Seedling performance, allocation patterns and phenotypic plasticity of two sympatric hawthorn species and their natural hybrid

Abstract

Hybridization followed by introgression is common between European Crataegus species, while hybrids of numerous sympatric taxa are abundant and widespread in several natural and seminatural environments. No study to the present day has compared the development of Crataegus species and their hybrids in their seedling stage. Our aim was to compare seedling performance, biomass allocation and phenotypic plasticity of two sympatric Crataegus species (C. monogya and C. rhipidophylla) with contrasting shade tolerance and their natural hybrid along light and water availability gradients in a controlled pot experiment. By this means we tested which of the existing models can successfully explain the field distribution and habitat preference of the hybrid. The performance of both species and their hybrid decreased in deep shade as well as under high water availability. Variation in light availability had a more pronounced effect on seedling development than that in water availability. These results evidence that habitat differentiation between the two species under natural conditions is probably mainly driven by competitive interactions. The shade tolerant forest species C. rhipidophylla had shorter, more dense stems compared to the shade intolerant C. monogyna, with the hybrid being intermediate in these characters. This suggests a steadier growth and higher resource allocation into stems in the case of the shade tolerant species. Furthermore, the hybrid was phenotypically more plastic in biomass allocation relative to at least one of its parental species under various water availabilities, suggesting an advantage on behalf of the hybrid in establishment under a wider range of environmental conditions. Our results support the “mosaic-model” that describes most accurately the hybrid zone.