On plastic and non-plastic variation in clonal plant morphology and its ecological significance

Morphological plasticity in clonal plants has received wide attention because localized plastic changes in spacer length, branching intensity and branching angle may enable clonal plants to place ramets selectively in the more favourable microhabitats within a heterogeneous environment. These responses have been interpreted in terms of foraging behaviour. Studies of morphological plasticity in clonal plants are usually carried out with one or two genotypes of a species, or with material of unknown genetic origin. Based on the concept of phenotypic plasticity, it is argued that such studies do not reveal whether plasticity in a population can be modified by natural selection. In addition, responses are often evaluated at two environmental conditions only, which may underestimate plasticity. Hence, our information on the ecological and evolutionary significance of morphological plasticity in clonal plants is still very incomplete. Two examples are given to show that stolon internode and rhizome lengths may vary considerably within an individual plant. Only a minor part of this variation may be plastic, i.e. the variation is hardly changed by the environmental conditions to which the plants are subjected. Hence, non-plastic variation in clonal morphology may exceed the degree of morphological plasticity. The non-plastic variation seems to originate from species-specific patterns of stolon and rhizome development. Marked non-plastic variation may obscure the effects of morphological plasticity on the placement pattern of ramets in the field, suggesting that plasticity in clonal morphology may not be very effective in terms of foraging for favourable patches. Possible reasons for the low levels of plasticity of clonal spacers are discussed.