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Universität Bayreuth (2019)

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.

Okach, Daniel Osieko

Titre : 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.

Auteur : Okach, Daniel Osieko :

Université de soutenance : Universität Bayreuth

Grade : Doctor of Natural Science (Dr. rer. nat.) 2019

Résumé partiel
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 < 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.

Mots clés  : Ambient rainfall ; CO2 exchange ; grazing exclusion ; herbaceous layer community ; species diversity ; soil properties

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