Abstract

Spiraea virginiana L. (Virginia spiraea) is a federally threatened riparian rose native to the Appalachians. Due to its predominantly asexual reproduction, S. virginiana relies on regular flooding to send broken stems downstream and establish new populations. Since the introduction of dams, however, opportunities for population expansion have greatly decreased. Sexual reproduction is uncommon in S. virginiana; however, the limiting factor(s) have yet to be identified. Reproductive constraints for this species could include pollen and ovule production or ratios; visitor type, frequency, and effectiveness; seed production; or seed viability. This study examined pollination behavior and seed traits in S. virginiana from two western North Carolina sites. Pollen counts made in 2017 from Ashe County were compared to ovule counts of 2018 to determine pollen:ovule ratios. In summer 2018, open-pollinated flowers were collected from one population made up of many subpopulations in Graham County. Seeds from each sample were counted and compared among subpopulations. Seeds were tested for viability using a 1% triphenyl tetrazolium chloride solution (TTC), and 2018 viable seed counts were also compared to those from 2010 and 2013. Flower visitors of S. virginiana were collected from a neighboring population in Macon County to identify pollination characteristics of this species. Pollen and ovule counts were significantly different among flowers (pollen) and corymbs (ovules) in Ashe County (P<0.0001 and P=0.0119, respectively). Pollen:ovule ratio was low (60.3) but consistent with biotic pollination. Seed count and seed viability did not differ among subpopulations in Graham County (P= 0.9353 and P=0.6410, respectively). Most visitors of Macon County subpopulations are classified as generalists, suggesting that S. virginiana may have a generalist pollination syndrome. These results suggest interannual variation in seed traits for S. virginiana due to either resource allocation in the form of light availability or pollination effectiveness based on pollination syndrome.

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