Population ecology of an afro-tropical savanna herb, Lapeirousia rivularis, in Zambia

Detailed demographic studies of herbaceuos plants in afro-tropical savannas are extremely rare in published literature. I studied phenology and population dynamics of a perennial herb, Lapeirousia rivularis Wanntorp, at a savanna site in Zambia over a 4-year period, from 1997 to 2001, using enumeration techniques in permanent and temporary quadrants. The age of the plants was accurately determined from persistent annual sheaths that accumulate around the corm throughout the life of the plant while the estimated survivorship of the 1998 cohort was developed from a mathematical model based on the observed mortality of the cohort over a 3-year period. L. rivularis completed its annual phenological cycle in a single rainy season. Plants sprouted from perennating corms in early December, flowered, fruited and dispersed seeds by end of January. Seeds germinated immediately after dispersal and seedlings produced small corms before aerial parts died early in the dry season. L. rivularis has two morphs: a vegetative morph and a reproductive morph; the latter bears several leaves and flower stalks. In the vegetative morph, the corm is renewed annually while the corms of the reproductive morph did not appear to be renewed. Instead corms grew larger and produced lateral daughter corms that became independent ramets the following rainy season. Most L. rivularis plants reached reproductive maturity when they were 6-10 years old. Reproductive success, seedling establishment and recruitment varied from year to year in the grassland plot perhaps because of fluctuations in weather conditions and heavy episodic insect herbivory. The survivorship curve of L. rivularis was characteristically concave due to high juvenile mortality (0.2-0.5) and very low adult mortality (0.03). Although about 3% of the plants live to be up to 30-35 years, the mean age of the population in 2001 was 6.4 years because of the predominance of juvenile plants. The production of a protective sheath at the end of the rainy season and the accumulation of old sheaths around the fleshy corm are apparent adaptations against desiccation during the long dry season drought when the topsoil remains below wilting point. The high juvenile mortality during the dry season is probably caused by inadequate protection by the few sheaths around the corm against desiccation. Population dynamics in L. rivularis were caused by variable annual recruitment and high juvenile mortality. Population density doubled in 1999 due to good fruiting success in the previous season that was associated with good weather conditions and negligible herbivory. Although cultivation had a significant negative effect on the population of L. rivularis, it increased consistency in fruiting success and production of fruits per plant, presumably because of improvement in soil-moisture status, reduction in plant competition and by providing temporal escape from insect herbivory through delayed flowering. The phenology and life history of L. rivularis exhibit adaptations to a savanna environment that is characterized by disturbance and stress caused by seasonal drought, fire, episodic herbivory and cultivation.