Rangeland ploughing and cultivation using dry land farming crops is a major reason for destruction of natural resources in the semiarid and sub-humid regions, which may significantly change the composition and reduce stability of the affected communities. In present research an abandoned ploughed site was compared with a nearby reference site, in the semiarid rangelands of Baharkish, Quchan, Iran, during spring-summer 2010. Frequency and canopy cover of all plant species were recorded within 40 quadrates of 1m2 area. Simpson, Shannon-Weiner, Hill and Macintosh indices were used for biodiversity, Margalof and Menhening for richness, and Camargo, Simpson, Modified Nee, and Smith and Wilson for evenness. Floristic composition, plant life forms, and important value of the major plant species were compared between the sites. Land ploughing and subsequent abandonment had increased total number of plant species (richness) but decreased species heterogeneity (evenness). It resulted to non-significant differences in species diversity between the ploughed and reference sites. Ploughing had increased (8%) theimportant values of respruting plant species. Therefore patchy distribution of clonal plants had reduced species evenness within the abandoned site. Furthermore, there were increases in the number of therophytes (100%), but reduction of hemicryptophytes (24%), chamaephyte (33%), and the phanerophyte (100%) species, in the abandoned site. In conclusion, lower evenness and high proportion of annual plants should make the abandoned site more fragile against the future environmental fluctuations.

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