The chalk barrens of the Niobrara Formation in the Arkansas River valley of Colorado exhibit a high degree of plant endemism and rarity. We examined their geochemistry and structure as well as the reproductive ecology of their signature species Mirabilis rotundifolius to assess factors influencing endemism. While no single component consistently identified the barren soils, the natural locations that support Mirablilis can be identified using elemental profiles from a cluster analysis of soil extracts. Because seeds germinate and plants proliferate in transplant gardens with non-calcic soils, bedrock and soil chemistry do not appear to be determinant components of endemism. Mirabilis rotundifoilus utilizes diverse pollinators but is capable of autogamy when insects are excluded; seed predation by harvester ants may be a limiting factor for population spread outside the barrens habitat. Mirabilis rotundifolius is a disturbance-tolerant species, part of a barrens cohort that shares traits of suffrutescence, woody caudices, or multibranched rhizomes penetrating the chalk strata. Its presence may be enhanced when disturbance inhibits the presence of other species that compete for limited water resources. We conclude that endemism is dictated not by unusual soil chemistry requirements but rather by structural components of the barrens. In a water-limited environment, shallow soils and thin chalk strata coupled with a functionally adaptive growth habit allow M. rotundifolius to survive when other species are excluded by disturbance or the physiochemical aspects of the barrens.
Kelso, Sylvia; Bower, Nathan W.; Heckmann, Kirsten E.; Beardsley, Paul M.; and Greve, Darren G.
"Geobotany of the Niobrara chalk barrens in Colorado: a study of edaphic endemism,"
Western North American Naturalist: Vol. 63
, Article 3.
Available at: http://scholarsarchive.byu.edu/wnan/vol63/iss3/3