Effects of Climate and Land-Use Change on the Establishment and Growth of Cembran Pine (Pinus cembra L.) over the Altitudinal Treeline Ecotone in the Central Swiss Alps

Tree growth is generally limited by temperature in cold climates and by water availability in arid zones. Establishment in altitudinal treeline ecotones depends on the temperature, but may be very sensitive to water availability as well. We studied the effect of climate and land use on the colonization and growth of Pinus cembra in the treeline ecotone of the dry Central Swiss Alps: one site was influenced by timber harvest and cattle activity and another one was undisturbed. Stands were sampled at three elevations: in the forest and the lower and upper parts of the treeline ecotone. The age structure was similar in all sites, ranging from unevenaged (forest) to more even-aged, with recent densification and upslope expansion of the treeline ecotone. However, recruitment started at the treeline around 1850 (the end of the Little Ice Age) in the undisturbed site, simultaneously with an increase of tree-ring growth, but about 60 years later at the other site, after cattle grazing decreased. These results, and the positive correlation of radial growth with summer and previous autumn temperatures indicated that, in this altitudinal treeline ecotone, growth and establishment are mainly linked to temperature. However, drought stress was visible in the lowest stands, with a positive correlation of growth with rainfall during the previous autumn and December, and in August of the growing season. This could limit growth in a future warmer climate. Introduction Since the 19 century, an altitudinal and latitudinal expansion of trees has been observed in many altitudinal or boreal treeline ecotones (e.g. Brink, 1959; Kullman, 1986; Taylor, 1995; Shiyatov, 2003; Camarero and Gutiérrez, 2004). This is generally attributed to the warmer conditions which followed the end of the Little Ice Age. However, such forest expansion was not observed everywhere, and tree recruitment is sometimes mainly a densification of the altitudinal treeline ecotone (Sceicz and MacDonald, 1995; Camarero and Gutiérrez, 1999; Mazepa, 2005). The low dispersal capacity of the species explains locally the absence of expansion (Mazepa, 2005), but microclimate conditions are often important. Shrubs, krummholz or adult trees keep a higher soil moisture, protect against wind abrasion and modify snow distribution, all conditions which may improve (nurse effects) seedling survival in the first years and favour establishment (e.g., Szeicz and MacDonald, 1995; Camarero and Gutiérrez, 1999; Holtmeier and Broll, 2005). Moreover, infrequent disturbances, like avalanches, fire, extreme snow cover or drought, may kill newly established seedlings or trees above the treeline ecotone and maintain a long-term stable position of the treeline (Noble, 1993).