Analysis of the Rate of Change of Mangrove Forest Ecosystem in Calabar South, Nigeria

Abstract

The study aimed at determining the magnitude of change of the mangrove forest ecosystem in the coastal communities of Calabar. Also, this study examined the volume of mangrove trees exploited by the inhabitants of the study area. Five communities highly noted for the exploitation of mangrove in the area were used for the study – Anantigha, Efut Obufa Esuk, Cobhan-Duke Town, Henshaw town and Mbukpa Akani Esuk orok. Volume of wood exploited and which was determined by the researchers using the single tree stumpage volume with the formula,

Where  p= 3.143 G= Girth at breast height (^M) and Ht = Tree height (M). However, Tariff rate/pole and merchantable tree (logs), height, were gathered from the Monthly tree Analysis Report of Forestry Department in Calabar. In order to establish trend of mangrove forest change over years, a multi-temporal image data of the mangrove vegetation covering the area (1970 – 2011) was acquired and processed. These Include: a Toposheet (1970) derived from an aerial photo of 1970, Landsat imagery 1970, 1986, 2011 at 30m resolution. These data sets were geo- referenced into a common framework in a G.I.S environment (Arc. GIS 9.3) after which they were digitized and delineated into mangrove class, and other Non – Mangrove classes (Fresh Water Swamp, Nypa palm, Farm land and so on.)  The Landsat imagery was also treated likewise. Intersection operations of the above data sets were carried out to determine the change in mangrove area coverage between the different time periods in consideration (1970, 1991 and 2011).

Indices of mangrove forest change which includes: Intensity Index, or potential trend which is given by;

T_? = U_bi – U_ai, (1, 2…8) was computed, dynamic index whose formula is K_i = U_bi – U_ai x ¹/_T x100%, (1, 2….8) was equally determined and finally the Contribution rate of change given as A_i = U_bi – U_ai ?(U_bi – U_ai) was analyzed.

The result of analysis revealed among other things.

1. The rate of areal change of mangrove ecosystem has been in the neighbourhood of -2.75km² between 1970 and 1990 and -32.12km² between 1991 and 2011.
2. The areal cover of Nypa palm has been on increase from 3.04km² between 1970 and 1990 to 31.86 between 1991-2011.
3. With mean value of total annual number of mangrove trees exploited was put at 6225. It was discovered that eleven years (2001-2011) recorded over exploitation.