HIERARCHICAL CONTROL OVER SEEDLING RECRUITMENT OF THE BUNCH-GRASS THEMEDA-TRIANDRA IN A SEMIARID SAVANNA

1. The persistence of populations of the economically important bunchg rass Themeda triandra in rangeland depends on seedling recruitment. A 2(5)-factorial field experiment in a single randomized block tested wh ether seedling recruitment of this grass in a semi-arid savanna would be enhanced by increased availability of seed. moisture. irradiance or gaps. or by decreased interference from vegetation: and evaluated whi ch of seedling emergence. growth or first-year survival was critical f or seedling establishment. 2. Availability of seed had an overwhelming influence on recruitment of T. triandra in a community where it was c onspicuous but not dominant. increasing recruitment 37-fold despite dr amatic effects of other treatments on the abiotic environment and swar d structure. 3. Provided seed was available, shading increased seedlin g recruitment 7-fold by enhancing both emergence and seedling survival , because it conserved available moisture. Supplementation of moisture further improved recruitment of seedlings growling under shading by i ncreasing their survival and growth during the first growing season. I n turn. larger size conferred a survival advantage during the followin g 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 surv ival influenced seedling establishment. 4. There was a strong hierarch y amongst factors influencing seedling recruitment of T. triandra beca use of the magnitude of individual effects and the dependence of some factors on others. The overriding effects of seed and moisture availab ility suggest that recruitment of T. triandra may be vulnerable to hea vy grazing, which would curtail seed input, during critical years of s ufficient rainfall for recruitment.