"Although temporal instability of tree-ring/climate relationships is a critical issue with potentially significant implications for the global carbon cycle, forest growth patterns, and climate reconstruction, a greater level of understanding is still needed. Two temporal instabilities identified in Boreal and Iberian forests, termed the “divergence problem” and “climate stress strength”, respectively, will be evaluated. Novel regional (aim 1) and local (aim 2) approaches will be used to understand and attribute their causes, and a multi-proxy approach will deal with their implications for climate research (aim 3). Aim 1 will assess climate change impacts on forests and will predict their response to future climatic conditions using a network of tree-ring chronologies. This regional approach will use tree-growth models for a better knowledge of growth limiting factors and to simulate forest growth under different climate scenarios. Aim 2 will explore stable isotopes as a key for the “divergence problem”, performing analytical work at an Alaskan site. This local approach will provide physiological insights and new data to improve accuracy in the calibration period. Aim 3 will develop more reliable paleoclimate reconstructions. This multi-proxy approach will combine tree-ring parameters (width, density and stable isotopes) and non tree-ring archives to reduce uncertainties in climate reconstructions. This multidisciplinary project will employ biogeochemical techniques using mass spectrometry, dendrochronological techniques and mechanistic models. The study of Boreal forests during the outgoing phase (USA) will develop new techniques and procedures that will be adjusted and implemented to study instabilities at Iberian forests and to reconstruct past climate in the western Mediterranean area, mainly during the return phase (Spain). The project covers disciplines such ecology, plant physiology, climatology and atmospheric sciences within the context of climate change."