Distribution and Structure of Treeline Species in Ericaceous Vegetation of Harren A Escarpment, Bale Mountains

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Addis Ababa University


TIw Bale 1II0untain range is located between the wet east Africml 1II0untains (propel~ and the (by northeast AfNcan 1II0untains, Southeast Ethiopia. TIlis 1II0untain range hosts sOllie of endelllic flora and fauna which are endangered of extinction. TIle 1II0St extensive Ericaceous vegetation in the continent is found in Bale Mountains. TIle southem slope of this 1II0untain range is Imown for its distinct vegetation zonation of the Aji'omontane forests. TIle Ericaceous vegetation between the montane forest and the afroalpille of this slope is relatively little disturbed than other similar Ericaceous vegetation elsewhere in Aji·ica. Stltdy on the distribution and structure of this vegetation was made all the sOllthem slope, Harrena escmpment. TIle vegetation north of Rira village, between 3000m and 4200m was sampled after selecting continuous homogenous sites systematically alollg the altitudinal gradient. Cover abundance of the species for vascular plants, frequency, height and DBH for woody treeline species were taken in 110 quadrats. TIle envirollmental parameters alollg the altitudinal gradiellt illcluding soil pH, texture, total lIitrogen, and soil moisture were measured. Altitude, slope, alld aspect were measured for all qudrats. All the envirollmelltal alld vegetatioll data were allalyzed with SYlltax, Calloco, Millitab and Sigma plot. TIlirteell commullity types were described and their distributioll showed a clear pattern at differellt parts of the Ericaceous vegetatioll. However some of the COIII/III11/ity types which were restricted to the Afroalpille belt were foulld ill the Ericaceous vegetation. TII;s might be a possible illdicatioll of the e;..pansion of the aji'oalpine belt to lower altitude, evell below 3400 m (Erica domillated Hagellia-Hypericum zOlle). TIle alia lysis of distribution of sevell tree alld shrub species showed that SchefJlera volkensii was restricted to the lower part of the Ericaceous vegetatioll (Erica domillated HagelliaHypericum zOlle) and H. revolutum, R. melallophloes alld D. pellllillervium occur both at lower alld celltral parts of the Ericaceous vegetation. E. trim era and E. arborea occur in all the three parts of the Ericaceous vegetation. TIle density of trees (propel~ was higher at lower parts of the Ericaceous vegetation than the celltral part.The analysis of distribution of seven tree and shrub species showed that SchefJIera volkel/sii lVas restricted to the 10IVer part of the Elicaceous vegetation (Erica dominated Hagel/iaH) 1Jericlim zone) and H. revollltllm, R. melanophloes and D. penninervillm occllr both at lower and cel/tral parts of tbe Ericaceous vegetation. E. trimera and E. arb area occllr in all tbe three parts of tbe Ericaceolls vegetation. The densily of trees (propel) was higber at lower parts of tbe Ericaceolls vegetation tban tbe central part. The height of the tree and sbrllb species has shown a decreasing tendency with increase in altill/de. This trend was velY gradllal for E. trimera. The species occllrs for about 1.2 km altiludinal range sbowing difference in heigbt and habit along altitudinal gradient. Tbe regression analysis (/=0.58) bas shown a consistent decrease in height along altilude. No abrupt transition was docllmellted in the systematically selected continuolls Ericaceous vegetation. Among the environmellfal parameters taken altitude was the strongest explanatOlY variable. Soil pH, and texture have shown stronger correlation wilh altilude. Wbile percent total nitrogen was showing more significant (p<0.01) correlation with microsite factors. E. trim era, as major constiluellt the vegetation under question showed a gradual transilion in height and life-forlll.