عنوان مقاله [English]
Nowadays, with increasing tendency to monoculture of medicinal and horticultural plants, the range of genetic diversity in agricultural ecosystems has decreased. Conventional agricultural activities with simplification of ecosystems makes ecosystems more unstable and more sensitive to environmental hazards. Therefore, the promotion of biodiversity in these systems is effective for their recovery and sustainability. The purpose of designing sustainable agricultural ecosystems is to integrate ecosystem components in such a way that maintains and strengthens the biodiversity, productivity and self-regulatory capacity of the agricultural ecosystem. Functional diversity is achieved by using different functional species with different roles in agroecosystems. The role of weeds in increasing diversity in agroecosystems is very important because they are closely associated with many of the crops and genetic exchange between them. The presence of weeds in the crop ecosystem reduces genetic vulnerability through strategy of environmental diversification as well as the occurrence of resistance mechanisms.
Materials and Methods
In order to investigate the effect of some medicinal plants on weed density and diversity, an experiment was conducted based on a complete randomized block design with three replications at the agricultural research station, Ferdowsi University of Mashhad, Iran, during 2012 and 2013. Treatments included 13 medicinal species (Origanum vulgar, Hyssopus officinalis, Tanacetum parthenium, Echinacea purpurea, Ruta graveolens, Althea officinalis, Hypericum perforatum,Trigonella foenum-graecum, Oenothera erythrosepala, Malva sylvestris, Securigera securidaca, Achillea millefolium and Digitalis purpurea).After tillage, land preparation and leveling, each plant was cultivated in strips of 10*3 meters. After complete establishment of the plants in order to achieve the desired density, thinning operations were performed on each strip. The distance between the test strips was 0.5 m. Seeds of all studied plants were sown in April 2012. In both cropping years to study the diversity of weeds, a part of each plot with dimensions of 0.5 m was separated and remained without weeding and in all plots of medicinal plants, density and dry weight of weeds was determined at two stages of vegetative and reproductive growth of medicinal plants. Also, physiological and morphological characteristics such as height, leaf area and dry weight of plants in each plot were measured. In order to calculate the diversity of weeds, the type of weed species and their density were determined in quadrates with dimensions of 40*40 cm. Then the weeds in each plot were harvested separately and transferred to the laboratory and were placed at 70 °C for 48 hours to determine the dry weight. Shannon, Margalf and Simpson diversity indices were used to quantify weed diversity.
Results and Discussion
The results indicated that the effect of medicinal species on total weed density and weight was significant. The highest and the lowest dry weed weight was obtained in Digitalis purpurea (39.4 g.m-2) and Tanacetum parthenium (3.2 g.m-2), respectively. The highest total weed density was observed in Malva sylvestris, Securigera securidaca, Digitalis purpurea and Hyssopus officinalis. Malva sylvestris had the highest weed richness (four species) that it was not significantly different with Securigera securidaca and Achillea millefolium. During different sampling years and stages, the highest Shannon index was obtained in Achillea millefolium (0.48) that it was not different with Malva sylvestris, Securigera securidaca, Hyssopus officinalis, Hypericum perforatum, Digitalis purpurea and Echinacea purpurea, significantly. Also, the highest margalof index was obtained in Malva sylvestris, Achillea millefolium and Hyssopus officinalis (4.05, 4.03 and 3.33, respectively). Studied medicinal species for weed diversity divided to 5 groups in 75% similarity level.
Despite the importance of biodiversity for the stability of natural ecosystems and agroecosystems, increasing species and functional diversity, simultaneously, is necessary to improve agroecosystem’s function. Keep the limited weed population in field increase functional diversity of agricultural landscapes but, this profit must reach equilibrium with the risk of decreasing crop production due to competition with weeds.