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

KITZBERGER, T.; STEINAKER, D.F.; VEBLEN, T.T.

ECOLOGY

ECOLOGICAL SOC AMER

Año: 2000 vol. 81 p. 1914 - 1924

0012-9658

Abstract: Facilitation of tree establishment by nurse shrubs, which ameliorate otherwise unfavorable microenvironmental conditions, is a widely studied phenomenon. However. relatively little is known about how facilitative influences change in relation to interannual climatic variability. In northern Patagonia, Argentina, we examined influences of potential nurse shrubs on the establishment of the conifer Austrocedrus chilensis and assessed 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, whereas 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 variability. 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.