EFFECT OF SLOPE GRADIENT ON VEGETATION AND SOIL ATTRIBUTES OF GARA ADES PROTECTED VEGETATION IN WESTERN HARARGHE ZONE OF OROMIA REGION, ETHIOPIA

Abstract

The study was conducted at Gara Ades protected vegetation of Doba woreda, Western Hararghe Zone, Ethiopia; from October to December 2017 with the objective of analysing the woody vegetation structure, diversity and regeneration status, as well as soil status potentials under three slope gradient classes. Primarily, slope classes were set purposively; where lower slope class was (LSC: <10 %); middle slope class was (MSC: >10 - 20 %) and upper slope class was (USC: >20 - <30%). Then, randomly 60 (20 m × 20 m) main plots were sampled along the three slope gradient classes. In each main plot, one 5m x 5m sub-plot was laid at the middle of main plot for shrub and climber species and 5 sub-plots of 1 m x 1 m were distributed inside the main plot (one at each corner and one at the center) laid down for herbaceous species and soil sample data collection. Soil samples were collected by auger at a depth of 30cm followed by composite soil sample preparation to analyse soil textural class and chemical nutrient contents such as potential of hydrogen (pH), electrical conductivity (EC), exchangeable acidity (EA), organic carbon (OC), available phosphorus (P), calcium (Ca), magnesium (Mg), available potassium (K), sodium (Na), cation exchange capacity (CEC) and total nitrogen (TN) at Bedele Soil Laboratory Research Center of Oromia Agriculture Research Institute (OARI). Vegetation and soil data were analysed in one-way ANOVA using SAS version 9.1 software packages and LSD was used for mean comparison. Analysis results indicated that, slope classes significantly (P < 0.05) affected woody species richness, diversity, evenness and density. Primarily, a total of 65 woody species that fall within 50 families were identified in the study area. Out of the 65 woody species 35 (53.85 %) were trees, 26 (40 %) were shrubs and 4 (6.15) were climbers. However, the average density of woody species of the study site was 975.8; where, highest density of woody species was recorded in the USC (1251.25 ha-1) followed by MSC (928.75 ha-1) and LSC (747.5 ha-1). Juniperus procera (Hochest.ex Endel) was the first dominant tree species with 217.5 individuals ha-1in LSC, 221.25 individuals ha-1 in MSC and 303.75 individuals ha-1in USC. Podocarpus falcatus (Thmb R.B. ex. Mirb) was the second dominant tree species with 131.25 individuals per ha in LSC, 110 individuals ha-1in MSC and 166.25 individuals ha-1in USC. Cupressus lucitanica (Mill.) was the third dominant tree species with 68.75 individuals’ ha-1in LSC, 118.75 individuals’ ha-1in MSC and 82.5 individual’s ha-1in USC. Woody species population distribution in the three slope gradient classes exhibited highest frequency of species in the lower diameter classes with a gradual decrease towards higher diameter classes. On the other hand, the total basal area of woody species was 8286.83 in the LSC, 10908.51 in the MSC and 10026.22 in the USC. Regeneration status of the study site was very poor; the overall average density of seedling, sapling and mature density of woody species was 117.9 ha-1 (8.6 %), 282.9 ha-1 (20.5 %) and 975.8 ha-1 (70.9 %) respectively. Lower slope class has an average density of 106.3 ha-1 seedling and 120 ha-1of sapling. Middle slope class has an average density of 358 ha-1 seedling and 286 ha-1 sapling; whereas, upper slope class has 747.5 ha-1 seedling and 928.8 ha-1 saplings. However, herbaceous dry biomass yield and herbaceous basal cover were also significantly (P < 0.05) affected by slope gradient where, LSC has higher mean herbaceous dry matter production and herbaceous basal cover than MSC and USC. Accordingly, the mean herbaceous dry biomass yield was 1.22 ton ha-1 in the LSC, 1.09 ton ha-1 in the MSC and 1.03 ton ha-1 in the MSC. The mean herbaceous basal cover was 75.95 % in LSC, 66.55 % in MSC xvi and 64.35 % USC. Though, soil textural classes of sand and clay were not significantly (P > 0.05) different in the LSC, MSC and USC which is predominantly loam and sandy loam contents but, textural class of silt of LSC was significantly (P < 0.05) different from two groups (MSC and USC). Silt textural class of the study soil was influenced by the difference in slope gradient classes. However, soil chemical parameter analysis shows that except phosphorus (P), all parameters have a significant (P < 0.05) difference among slope classes with different mean values. Soil pH, EC, Ca, Mg, Na and TN parameter mean values were higher in the LSC and have significant difference from MSC and USC. Soil OC, OM and CEC parameters have higher mean values in MSC (MSC was significantly different from LSC and USC). LSC has lower Soil K mean value which was significantly different from MSC and LSC. Soil EA was higher in USC and the mean value of USC was significantly different from MSC and LSC. Results of this study revealed that, slope gradient showed variation in vegetation and soil measurement parameters. Generally, LSC has lower woody vegetation mean values than MSC and USC; where slope gradient increase had a positive effect on woody species diversity and composition. On the other hand; LSC has higher herbaceous plant dry matter yield, herbaceous plant basal cover and soil physico-chemical property mean values than MSC and USC; hence slope gradient increase had a negative effect on herbaceous plant dry matter yield, herbaceous plant basal cover and soil physico-chemical property parameters. More than woody vegetation herbaceous and soil parameters are more influenced with an increase in slope gradient. In general, disparity of slope gradient should have to be considered during further ecological studies in the study area to reduce variations that occur due to slope gradient variation. Additionally, human and livestock intervention were also expected to be serious challenges in vegetation structure and composition of Ades protected vegetation. Therefore, better ecological, socio economic and policy concerns should have to be adopted and established in the study area for a better management to increase the productivity of the forest without depletion of the available vegetation and soil resources.