Experimental investigation of the origin of fynbos plant community structure after fire

In an ongoing investigation of the relationship between plant communities and hydrology in fynbos habitat in the Cape of South Africa (www.open.ac.uk/fynbos)., we are testing the hypothesis that plant species in fynbos communities segregate on fine-scale hydrological gradients. We have already shown that this occurs in meadow plant communities in England (Silvertown et al. 1999. Nature 400: 61-63) and have now demonstrated the same phenomenon also occurs in fynbos. This is an important discovery because it shows that quite different plant communities (with almost no plant families in common) are structured by the same fundamental mechanism. It is also surprising because fynbos habitats are fire-driven ecosystems that burn and regenerate naturally after fire and one might expect this disturbance to over-ride any fine-scale hydrological influence. Fires occur in the S Hemisphere summer (Nov - Apr) with a periodicity of 15 - 50y, but are unpredictable. A fire has just occurred at one of our field sites and we wish to take advantage of this unpredicted event to set-up a long-term experiment to test two hypotheses: A. Within-plot hydrological differences influence the structure of communities by controlling seedling establishment after fire, B. Seedling response to hydrology is influenced by root competition. The hypotheses will be tested by extracting soil monoliths (blocks of soil) with intact soil seed banks from different locations along the known hydrological gradient and transferring these to wetter and drier locations within the site. Control monoliths will be dug up and replaced in their original locations. A root barrier treatment (with controls) will be used in the experiment to determine the effect of root competition on plant performance along the soil moisture gradient. If hypothesis A is correct, then seedling mortality ought to be greater in translocated monoliths than controls and the ultimate species composition of monoliths should be influenced by their post-fire location on the hydrological gradient. Hypothesis B will be supported if competition treatments alter the effect of hydrology on distribution. Results will be incorporated into a spatial model that will be used to determine whether the effects measured in the experiment are sufficient to generate the patterns of species distribution observed late in post-fire succession. The sensitivity of the model outputs to a range of patterns of seed dispersal and changes in hydrology will be tested.