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40The interaction between xylem phenology and climate assesses forest growth and productivity 41 and carbon storage across biomes under changing environmental conditions. We tested the annual temperature, from 83.7 days at -2 °C to 178.1 days at 12 °C, at a rate of 6.5 days °C -1 . 54April-May temperatures produced the best models predicting the dates of wood formation. 55Our findings demonstrated the uniformity of the process of wood formation and the 56 importance of the environmental conditions occurring at the time of growth resumption. 57Under warming scenarios, the period of wood formation might lengthen synchronously in the 58

Wood formation consumes around 15% of the anthropogenic CO2 emissions per year and plays a critical role in long-term sequestration of carbon on Earth. However, the exogenous factors driving wood formation onset and the underlying cellular mechanisms are still poorly understood and quantified, and this hampers an effective assessment of terrestrial forest productivity and carbon budget under global warming. Here, we used an extensive collection of unique datasets of weekly xylem tissue formation (wood formation) from 21 coniferous species across the Northern Hemisphere (latitudes 23 to 67°N) to present a quantitative demonstration that the onset of wood formation in Northern Hemisphere conifers is primarily driven by photoperiod and mean annual temperature (MAT), and only secondarily by spring forcing, winter chilling, and moisture availability. Photoperiod interacts with MAT and plays the dominant role in regulating the onset of secondary meristem growth, contrary to its as-yet-unquantified role in affecting the springtime phenology of primary meristems. The unique relationships between exogenous factors and wood formation could help to predict how forest ecosystems respond and adapt to climate warming and could provide a better understanding of the feedback occurring between vegetation and climate that is mediated by phenology. Our study quantifies the role of major environmental drivers for incorporation into state-of-the-art Earth system models (ESMs), thereby providing an improved assessment of long-term and high-resolution observations of biogeochemical cycles across terrestrial biomes.

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.

The easternmost parts of the Hercynian mid-mountains of Central Europe, namely Hrubý Jeseník and Králický Sněžník, are characterised by extensive alpine areas at altitudes above approximately 1300 m a.s.l. In order to determine the contribution of human activities to the extent of these summit grasslands we analysed charcoal assemblages and pollen profiles taken from high elevation sites. The first burn was dated to the Iron Age (about the first to second centuries BC), with successive fire events recorded in the early Mediaeval epoch from about AD 670. Significant human influence as recorded in pollen diagrams was detected as late as during the High Middle Ages (about the twelfth to thirteenth centuries AD). Charcoal assemblages reveal similar trends in species composition. The oldest and/or deepest samples are represented by charcoal fragments of Picea abies and various broadleaf trees and shrubs such as Betula sp., Sorbus sp., Juniperus sp. and Salix sp. Towards the surface, Picea abies gradually becomes dominant and then Vaccinium charcoal particles dominate the charcoal pool. Radiocarbon data of individual charcoal fragments did not, however, confirm a stratification of charcoal in the soil. According to anthracomass, pollen and macrofossils, the pattern of forest-free areas was originally determined by terrain morphology. While forest-free patches occurred on exposed summits and the convex edges of summit plateaus, open canopy tree growths dominated high elevation summit flats, and closed canopy forests occurred on adjacent slopes.