Effects of microsite on growth of Pinus cembra in the subalpine zone of the Austrian Alps

319 Ann. For. Sci. 61 (2004) 319–325 © INRA, EDP Sciences, 2004 DOI: 10.1051/forest:2004025 Original article Effects of microsite on growth of Pinus cembra in the subalpine zone of the Austrian Alps Mai-He LIa,b*, Jian YANGc a Institute for Forest and Mountain Risk Engineering, Vienna University of Agricultural Sciences, Peter Jordan-Strasse 82, 1190 Vienna, Austria b Current address: WSL, Swiss Federal Institute for Forest, Snow and Landscape Research, Zuercherstrasse 111, 8903 Birmensdorf, Switzerland c Applied Environmental Geoscience, University of Tübingen, Sigwart Strasse 10, 72076 Tübingen, Germany (Received 7 March 2003; accepted 20 August 2003) Abstract – We examined growth in Pinus cembra L. (28 years old) across the treeline ecotone from 1900 to 2100 m elevation in the Alps. Eighteen plots were chosen at different microsites which are defined as a combination of elevation and steepness (gentle vs. steep slope) on a south-facing slope in the Schmirn Valley, Tyrol/Austria. Over the range of altitudes studied, elevation and steepness had influences on growth depending on tree size: (1) Elevation and steepness had little effect on growth as long as trees were very small (< 0.5 m in height); (2) Both elevation and steepness affected tree growth significantly when the tree height was between 0.5 and 3 m; (3) As trees exceeded 3 m in height, tree canopies fully covered the ground surface and created a forest microclimate causing growth to decline with increasing elevation, irrespective of steepness. We conclude that the microsite related to microclimate, controls growth during the early life stages of trees, but following canopy closure the local climate (mesoclimate) associated with topography begins to determine tree growth. growth responses / high altitude / micro-environmental conditions / tree ecology / treeline ecotone Résumé – Effets de la microstation sur la croissance de Pinus cembra dans la zone subalpine des Alpes autrichiennes. Nous avons examiné la croissance de Pinus cembra L. (28 ans) dans l’écotone de la limite forestière entre 1900 et 2100 m d’altitude dans les Alpes. Dixhuit placettes ont été choisies dans différentes microstations définies selon l’altitude et la déclivité du terrain sur une pente exposée au sud dans la vallée de Schmirn, dans le Tyrol autrichien. Dans toute la zone étudiée, l’altitude et la déclivité ont exercé une influence qui dépendait de la taille de l’arbre: (1) elles avaient peu d’effet sur la croissance des arbres de très petite taille (< 0.5 m de haut); (2) elles avaient un effet significatif sur les arbres d’une hauteur entre 0,5 et 3 m; (3) à partir de 3 m de haut, la canopée couvrait complètement la surface du sol et créait ainsi un microclimat forestier qui entraîne un ralentissement de la croissance avec l’augmentation de l’altitude, indépendamment de la déclivité. Nous en concluons que la microstation liée au microclimat détermine la croissance des arbres durant leur jeune âge, mais après la fermeture de la canopée, le climat local (mésoclimat) associé à la topographie commence à influencer la croissance des arbres. réactions à la croissance / altitude / conditions microenvironnementales / écologie des arbres / écotone de la limite forestière 1. INTRODUCTION Cembran pine (Pinus cembra L.) is an important species of forests in the subalpine zone of the Alps, where forests have been depressed from the natural climatic treeline by land use over several centuries [22, 24, 36]. As a consequence avalanche risk has enhanced. Hence, programs of forest restoration have been initiated in the Alps several decades ago, to prevent and avoid such damages. The objective of this study addressed to a better understanding of tree growth in this area. The slower growth rate of subalpine trees is a documented phenomenon in forestry literature. Many authors have given a common description of decreasing growth of subalpine trees with increasing elevation (e.g. [5, 15, 16, 18, 19, 28, 37]). In * Corresponding author: the Swiss and Austrian Alps, the reduction of tree height with increasing elevation was site-specific and varied between 2 and 17 m per 100 m [31]. At elevations between 1700 and 1900 m in the Sellrain Valley (47° 13’ N, 11° 06’ E) in Tyrol, Austria, annual height growth of Pinus cembra L. decreased with increasing elevation by approximately 5 to 6% per 100 m, corresponding to the decrease in length of the growing season [19]. Paulsen et al. [31] found that annual radial increments of Pinus cembra linearly decreased with increasing elevation in the first part of the 19th century, and after 1940, average tree-ring width within the subalpine zone was similar, irrespective of the elevation. Rolland et al. [34] also reported that macroclimate change induced an increase in radial growth of four coniferous species (Picea abies (L.) Karst., Larix decidua Mill., Pinus 320 M.-H. Li, J. Yang cembra L. and Pinus uncinata Mill. ex Mirb.) growing in the French Alps near the upper treeline since 1750. Innes [13] related the worldwide increase in radial growth of subalpine trees to the macroclimate change since 1850. However, to our knowledge no studies have investigated the effects of microsite related to microclimate on tree growth within the subalpine zone. Indeed, tree life/growth is strongly controlled by the micro-environmental conditions at and near the ground surface (e.g. microclimate) at high altitudes [3]. On the other hand, elevation, slope angle and aspect have a strong influence on radiation, temperature, evaporation, wind speed and snow accumulation (e.g. [2, 3]), as well as on soil erosion and transport, local water balance, etc. In other words, elevation and steepness (depression is not studied, see [23]) seem to be a substitute for the complexity of local environmental elements on a given aspect. Hence, our microsites were a combination of elevation and steepness. Therefore, we examined tree growth responses to microsite, across a 200 m transect in the subalpine zone of the Austrian Alps (in the summer of 1997), to answer: (1) Whether increasing elevation similarly affects tree growth in height, diameter and biomass; (2) Whether different microsites affect tree growth significantly; and (3) Whether tree responses to microsite change with tree size (age)? 2. MATERIALS AND METHODS 2.1. Site location and description The study forests were located in the Schmirn Valley (11° 30’ E, 47° 07’ N) in Tyrol, Austria, and extended from an elevation of 1900 to 2100 m on a south-facing slope. The uppermost native adult trees (larch, spruce) in this area are found at ca. 2000 m elevation. The original larch-spruce forest was heavily exploited between the 12th and 19th century [36]. Before the afforestation, the sites were used historically for grazing (H. Aulitzky, 1997, personal communication). The characteristic vegetation at the onset of the afforestation consisted of Rhododendretum ferrugineum L., Vaccinieta and Callunet (...truncated)