Effects of atmospheric and climate change at the timberline of the Central European Alps

Ann. For. Sci. 66 (2009) 402 c INRA, EDP Sciences, 2009  DOI: 10.1051/forest/2009023 Available online at: www.afs-journal.org Review article Effects of atmospheric and climate change at the timberline of the Central European Alps Gerhard Wieser1 *, Rainer Matyssek2, Roland Luzian1 , Peter Zwerger1 , Peter Pindur3 , Walter Oberhuber4 , Andreas Gruber4 1 Dept. Alpine Timberline Ecophysiology, Federal Research and Training Centre for Forests, Natural Hazards and Landscape, Rennweg 1, 6020 Innsbruck, Austria 2 Dept. of Ecology/Ecophysiology of Plants, Technische Universität München/Weihenstephan, Am Hochanger 13, 85354 Freising, Germany 3 Institut für Stadt- und Regionalforschung, Österreichische Akademie der Wissenschaften, Postgasse 7, 1010 Wien, Austria 4 Institute of Botany, University of Innsbruck, Sternwartestraße 15, 6020 Innsbruck, Austria (Received 21 July 2008; accepted 19 December 2008) Keywords: Alpine timberline / treeline / global warming / CO2 / ozone / water balance / carbon gain Abstract • This review considers potential effects of atmospheric change and climate warming within the timberline ecotone of the Central European Alps. After focusing on the impacts of ozone (O3 ) and rising atmospheric CO2 concentration, effects of climate warming on the carbon and water balance of timberline trees and forests will be outlined towards conclusions about changes in tree growth and treeline dynamics. • Presently, ambient ground-level O3 concentrations do not exert crucial stress on adult conifers at the timberline of the Central European Alps. In response to elevated atmospheric CO2 Larix decidua showed growth increase, whereas no such response was found in Pinus uncinata. Overall climate warming appears as the factor responsible for the observed growth stimulation of timberline trees. • Increased seedling re-establishment in the Central European Alps however, resulted from invasion into potential habitats rather than upward migration due to climate change, although seedlings will only reach tree size upon successful coupling with the atmosphere and thus loosing the beneficial microclimate of low stature vegetation. • In conclusion, future climate extremes are more likely than the gradual temperature increase to control treeline dynamics in the Central European Alps. Mots-clés : limite supérieure alpine de la forêt / limite de croissance des arbres / réchauffement global / CO2 / ozone / bilan hydrique / gain de carbone Résumé – Effets des changements atmosphériques et des changements climatiques à la limite supérieure de la forêt en Europe dans les Alpes centrales. • Cette étude examine les effets potentiels des changements atmosphériques et du réchauffement climatique au sein de l’écotone que constitue la limite supérieure de la forêt dans les Alpes centrales en Europe. Après avoir mis l’accent sur les effets de l’ozone (O3 ) et de l’augmentation des concentrations atmosphériques du CO2 , les effets du réchauffement climatique sur le bilan de carbone et le bilan hydrique des arbres et des forêts à la limite supérieure de la forêt seront présentés en vue de tirer des conclusions sur l’évolution de la croissance des arbres et sur les dynamiques de la limite supérieure de la forêt. • Actuellement, les concentrations en O3 de l’air ambiant au niveau du sol n’exercent pas un stress critique sur les arbres adultes à la limite supérieure de la forêt dans les Alpes centrales en Europe. En réponse à des concentrations élevées en CO2 Larix decidua a montré une augmentation de la croissance, alors qu’une telle réponse n’a pas été trouvée chez Pinus uncinata. Globalement, le réchauffement climatique apparaît être le facteur responsable de la stimulation de la croissance observée chez les arbres à la limite supérieure de la forêt. • Toutefois, l’augmentation de la réinstallation des semis dans les Alpes centrales en Europe est le résultats de l’invasion d’habitats potentiels plutôt qu’une migration en altitude due aux changements climatiques, bien que les semis atteindront seulement la taille des arbres après un couplage réussit avec l’atmosphère et donc perdant le microclimat favorable d’une végétation basse. • En conclusion, les phénomènes climatiques extrêmes futurs sont plus susceptibles de contrôler les dynamiques de limite supérieure de la forêt, que l’augmentation progressive de la température dans les Alpes centrales en Europe. * Corresponding author: Article published by EDP Sciences Ann. For. Sci. 66 (2009) 402 G. Wieser et al. Figure 1. Top: Monthly average (solid line), maximum and minimum (grey bars) air temperature from January 1963 throughout December 2007 and (bottom) mean annual air temperature at the Mt. Patscherkofel Klimahaus Research Station, Austria, 1950 m a.s.l. 1. INTRODUCTION The alpine timberline designates the limit of forest growth as imposed by the environmental specificities of this ecotone. Rather than being an abrupt boundary, this timberline typically forms a transition zone between the forest line (i.e. the upper limit of closed-canopy conditions) and the tree line (disappearance of life form tree) mediating to the krummholz zone above (Däniker, 1923; Holtmeier, 2003; Slatyer and Noble, 1992; Tranquillini, 1979; Wardle, 1974; Wieser and Tausz, 2007). The tree limit is defined by tree heights of 2 to 3 m (Körner, 2007; Wieser and Tausz, 2007), which ensure tree crowns to be well coupled with the atmosphere (also in winter, as protruding above the snow cover). Above the tree line, tree and shrub species are deformed to krummholz, characterized by stunted habit, either hugging the ground or attaining, at most, the height of the snow cover (Tranquillini, 1976). Hence, such plants experience microclimates similar to dwarf plants which dominate the alpine zone at the next higher vegetation belt (Grace et al., 2002). Although at a given site the position of the timberline depends on a multitude of factors, in particular, temperaturecaused limitations in growth (Körner, 1998), such restrictions are interrelated with incomplete tissue maturation, sometimes inadequate resistance to climatic stress, which altogether determines the altitudinal distribution limit of trees (Wieser and Tausz, 2007). A number of hypotheses have been posed to functionally explain the occurrence of timberlines (Körner, 2003). The multi-factorial interactions govern the transition from the life form tree to low-stature vegetation such as dwarfshrubs and alpine grasslands. Awareness currently increases about timberline-associated forest ecosystems because these are expected to undergo significant alterations due to climate warming and changes in the chemical composition of the lower atmosphere (Grace et al., 2002; Holtmeier and Broll, 2007; Walther et al., 2005, Wieser and Tausz, 2007). Taking into account that growth and regeneration of trees within the timberline ecotone is mainly controlled by temperature (Körner, 2003), global warming is supposed to cause both an upwa (...truncated)