Why Perennial Grass Species Disappear and Annual Grass Species Become Abundant in Soil Seed Banks of Semi-Arid Savannas: A Seed Burial Experiment

Background Buried seeds can play a pivotal role in the restoration of degraded ecosystems, where plant species have disappeared from the established vegetation but survive in the soil seed banks. Some studies suggested that the return of aboveground vegetation disappeared due to heavy grazing can be facilitated by the soil seed bank, and trigger the vegetation recovery since the soil seed bank serves as a reserve to return to the original vegetation. However, semi-arid savannas did not recover from heavy grazing, even after many years of grazing exclosures that might be the depletion of seeds due to germination and mortality in the soil, and/or differences in longevity and germination of seeds between species in the soil seed banks. Therefore, we studied the fate of grass seeds buried in the soil within mesh bags over time, from June to October 2019, in two locations, in a semi-arid Ethiopian savanna. Results There were significant differences in germination, viability and mortality rates of seeds among grass species buried in the soil. Perennial grasses lost their longevity due to germination (46%) and mortality (28%), whereas annual grasses had lower germination (24.3%) and mortality rates (13.7%) while buried. Moreover, perennial grass seeds had a lower germination rate at the beginning of the burial time, peaked at the middle of the study period and declined thereafter, whereas annual grass showed an increasing trend in germination until the end of the burial experiment. The seed viability of perennial grasses was higher at the beginning of the burial experiment, but rapidly declined thereafter, whereas annual grass species maintained their seed viability at the beginning and remained stable over time. The proportion of dead seeds in all grass species increased with increasing burial time. Due to the lower germination, higher viability, and lower mortality rates, annual grasses had a longevity of 62%, considerably larger than the 28% for perennials. Conclusions We concluded that seeds of perennial grasses can rapidly disappear in the soil, whereas annual grasses form persistent soil seed banks in semi-arid savannas. Hence, conservation and better management of aboveground vegetation in semi-arid savannas are suggested as recovery of perennial grass species might be unlikely due to poor soil seed bank formation coupled with overgrazing and trampling by large herbivores under the changing climate and global warming.