Abstract
Encroachment of woody plants has negative effects on grass species and seriously challenging livestock production and pastoral livelihood in semi-arid savanna. The effect of thinning and season on grass species composition and biomass was investigated in lowland grazing area for three seasons. The study consisted of 15 plots (each 50 m × 50 m), thinned to differing intensities. The plots were located next to each other on a homogeneous area of 3.8 ha. Five thinning intensities (0, 25, 50, 75, and 100%) were allocated randomly to the plots and replicated three times. The 0% plot was left unthinned referred to as the control plot. The remaining plots were thinned to the equivalents of 25, 50, 75, and 100% of the tree density of that of the control plot. A two-way analysis of variance and Tukey Honest Significant Differences tests were used to compare responses of grass species. Ordination of variables was carried out by multivariate technique using correspondence analysis to explore the similarity relationship of plots, seasons and species. The differences in species composition and biomass in the control and the thinned plots were significant (p< 0.05). Higher grass species composition and biomass were documented in thinned plots than in the control plot. Species abundance in thinned plots ranged from 11 to 12, while that of the 0% plot was six species. Grass biomass (Kg ha-1) varied from 2,688.4 ± 736.8 to 5,035.8 ± 743.9 across thinned plots, whereas in the control plot the yield was 885.8 ± 369.1. The result showed that grass species composition and biomass increased with the increasing intensity of thinning. Grass values appeared to be optimum at the heavy intensity of thinning woody plants. Seasons were also significant with the highest values recorded in the rainy season. Rainfall played an important role by interacting with thinning and influenced total grass values. Overall, thinning that combined improved soil moisture negatively influenced woody cover and enhanced grass species composition and biomass production. Thinning of bushes to the 75% of tree density would sustain the original savanna and maximize forage production for grazers.
Highlights
Savanna vegetation consists of diverse tree-grass mixtures with continuous herbaceous cover and a discontinuous cover of woody plants (Scholes and Archer, 1997)
The negative effect of densely populated A. drepanolobium trees was evident in the 0% plot which had more (60%) rare species indicating that this limited abundance of desirable forage grasses in the dense plot may be one indicator of the deteriorating condition of savanna rangelands
The study of the influence of thinning and season on grass species composition and biomass was carried out in semi-arid savanna where bush encroachment is the major threat to livestock production and people’s livelihood
Summary
Savanna vegetation consists of diverse tree-grass mixtures with continuous herbaceous cover and a discontinuous cover of woody plants (Scholes and Archer, 1997). Encroachment of woody species is a type of range degradation where an excessive and undesirable increase in woody plant abundance results in the suppression of palatable grasses by encroaching woody species and herbaceous forbs often unpalatable to domestic livestock (Ward, 2005; Angassa and Oba, 2008). The encroaching woody plants can be non-native species that were introduced or native species that have increased in density within their historic geographic ranges (Van Auken, 2009; Archer et al, 2017). In the savanna of the study area, the spread of woody plant species was noticed after the 1960s and 83% of the communal grazing lands are threatened by a combination of thorny woody species and unpalatable forbs (Angassa, 2007). It has been stated that a woody cover of 40% which is approximately equal to a density of 2,400 plants per hectare (ha−1) is considered to be at equilibrium between encroached and non-encroached (Roques et al, 2001)
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