Association between competition and facilitation processes and vegetation spatial patterns in alpha steppes

In semiarid ecosystems, the self-organized spatial patterns of plants associated with catastrophic shifts can emerge from a variety of processes. In this study, on moderate slopes where Stipa tenasissima cover was high, the self-organization of some of the typical species of semiarid Mediterranean matorral (Phlomis purpurea, Sideritis oxteosylla, Helianthemum almeriense, and Brachypodium retusum) was negatively correlated with Stipa cover. The extent of Stipa cover did not affect desert pioneer species, such as Artemisia herba-alba, Fagonia cretica, and Launaea lanifera. On pronounced slopes, the self-organizing structure of brushwood vegetation did not vary predictably with the amount of Stipa cover. We examined the competition/facilitation processes associated with self-organizing patterns in the dwarf shrub (Phl. purpurea) and the half shrub (H. almeriense). The developmental stability of H. almeriense was positively correlated with Stipa cover, which was expected because they are associated species in this seral thyme brushwood community. Indeed, facilitation processes were manifested by the developmental stability increases under the Stipa canopy, particularly on high slope areas, where Stipa is less competitive. In Phl. purpurea, negative feedback processes from competition with Stipa were manifested where Stipa cover was high and on low slopes (developmental instability increased). In general, competition with Stipa on low slopes tended to decrease plant self-organization. (c) 2006 The Linnean Society of London.