Seed viability of Afromontane tree species in forest soils

Abstract

The fate of seeds of eight tree species was followed during 4 y of storage in the soil of an Afromontane forest at Gara Ades in the eastern highlands of Ethiopia. Seeds were enclosed in nylon mesh bags and buried at 5 cm soil depth. The bags were exhumed at intervals and the viability of the seeds was assessed by germination and cutting tests. Seeds ofBersama abyssinicaandEkebergia capensisgerminated in the soil almost completely within a year after burial. The seeds ofJuniperus procera, Olea europaeaandPodocarpus falcatusalso germinated to a substantial degree in the soil but with a distribution over several years, and some seeds of these species remained viable at the end of the 4-y period. Germination in the soil was very low in seeds ofAcacia abyssinicaandCroton macrostachyusthroughout the whole burial period and the seeds kept their viability. InC. macrostachyusfresh seeds were highly dormant, but after 3 y or more in the soil they germinated readily in the laboratory suggesting an altered dormancy with time in the soil. Dormancy in seeds ofA. abyssinicaandIndigofera rothiiwas not altered throughout the study period as evidenced by marginal or no germination during incubation in the laboratory. The differential seed behaviour observed during storage in the soil can be an indicator of the regeneration strategy of the species studied.B. abyssinica, E. capensis, J. procera, O. europaeaandP. falcatusform seedling banks on the forest floor and lack persistent soil seed reserves in contrast toA. abyssinica, C. macrostachyusandI. rothiiwhich accumulate reserves of long-lived seeds in the soil. The generally high levels of dormancy and somewhat extended viability in the soil, even in several of the species producing seedlings in undisturbed forest, may have been selected for under a climate of seasonal drought and unreliable rainfall that characterizes this region.