واکاوی زوال جنگل‌های استان ایلام و ارتباط آن با تغییرات بارش

نوع مقاله : علمی- پژوهشی

نویسندگان

1 دانشیار، دانشکده جغرافیا، دانشگاه تهران

2 دانشجوی دکتری اقلیم‌شناسی، دانشکده جغرافیا، دانشگاه تهران

3 استاد، دانشکده جغرافیا، دانشگاه تهران

4 دانشیار پژوهش، مؤسسه تحقیقات جنگلها و مراتع کشور

چکیده

 امروزه به ­دلیل بروز پدیده زوال بلوط، گستره قابل ­توجهی از جنگل­های زاگرس دچار خشکیدگی شده است. هدف اصلی پژوهش پیش ­رو تعیین گستره پوشش‌های جنگلی استان‌ ایلام و بررسی روند تغییرات زمانی آنها طی دوره 14ساله (2000 الی 2013)‌ در ارتباط با تغییرات بارش می‌باشد. برای این منظور از تکنیک‌های سنجش از دور و سیستم اطلاعات جغرافیایی استفاده شد. داده‌های مورد استفاده دربرگیرنده تصاویر NDVI سنجنده مادیس با فواصل 16روزه، تصاویر موجود در آرشیو Google earth و نقاط کنترل زمینی، داده‌های اقلیمی ایستگاه‌های همدید، داده­‌های درازمدت GPCC و داده‌های بارش ماهواره TRMM بودند. تعیین تعداد طبقات مطلوب برای شناسایی پوشش‌های منطقه و درنهایت جنگل‌ها با استفاده از روش نظارت‌­نشده و تکنیک ISODATA و شاخص‌های تفکیک واگرایی کمترین و میانگین انجام شد. نتایج به‌دست‌آمده از طبقه‌بندی نشان داد که تعداد طبقه 35 برای بررسی سالانه و طبقه 30 برای بررسی درازمدت، منطقی­ترین تعداد طبقه مطلوب برای پوشش‌های این استان هستند. نمودار طیفی نیز بیانگر دوره رشد جنگل‌های استان ایلام از اواخر بهمن تا اوایل مرداد بود که اوج سبزینگی آنها نیز در اواسط اردیبهشت بود. نمودار روند تغییرات NDVI مناطق جنگلی طی دوره 14ساله بیانگر اولین کاهش قابل ­توجه سبزینگی در سال 2005 (1383) و به میزان بیشتری در سال 2008 به‌همراه روند نزولی آن برای سه سال انتهایی دوره بود. نتایج به‌دست‌آمده از بررسی داده‌های بارش ایستگاهی و شبکه‌­ای نشان­دهنده روند منفی بارش در مقیاس‌های زمانی سالانه و ماهانه بر پوشش‌های مختلف استان ایلام و جنگل‌های منطقه بود. در مناطق جنگلی استان، روند منفی بارش در سطوح اطمینان 90 و 95 درصد معنی‌دار بود. طی فصل رشد جنگل­ها، بیشترین روند منفی بارش در اسفند (هم‌زمان با اوایل دوره رشد جنگل) مشاهده شد. بررسی ارتباط بین میزان NDVI و روند بارش نشان داد که یکی از عامل‌های اصلی خشکیدگی و کاهش سبزینگی در جنگل‌های این منطقه، کاهش معنی‌دار بارش و ایجاد تنش خشکی در مقیاس سالانه و به ­ویژه فصل رشد می‌باشد. 

کلیدواژه‌ها

عنوان مقاله [English]

Analysis of relationship between forest decline and precipitation changes in Ilam Province

نویسندگان [English]

  • Ghasem Azizi 1
  • Morteza Miri 2
  • Hossein Mohammadi 3
  • Mehdi Pourhashemi 4
1 Associate Prof., Faculty of Geography, University of Tehran, Tehran, Iran ‎
2 Ph.D. Student, Faculty of Geography, University of Tehran, Tehran, Iran‎
3 Prof., Faculty of Geography, University of Tehran, Tehran, Iran ‎
4 Associate Prof., Research Institute of Forests and Rangelands, Agricultural Research, ‎Education and Extension Organization (AREEO), Tehran, Iran
چکیده [English]

    Oak decline phenomenon recently led to considerable dieback within Zagros forests. This study was designed to assess the temporal trends of changes in forest cover of the Ilam province in association with precipitation variations during a 14-year period (2000-2013). To this aim remote sensing and GIS techniques were used. The remote sensing data included 16-day NDVI products of MODIS data as well as Google-Earth archive and the related control points. Additionally climatic data from synoptic stations, GPCC long-term data and TRMM rainfall data were applied. The optimal number of land cover and forest classes was defined by means of an unsupervised ISODATA method and mean and minimum divergence separation indexes. The result indicated 35 and 30 land cover classes to be appropriate for annual and long-term assessments, respectively. Spectral graph revealed a growth period from mid-February to early-May within Ilam forests. NDVI trend revealed the first significant reduction in forest cover in 2005 and the second one in 2008, which was followed by a descending trend in forest cover for the last 3-years of the investigated 14-year period. The data from ground stations and rainfall grid point indicated negative annual and monthly rainfall trends. The trend was significant within forests in 90% and 95% confidence levels. The peak of the declining precipitation trend was observed in the beginning of growth season in March.  Correlating the NDVI with and precipitation trend revealed significant reduction of annual precipitation and drought stress during the growth season are particularly responsible for considerable amounts of forest dieback and chlorophyll decline within forests of Ilam.

کلیدواژه‌ها [English]

  • Separability
  • oak decline
  • GPCC
  • satellite TRAA
  • Satellite TRMM

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