An ecological, genetic, and reproductive study of big bluestem (Andropogon gerardii) populations in the Carolinas

Andropogon gerardii (Big Bluestem) is a dominant grass of the North American tallgrass prairie and is also found in remnant populations in the eastern U.S., including North and South Carolina. This research included a systematic study of the ecology, genetics, and reproductive potential within and among A. gerardii populations in the Carolinas. Floristic composition and edaphic (soil) features were characterized for eight A. gerardii populations across various physiographic regions of North and South Carolina. A total of 362 quadrats (1 m x 1 m) were sampled during the 2006-2008 growing seasons. A total of 306 vascular plant species was identified comprising 64 families, including 99 (32%) graminoids. Andropogon gerardii had the highest Commonness Index (CI) value (5900), followed by Rubus spp. (1260). Community Coefficient (CC) values were <0.5 for all pairings between sites, indicating high dissimilarity in species composition among sites. Soil pH values varied among the sites from 4.8 to 6.9. Calcium and Mg nutrient values were also highly variable. An outcrossing reciprocity study was conducted with five A. gerardii populations. Fifteen treatments were established at four garden sites. Seed germination was low for both outcrossed (4.8%) and selfed (2.4%) pairings. However, germination was significantly higher for both out-crossed and selfed seeds from ramets from the Suther Prairie population. There were no significant differences in seed set among plants from the five populations. Protein electrophoresis was used to assess genetic variation within and among nine local A. gerardii populations from various physiographic regions of both North and South Carolina. Twenty-seven polymorphic loci and one monomorphic locus were resolved. Populations contained high levels of genotypic diversity. Genetic diversity was high at both the pooled species-wide level and at the mean within-population level. For the species-wide sample, percent polymorphic loci (Ps) was 96.7%, the number of alleles per polymorphic locus (APsp) was = 4.07, the mean number of alleles per locus (A s) was = 3.97, and the genetic diversity (Hes) was 0.422. For within- populations, the mean Pp = 81.2%, APp, = 2.63, Ap = 2.38, and Hep = 0.340. Levels of within population (Hep) genetic diversity ranged from 0.181 for the Sumter National Forest I population to 0.462 for the Bowman Barrier population. Genetic structure among populations (Gst) was 0.175, indicating that 83% of the genetic variation is found within-populations. The level of genetic diversity for the three larger populations (He = 0.387) and for the six smaller populations (He = 0.330) was not significantly different (p = 0.5535). Nei's unbiased genetic identity between pairs of populations ranged from 0.6386 (populations BlackJacks Heritage Preserve and Sumter National Forest I) to 0.9692 (populations Buck Creek Serpentine Barrens and Suther Prairie). Of the population sites, the Blackjacks Heritage Preserve population had the lowest measure of genetic identity with the other population sites, while the Suther Prairie population had the highest. The Mantel test showed no significant genetic isolation by geographic distance (r = 0.065; p =0.614). Allelic richness values were high for the populations. Banding patterns were consistent with disomic inheritance. However for two loci (PGI3; UGPP1), tetrasomic patterns were indicated.