The relationship between irrigated wheat field's area and species diversity, dominance and similarity of weeds in central region of Arak

By understanding the diversity and dominance of weed species, which is affected by changes crop management systems and weed control practices, these plants can be managed more efficiently. In this study, the diversity of weed species of irrigated wheat fields was studied in an area of ​​less than one, one to two, two to four, four to eight and more than eight hectare in the central region of Arak, Iran, in 1919. A total of 74 weed species were identified from 25 plant families, of which 61 species were dicotyledons and 13 species monocotyledons, 78.38% annual and 21.62% perennial as well as 95.95% C3 and 4.05% C4. Also, Asteraceae and Brassicaceae families among dicotyledons and Poaceae family among monocotyledons had the highest species diversity. According to the dominance index, ten dominant weed species were flixweed (Descurainia Sophia), binweed (Convolvulus arvensis), foxtail grass (Alopecurus myosuroides), common thistle (Cirsium arvense), Iranian knapweed (Centaurea depressa), lowgrass (Polygonum aviculare), corn cleavers(Galium tricornutum), Syrian mustard (Eeuclidium syriacum), goldbachi (Goldbachia laevigata) and nodding hypecoum(Hypecoum pendulum). There was a significant inverse relationship between field size with Shannon-Wiener and Pielou's uniformity indices and their highest values (2.58 and 0.84, respectively), was obtained in the fields less than 1 ha and the lowest values (1.48 and 0.61, respectively), in the fields more than 8 ha. In contrast, the relationship between field size and Simpson's dominance Index was positive and significant and its value was calculated 0.1 and 0.33 for fields less than 1 ha and more than 8 ha, respectively. In total, the results of this study showed that a few weed species in most wheat fields in this region were well distributed and had high density, abundance, uniformity and dominance index and if the usual farm management methods continue, there is a possibility that species diversity will decrease and the problematic weeds will dominate.