Reproductive and Genetic Patterns in Virginia Spiraea (Spiraea virginiana Britton; Rosaceae) Reveal a Fragmented Species, Frozen in Place and Time

Spiraea virginiana Britton (Virginia spiraea), a threatened riparian shrub whose range extends through the Appalachian and Blue Ridge Mountains, reproduces primarily asexually. Successful seed production is rare under field conditions, but the relative contributions of intrinsic factors and pollinators are unclear. Open, selfed, inbred, and outbred pollination treatments were applied to populations of S. virginiana from three western North Carolina drainages, and stigmas from open pollinated treatments were collected to quantify pollen loads under light microscopy. While viable pollen production was high, seed set was low under both open and hand-pollinated treatments and was not correlated with pollen load. Pollen:ovule ratios suggested a generalist pollination syndrome, as did pollen loads on abundant floral visitors. Thus, rather than pollen limitation, reproductive success could be limited by factors like depressed genetic diversity among populations. Microsatellite analysis was used to assess population genetic patterns across the species' range. Spiraea virginiana demonstrated high heterozygosity and little genetic structuring, and frequency of clones was high. In addition, polyploid individuals were found at all sites, and potential mixing of ploidies within populations might also limit sexual reproduction. Management steps should be taken to allow this clonal species to continue successful rhizomatous growth and fragmentation. In addition, restoration work should consider the lack of genetic structure when choosing appropriate vegetative propagules for out-planting, understanding that the goal of augmentation is to increase genetic diversity via sexual reproduction, which is uncommon is Virginia Spiraea.