Does global warming increase or decrease tree growth at the alpine tree line?
In the 21st century, there is concern about how shifting climate conditions may affect tree growth, with consequences ranging from stand productivity to forest ecological diversity and function. The alpine tree line climate regime is assumed to be driven by mainly low temperatures, and therefore to be likely to experience a potential upward shift under global change. As global temperature rises under a general framework of climate change, tree growth at the tree line are likely to either increase if the trees are temperature limited or decrease if climate change driven droughts trigger a detrimental loss of moisture. Here we propose to use existing European larch (Larix decidua) tree-ring samples collected throughout the Alps to parameterize statistical climate-growth relationships and determine whether interannual climate–growth responses in Alpine tree line forests are stationary over time. We use dendrochronological techniques to investigate the dominant climate controls of alpine tree line ecotones on the South Eastern part of the Alps, focusing on one specie. Tree rings are widely used as long-term proxy data to provide robust information on global climate variability, moreover we decided to work exclusively on Larch because Larix decidua is well documented to be competition intolerant and cold adapted which make it a good candidate for strong climate signals. Ring-width measurements from 11 sites throughout the Eastern Alps between 1600m and 2400m were significantly and positively correlated with May-June-July-August temperature means. Our preliminary results highlight the sensitivity of tree growth to summer temperature occurring at the tree line ecotone which means also that larches are experiencing a longer growth season. These findings may indicate that low temperatures are likely to be the sole driving force limiting larch growth. Nevertheless, these results remain un-applicable to the range dynamics of alpine tree lines.