The alien weed flora of the argentine pampas: disentangling the ecological and historical patterns involved in its formation

POGGIO, SANTIAGO L; MOLLARD, FEDERICO P O

Kaposvár, Hungría

Congreso; 15th European Weed Research Society Symposium; 2010

European Weed Research Society

Naturalization of plant species introduced into a region is limited by several geographic, biotic, and abiotic barriers that regulate their colonization, perpetuation and dispersion. In agro-ecosystems, these ecological filters define the size of the regional species pool of alien weeds that may assemble communities at field scale. We studied how the species richness of alien weeds has increased in the Pampas of Argentina from early agriculture expansion to the present (1877-2007). The Pampas are one of the most extensive and productive areas of agricultural commodities in the world, thanks to their favorable climate and fertile soils. Our study is based on a comprehensive biographical revision including floras, handbooks, scientific and technical publications. We only listed alien species that have been reported to occur in annual field crops, horticultural crops, and temporary, implanted pastures. Since we focused our study on arable weed, alien species occurring in fruit plantations and woodlands were not included in the list. Species number was accumulated according to the year of first report of an alien species to occur as an arable weed in the region. The accumulated species richness of alien weeds was 260 for all the study period. Most alien weeds were annuals (153, 53% of all aliens) and native from Eurasia (203; 78% of all aliens from all continents). We have identified three periods of richness accumulation of alien weeds. Richness first sigmoidally increased (1877-1935). These periods coincides with the notable increase of rural population from ca. 270,000 people to ca. 1.3 millions between 1869 an 1914 by mostly European farmers, developing of railroad network, and introduction of row-crop area in the Pampas, suggesting high propagule pressure, dispersal, and open-site availability. Simultaneously, arable land devoted to sown cereals and linseed also increased from 4.7 to 13.6 million ha between 1899 and 1914, whereas alfalfa area increased from 1.3 to 7.4 million ha during the same interval. This extraordinary increase of the arable land area provided safe sites for the colonization of weed seed contaminants that were unintentionally sown with crops. Nowadays, an important share of the arable weed flora of the Pampas is comprised by many weed species belonging to the arable flora of Mediterranean Europe, which had been entered the country as seed contaminants. Alien weed richness continued to linearly increase during the second phase (1937-1969: 1.6 species per year), while there were not significant change during the third stage (1970-1999: 0.3 species per year, P = 0.06). No new alien weed species has been documented after 1999. The turning point (1969 CI 95% = 4.9) coincide with the start of agriculture intensification, which have implied the increase of herbicide use and the implementation of phytosanitary measures to prevent the presence of weed seed contaminants in the seeds of annual field crops, horticultural crops, and pastures entering the country and in national commercialization as well. These preventive practices have hence constrained propagule pressure and receptivity of cropped areas. Our results highlight the importance of anthropic factors in modulating the introduction of alien plant species into agricultural regions and their naturalization as arable weeds.