title: Changes in grass plant populations and temporal soil seed bank dynamics in a semi-arid African savanna: Implications for restoration.
abstract: The re-colonization or recovery of grass species after disappearance due to heavy grazing depends on the presence of persistent soil seed banks that might be accumulated over time from the aboveground vegetation. Moreover, successful plant recruitment is a function of seed production, seed germination and seedling survival, which can be mechanistically understood through studying the life cycle processes of grass species populations under field conditions. Therefore, we studied the number of germinable seeds, species richness and life-forms in the soil seed banks under light and heavy grazing conditions, and the changes in grass species populations in a semi-arid savanna of Ethiopia. Accordingly, a total of 103 species (15 perennial and 29 annual grasses, 6 legumes, 52 forbs and 1 woody species) emerged from the soil samples collected. Lightly grazed sites had a higher seed density compared with heavily grazed sites. The seed density increased over the first three months of soil sampling and decreased thereafter. Perennial grasses dominated the light grazing sites, whereas annual species dominated the heavily grazed sites, indicating that perennial grasses were replaced by annual species in the soil seed bank through grazing. The mean mortality rate from the seedling stage to adult plants was 65%. The seed-to-seedling stage was found to be the most critical transitional stage for grass survival. High seedling mortality in the aboveground vegetation and depletion of seeds in the soil seed banks as a result of sustained heavy grazing can lead to local extinction and disappearance of perennial grasses in semi-arid Ethiopian savannas.
authors.affiliation: Rangeland Ecology and Biodiversity Program, School of Animal and Range Sciences, Haramaya University, PO Box 138, Dire Dawa, Ethiopia. Electronic address: tessemaz@yahoo.com.
authors.forename: Zewdu K
authors.initials: ZK
authors.lastname: Tessema
authors.affiliation: Resource Ecology Group, Wageningen University, Droevendaalsesteeg 3a, 6708PB, Wageningen, The Netherlands.
authors.forename: Willem F
authors.initials: WF
authors.lastname: de Boer
authors.affiliation: Resource Ecology Group, Wageningen University, Droevendaalsesteeg 3a, 6708PB, Wageningen, The Netherlands.
authors.forename: Herbert H T
authors.initials: HHT
authors.lastname: Prins
mesh_terms: D003247:Conservation of Natural Resources; D055864:Droughts; D017753:Ecosystem; D005002:Ethiopia; D065948:Grassland; D006801:Humans; D006109:Poaceae / Q000145:classification / Q000502:physiology*; D000068098:Seed Bank; D012639:Seeds / Q000502:physiology; D012987:Soil / Q000737:chemistry*
chemical_list: D012987:Soil
keywords: Grass plant population; Grazing intensity; Growth stages; Life–forms; Seed density; Species richness