Recovery of rangelands : the functioning of soil seed banks in a semi-arid African savanna

Rangelands in Africa provide important forage resources for herbivores; particularly perennial grasses provide grazing for domestic and wild herbivores. However, semi-arid African rangelands experience severe vegetation and soil degradation due to heavy grazing, causing negative impacts on the ecosystems, livestock production and livelihoods of the people. Semi-arid African rangelands can be described by a state-and-transition models, often with three stable states, the first one being a state with ample herbaceous cover, perennial grasses and scattered trees, the second one as a state with a poor cover of annual grasses, absence of perennial grasses, and the third state with a high proportion of bare soil and/or often bush encroached. This thesis aims to fill important information gaps concerning soil seed banks that play in the recovery and possible restoration of degraded semi-arid African rangelands with particular emphasis on Ethiopia. This was done through investigating the mechanisms of how heavy grazing affects the soil seeds bank dynamics so as to understand stable states and transition processes of aboveground vegetation. In this thesis, aboveground vegetation and soil seed bank dynamics were studied under heavy and light grazing pressures in a field and experimental conditions. Results show that heavy grazing resulted in the disappearance of perennial grasses, a reduction in herbaceous species diversity and their plant abundance, standing biomass and basal cover, as well as a decrease in the soil nutrient conditions. The soil seed banks was correlated to differences in grazing pressure, with a greater seedling density under light grazing compared with heavy grazing. Immediately after seed dispersal, the seedling density increased over the first first three months until the eight months of soil sampling, and decreased thereafter. Under light grazing, perennial grass species dominated, whereas annual species were abundant at the heavily grazed sites, indicating that perennial grasses, with good fodder value, are replaced by annual species in the soil seed banks due to heavy grazing. With increasing soil depth, the seedling density and its species richness declined. Moreover, the seeds of perennial grasses were less abundant in the soil seed banks under heavy grazing. The similarity in species composition between the soil seed banks and aboveground vegetation was low under heavy grazing. Results also show that annual grasses had a lower germination and mortality, and higher viability, leading to a longevity of 62% over the 120 days burial in the soil, which was high compared to the 28% for perennials. Moreover, most perennial grasses germinate rapidly after initial seed dispersal at the first rains early in the year, whereas annual grasses show a linear germination pattern over time, indicating that perennial grasses have different survival strategy in semi-arid Africa. As a result, annual species are expected to dominate the soil seed banks, whereas most perennial grass species do not form persistent soil seed banks.The mean mortality from the seedling stage to adult plants in grass species was 65%, and the seed–to–seedling stage was found the most critical transitional stage for grass survival on these rangelands, suggesting that exclusion from grazing and trampling in the early germination stage is important to facilitate the transition from seedling to established plants. Depletion of perennial grass seeds in the soil due to heavy grazing coupled with high seedling mortality leads to a strong decrease in perennial grasses both in the soil seed banks, as well as in the aboveground vegetation. I found that the positive relationship between plant cover and differences in soil seed bank dynamics, i.e., seed density, seed germination rate and longevity, trigger the transition from perennial grasses to annuals and from annual plant cover to bare soil under heavy grazing. I hypothesize that the restoration of perennial grasses from the soil seed banks in heavily grazed areas in semi-arid African rangelands cannot be successful without an extraneous source of perennial grass seeds and without protecting the young plant’s regrowth from trampling and grazing. Therefore, the persistence of species and maintenance of biodiversity in semi-arid rangelands depends mainly on the recruitment of seedlings from annual species, and on vegetative reproduction of perennial grasses and woody species. These findings have important implications for the management, conservation and restoration of semi-arid African rangelands. Keywords:Africa; Aboveground vegetation; Ethiopia; Pastoral system; Rangeland; Restoration; Soil seed bank; Savanna; Semi-arid ecosystem; Vegetation and soil degradation