title: Effects of livestock grazing and rainfall variability on the structure and function of the herbaceous layer community of a humid savanna ecosystem in Lambwe Valley - Kenya
Creators: name: Okach, Daniel Osieko
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: Ambient rainfall
subject: CO2 exchange
subject: grazing exclusion
subject: herbaceous layer community
subject: species diversity
subject: soil properties
subject: 570

description: Savanna ecosystems have undergone structural and functional transformations in the last two centuries, following a shift in the grazing communities from predominantly migratory wild mega-herbivores, to sedentary domestic herbivore populations (livestock). The dominance of livestock and their sedentary grazing patterns have increased pressure on the herbaceous layer communities. Changes in rainfall regimes, characterized by shorter but more intense rainy seasons (flooding) and prolonged dry seasons (drought) have also occurred during the same period. This transformation threatens the sustainability of the herbaceous layer, but has rarely been studied. Rapid increase in livestock populations in Lambwe Valley, a typical savanna in Western Kenya, has increased grazing pressure on the herbaceous vegetation during the last three decades. Extended dry periods and shorter, but more intense, rainy periods have also become common in the region. Phenotypic and structural alterations caused by the ongoing grazing pressure and increased rainfall variability are likely the visible ramification of the changing ecosystem processes, taking place at the background, and which have rarely been investigated. Lambwe valley is, therefore, a suitable study object for understanding how savanna ecosystems respond to the ongoing changes in grazing and rainfall variability. This study investigated how two levels of livestock grazing (grazed and ungrazed) and three levels of rainfall amounts (50%, 100%, and 150% of ambient) affected (i) soil properties; (ii) plant species diversity, and (iii) CO2 fluxes in the herbaceous layer community during dry and wet periods. Study plots were abbreviated using first letters of the grazing levels (grazed - G or ungrazed - U) followed by the amounts of rainfall (50%, 100% and 150%) i.e. G50%. Volumetric soil water content (VWC) increased along a rainfall gradient of 50–150% (RMSE = 3.55; r2 = 0.61; p &lt; 0.05), from an average amount of 17.5 ± 4.9% to 26.9 ± 4.9% in the G50% and U150% plots, respectively. This shows that rainfall was the main source of water at the 20 cm soil depth during the measurement period. The grazed plots had higher soil bulk density of 3.2% and nitrogen (N) content of 7.8% compared to the ungrazed, however, VWC and soil carbon (C) content decreased by 19.64% and 11.45%, respectively in the grazed plots compared to the ungrazed. Reducing the ambient rainfall by 50% had a significant effect (p &lt; 0.05) on the soil properties compared to a 50% increase (150%). VWC significantly correlated to soil bulk soil density (r2 &gt; 0.62; p = 0.05). Livestock grazing increased species diversity (H), but decreased aboveground biomass (AGB). Hyparrhenia fillipendulla (Hochst) Stapf. and Brachiaria decumbens Stapf, dominated the ungrazed plots, making up &gt; 70% of the relative abundance and a larger proportion of AGB. Mean H did not vary amongst G50%, G100%, and G150% plots. In the ungrazed plots, however, H was higher in the U100% than both U50% and U150% plots. The AGB and species abundance mediated (indirect-only mediation) the effect of VWC on H. The highest mean net ecosystem exchange (NEE) of –8.80 ± 2.26 µmol m–2 s–1, AGB of 1208.41 g m–2, and total biomass of 1589.06 g m–2 were reported in the U150% plot during the wet months. VWC significantly correlated (p &lt; 0.05) to the changes in NEE (r2 ≥ 0.65), ecosystem respiration (Reco) (r2 ≥ 0.52), gross primary production (GPP) (r2 ≥ 0.71), and total biomass (r2 ≥ 0.61) across plots. CO2 fluxes and the total biomass of the herbaceous layer community decreased due to: grazing by livestock; 50% reduction of ambient rainfall; and during the dry months, when VWC was lower. During the dry months, NEE in the G50% plot declined to 3.68 ± 0.81 µmol m–2 s–1, from –3.06 ± 1.56 µmol m–2 s–1, observed when soil moisture was high. The interaction among grazing, rainfall manipulation, and seasonality in rainfall significantly regulated AGB, total biomass, and root to shoot biomass ratio (R:S). The total amount of rainfall received during the entire period of measurement directly affected soil moisture availability, however, variations in VWC among the plots resulted from the interaction of livestock grazing and rainfall manipulation. Grazing and reduction of ambient rainfall decreased the soil C content, species diversity, CO2 uptake, and biomass production, particularly, during the dry months when the negative effects were more pronounced. By reducing biomass of the most dominant species, grazing increased species diversity. This study demonstrated that the Lambwe Valley ecosystem has adjusted to the current grazing levels and rainfall amounts. Future decrease or increase in current ambient rainfall or grazing exclusion will lower herbaceous species diversity, due to adaptation of the current vegetation types to grazing under ambient rainfall amount. Interaction between livestock grazing and variation in rainfall amount will inform sustainable management strategies to regulate the herbaceous diversity and productivity.