تأثير تغيير اقليم بر كشاورزي ايران: 2- پيش‌بيني توليد محصولات زراعي و راهكارهاي سازگاري

Climate Change Effects on Agricultural Production of Iran: II. Predicting Productivity of Field Crops and Adaptation Strategies

شواهد موجود نشان می‌دهد تغییر جهانی اقلیم بسیاری از متغیرهای اقلیمی كشاورزی ایران را در چند دهه آینده تحت تأثیر قرار خواهد داد. با این حال مطالعات اندكی در مورد پی‌آمدهای آن بر تولید گیاهان زراعی در سطح ملی انجام شده است. در این تحقیق عملكرد گندم، ذرت، نخود و چغندرقند به‌عنوان 4 گونه مهم زراعی در شرایط اقلیمی سال 2050 میلادی با مقادیر فعلی مقایسه و تغییرات آن مورد ارزیابی قرار گرفته است. شرایط اقلیمی سال هدف براساس یك سناریوی استاندارد به‌وسیله مدل گردش عمومی GFDL برآورد شد و نتایج به‌عنوان داده‌های آب و هوایی در مدل سوكروز كه قبلاً به‌وسیله داده‌های آزمایشی برای گونه‌های تحت بررسی تعیین اعتبار شده بود مورد استفاده قرار گرفت. نتایج نشان داد كه میانگین عملكرد گندم، ذرت، نخود و چغندر در سال هدف در مناطق مختلف كشور به‌ترتیب 6/18، 1/19، 6/6 و 20 درصد كاهش خواهد یافت. این كاهش عمدتاً به دلیل افزایش دما بوده و افزایش غلظت دی اكسید كربن تنها در حدود 6/5 درصد از اثرات منفی دمای بالا را جبران می‌كند. آنالیز رگرسیون بر روی نتایج نشان داد كه عملكرد گیاهان تحت بررسی به‌ازای هر C°1 افزایش دما در طی كل دوره رشد بین 9 تا 17 درصد كاهش می‌یابد. البته در مورد گندم و ذرت كاهش عملكرد تا حد زیادی تابع میزان افزایش دما در طی دوره پر شدن دانه خواهد بود. افزایش دما، طول كل دوره رشد و نیز طول دوره پر شدن دانه را در گیاهان دانه‌ای كاهش خواهد داد. نتایج حاكی از این بود كه در ذرت و گندم كاهش طول دوره پر شدن دانه و در نخود و چغندر كاهش طول دوره رسیدگی (كاشت تا برداشت) عامل اصلی كاهش عملكرد می‌باشد. با توجه به مقدار ضریب تبیین بین متغیرهای مختلف، به‌طور متوسط در حدود 60 درصد از تغییرات عملكرد 4 محصول تحت بررسی با افزایش دما توصیف می‌شود و به همین دلیل تغییرات مكانی عملكرد این محصولات در مناطق مختلف كشور با الگوی مكانی تغییرات دما در سال 2050 منطبق است. بنابراین راهكارهای سازگاری به شرایط اقلیمی سال هدف باید به‌گونه‌ای طراحی شوند كه اثرات سوء دماهای بالا به‌ویژه در مراحل حساس رشد را كاهش دهند. ارزیابی روش‌های سازگاری از طریق شبیه‌سازی نشان داد كه تغییر تاریخ كاشت و افزایش مقاومت به گرما روش‌های مؤثری برای سازگاری به شرایط اقلیمی آینده در محصولات تحت بررسی می‌باشد.

<strong >Introduction </strong > Recent evidences confirm that during the next few decades, many agroclimatic indices of Iran would be affected by global climate change. Koocheki et al. using two General Circulation Models showed that the mean annual temperature of the country will increase between 3.5-4.5°C while mean precipitation will reduce by 7-15% to 2050. It is well established that crop growth and development would drastically affect by the future global warming and its consequences because yield determining processes such as photosynthesis and crop phenology are directly related to temperature. On the other hands, the combined effects of CO2 enrichment and temperature rise on crop growth are complicated and should be studied using crop simulation models. Furthermore, adapting to climatic variability will have a substantially greater effect in reducing impacts than willing mitigation. However, such impacts on crop productivity at national scale and adaptive measures for future conditions are rarely studied in Iran. In this research crop development and yield of wheat, corn, chickpea and sugar beet were simulated for the target year of 2050 and the results are compared with the current yield as the baseline. <strong >Materials and Methods </strong > Future climatic variables were predicted using A1f (business as usual) scenario by GFDL general circulation model and the results were used as weather inputs in the SUCROS model which was previously validated against measured data of the four crops. To account for the effect of CO2 enrichment on crop growth the photosynthesis routine of the model was adopted for increased CO2 concentration using a scaling factor. Changes in developmental stages of each crop were estimated for the future conditions and the relation between duration of these stages and yield was determined. Predicted crop yields for the year 2050 were compared with the current potential yields considering some adaptation strategies. <strong >Results and Discussion </strong > Results indicated the average over the different regions, yield of wheat, corn, chickpea and sugar beet will be decreased by 18.6, 19.1, 6.6, and 20%, respectively. This decrease is mainly due to temperature rise and only 5.6% of the negative effects of temperature will be compensated by increased CO2 concentration. Regression analysis showed the yield of the studied crops will be reduced between 9 to 17% per 1°C increase in mean temperature during the growing season. For wheat and corn such a reduction would also be expected for the duration of the grain filling period. Based on these results, reduction of the total growing period (planting to maturity) as well as shortened grain filling period are the main causes of the predicted crops yield decline by the target year. On average up to 60% of the yield reduction over the country could be described by global warming and the pattern of the crops yield variation follows the same spatial pattern predicted for temperature rise in 2050. Therefore, adaptation strategies for the future climate should be designed to prevent the negative effects of increased temperature during the sensitive development stages. Simulation results obtained by SUCROS model showed that among different studied adaptation strategies, using new wheat cultivars with higher heat tolerance and changing planting dates are the most efficient management practices and drastically prevent yield reduction under defined future climate scenarios. Use of these readily implemented adaptation strategies is profitable based on the available resources by improving national wheat production compared to the same climatic scenarios without adaptation. <strong >Conclusions </strong > Despite of uncertainties in the predicted results of simulation models, the results of this research indicated that during the next few decades crop yields in different regions of Iran will be reduced due to climate change and more specifically temperature rise. This yield reduction could be more drastic under actual conditions where the effects of climate change will intensify due to biotic and abiotic stresses. However, these negative impacts could be avoided by using proper adaptation strategies.