Positive associations between the cushion plant Azorella monantha (Apiaceae) and alpine plant species in the Chilean Patagonian Andes M.T.K. Arroyo 1 , L.A. Cavieres 2, *, A. Peñaloza 3 and M.A. Arroyo-Kalin 4 1 Laboratorio de Sistemática y Ecología Vegetal, Facultad de Ciencias, Universidad de Chile, Chile; 2 Departamento de Botánica, Facultad de Ciencias Naturales y Oceanográficas, Universidad de Concepción, Chile; 3 Centro de Ecología Aplicada, Santiago, Chile; 4 Department of Archaeology, University of Cambridge, Cambridge, CB2 3DZ, UK; *Author for correspondence (e-mail: lcaviere@udec.cl) Received 26 January 2001; accepted in revised form 27 May 2002 Key words: Alpine ecology, Andes, Cushion plants, Positive associations Abstract Low growing, compact cushion plants are a common and often dominant life form in temperate and subpolar alpine habitats. The cushion life-form can modify wind patterns, temperature and water availability and thus cushion species could be expected to act as nurse-plants facilitating the establishment of other alpine plant spe- cies on their surfaces. It has been suggested that the nurse effect should be most pronounced under more stressful environmental conditions, as found with increasing elevation in the alpine. One of the approaches used to detect the nurses has been the study of spatial associations among species, in which extreme clumping within or be- neath one species has been interpreted as evidence of nursing. We characterized microclimatic conditions (soil and air temperature) within and outside cushions of Azorella monantha at two elevations (700 m a.s.l., corre- sponding to an elevation just above treeline, and 900 m a.s.l., corresponding to the upper limit of the cushion belt zone) on Cerro Diente in the Patagonian alpine of southern South America (50° S) and recorded all plant species growing upon cushions of various sizes and for paired sampling areas of equivalent sizes outside cushions. At 5 cm depth, soil temperature was slightly higher under cushions than under bare ground, but only significantly so at 900 m. Air temperature at ground level was significantly higher in the cushion microhabitat at both 700 m and 900 m, with the difference being more exaggerated at the highest elevation. At 700 m, a total of 27 species were recorded growing within cushions as compared to 29 outside cushions. At 900 m the corresponding numbers were 34 and 18. At the highest elevation, significantly more species grow within cushions than for equal areas outside cushions. Here moreover, 17 (48.6%) species grew preferentially within cushions, with eight of the latter being limited to the cushion microhabitat at this elevation. However, at 700 m there was no significant difference in species richness in the two microhabitats, and only one species (3.1%) grew preferentially on cushions. Con- sidering individual species, nine occurring at both elevations showed non-preferential recruitment on cushions at 700 m, but significantly higher frequencies on cushions at 900 m. Results suggest striking altitudinal variation in the association with Azorella monantha on Cerro Diente, ranging from a very strong at 900 m to near absence at 700 m. Milder air and soil temperatures, shelter from wind, and greater water availability within cushions as opposed to outside cushions are discussed as possible factors favoring strong plant recruitment on cushions at higher elevations in the harsh Patagonian alpine environment. Introduction Nurse plants are those that facilitate the establishment of other plant species beneath their canopy, because they offer microhabitats that are more favorable for seed germination and/or seedling recruitment than their surrounding environment (Franco and Nobel 1988). The nurse effect can act through protection against seed predators or herbivores, protection against extreme temperatures, accumulation of or- 121 Plant Ecology 169: 121–129, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.