برآورد میزان فرسایش ورقه‌ای، با استفاده از تحلیل دندروژئومورفولوژیکی ریشه‌های درخت در حوضه قره‌چای (رامیان)

نویسندگان

1 استادیار گروه جغرافیا، دانشگاه تربیت معلم سبزوار

2 کارشناس ارشد ژئومورفولوژی، دانشگاه تربیت معلم سبزوار

3 کارشناس¬ارشد سنجش از دور، دانشگاه تربیت معلم سبزوار

4 دانشجوی کارشناسی¬ارشد ژئومورفولوژی، دانشکده جغرافیا، دانشگاه تهران

چکیده

دندروژئومورفولوژی یکی از رشته‌های زیرمجموعه دندروکرونولوژی است که بر اساس تجزیه و تحلیل حلقه‌های رشد سالانه درختان و مورفولوژی رشد آنها به بررسی جنبه‌های فضایی و مکانی فرایندهای سطحی زمین در دوره‌های هولوسن می‌پردازد. هدف این تحقیق تحلیل دندروژئومورفولوژیکی حلقه‌های ریشه درخت برای برآورد نرخ فرسایش ورقه‌ای در حوضه قره‌چای (رامیان) است. به منظور تعیین میزان فرسایش ورقه‌ای بر اساس تحلیل دندروژئومورفولوژیکی ریشه‌های درختان، بعد از شناسایی ریشه‌های برونزد یافته بر اثر فرسایش، مجموعا تعداد 42 مقطع از ریشه درختان سوزنی‌برگ و پهن‌برگ تهیه شد. هنگام برداشت نمونه‌ها، اطلاعاتی مانند قسمت رو به آفتاب ریشه رخنمون یافته، محل تماس ریشه با سطح خاک، فاصله سطح خاک تا سطح فوقانی ریشه و اطلاعات مورد نیاز دیگر برای هر مقطع ثبت شد. سپس مقاطع تهیه شده خشک گردیدند و سمباده زده شدند. بعد از تعیین اولین سال رخنمون ریشه‌ها، بر اساس معادله 1 مقدار کلی فرسایش(Er) بدست آمد و سپس فرسایش سالیانه به میلی‌متر محاسبه گردید. متوسط فرسایش ورقه‌ای منطقه مورد مطالعه براساس مقاطع ریشه-های برونزد یافته، 541/0 میلیمتر در سال برآورد شده است. بررسی داده‌ها نشان می‌دهد که میانگین فرسایش سالانه در مقاطع دارای سن کمتر از 50 سال ،50 تا 100 سال، 100 تا 150 سال، 150 تا 200 سال و 200 تا 250 سال به ترتیب 86/0 ، 48/0 ، 4/0 ، 3/0 و 25/0 میلی‌متر است که این موضوع، افزایش میزان فرسایش از 250 سال پیش تا زمان حال را نشان می‌دهد. بررسی فرسایش در انواع پوشش گیاهی بیانگر آن است که مقدار متوسط فرسایش سالانه در جنگل‌های متراکم کمتر از جنگل‌های نیمه‌متراکم و کم‌تراکم است. ارزیابی مقدار فرسایش در سازندهای زمین‌شناسی نشان می‌دهد که مقدار فرسایش در خاک‌های واقع بر روی سازندهایی که دارای جنگل‌های متراکم کمتری هستند (مانند سازندهای شمشک پایینی و شمشک میانی)، بیشتر است. تحقیق حاضر نشان می‌دهد که گونه‌های سوزنی‌برگ مانند سرخدار و زربین، به علت دارا بودن حلقه‌های واضح‌تر و قابل شمارش‌تر، دارای کارایی بهتری در برآورد میزان فرسایش، در قیاس با گونه‌های پهن‌برگ مانند بلوط و زبان‌گنجشک هستند.

کلیدواژه‌ها


عنوان مقاله [English]

Estimating the rate of sheet erosion by dendrogeomorphological analysis of tree roots in Gharechai (Ramian) Catchment

نویسندگان [English]

  • Sh Shahram Bahrami 1
  • Fatemeh Mahboobi 2
  • Javad Sadidi 3
  • Mariam Jafari Aghdam 4
1
2
3
4
چکیده [English]

Introduction
Dendrogeomorphology, which was first introduced by Alestalo (1971), is a subdiscipline of dendrochronology that, based on the analysis of annual tree rings and their growth morphology, addresses the spatial and temporal aspects of earth surface processes in Holocene period.
The applications of dendrogeomorphology is dating and determining changes of various geomorphic processes such as floods, glaciers, storms, river channel migrations, mass wastings, earthquakes, volcanism and soil erosions, etc. Dendrogeomorphological research has mainly concentrated on tree trunks. Only to a lesser extent, roots have been studied for evaluation of sheet erosion rate and gully erosion. Estimating the rates of erosion based on the exposure of roots is based upon the changes in the wood anatomical pattern which occurs as soon as a root is exposed.

Materials and Methods
The study area is located in southern part of Gharachay catchment in Ramian County (Golestan province, Iran), between 36ْ 48ََ to 36ْ 57 ََ N lattitude and 55ْ 6ََ to 55ْ 17 ََ W longitude. Gharachay catchment drains an area of 143 km2. The altitudinal extend of the area ranges from 560 m in the northwest to 2650 m in the southwest.Geologically the studied area is a part of the Gorgan-Rasht zone in the Alborz mountains. The geological formation of this area includes Khosh Yeylagh (Kh), Lower part of Khosh Yeylagh (LKh), Mobarak formation (Cm1), Ghezel Ghaleh formation (Gh), Doroud formation (D), Lower Shemshak (Js1), Middle Shemshak (Js2), Upper Shemshak (Js3), and Old Alluvial Fans (Qt1).
The aim of this research is a dendrogeomorphological analysis of tree roots for estimating the rate of sheet erosion in Gharechai (Ramian) Catchment. At first, DEM, slope and aspect map of the study area have been generated based on the topographic map at a scale of 1:50000 from Iranian National Geography Organization toposheets. Lithological formations were derived from the Geologic map of the studied area at a scale of 1:100000 by means of GIS (Integrated Land and Water Information System (ILWIS)).
Before sampling of tree roots, the location of the exposed roots recognized by field observations and a detailed geomorphic mapping of the mentioned area.
Before cutting each section, a detailed delineation of the spatial and morphological characteristics of the surroundings of the root-such as tree type, geographical location (UTM coordinates), altitude, aspect; slope of the specific root-was made.
A total of 42 samples of tree roots from conifers and broadleaf species that were nearly and uniformly distributed in the catchment (Fig.5) were obtained. Following detection of exposed roots, 38 sections of conifers (33 sections from yew, 1 section from cypress, 3 sections from juniper, and 1 section from pine) and 4 sections of broadleaves (2 sections from ash and 2 sections from oak) were provided during August 2008.
During the cutting of tree root samples, sun-exposed part of outcropped root, contact of root and soil surface, the vertical distance between the upper part of the root and the present soil surface were measured for every section.

Results and Discussion
In this study, the height of eroded soil layer since the time of exposure (Er) has been calculated by Gartner method (2007, P. 248). Several parameters have been calculated for reconstruction of Er. At first, the exposed part (height) of the root (Ex) for samples were recorded. Reconstructing the first year of root exposure was carried out by analyzing the changes in the ring-growth pattern (from concentric to eccentric) and thereby the size of the root (exluding bark) at the time of exposure (R2) was determined. Then, the size -or height- of whole root (including bark) in cross section (R1), thickness of the upper (B1) and lower (B2) part of the bark were measured.
Based upon the Gartner method (2007), Er has been calculated as follows:
Er=R2+(B1+B2)/2-R1+Ex (1)
The value of Er were divided by the number of rings growing since the year of exposure (NRex) to calculate the annual erosion rate(Era):
(2)
Whereas conifers show more observable growth rings in comparison with broadleaves, most of the root cross sections of this study have been derived from conifers such as yew that have distinguishable growth rings. The mean sheet erosion of study area, based on sections of exposed tree roots, has been estimated 0.541 mm/y. erosion rates in sections with 200-250, 150-200, 100-150, 50-100 and less than 50 years old are 0.25, 0.30, 0.40, 0.48 and 0.86 mm/y respectively.

Conclusion
This represents that the rate of erosion has been increased since the past 250 years. Analyzing the erosion rate in vegetation types reveals that the mean erosion rate is lower in denser forests than sparse forests. Evaluation of erosion rate in geological formation reveals that the mean erosion rate in soils over geological formations that have less dense forests, such as Lower Shemshak(Js1) and Middle Shemshak(Js2), is higher. This study also shows that conifers like Yew and Cypress that have more obvious and countable rings, have more capabilities in estimating erosion than broadleaf species like oak and ash.

کلیدواژه‌ها [English]

  • dendrogeomorphology
  • Gharechai
  • Ramian
  • Sheet erosion
  • Tree root