Environmental Erosion Research

fa پایش روند تغییرات گستره و مرز رویشگاه های جنگلی خلیج گواتر Monitoring the Trends of Extent and Boundary Changes in the Mangrove Forests of Gwadar Bay روشهای نوین و دقیق در تهیّه نقشه فرسایش (سنجش از دور و سیستم‌های اطلاعات جغرافیایی) پژوهشي Research پایش تغییرات گستره و مرز رویشگاه‌های مانگروها در طول زمان یکی از الزامات اساسی برای ارزیابی میزان آسیب‌پذیری این اکوسیستم‌ها در برابر پیامدهای تغییرات اقلیمی به‌شمار می‌آید. این اکوسیستم‌ها علاوه بر نقش مهم در حفظ تنوع زیستی، در تثبیت سواحل، کاهش فرسایش و تعدیل تغییرات خط ساحلی نیز نقش مؤثری ایفا می‌کنند. این پژوهش به تحلیل تغییرات 39 ساله (2024-1986) جنگل‌های حرا خلیج گواتر در استان سیستان و بلوچستان پرداخته است. با بهره‌گیری از سری زمانی تصاویر ماهواره‌ای لندست در بازه زمانی مذکور و استفاده از ابزارهای سامانه اطلاعات جغرافیایی (GIS) و تجزیه و تحلیل رقومی خط ساحل (DSAS)، میزان تغییرات وسعت و مرز جنگل‌های حرا در سمت دریا محاسبه و نقاط عطف در روند تغییرات شناسایی شدند. یافته‌ها حاکی از آن است که سال ۱۹۹۸ به‌عنوان نقطه کلیدی تغییرات وسعت جنگل‌های حرا خلیج گواتر شناخته می‌شود. پیش از سال ۱۹۹۸، وسعت این رویشگاه‌ها سالانه به‌طور متوسط 02/3 هکتار افزایش داشت، اما پس از این سال، وسعت آن‌ها با نرخ کاهشی 8/1 هکتار در سال کاهش یافته است. نرخ کلی تغییر وسعت در طول دوره مورد بررسی منفی (9/۰- هکتار در سال) بوده و بر روند نزولی این تغییرات دلالت دارد. افزون بر این، کاهش سالانه وسعت همراه با پسروی مرزهای این رویشگاه در سمت دریا، به میزان 141/۰ متر در سال، مشاهده شد. روند کاهشی وسعت و پسروی مرزهای جنگل‌های حرا خلیج گواتر در سه دهه گذشته بیانگر حساسیت بالای این اکوسیستم نسبت به پیامدهای تغییرات اقلیمی، از جمله خشکسالی و افزایش سطح آب دریا، است. نتایج این تحقیق به‌عنوان مبنای علمی برای توسعه راهکارهای حفاظتی و سازگاری با تغییرات اقلیمی در این رویشگاه‌های ارزشمند مفید خواهد بود.   1- Introduction Mangrove ecosystems are highly sensitive to both natural and anthropogenic pressures, including climate change, sea-level rise, prolonged droughts, and unsustainable human activities such as oil pollution, coastal development, and overexploitation. In the face of these threats, monitoring changes in the extent and boundaries of mangrove forests over time is essential for assessing their vulnerability and informing conservation strategies. In Iran, mangrove forests, particularly those along the northern coasts of the Persian Gulf and the Sea of Oman, face significant challenges from natural hazards like drought and sea-level rise, as well as human-induced pressures such as unsustainable tourism, aquaculture development, and industrial pollution. The mangrove forests of Gwadar Bay, located in Sistan and Baluchestan Province, are no exception. These forests, dominated by Avicennia marina, are integral to the ecological integrity of Gwadar Bay, a part of the internationally recognized Gando Protected Area. Despite their ecological significance, these mangroves have experienced notable changes in extent and shoreline boundaries over recent decades, driven by climatic and anthropogenic factors. Previous studies in Iran have explored mangrove dynamics, often focusing on either extent or boundary changes independently. However, simultaneous analysis of both aspects provides a more comprehensive understanding of ecosystem responses to environmental stressors. 2-Material and Methods This study utilized a 39-year time series of Landsat satellite imagery (1986–2024) to monitor changes in the extent and seaward boundaries of mangrove forests in Gwadar Bay. Thirty-two summer-season images (path 156, row 43) were selected to minimize phenological and tidal variations, ensuring consistent data quality. Images were validated using aerial photographs (1993, 2005) and 48 ground-truth samples (30×30 m) collected in 2016. Geometric corrections were performed using image-to-image methods, with 2017 Landsat 8 images aligned to a 1:25,000 topographic map. Radiometric corrections were conducted in ENVI software to standardize illumination and atmospheric conditions. Mangrove extent was extracted using the maximum likelihood classification method, leveraging Normalized Difference Vegetation Index (NDVI) and false-color composites (green, red, near-infrared bands). Classification accuracy was assessed via stratified random sampling, calculating overall accuracy, user/producer accuracy, and Kappa coefficient. Temporal trends in mangrove extent were analyzed using the Pettitt-Mann-Whitney test (α=0.05) to detect trend changes, with the CUSUM method in Change Point Analyzer (CPA) software identifying the primary change point. A t-test evaluated the significance of extent differences before and after the change point. Seaward boundary changes were analyzed using DSAS in a GIS environment. A baseline was manually delineated based on the 1998 mangrove shoreline, buffered by 100 meters. A total of 376 transects, spaced 30 meters apart, were drawn perpendicular to the baseline to measure boundary shifts in 1986, 2004, and 2024. Linear regression calculated annual rates of boundary advance or retreat, with positive values indicating seaward expansion and negative values indicating landward retreat. This methodology ensured precise quantification of spatial-temporal mangrove dynamics, supporting robust vulnerability assessments. 3- Results The analysis of the 39-year Landsat imagery revealed significant fluctuations in the extent of Gwadar Bay’s mangrove forests. From 1986 to 1998, the mangrove extent exhibited an increasing trend, growing from 137.9 hectares to 177.2 hectares, with an average annual increase of 3 hectares. This expansion likely reflects favorable environmental conditions, such as adequate rainfall and limited human interference during this period. However, post-1998, a marked decline was observed, with the extent decreasing to 121.7 hectares by 2024, at an average annual reduction of 1.8 hectares. The overall net change rate over the 39-year period was negative, at -0.9 hectares per year, indicating a persistent decline in mangrove coverage. Statistical analyses, including the Pettitt-Mann-Whitney test and CUSUM method, identified 1998 as a critical turning point, with a 99% probability of being the primary change point in the time series. The t-test confirmed a significant difference in mean mangrove extent before and after 1998 (p = 0.004), underscoring the shift from expansion to contraction. Spatially, the mangrove patches showed dynamic changes. Between 1986 and 1998, new mangrove patches emerged, particularly along the margins of tidal creeks and adjacent to larger mangrove stands, indicating seaward expansion. Conversely, from 1998 to 2024, smaller peripheral patches diminished, and larger central patches contracted, particularly on the landward side. Despite this overall reduction, some new patches formed in muddy coastal areas near tidal creeks, suggesting localized resilience. The seaward boundary analysis, conducted using DSAS, revealed an average annual retreat of 0.141 meters, reflecting erosion and landward migration of mangrove boundaries. Classification accuracy assessments confirmed the reliability of these findings, with overall accuracies exceeding 91% and user/producer accuracies above 85% for all classified images. 4- Discussion & Conclusions The observed trends in Gwadar Bay’s mangrove forests highlight their high vulnerability to climate change impacts, particularly prolonged droughts and sea-level rise, compounded by human activities. The pre-1998 expansion aligns with periods of higher rainfall and milder environmental conditions, as noted in prior studies of Iranian mangroves. The post-1998 decline corresponds with intensified drought cycles and rising sea levels, which have been identified as primary stressors for mangrove ecosystems globally. The significant reduction in extent and the seaward boundary retreat underscores the combined effects of climatic stressors and anthropogenic pressures, such as coastal development and pollution, which disrupt sediment dynamics and exacerbate erosion. The identification of 1998 as a turning point is consistent with regional studies that link mangrove decline to prolonged droughts and climatic shifts in the late 1990s. The negative net change rate and boundary retreat indicate that Gwadar Bay’s mangroves are undergoing significant stress, threatening their ecological functions, including coastal stabilization and biodiversity support. However, the emergence of new patches in some areas suggests potential for natural regeneration under favorable conditions, highlighting the need for targeted conservation efforts.   تجزیه و تحلیل رقومی خط ساحلی, بالا آمدن سطح آب دریا, مدل‌سازی تغییرات. Digital shoreline analysis, sea-level rise, change modeling. 140 156 http://magazine.hormozgan.ac.ir/browse.php?a_code=A-10-1039-1&slc_lang=fa&sid=1 Akram Nouri-Kamari اکرم نوری کمری a.nourikamari@cfu.ac.ir 10031947532846009759 10031947532846009759 Yes Department of Geography Education, Farhangian University, Tehran, Iran. گروه آموزش جغرافیا، دانشگاه فرهنگیان، تهران، ایران. Fariba Karami فریبا کرمی f.karami@cfu.ac.ir 10031947532846009760 10031947532846009760 No Department of Geography Education, Farhangian University, Tehran, Iran. گروه آموزش جغرافیا، دانشگاه فرهنگیان، تهران، ایران.