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Lötschental offers an ideal setting to study the influence of global warming on tree-growth


Global warming: a certainty

The anthropogenic influence on the earth’s climate system and in particular the temperature increase observed over the past century is no longer disputed (IPCC 2007). Even though considerable uncertainty exists in future projections, all General Circulation Models predict that temperatures will continue to increase during the 21st century and beyond. Impacts of the recent warming trend are currently recognized for both abiotic (e.g., glacier shrinkage, sea-ice thinning) and biotic (spring greening, treeline advance) systems. Even without consideration of complicating feedbacks, potential impacts to biological systems and ecosystem function related to projected temperature increases are more difficult to predict. Forested ecosystems, which contain about 90% of the living terrestrial biomass and regulate most of the land-atmosphere flux of water vapor, represent one of the most important biosphere systems. The impacts of climate change on tree-growth represent a critical shift to an important feedback to the earth’s climate system, carbon cycle, and societies economic interests.

The Lötschental is an east-west oriented valley situated right in the middle of the Alps, in the canton of Valais in Switzerland.The picturesque valley extends about 27 km from the Lötschenlücke (3178 m) at the top of the Langgletscher to the mouth of the valley at Gampel-Steg (630 m). Surrounded by 3000 meter high mountains, the valley bottom (1300) m contains small ancient villages and the glacial-fed river Lonza. Both the south and northern facing mountain slopes are covered up to about the theoretical maximal treeline (about 2200 m) by the typical subalpine forest consisting of two conifer species, the deciduous larch (Larix decidua) and the evergreen spruce (Picea abies). The timbers from living trees and historical buildings have been used to reconstruct summer temperatures and larch budmoth population dynamics over the past Millennium.

The “transect” project

The air temperature difference (approx. 3 degrees C) between the treeline and the valley bottom, which roughly coincides with the predicted deviation from current temperatures over the next 100 years, and the two ecologically different tree species make this site an ideal place in which to monitor the effects of climate change on tree growth. In order to further understand and perhaps predict the effects of projected temperature increase in forest ecosystems, the Dendro Sciences Unit of the WSL has begun observations of forest growth along both the south and north facing slopes of the Lötschental valley.

A total of 8 sites with trees of both species have been selected along an elevational transect. Seven sites are located in the Lötschental, one at the valley bottom, and three placed at 300 m elevational steps on both the north and south facing slopes. To represent even lower (warmer) conditions, an additional site has been selected at lower altitude (800 m) close to Gampel-Steg in the Rhône valley.

The project has a modular and open structure. This means that there is a general setting given (site selection and basic measurements such as ground temperature) to which other specific research questions can be additionally inserted. The project started in 2007 and will run for several years. The first field campaign (2007) was especially focused on monitoring temperature (air and soil) and growth (season length and intra-annual growth variation and spring phenophases) differences along the transect (module growth). Data collected so far include soil and air temperature measurements, stem growth fluctuations, timing of cambial activity and tracheid development, and phenological observations. In addition to these observations, the 2008 field campaign has a new module focusing on isotope cycles.



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