Growth responses of Picea abies and Larix decidua to elevation in subalpine areas of Tyrol, Austria Mai He Li, Jian Yang, and Norbert Kräuchi Abstract: Microsites related to microenvironmental conditions, including microclimate, seem to be a key factor for the restoration of forests in the subalpine area. Tree growth was studied in Picea abies (L.) Karst. (Norway spruce) and Larix decidua Mill. (European larch) on 30 plots located at different microsites (i.e., different elevations and microtop- ographies combined) within the subalpine zone (1680–1940 m) of the Schmirn Valley (Tyrol, Austria). The age of the trees studied was 27 years for larch and 28 years for spruce. The mean height and biomass growth decreased signifi- cantly with increasing elevation. The effect of elevation and microtopography on growth varied with tree size (age): (1) elevation had little effect on growth of trees less than 0.5 m in height; (2) both elevation and microtopography af- fected tree growth significantly when the tree height was between 0.5 and 3 m; (3) as trees exceed 3 m in height, tree canopies can fully cover the ground surface and create a forest microclimate causing growth to decline with increasing elevation, irrespective of microtopography. We conclude that the microclimate, associated with microsite, controls growth during the early stages of tree development, but following canopy closure, the local climate (mesoclimate) asso- ciated with topography begins to determine tree growth. Résumé : Les microsites ayant un rapport avec les conditions micro-environnementales, incluant le microclimat, sem- blent être un facteur-clef de la restauration des forêts dans la région subalpine. La croissance de Picea abies (L.) Karst. (épicéa commun) et de Larix decidua Mill. (mélèze d’Europe) a été étudiée dans 30 placettes situées dans différents microsites (c.-à-d. une combinaison d’altitude et de microtopographie différentes) dans la zone subalpine (1680– 1940 m) de la vallée de Schmirn (Tyrol, Autriche). L’âge des arbres étudiés était de 27 ans pour le mélèze et de 28 ans pour l’épicéa. La hauteur moyenne et la croissance en biomasse diminuait de façon significative avec l’altitude. L’effet de l’altitude et de la microtopographie sur la croissance variait avec la taille de l’arbre (l’âge) : (1) l’altitude avait peu d’effet sur la croissance des arbres hauts de moins de 0,5 m; (2) à la fois l’altitude et la microtopographie af- fectaient la croissance de façon significative lorsque la hauteur des arbres se situait entre 0,5 et 3 m; (3) lorsque les ar- bres dépassaient 3 m de hauteur, le couvert des arbres pouvait recouvrir entièrement la surface du sol et créer un microclimat qui ralentissait la croissance à mesure qu’augmentait l’altitude, indépendamment de la microtopographie. Nous concluons que le microclimat, associé au microsite, contrôle la croissance pendant les stades précoces du déve- loppement de l’arbre mais que, suite à la fermeture du couvert, le climat local (le mésoclimat) associé à la topographie commence à déterminer la croissance des arbres. [Traduit par la Rédaction] Li et al. 662 Introduction Mountain peaks covered in ice and rocks, steep slopes, wild torrents, and fragile soil and vegetation cover character- ize the topography of the Central Alps, where the environ- mental risks are high and natural disasters (e.g., flood, erosion, rock and land slides, avalanches etc.) are frequent. The subalpine vegetation belt is a very important buffer zone limiting potential threats in mountainous areas, and un- disturbed subalpine forests can efficiently reduce the risks of natural disasters (Hamilton et al. 1997). However, this vege- tation belt is also a sensitive area, and the upper limits of subalpine forests are lower than the natural climatic tree line as a result of land use over several centuries in the Alps (Stern 1966; Mayer and Ott 1991; Li 1999). As a conse- quence, the risks of avalanches and other natural damages are enhanced. For decades, foresters have attempted to re- store the forests in this area. After several disastrous ava- lanches in the Schmirn Valley in 1951 and 1968, a forest restoration project was initiated in the subalpine zone of this valley in 1969. The goals of the project were to develop ade- quate methods for a successful establishment of forest stands on steep, warm aspects to minimize the risk of damaging av- alanches. This investigation was initiated in 1997 to deter- mine the progress and success of this afforestation. The zone between the closed upper montane forest and the uppermost limits of the distribution of small individuals Can. J. For. Res. 33: 653–662 (2003) doi: 10.1139/X02-202 © 2003 NRC Canada 653 Received 15 February 2002. Accepted 24 October 2002. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on 20 March 2003. M.H. Li 1,2 and N. Kräuchi. Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland. J. Yang. Applied Environmental Geoscience, University of Tübingen, Sigwart Strasse 10, D-72076 Tübingen, Germany. 1 Corresponding author (e-mail: maihe.li@wsl.ch). 2 Present address: Institute for Forest and Mountain Risk Engineering, Vienna University of Agricultural Sciences, Peter Jordan-Strasse 82, A-1190 Vienna, Austria.