Phenotypic plasticity in leaf morphology of Crataegus monogyna (Rosaceae): an experimental study with taxonomic implications

Many leaf characters are considered in the taxonomy of Crataegus in Europe, and several have been used in studies of the extent of hybridization in populations of northwest Europe. In such analyses it is assumed that the environmental component of phenotypic variation in such characters is insignificant. We tested this assumption by analysing the variation in the size and shape of leaves borne on clone cuttings of Crataegus monogyna maintained under identical conditions apart from the availability of soil nutrients. The resulting variation among leaves from this single genotype was as great as that observed previously across populations. Furthermore, although most of this variation could not be explained, a part could be attributed to differences in nutrient availability; of nine leaf characters investigated, eight showed significant variation due to this source, and in seven, the variation covaried significantly with nutrient level. The systematic implications of this are briefly explored.