David Draper & José María Iriondo Spatial patterns of vegetative and sexually reproducing individuals of Marsilea strigosa Willd. Abstract Draper, D. & Iriondo, M. J.: Spatial pattern of vegetative and sexually reproducing individuals of Marsilea strigosa Willd. — Bocconea 21: 000-000. 2007. — ISSN 1120-4060. We analyzed the spatial distribution of Marsilea strigosa, an aquatic fern, considering two life stages (vegetative and sexual) and interpreted results taking into account possible facilitation or competition effects among individuals as well as existing colonization strategies. Marsilea strigosa is distributed from Spain to the Black Sea. It is considered a rare species due its high habitat specificity and small fragmented populations. We surveyed 95% of the Balearic popu- lation of M. strigosa, covering a surface of approximately 1000 m2. Single life stage and bivari- ate distribution patterns were analyzed using second-order spatial analysis based on Ripley’s K- function. The relationship between vegetative and sexual node types is discussed. Aggregation of vegetative and sexual nodes was the only spatial point pattern found. The same results were obtained when both stages were analyzed together with bivariate function. We discuss the effect of life stage on the balance between competition and facilitation in dynamic environments. The importance of these two processes in colonization sites can best be understood by comparing them between different life stages. Introduction Marsilea strigosa is an amphibious rhizomatous geophyte (Boudrie 2004). It can pro- duce very slender runners (2-25 cm long) that bear leaves and roots at each node. The nodes have a high capacity of vegetative reproduction, while some also produce macro- scopic reproductive structures (3-5 mm) which can be easily identified during the dry sea- son. The roots need at least a thin layer of clay. This aquatic fern species is scattered from Spain to Crimea. Although it is widely dis- tributed, it presents high habitat specificity, and the population sizes are frequently low. As a result, M. strigosa is considered a rare species belonging to Category V of the Rabinowitz (1981) classification. Marsilea strigosa is a hydrophyte that requires a combi- nation of geophysical and weather conditions: small depressions in sites where a watershed appears during the rainy season followed by a dry season in which the pond dries up. This cyclic regime is probably one of the most characteristic features of the Mediterranean cli- mate. Marsilea strigosa has no soil type restrictions and tolerates dramatic water level