The regeneration niche of trees at the alpine treeline

Climatic constraints on germination and seedling establishment

Hannah Loranger (PhD student), Maaike Bader, Gerhard Zotz

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. This may explain why few alpine treelines currently reach the potential tree-growth limit, and establishment limitations also have important bearings for treeline responses to climatic changes.

While most ecophysiological treeline research currently focuses on growth limitations for adult trees, we focus on the earliest life stages, germination and seedling performance, which represent strong demographic bottlenecks determining treeline dynamics and positions.

The goal of the project is to understand the processes that determine treeline dynamics in different tree species, distinguishing generalised and species-specific responses. We are particularly interested in the relative importance and the interactions of growing season warmth, growing season length, and frost and radiation stress for the initial establishment of trees in the alpine life zone. We use germination experiments and manipulative field experiments using seeds and small seedlings of various tree species, measuring growth and performance through the seasons. Field sites are located at the Lautaret Alpine Station in the French Alps.