Effects of climatic variability on facilitation of tree establishment in northern Patagonia

Facilitation of tree establishment by nurse shrubs, which ameliorate otherw ise unfavorable microenvironmental conditions, is a widely studied phenomen on. However. relatively little is known about how facilitative influences c hange in relation to interannual climatic variability. In northern Patagoni a, Argentina, we examined influences of potential nurse shrubs on the estab lishment of the conifer Austrocedrus chilensis and assessed the significanc e of those influences to establishment during years of contrasting climate. We also experimentally investigated the effects of nurse shrubs and differ ent water availability on tree seedling emergence and survival. A strong spatial association of Austrocedrus juveniles with shrubs, both be neath shrub canopies and near shrub canopies, indicates that shrubs favorab ly influence tree regeneration and that in some habitats and time periods n urse plants appear to be required for successful tree seedling establishmen t. Protection from direct sunlight was the main factor contributed by shrub s that enhanced the germination and survival of Austrocedrus. During the 19 95-1996 experiment, no seedlings survived in the unwatered interspaces betw een shrubs, whereas maximal survival was obtained by watering seedlings at shaded sites. The results of this study indicate that in the Patagonian ecotone the Stren gth of facilitative associations between shrubs and Austrocedrus juveniles closely tracks annual climatic variability. During extremely warm dry years , recruitment of Austrocedrus is nil with or without protection by nurse sh rubs. During cool wet years, establishment may occur both beneath shrubs an d in open interspaces: however, during average years, which are still years with substantial drought stress, establishment of Austrocedrus appears to require nurse shrubs.