Effects of bush encroachment intensity on herbaceous vegetation and soil quality indicators in semi-arid savannah rangelands of the Eastern Cape province, South Africa
Loading...
Date
2024-07
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Fort Hare
Abstract
Bush encroachment reduces herbaceous productivity and diversity, leading to a decline in grazing capacity and soil quality, thereby causing rangeland degradation. Therefore, this study set out to investigate the response of rangelands released from bush encroachment (specifically from Vachellia species), in the semi-arid savannah ecosystems of the Eastern Cape Province, South Africa. This was achieved by first examining the effects of bush thinning intensity on herbaceous vegetation and then assessing its impact on soil quality indicators. The research was conducted at the Napier Camp bush control experimental site within the University of Fort Hare Honey Dale Research Farm in Alice, Eastern Cape Province of South Africa. The bush control experimental trial covered two hectares and was divided into 15 sub-plots with bush thinning at five levels applied using a selective bush thinning method. The bush encroachment levels (BEL) replicated three times were 12% (Light), 25% (Moderate), 50% (Heavy), and 75% (Severe), with 100% as the Control (no thinning applied). Herbaceous species identification and mean point-to-tuft distance (estimate of basal cover) were conducted using the step point method, while the quadrat method was used to measure grass biomass and tuft density within sub-plots of different BELs. Soil chemical properties (i.e., organic carbon, total nitrogen, phosphorus, and potassium) and physical properties (dry bulk density, temperature, and moisture) were measured by collecting thirty composite soil samples randomly at each BEL under canopy and open zones. Moisture and temperature were measured directly at each subplot. Sixty soil samples were collected randomly from open and under canopy zones to determine soil dry bulk density. A total of 15 herbaceous species were identified including 13 perennial grasses and two species classified as 'Others' (Forb and Karroo species). Among the perennial grasses, nine were Increaser species and four were Decreaser species. Decreaser species increased while Increaser species declined with rising BELs. Highly nutritious species such as Panicum maximum and Panicum stafianum were more abundant at BELs ≥50%.
Statistical analyses were conducted using SAS, with ANOVA used to assess differences among BELs for each parameter, and the Least Significant Difference (LSD) test was applied to compare and separate means where significant differences were detected. Grass biomass production (GBP) was significantly affected by BELs (p < 0.05), decreasing linearly with increasing BELs. GBP was significantly higher (p < 0.05) at 12% BELs (1533 kg/ha) than in other BELs, with 25%, 50%, and 75% BELs exhibiting similar GBP. Mean point-to-tuft distance (MPTD) increased proportionally (r² = 0.96) with BELs (p < 0.05), ranging from 2.3 to 4.7 cm from 12-100% BELs. Tuft density (TD) significantly decreased (r² = 0.75) with increasing BELs (p < 0.05), with 12% and 25% BELs having the highest densities (52 tufts/m² and 55 tufts/m²¸respectively). Soil data indicated fluctuating chemical and physical properties. Across BELs, soil organic carbon (SOC), total nitrogen (TN), and potassium (K) did not differ significantly (p > 0.05), except for soil phosphorus (P) (p < 0.05). The SOC, P, and K were similar between open and under canopy zones (|x̄1-x̄2| < LSD), while TN was significantly higher under the canopy zone (|x̄1-x̄2| > LSD). Soil dry bulk density, temperature, and moisture did not vary significantly with BELs (p > 0.05). However, dry bulk density and moisture were lower (|x̄1-x̄2| > LSD) under the canopy zone, while soil temperature remained similar (|x̄1-x̄2| < LSD) across treatments. Herbaceous vegetation responses to bush encroachment indicated reduced rangeland productivity and increased soil bareness. The results suggest that bush thinning between 12% and 25% BEL is necessary to restore herbaceous vegetation. The dominance of Decreaser species at lower BELs highlights the importance of controlled bush management in maintaining species composition. In terms of soil quality, most chemical and physical properties remained unaffected by bush encroachment levels, except for soil phosphorus. Improved total nitrogen under canopy zones suggests a positive effect of bush presence on nitrogen cycling. However, reduced soil bulk density and moisture under the canopy zone may have implications for water retention and soil stability in encroached areas.
Overall, individual bush species and bush thinning between 12% and 25% BEL had no detrimental effects on herbaceous vegetation and on some soil quality indicators, instead appeared beneficial for maintaining herbaceous cover and promoting desirable species. These findings highlight the need for targeted bush management strategies to sustain rangeland productivity while minimizing soil degradation.
Description
MSc dissertation
Keywords
Plant canopies -- South Africa -- Eastern Cape, Plant biomass -- South Africa -- Eastern Cape, Siols -- Quality -- South Africa -- Eastern Cape
Citation
Mgushelo, I. (2024) Effects of bush encroachment intensity on herbaceous vegetation and soil quality indicators in semi-arid savannah rangelands of the Eastern Cape Province, South Africa. MSc Agriculture (Pasture Science) dissertation. Alice, South Africa: University of Fort Hare.