5 years ago

Die Wirksamkeit von Boden

Die Wirksamkeit von Boden

Performance of exclosure

Performance of exclosure in restoring soil fertility landscape position; ��� �ij = effect of interaction of agro-ecological zones and age; ��� �ik = effect of interaction of agro-ecological zones and landscape position; ��� �jk = effect of interaction of age and landscape position; � �ijk ��� = effect of interactions of agro-ecological zones, age and landscape position, � ijk = random error. The data were statistically analyzed by use of the above nested model. The interaction of fixed factors revealed insignificant differences throughout. The data were also examined for likelihood ratio test before rejection of the null hypothesis (nested model) using STATA (version 12). The test revealed lower probability values, which implies that the observed outcome was much less likely to occur under the null hypothesis than a simplified model, which disregards interactions of the fixed factors. Therefore, the above nested model (null hypothesis) was reduced to the following simple model: y � � � � � � � � � � (7.2) ijk Note: for the legend see the nested model. Furthermore, to distinguish the relation of soil properties with each other, the soil analysis data were also checked for statistical correlation using a bivariate correlation analysis in SPSS version 17. 7.3 Results and discussion 7.3.1 Changes in biophysical features of exclosures The vegetation cover, structure and composition of the exclosures showed clear differences in relation to age and agro-ecological zone, but there were no regular trends across the slope positions. Vegetation type and cover changed with increasing age of the exclosures (Table 7.1, Table 7.2, and Figure 7.5). As a result, the 10-year-old exclosures were largely dominated by grasses and bushes (Figure 7.5 a and c) where the average ground and canopy covers accounted for 60.5% and 19%, respectively. The 27-year-old exclosures were dominated by large trees (Figure 7.5 b and d), and had 67% ground and 69% canopy cover (Table 7.1). On the other hand, marginal lands left open for free grazing showed high biomass degradation (Figure 7.5 e and f), i.e., the canopy and ground cover were less than 5% and 25%, respectively (Table 7.1). Degraded land was i j 108 k ijk

Performance of exclosure in restoring soil fertility first inhabited by opportunistic herbaceous vegetation and grasses, which were followed by relatively higher-layer vegetation (shrubs) succeeded by bushes and/or trees (Table 7.2). Similarly, gradual replacement of lower-layer by higher-layer vegetation with increasing age of the exclosures was reported in various studies (Khater et al. 2003; Descheemaeker et al. 2006; Oba et al. 2006). Vegetation structure and composition variation play an important role in soil fertility restoration. Table 7.1 Vegetation cover of exclosures and control sites by age, agro-ecology and landscape position Agro- Ground cover (%) Canopy cover (%) Sampling site Age ecology LP MP UP LP MP UP Abo-Dur 27 Cool 75 63 67 80 87 80 Zendo-Girat 10 Cool 55 58 55 20 32 17 Open site 0 Cool 22 25 20 0 0 2 Ayda-Tig 27 Mild 68 77 52 63 53 50 Lenche-Dima 10 Mild 60 65 70 15 10 18 Open site 0 Mild 20 25 20 0 1 3 Table 7.2 Vegetation types and SWC structures in exclosures and control sites Sampling site Vegetation type SWC structures Abo-Dur Trees, bushes, grass, shrubs, herbs Hillside terraces, cut-off 109 drains Zendo-Girat Grasses, shrubs/herbs, bushes, Hillside terraces, micro- trees basins, check-dams, trenches Open site Grasses, bushes, shrubs and trees Non Ayda-Tig Trees, bushes, grass, shrubs, herbs Hillside terraces, cut-off drains Lenche-Dima Grasses, shrubs/herbs, bushes, Hillside terraces, micro- trees basins, check-dams trenches Open site Bushes, shrubs, grasses and trees Non Note: Vegetation types are listed in the order of abundance from higher to lower. Physical SWC structures in the older exclosures have been deteriorating due to regular sedimentation within successive structures Climate, particularly rainfall and temperature, influences vegetation production, types and structure. Hence, the agro-ecological zone of an exclosure determines not only vegetation cover but also vegetation type (Table 7.2). The two agro-ecological zones in the study area have specific climatic features and are located in different altitude ranges. Exclosures vary due to the differences in climatic conditions and other environmental attributes (Descheemaeker et al. 2006; Aynekulu et al. 2009; Mekuria et al. 2011).

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