmottled leaves. Mottled plants are leaves among tropical forest herbs are discussed relatively more frequent in partial or complete and parallels with other recently proposed clearings than under closed canopies and trans- mechanisms of vegetative crypsis in angiosperms plant studies suggest that the degree of leaf mot- based on leaf size, shape, texture, colour, and/or tling is genetically determined. Leaf production by movement (i.e. Mimosa pudica) are outlined. transplants in the absence of herbivory indicates Key-words: Anti-herbivore defence, camouflage, forest herbs, leaf mottling, leaf phenology that the average amount of variegation per leaf is correlated positively with growth in open habitats and negatively with growth in shaded sites; Smith Introduction interprets this as tentative support for hypotheses 463 Leaf variegation entails the local absence, reduc- (ii) and (v). Furthermore, when open and shaded This content downloaded from 130.223.51.59 on Wed, 27 Apr 2016 13:23:21 UTC All use subject to http://about.jstor.org/terms 464 sites are pooled, the mottled morph is less heavily preference for shady conditions or for plants T. J. Givnish attacked than the unmottled morph when they grown under shady conditions. To exclude this occur at the same frequency (the extent of leaf possibility, we would need studies of miner attack mining on a given morph increases with its rela- as a function of leaf variegation in sunny and tive frequency). Smith (1986) interprets the latter shady field sites in which the frequencies of finding as tentative support for hypothesis (i). mottled and unmottled vines have been experi- mentally manipulated. 3 Finally, Smith's (1986) proposal that the Critique of Smiths's model for leaf combined effects of leaf variegation on leaf miners, variegation light interception and heat load favour mottled Smith's (1986) empirical findings are important leaves in open or partially shaded sites contradicts and appear to provide the first experimental evid- qualitative observations by Richards (1952) and ence for the adaptive significance of leaf varie- Givnish (1988a) that species characterized by such gation in sunny vs shady environments. Yet there leaves are relatively most numerous among herbs are some difficulties with the interpretations he native to temperate and tropical forest under- advances: storeys. Smith's proposal may thus have no gen- 1 The growth of transplanted Byttneria vines is in eral applicability and the correlation between leaf accord with hypotheses (ii) and (v) that mottled mottling and open or partially shaded habitats (and hence more reflective) leaves would be cooler seen in Byttneria may belie the pattern seen across and achieve lower rates of transpiration in sunny plant species elsewhere. sites, but achieve substantially lower rates of photosynthesis in shady sites. Yet other plausible Camouflage hypothesis interpretations of this result exist. For example, a As an alternative hypothesis, I propose that mot- correlation between the degree of leaf variegation tling may serve to camouflage the foliage of certain and growth rate in forest openings may arise if groups of forest herbs, by disrupting their outline variegation in Byttneria is genetically linked to as perceived by colour-blind vertebrate herbivores other, functionally unrelated traits (e.g. Rubisco (e.g. deer) in sun-dappled understoreys. It is expli- level, stomatal conductance, leaf thickness, leaf- citly assumed that, when viewed in black-and- root allocation, anti-herbivore defences) that white, mottling acts as a dappled camouflage determine photosynthetic performance and/or pattern against the sunlight-dappled forest floor competitive ability in sunny sites (see Bj6rkman, and would tend to reduce the probability that a 1981; Givnish, 1988a). mottled leaf or spatial cluster of such leaves would 2 More importantly, even if hypotheses (ii) and (v) be consumed. Such crypsis is possible only for were both correct, neither would support an adap- leaves held close to the ground, because even tive value for leaf mottling per se, because leaves of foliage that matches the forest floor light pattern uniformly reduced absorptance can reflect or perfectly could be detected visually if it were held transmit as much light, on average, as mottled much above the ground, through differences in the leaves. The same is true of hypothesis (iv): it is not focal planes of leaves vs ground relative to a clear why aposomatic leaves should be mottled, stationary herbivore or differences in parallax rather than strikingly marked in bold patterns. No relative to a moving herbivore. evidence is presented to support hypothesis (iii) If this hypothesis is correct, then leaf mottling and it is not clear why visual divergence through should be most common in forest herbs. An leaf mottling (vis-&-vis leaf size or shape) should important corollary of this hypothesis, indepen- be particularly common in shady environments. dent of previous observations, would be that Of Smith's hypotheses, only (i), involving among forest herbs leaf mottling should be most supposed mimicry of