Spatial pattern analysis based on Ripley's K-function is a second-order analysis of point patterns in a twodimensional space. The method is increasingly used in studies of spatial distribution patterns of plant communities, but the statistical methods involved are sometimes poorly understood or have been modified without evaluating the effects on results. The procedures of field data acquisition, statistical analysis, and the test for the null hypothesis of complete spatial randomness are described and the presentation of results is discussed. Different methods of edge correction were tested on a computer-generated random pattern and a mapped distribution of a Mediterranean shrubland. The inclusion of buffer zones around mapped plots describes the spatial pattern most accurately, but may not warrant the additional labour involved. Three variations of the weighted edge correction yielded comparable results for the distribution patterns tested. The toroidal edge correction may give biased results for nonrandom patterns. Recommendations for standardisation of the statistical procedures and data presentation are given.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract. A major role of facilitation between higher plant species, particularly in stressful environments, has recently been reported in several papers that suggest that beneficial interactions could be more important in the dynamics of plant communities than has been recognized to date. In a semiarid region in southeastern Spain we determined the effect of a leguminous shrub, Retama sphaerocarpa, on its environment, testing the hypothesis that facilitation by the shrub of one species of its understory, Marrubium vulgare, is reciprocal and that the shrub benefits from sheltering herbs beneath its canopy.Marrubium plants under Retama had greater specific leaf area, leaf mass, shoot mass, leaf area, more flowers, a higher nitrogen (N) concentration in leaf tissue, and more N per plant than isolated plants, suggesting a facilitative effect of Retama on Marrubium including increased availability of resources. Biomass of 1-yr-old cladodes, total biomass, total nitrogen content of 3-yr-old branches, and shoot water potential at midday were higher for Retama shrubs with Marrubium beneath them than in shrubs without Marrubium.Retama strongly improved its own environment, facilitated the growth of Marrubium and other species underneath its canopy, and at the same time obtained benefits from sheltering herbs underneath. The interaction between these two species was indirect, associated with differences in soil properties and with improved nutrient availability under shrubs compared with plants growing on their own.We propose that the mutual benefit of the association between Retama and Marrubium is best termed a facultative mutualism. Each partner benefits from greater availability of resources in the "island of fertility" that results from their association. If beneficial plantplant interactions as described here are widespread, positive relationships may have a major role in determining the pattern and structure of plant communities.

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