Seed dynamics and the ecological restoration of hill slopes of Kondoa Irangi, Central Tanzania

Sammanfattning: Improper agricultural practices such as bill slope cultivation, extensive clearing of vegetation using fire over a long period, and to some extent overgrazing have converted the once fertile hill slopes of Kondoa Irangi in central Tanzania into unproductive marginal land. It is 25 years now since reclamation measures were introduced to save the hills from further degradation, but the regeneration process, has been rather slow. The present study was initiated with the objective to explore alternative approaches to facilitate vegetation recovery in the hills. It consisted of a description of an Afromontane dry forest, soil seed bank, seed rain and seed longevity studies, and experiments on the influence of different soil treatments on regeneration.There was significant seasonal variation in the seed bank (p < 0.001), with dry-season samples having higher densities than wet-season samples. Annual species dominated the seed bank samples in most of Kondoa Irangi Hills, but perennial species were dominant in the soil seed bank of the Afromontane dry forest. In the seed rain, both density and species richness were negatively correlated with vegetation cover. This points to the important role of anemochory and active seed rain in disturbed sites.The intermittent and extended germination of buried seeds shown by i.e., Acacia tortilis, is an adaptation to prevent synchronous germination in unpredictable, harsh environments, whereas prompt germination after rains of e.g., Acacia seyal, is a strategy to avoid seed predation.Inorganic fertilisation is not an efficient means of improving short-term productivity in poor savanna soils. Nutrient-deficiency should be corrected by moderate grazing and controlled fires.Permanent and speedy vegetation recovery of degraded savannas such as in the Kondoa IrangiHills, may require (1) enhanced nitrogen mineralization and seed dispersal by grazing livestock, (2)selective soil seed bank enrichment, and (3) transplanting seedlings with desired traits.