The Effect of Ecological Factors on Plant Species Biodiversity of Natural Ecosystem in Quchan Baharkish

Biodiversity as a key factor of sustainability of natural ecosystems have been largely used in studies on natural vegetations (e.g. rangelands) at different spatial scales. Baharkish pasture with area of 1035 hectare is situated in 60 km distance southern of the city Quchan in Khorasan Razavi province. The lowest and the highest altitudes of the pasture are 1740 and 2440 meters a.s.l., respectively. Long-term (25-year) mean annual precipitation and temperature of the region is 337 mm and 9.4°C with 998.2 mm evaporation. In this study, about 200 hectares of Baharkish pasture with moderate grazing pressure was chosen by the clipping and weighing method to study the effects of environmental factors of plant species richness and diversity.

In order to evaluate the effect of ecological factors on the biodiversity of plant species in Quchan Baharkish pasture, at first all topographic maps (1:25,000), geological maps (1:100,000), aerial photos (1:20000) and satellite images of Quchan Baharkish were collected. Then, maps of altitude classes, slope, aspect and geological formations were prepared and merged by overlapping in ArcGIS 10.2. The generated maps were coded and 13 homogenous working units and their maps were prepared for further studies, field visits and pasture vegetation measurements. Finally,  four homogeneous vegetation types were determined for sampling based on the structure, distribution, and presence of dominant species. Hence, in each vegetation type 35 plots (2m2 each) were systematic-randomly selected and used for composite soil sampling (0-30 cm depth) and estimation of  vegetation indices. The species number and percentage of canopy cover were estimated in each plot and the plot locations were recorded with GPS devices in U.T.M system.Plant species diversity per plot was estimated using Shannon-Wiener and Simpson diversity indices, richness index of Margalef and evenness index of Smith-Wilson in Ecological Methodology software. One-way ANOVA was used to examine the components of biodiversity and Tukey test was performed for multiple mean comparisons. Principal component analysis (PCA) was conducted to classify vegetation types and to determine the contribution of ecological factors on biodiversity of the studies pasture.

The results indicated that altitude had significant effect on species diversity, richness and evenness and the highest values of Simpson and Shannon indices were obtained at 1900-2200 m a.s.l. Indirect effect of soil formation and direct influence of temperature were led to variation in distribution of plant species and rangeland community structure. Amount and type of precipitation and consequently vegetation type was changed with altitude. Species richness and diversity was significantly affected by slope, and lower slopes (<15%) showed the highest values of richness and diversity index. However, evenness was higher in 15- 45% slopes. Overall, northern slopes had significantly higher species diversity, suggesting that plant available water is drastically influenced by slope through infiltration rate and land index. Results of PCA indicated that the first PC containing soil organic carbon, sand content and slope had the highest effect on species diversity.

In general, it can be concluded that plant biodiversity in natural ecosystems was affected by topographic and edaphic factors particularly in high altitudes. The results of PCA showed that the PC1 including percentage of soil organic carbon, organic matter and clay and PC2 including electrical conductivity and elevation, percentage of soil moisture, silt, sand and slope had the greatest impact on species diversity. In addition, getting insight in to the relationship between diversity and ecological, topographic and edaphic factors can be proposed as an ecological approach for sustainable rangeland management.