تأثير عوامل بوم‌شناختي بر تنوع زيستي گونه‌هاي گياهي بوم‌نظام طبيعي بهاركيش قوچان

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

به‌منظور دستيابي به توسعه پايدار و هم‌چنين حفاظت از اكوسيستم‌هاي طبيعي و تنوع زيستي آن‌ها، لازم است نقش عوامل بوم‌شناختي و تأثير آن­ها بر تنوع گونه ­هاي گياهي مورد ارزيابي قرار گيرد. در اين راستا، در پژوهش حاضر تأثير عوامل بوم‌شناختي بر تنوع زيستي گونه‌هاي گياهي در بوم‌نظام طبيعي بهاركيش قوچان در بهار سال 1393 بررسي شد. ابتدا با روي‌هم‌اندازي چهار نقشه شيب، جهت جغرافيايي، طبقات ارتفاعي و سازندهاي زمين‌شناسي نقشه واحدهاي كاري تهيه شد، سپس بر اساس ساختار، نحوه توزيع و حضور گونه­ هاي غالب، چهار تيپ همگن مشخص شده و 35 پلات در هر تيپ گياهي به‌روش سيستماتيك- تصادفي مسقر گرديد و پلات­هاي با سطح نمونه دو مترمربعي (بر اساس روش تعيين حداقل سطح) جهت برآورد پوشش گياهي به‌كار گرفته شد. نمونه‌برداري از پوشش گياهي در كليه پلات‌ها و نمونه‌برداري از خاك به‌صورت نمونه مركب از هر واحد كاري به‌وسيله اوگر از عمق صفر تا 30 سانتي‌متري انجام شد. در هر پلات گياهان موجود فهرست و درصد تاج‌پوشش ثبت و كليه ويژگي‌هاي خاك شامل: درصد رس، سيلت، شن، ماده آلي، رطوبت اشباع، كربن آلي و ميزان اسيديته و هدايت الكتريكي و هم‌چنين ويژگي‌هاي توپوگرافيك از جمله: شيب، جهت جغرافيايي و ارتفاع از سطح دريا اندازه‌گيري شد. از طرفي، تغييرات تنوع در سازند­هاي مختلف زمين‌شناسي نيز مشخص شد. مطالعه تنوع زيستي گونه­هاي گياهي با استفاده از شاخص ­هاي تنوع شانون- وينر و سيپمپسون، شاخص غناي مارگالف و شاخص يكنواختي اسميت- ويلسون انجام شد. هم‌چنين به‌منظور تعيين مهم‌ترين عوامل تأثيرگذار بر تغييرات تنوع گونه‌اي از آناليز تجزيه به مؤلفه‌هاي اصلي و در محيط PCA استفاده شد. نتايج پژوهش نشان داد كه ارتفاع از سطح دريا بر تنوع، غنا و يكنواختي گونه‌هاي گياهي منطقه تأثير معني‌داري داشته و در دامنه ارتفاعي مياني2200-1900 متر، شاخص‌هاي تنوع سيمپسون و شانون– وينر، غناي مارگالف و يكنواختي اسميت- ويلسون بيش‌ترين مقدار را داشتند. هم‌چنين جهت دامنه شمالي و شيب دامنه، تنوع و غناي گونه‌اي را به‌طور معني‌داري تحت تأثير قرار داده و در شيب­هاي تا 15 درصد، تنوع شانون- وينر و غناي گونه‌اي مارگالف بالاترين مقدار را داشته و شاخص يكنواختي اسميت- ويلسون در شيب 15 تا 45 درصد، بيش‌ترين مقدار را داشت. نتايج حاصل از تجزيه به مؤلفه‌هاي اصلي نشان داد كه با توجه به قدر مطلق ضرايب، مؤلفه اول شامل: درصد كربن آلي خاك، درصد ماده آلي و درصد رس و مؤلفه دوم شامل: هدايت الكتريكي و ارتفاع و درصد رطوبت اشباع، درصد سيلت، درصد شن و درصد شيب، بيش‌ترين تأثير را بر تنوع گونه‌اي منطقه مورد مطالعه داشتند.

Introduction 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. Materials and Methods 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. Results and Discussion 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. Conclusion 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.