1914 Ecology, 81(7), 2000, pp. 1914–1924  2000 by the Ecological Society of America EFFECTS OF CLIMATIC VARIABILITY ON FACILITATION OF TREE ESTABLISHMENT IN NORTHERN PATAGONIA THOMAS KITZBERGER, 1,4 DIEGO F. STEINAKER, 2 AND THOMAS T. VEBLEN 3 1 Departamento de Ecologı ´a, Universidad Nacional del Comahue, Quintral 1250, 8400 Bariloche, Argentina 2 Estacio ´n Experimental Agropecuaria San Luis, Instituto Nacional de Tecnologı ´a Agropecuaria, Casilla de Correo 17, 5730 Villa Mercedes, San Luis, Argentina 3 Department of Geography, University of Colorado, Boulder, Colorado 80309 USA Abstract. Facilitation of tree establishment by nurse shrubs, which ameliorate other- wise unfavorable microenvironmental conditions, is a widely studied phenomenon. How- ever, relatively little is known about how facilitative influences change in relation to in- terannual climatic variability. In northern Patagonia, Argentina, we examined influences of potential nurse shrubs on the establishment of the conifer Austrocedrus chilensis and as- sessed the significance of those influences to establishment during years of contrasting climate. We also experimentally investigated the effects of nurse shrubs and different water availability on tree seedling emergence and survival. A strong spatial association of Austrocedrus juveniles with shrubs, both beneath shrub canopies and near shrub canopies, indicates that shrubs favorably influence tree regeneration and that in some habitats and time periods nurse plants appear to be required for successful tree seedling establishment. Protection from direct sunlight was the main factor contributed by shrubs that enhanced the germination and survival of Austrocedrus. During the 1995– 1996 experiment, no seedlings survived in the unwatered interspaces between shrubs, where- as maximal survival was obtained by watering seedlings at shaded sites. The results of this study indicate that in the Patagonian ecotone the strength of facilitative associations between shrubs and Austrocedrus juveniles closely tracks annual climatic var- iability. During extremely warm dry years, recruitment of Austrocedrus is nil with or without protection by nurse shrubs. During cool wet years, establishment may occur both beneath shrubs and in open interspaces; however, during average years, which are still years with substantial drought stress, establishment of Austrocedrus appears to require nurse shrubs. Key words: Argentina; Austrocedrus chilensis; climatic variability; dendroecology; ecotones; experimental microsites; facilitation; nurse shrubs; Patagonia; recruitment; seedling survivorship. INTRODUCTION In mesic temperate forests, regeneration dynamics are often studied under favorable site conditions where competition is likely to be the dominant biotic inter- action, and where interannual climatic variation may be relatively unimportant in determining regeneration success in comparison to disturbance and biotic influ- ences (e.g., White 1979, Oliver 1981, Whitmore 1982). However, under conditions of greater abiotic stress, such as at sites near ecotones, establishment of plant species is more strongly related to positive plant in- teractions (Bertness and Callaway 1994) and climatic fluctuation (e.g., Brubaker 1986, Kullman 1993). De- spite the recognition of the importance of nurse plants and climatic variation to tree establishment in many semiarid environments, the interactions between these two determinants of regeneration success have received little research attention. Facilitation, or positive interactions among plants, has long been recognized as a factor shaping plant com- Manuscript received 17 August 1998; revised 7 May 1999; accepted 15 June 1999; final version received 6 July 1999. 4 E-mail: tkitzber@crub.uncoma.edu.ar munities and has recently received renewed attention (Bertness and Callaway 1994, Callaway 1995, Kareiva and Bertness 1997). Plants may facilitate other plants directly by increasing availability of a key resource, improving substrate conditions, or ameliorating some unfavorable environmental characteristic; indirect fa- cilitative interactions include introduction of beneficial organisms (e.g., pollinators, mycorrhizae, or soil mi- crobes) or protection from herbivores (Callaway 1995). There may be an interplay between facilitative and competitive mechanisms between two plants. Net fa- cilitation occurs when the improvement of a key re- source under the canopy exceeds the combined cost of the deterioration of other factors (Holmgren et al. 1997). Changes in the relative costs, which result in switches from interference to facilitation, have been detected along gradients of abiotic stress (Hillier 1990) and microgradients around shrubs (Aguiar and Sala 1994), as well as by experimentally manipulating shade (e.g., Smith and Shackleton 1988, Kellman and Kading 1992), water (de Jong and Klinkhammer 1988), or com- petitors (Aguiar et al. 1992, Aguiar and Sala 1994). Such relative changes have also been associated with climatic fluctuation (de Jong and Klinkhammer 1988,