Demographic implications of life-history stage characteristics in two African acacias at a Makeni savanna plot in Zambia

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Abstract

Aims

In spite of the importance of African acacias in vegetation succession and provision of goods and services, little is known about life-history variations within and among species. Much of the work done on African acacias has focused on seed predation and germination and seedling establishment, especially of Acacia tortilis, Acacianilotica and Acaciakarroo. The primary aim of the present work is to investigate differences in the demography of Acaciapolyacantha and Acaciasieberiana and the relationship between life-history characteristics and population size. A secondary objective is to assess how fire, an important ecological factor in savanna vegetation, might modify the growth and demographic dynamics of the two acacias.

Methods

The study was conducted at the Makeni savanna plot in central Zambia, southern Africa. Seedling emergence from both non-scarified and scarified seeds sown at different times in the wet season and the fate and growth of seedlings and saplings were monitored over a period of 4 years. Annual growth of permanently marked sample trees in annually burnt and fire-protected blocks was recorded over a 6-year period (2002–08) in order to assess inter-specific differences and how fire modifies tree growth patterns. Censuses of natural saplings and trees were conducted periodically in sample blocks to determine recruitment into these life-history stages.

Important findings

Seedling emergence and sapling survival rates were much higher in A. sieberiana than in A. polyacantha. However, both seedling and sapling growth rates were higher in A. polyacantha than in A. sieberiana but tree growth rates were similar in the two species. Under fire protection tree growth was significantly influenced by tree size and year while under annual burning only tree size significantly affected tree growth.

Important findings

The dominance of A. sieberiana over A. polyacantha at the study site was attributed to higher seedling emergence rate, higher sapling survival rate and a large sapling bank that forms a reliable source of tree recruitment. The life-history stage characteristics of A. polyacantha suggest that this is an early successional species.

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