Potential Geographic Distribution of Palmer Amaranth under Current and Future Climates

Abstract

P almer amaranth [Amaranthus palmeri (S.) Wats.] is a C 4 summer annual native to Sonoran Desert regions of northern Mexico and the southwestern United States (Sauer, 1957). Beginning in the early 20th century, this weed started to expand beyond its native range due to a com-bination of human-assisted seed dispersal and new habitat creation through agricultural expansion (Ward et al., 2013). Despite its invasive tendencies and long history of range expansion, Palmer amaranth has only recently emerged as one of the most economically damaging weeds in soybean [Glycine max (L.) Merr], cotton (Gossypium hirsutum L.), and corn (Zea mays L.) (Webster and Nicholas, 2012). In the United States, yield losses have been reported up to 91% in corn (Massinga et al., 2001), 79% in soybean (Bensch et al., 2003), and 65% in cotton (Rowland et al., 1999). In the last decade, substantial range expan-sion across the midwestern United States has occurred via movement of con-taminated animal feed, manure, and harvest equipment, as well as conservation seed plantings. To date, this weed has been documented in 28 US states, most recently in Minnesota (Hensleigh and Pokorny, 2017). Palmer amaranth has also invaded portions of Europe, Africa, and South America. The competitiveness of Palmer amaranth is due to its extended period of germination, rapid growth rate, prolific seed production, and large root volume (Ward et al., 2013). Additionally, Palmer amaranth is a wind-pollinated dioe-cious species, which enables rapid development of herbicide resistance and other adaptive traits that improves its survival in different agroecosystems. Consequently, populations have developed resistance to multiple classes of her-bicides, including glyphosate, making management of this weed both difficult and expensive (Heap, 2015). The distribution and growing season of a weed species such as Palmer ama-ranth will undoubtedly change as a consequence of climate change (Ziska et