Hierarchical control over seedling recruitment of the bunch-grass Themeda triandra in a semi-arid savanna.
The persistence of populations of the economically important bunchgrass Themeda triandra in rangeland depends on seedling recruitment. A 25-factorial field experiment in a single randomized block tested whether seedling recruitment of this grass in a semiarid savanna would be enhanced by increased availability of seed, moisture, irradiance or gaps, or by decreased interference from vegetation; and evaluated which of seedling emergence, growth or first-year survival was critical for seedling establishment. Availability of seed had an overwhelming influence on recruitment of T. triandra in a community where it was conspicuous but not dominant, increasing recruitment 37-fold despite dramatic effects of other treatments on the abiotic environment and sward structure. Provided seed was available, shading increased seedling recruitment 7-fold by enhancing both emergence and seedling survival, because it conserved available moisture. Supplementation of moisture further improved recruitment of seedlings growing under shading by increasing their survival and growth during the first growing season. In turn, larger size conferred a survival advantage during the following dry season. Interference from established vegetation generally had a small effect on seedling recruitment, but decreased survival through the growing season of seedlings under shading. Both emergence and survival influenced seedling establishment. There was a strong hierarchy amongst factors influencing seedling recruitment of T. triandra because of the magnitude of individual effects and the dependence of some factors on others. The overriding effects of seed and moisture availability suggest that recruitment of T. triandra may be vulnerable to heavy grazing, which would curtail seed input, during critical years of sufficient rainfall for recruitment.