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.

Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes.

Key message Onset of cambial activity in Lebanon Cedar is triggered by stem temperature but may be delayed by high site-LAI. Higher growth rates and treering widths were observed under better water and light availability. Daily stem radius variations were sensitive to humid conditions. Abstract Studies on intra-annual dynamics of stem growth provide useful information on tree growth responses to environmental conditions, but are fragmentary for species from Mediterranean Mountain ranges. Lebanon Cedar is a frost and drought tolerant species, growing between 1000 and 2000 m a.s.l in the Taurus Mountains (Turkey). Foresters see it as a potential candidate for plantation in Central European forests facing global warming. To describe the natural variability of Lebanon Cedar growth dynamics, five study sites were established: four along an altitudinal gradient at a natural site in SW-Turkey and one in a Lebanon Cedar plantation in Central Germany. Two stem growth monitoring methods were used: (1) biweekly microcoring during 2013 growing season and (2) point dendrometers during 2013 and 2014. Histological analyses were used to calibrate dendrometer records and to describe cambium phenology. Seasonal dynamics of xylem and stem radial increments were modelled by Gompertz functions. Onset of cambial activity was observed 1-2 weeks after stem temperatures reached a threshold of 5°C but could be delayed by high site-LAI. Cedars growing under better light and water availability showed higher growth rates and wider tree rings. Daily stem radius variations (dSRV) extracted from dendrometer records were negatively related to vapor pressure deficit and global radiation; multiple linear regressions explained 30-52% of dSRV variance being dominated by relative humidity, precipitation, and soil water content. Best growth performance was observed at the German site, likely for a continuous water supply throughout the year, underlining the potential of Lebanon Cedar for Central European Forestry. Keywords Lebanon Cedar Á Point dendrometer Á Microcoring Á Cambial activity Á Tree-ring formation Á Microclimate Abbreviations A Final annual tree-ring width bE Onset of the enlarging phase bL Onset of the cell-wall lignification phase bM Onset of the mature phase CA Cambial activity cE Cessation of the enlarging phase cL Cessation of the cell-wall lignification phase CRF Cedar Research Forest d Time required for major period of tree-ring formation dE Duration of the enlarging phase dL Duration of the lignification phase DOY Day of year dSRV Daily stem radius variation dX Duration of xylogenesis Communicated by E. Liang.

Considering climate change, the discussion intensifies whether and to what extent exotic tree species should be taken into account for forest cultivation, especially when indigenous species are no longer able to fulfill essential forest functions. In this study, for the first time growth potential of Cedrus libani was evaluated under climatic conditions in Central Europe (Bayreuth, Germany). The sampled trees exhibited extraordinary growth with tree ring widths averaging 4.9 mm year -1 during the past 23 years. A continuously available soil water supply enhanced radial stem growth. Thus, growth declined during the dry year of 2003, but recovered to average values the following year. Our results confirm that C. libani is a lightdemanding species which is sensitive to competition and which shows a typical age trend. In a second study, we compared cambial growth in Bayreuth with a natural stand in Elmali (Turkey) in 2009. Cambial growing season in Bayreuth was 45 days longer, and radial growth rates in Bayreuth were four times higher than in Elmali. Interestingly, C. libani maintained a slow but continuous radial growth at Elmali even during the dry summer period, confirming its exceptional drought tolerance. Our results indicate a high adaption of C. libani to current and future climate conditions in Central Europe. It tolerates extreme cold in winter and prolonged droughts during summer. Thus, its promising potential for establishing stable and productive forest stands in Central Europe under a changing climate should be confirmed in further studies.