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



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):
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.

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.