Alien weeds unintentionally introduced in Argentina through alfalfa seed importation: contributions for a historical study of weed flora formation

POGGIO, SANTIAGO L; MOLLARD, FEDERICO P O; SALERNO, CECILIA M B

Kaposvár

Congreso; 15th European Weed Research Society Symposium; 2010

European Weed Research Society

Weed seed contaminants of crop seeds are important sources of alien weeds entering a region. Hence studying the alien weed species contaminating crop seeds imported to Argentina may contribute to the historical reconstruction of the weed flora formation in the Pampas since the agriculture expansion during the late 1800s. This aim may also help to identify the ecological factors that had been involved in the regulation of both the colonization and the naturalization of alien species of weed inadvertently introduced as crop seed contaminants. Here, we have reviewed the literature on weed seed contaminants of crop seeds; particularly focusing on alfalfa. Seeds of 102 species were detected in alfalfa imported mainly from Italy in early 1900s, most of them were recognized as weeds of field crops and invasive species into semi-natural habitats and wastelands. The most frequent and abundant species observed as seed contaminants of alfalfa were Ribwort plantain (Plantago lanceolata L.) and green foxtail (Setaria viridis (L.) P. Beauv.). Concurrently, both species were also conspicuous contaminant of alfalfa seeds in the United States in the same period. Ribwort plantain and green foxtail had disparate success of naturalization in the Pampas. During the first two decades of the 20th century, both species were not observed neither as weeds of field crops and pastures nor in wasteland and semi-natural habitats. First collected in a wasteland in the surroundings of Buenos Aires in 1910, ribwort plantain was first reported as a weed in an alfalfa field in the northern Pampas in 1920. Interestingly, seeds of ribwort plantain contaminating alfalfa seed exported from Argentina increased from 13% to 38% between 1932 and 1938. Perpetuated by sowing contaminated seeds, ribwort plantain became abundant in alfalfa crops during the 1950s and was recognized as a troublesome weed in this crop. Although herbicides and seed-cleaning have importantly reduced its abundance as weed of pastures, ribwort plantain is now widespread naturalized in semi-natural habitats since the late 1960s from the north-western, Andean tropics to Tierra del Fuego in southern Patagonia. Conversely, green foxtail, first collected in 1898, is nowadays considered casual for the flora of central Argentina, from the Pampas to the Andes and northern Patagonia, mostly occurring in ruderal habitats, such gardens, road verges, and wastelands, and rarely observed as common weed. Our findings suggest that the naturalization of alien weeds is mainly determined by biotic factors, such as life history, germination and reproductive output, after the propagule flux was reduced or even stopped.