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Laureto, P., Barkman. 2011. Nuclear and chloroplast DNA suggest a complex single origin for the threatened allopolyploid Solidago houghtonii (Asteraceae) involving reticulate evolution and introgression. Systematic Botan. 36: 209-226. Pimm, S.L., Raven, P. 2000. Extinction by the numbers. Nature. 403: 843-845 Schwartz, M.W., Iverson, L.R., Prasad.A.M., Matthews, S.N., O’Connor, R.J. 2006. Predicting extinctions as a result of climate change. Ecology 87:1611-1615. USDA, NRCS. 2014. The PLANTS Database. National Plant Data Team. http://plants.usda.gov. Accessed 5 March 2014. USFWS, 1997. Recovery plan for Houghton’s goldenrod (Solidago houghtonii A. Gray). Ft. Snelling, Minnesota. USFWS, 2011. Houghton’s Goldenrod (Solidago houghtonii A. Gray, Asteraceae): 5-Year Review. East Lansing, Michigan