Biomass and population structure of a geoxyle, Lannea edulis (Sond.) Engl., at a savanna woodland site in Zambia

Abstract

Geoxyles are plants with massive underground and diminutive aboveground parts and in sub-Saharan Africa this life form is most diverse in south-central Africa. However, little is known about the population ecology of these plants, especially in south-central Africa. The purpose of this research was to investigate the demography and biomass structure of Lannea edulis, a widespread geoxyle in south-central Africa, using census, phenology and biomass data obtained over a 20-year period at a site in central Zambia, southern Africa. Mast fruiting in L. edulis is rare and seed germination rate and seedling mortality vary from year to year, resulting in episodic recruitment of juveniles into the population. Juvenile plants have a tuberous fleshy root that is transformed into a thick-barked woody taproot in adults. Frequent dieback of aerial shoots due to fire and herbivory induces the formation and growth of subsurface stems. The average oven-dry biomass of a L. edulis plant was 370 g of which shoots and roots represented 10% and 90%, respectively, thereby confirming that this geoxyle has indeed massive belowground parts. The demography of L. edulis is greatly influenced by variable annual recruitment of juveniles into the population and large individuals were estimated to live for more than 60 years.