Consequences of a severe drought on spatial patterns of woody plants in a two-phase mosaic steppe of Stipa tenacissima L.

The establishment and development of the plants of S. tenacissima L. organize a source–sink pattern of water and sediments from bare soil areas to tussocks on mountain slopes forming small terraces or terracettes upslope of tussocks. We hypothesized that terracettes enhance woody plant survival compared with woody plants on bare soil. We examined this hypothesis at three locations in south-eastern Spain after a severe drought from 1993 to 1995. The spatial association of individual plants and the pattern of plant mortality support the hypothesis that the terracettes formed byS. tenacissima tussocks facilitate woody plants. The prediction that facilitation dominates interference during stress periods such as drought was supported by mortality patterns. Drought effects alone do not explain spatial patterns of woody plants in the two-phase mosaic steppe of Stipa tenacissima.