Evolutionary and organismic constraints on the relationship between spacer length and environmental conditions in clonal plants

The rate of lateral spread of clonal plants is considered a plastic trait that responds directly to environmental conditions. However, this response is likely constrained by evolutionary history and other species attributes. Here we ask how the relationships between lateral spread and the distribution of herbaceous plant species in different environments are influenced by the type of spacer (epigeogenous versus hypogeogenous rhizome), its persistence (integrator versus splitter), and the height and phylogeny of the plant. Analysis of spacer length of 367 species from temperate to arctic Europe revealed that other plant characteristics modulate the relationship between spacer length and environmental conditions. While plants with epigeogenous rhizomes, which have usually shorter spacers than hypogeogenous rhizomes, were associated with illuminated habitats, plants with hypogeogenous rhizomes were associated with warm habitats. These relationships were also shown within the specific rhizome type, as within the group of species with epigeogenous rhizomes, those having short spacers were associated with more light. The trend toward long spacers in warm environment was detected within both epigeogenous and hypogeogenous rhizome bearing species. Splitters were found to be associated with wet, nutrient rich habitats irrespective of rhizome type. When plant height was accounted for by using the ratio of spacer length to plant height (L: H) instead of absolute spacer length, no relationship with environmental variables was found until phylogenetic relationships among the species were taken into account. This implies that overall variability in L: H ratio in the dataset masked trends common for different taxonomic groups. Future comparative studies relating particular clonal growth forms or individual traits to environmental conditions should consider the constraints arising from other growth characteristics and evolutionary history of the subject species.