The phenology of wood formation is a critical process to consider for predicting how trees from the temperate and boreal zones may react to climate change. Compared to leaf phenology, however, the determinism of wood phenology is still poorly known. Here, we compared for the first time three alternative ecophysiological model classes (threshold models, heat-sum models and chilling-influenced heat-sum models) and an empirical model in their ability to predict the starting date of xylem cell enlargement in spring, for four major Northern Hemisphere conifers (Larix decidua, Pinus sylvestris, Picea abies and Picea mariana). We fitted models with Bayesian inference to wood phenological data collected for 220 site-years over Europe and Canada. The chilling-influenced heat-sum model received most support for all the four studied species, predicting validation data with a 7.7-day error, which is within one day of the observed data resolution. We conclude that both chilling and forcing temperatures determine the onset of wood formation in Northern Hemisphere conifers. Importantly, the chilling-influenced heat-sum model showed virtually no spatial bias whichever the species, despite the large environmental gradients considered. This suggests that the spring onset of wood formation is far less affected by local adaptation than by environmentally driven plasticity. In a context of climate change, we therefore expect rising winter-spring temperature to exert ambivalent effects on the spring onset of wood formation, tending to hasten it through the accumulation of forcing temperature, but imposing a higher forcing temperature requirement through the lower accumulation of chilling.

Biotechnical Faculty University of Ljubljana Ljubljana Slovenia

Département des Sciences Fondamentales Université du Québec à Chicoutimi Chicoutimi QC Canada

Département des Sciences Fondamentales Université du Québec à Chicoutimi Chicoutimi QC Canada Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems of the Chinese Academy of Sciences Guangdong Provincial Key Laboratory of Applied Botany South China Botanical Garden of the Chinese Academy of Sciences Guangzhou China

Department of Botany University of Innsbruck Innsbruck Austria

Department of Geography and Regional Planning University of Zaragoza Zaragoza Spain

Department of Physical Geography and Geoecology Faculty of Science Charles University Prague Czech Republic

Department of Wood Science Faculty of Forestry and Wood Technology Mendel University in Brno Brno Czech Republic

Ecologie Systématique Evolution Univ Paris Sud CNRS AgroParisTech Université Paris Saclay Orsay France

Ecologie Systématique Evolution Univ Paris Sud CNRS AgroParisTech Université Paris Saclay Orsay France College of Urban and Environmental Sciences Laboratory for Earth Surface Processes of the Ministry of Education Peking University Beijing China

Ecologie Systématique Evolution Univ Paris Sud CNRS AgroParisTech Université Paris Saclay Orsay France Département des Sciences de la Vie et de la Terre Faculté des Sciences Section 4 Université libanaise Hoch Al Oumara Zahlé Liban

Institut National de l'Information Géographique et Forestière Champigneulles France

Instituto Pirenaico de Ecología CSIC Zaragoza Spain

Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems of the Chinese Academy of Sciences Guangdong Provincial Key Laboratory of Applied Botany South China Botanical Garden of the Chinese Academy of Sciences Guangzhou China

Natural Resources Institute Finland Espoo Finland

Slovenian Forestry Institute Ljubljana Slovenia

Swiss Federal Research Institute WSL Birmensdorf Switzerland

Theoretical Ecology University of Regensburg Regensburg Germany

Université de Lorraine AgroParisTech INRA UMR Silva Nancy France

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