• Intra-annual radial growth rates and durations in trees are reported to differ greatly in relation to species, site and environmental conditions. However, very similar dynamics of cambial activity and wood formation are observed in temperate and boreal zones. • Here, we compared weekly xylem cell production and variation in stem circumference in the main northern hemisphere conifer species (genera Picea, Pinus, Abies and Larix) from 1996 to 2003. Dynamics of radial growth were modeled with a Gompertz function, defining the upper asymptote (A), x-axis placement (β) and rate of change (κ). • A strong linear relationship was found between the constants β and κ for both types of analysis. The slope of the linear regression, which corresponds to the time at which maximum growth rate occurred, appeared to converge towards the summer solstice. • The maximum growth rate occurred around the time of maximum day length, and not during the warmest period of the year as previously suggested. The achievements of photoperiod could act as a growth constraint or a limit after which the rate of tree-ring formation tends to decrease, thus allowing plants to safely complete secondary cell wall lignification before winter.

Xylogenesis was monitored during 2003 and 2004 in a timberline environment in southern Italy to assess links between temperature, cambial phenology and wood formation on a short-time scale. Wood microcores were collected weekly from May to October from 10 trees of Pinus leucodermis Ant., histological sections were cut with a rotary microtome and anatomical features of the developing and mature tracheids were observed and measured along the growing tree ring. Spring 2003 was hotter than spring 2004, with temperatures up to 2.6 °C above historical means. The hotter conditions resulted in an earlier onset of cambial activity and all differentiation phases of about 20 days, resulting in an increased duration of xylogenesis of about 23 days. Air and stem temperatures at which xylogenesis had a 0.5 probability of being active were calculated with logistic regressions fitted on binary responses. In both years, similar thresholds were estimated with daily mean values of 8.2 and 9.5 °C for air and stem temperatures, respectively. The observed convergent responses of cambium phenology to temperature during the two contrasting springs confirm the key role of this environmental factor in determining the onset and duration of wood formation in timberline areas. The intra-annual dynamics of ring-width increase differed between years, with significantly narrower rings formed in 2004 than in 2003. These differences were mainly related to cell size because larger earlywood tracheids were produced in 2003. This study demonstrates the plasticity of tree-ring formation in response to high temperatures as a result of modifications in the onset and duration of differentiation.

Significance: Forest trees can live for hundreds to thousands of years, and they play a critical role in mitigating global warming by fixing approximately 15% of anthropogenic CO ₂ emissions annually by wood formation. However, the environmental factors triggering wood formation onset in springtime and the cellular mechanisms underlying this onset remain poorly understood, since wood forms beneath the bark and is difficult to monitor. We report that the onset of wood formation in Northern Hemisphere conifers is driven primarily by photoperiod and mean annual temperature. Understanding the unique relationships between exogenous factors and wood formation could aid in predicting how forest ecosystems respond and adapt to climate warming, while improving the assessment of long-term and high-resolution observations of global biogeochemical cycles.

We present the first European network of tree ring δ¹³C and δ¹⁸O, containing 23 sites from Finland to Morocco. Common climate signals are found over broad climatic‐ecological ranges. In temperate regions we find positive correlations with summer maximum temperatures and negative correlations with summer precipitation and Palmer Drought Severity Indices (PDSI) with no obvious species‐specific differences. Regional δ¹³C and δ¹⁸O chronologies share high common variance in year‐to‐year variations. Long‐term variations, however, exhibit differences that may reflect spatial variability in environmental forcings, age trends and/or plant physiological responses to increasing atmospheric CO₂ concentration. Rotated principal component analysis (RPCA) and climate field correlations enable the identification of four sub‐regions in the δ¹⁸O network ‐ northern and eastern Central Europe, Scandinavia and the western Mediterranean. Regional patterns in the δ¹³C network are less clear and are timescale dependent. Our results indicate that future reconstruction efforts should concentrate on δ¹⁸O data in the identified European regions.