Do deer and raccoons defecate in the right place? Fitness consequences of vertebrate seed dispersal for a deciduous forest herb

Molecular phylogeography and historical demography of a widespread herbaceous species from eastern North America, Podophyllum peltatum

PREMISE

Glacial/interglacial cycles and topographic complexity are both considered to have shaped today's diverse phylogeographic patterns of taxa from unglaciated eastern North America (ENA). However, few studies have focused on the phylogeography and population dynamics of wide-ranging ENA herbaceous species occurring in forest understory habitat. We examined the phylogeographic pattern and evolutionary history of Podophyllum peltatum L., a widely distributed herb inhabiting deciduous forests of ENA.

METHODS

Using chloroplast DNA (cpDNA) sequences and nuclear microsatellite loci, we investigated the population structure and genetic diversity of the species. Molecular dating, demographic history analyses, and ecological niche modeling were also performed to illustrate the phylogeographic patterns.

RESULTS

Our cpDNA results identified three main groups that are largely congruent with boundaries along the Appalachian Mountains and the Mississippi River, two major geographic barriers in ENA. Populations located to the east of the Appalachians and along the central Appalachians exhibited relatively higher levels of genetic diversity. Extant lineages may have diverged during the late Miocene, and range expansions of different groups may have happened during the Pleistocene glacial/interglacial cycles.

CONCLUSIONS

Our findings indicate that geographic barriers may have started to facilitate the population divergence in P. peltatum before the Pleistocene. Persistence in multiple refugia, including areas around the central Appalachians during the Quaternary glacial period, and subsequent expansions under hospitable climatic condition, especially westward expansion, are likely responsible for the species' contemporary genetic structure and phylogeographic pattern.

Spatial structure develops early in forest herb populations, controlled by dispersal and life cycle

Fine-scale spatial structure is an essential feature of plant populations, controlling pollination, herbivory, pathogen spread, and resource partitioning. Origins of spatial distribution are often obscure in long-established forests, but successional stands offer insight through their physical and compositional simplicity. We tested the hypothesis that spatial structure in forest herb populations arises through a nucleation process in which colonizing species transition from random to clustered distributions through clonal expansion, seed dispersal, and conformity to environmental gradients. Spatial structure was examined in a chronosequence of 40 s growth stands in southeast Ohio, USA. Herbaceous vegetation was recorded in nested plots to describe the evolution of pattern across multiple scales. Spatial distribution was described as the variance:mean ratio of stem number plot-1, and compared between age classes and functional groups. Environmental influence was assessed as the marginal R² value of environmental models predicting stem number. Herb species responded individualistically to stand age and environmental gradients, although all were to some degree clustered across age classes. Dispersal-limited, non-clonal, and annual species were most strongly clustered, suggesting the importance of seed dispersal range and population growth rate in determining spatial structure. Spatial distribution was weakly related to environmental variables. Clustered distributions established early in succession and remained stable for at least 80 years. Pattern formation can be interpreted in terms of nucleation, as we hypothesized, but clusters form earlier than expected. The spatial structure of herb populations in deciduous forests appears to be governed by patterns established during colonization; environmental filtering appears to play a minor role.