The regeneration niche of trees at the alpine treeline: competition and facilitation by alpine vegetation during germination and seedling establishment

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The potential altitudinal limit for tree growth is probably set by a lack of warmth during the growing season. However, the altitudinal limit for tree establishment is probably set by quite different climatic factors, such as freezing temperatures and interacting stressors such as excess solar radiation, which may explain why few alpine treelines currently reach the potential tree-growth limit. Establishment limitations also have important bearings for treeline responses to climatic changes. While most current ecophysiological treeline research focuses on growth limitations for adult trees, our project focuses on the earliest life stages, germination and seedling performance, which represent strong demographic bottlenecks determining treeline dynamics and positions. Our aim is to clarify the relative importance and the interactions of growing-season warmth, growing-season length, and frost and radiation stress for the initial establishment of treeline-forming tree species in the alpine life zone. Our preliminary results suggest that protection from strong solar radiation prevents photoinhibition and increases survival but decreases growth and carbohydrate storage. These effects show the ambiguous role of established vegetation for regeneration: on the one hand it provides protection from the harsh alpine climate and on the other hand it competes for resources.In this one-year extension of the on-going project we will address this ambiguity and finish our experiments on microclimatic restrictions for seedling establishment. These experiments have been in place since autumn 2011/spring 2012 (the experiments installed autumn 2010/spring 2011 were discontinued due to a change of PhD student). Continuing them to throughout the 2014 summer will provide a control for weather effects, such as the dry conditions in the 2012 summer, and allow a monitoring of seedlings into their second and third year, including a detailed performance monitoring throughout all seasons with our new chlorophyll fluorescence monitoring equipment (in use since middle 2013). Two new experiments on vegetation interactions, one of which integrated in a collaborative global treeline experiment started this year (2013), while a third experiment is planned for next year.By focusing on seedlings and by explicitly integrating and ranking multiple causes, this project will be an important step to resolve many of the current controversies concerning treeline dynamics.