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.

Cambial activity is influenced by many environmental and physiological factors and among them, carbon acts as a source of energy for the growing meristems. This work has focused on the intra-annual stem growth of poplar compared with the carbon available for xylogenesis processes in cambium and outer wood. The major stages of xylem production and differentiation in two poplar genotypes with different growth performances were considered. Monitoring of stem growth and leaf phenology combined with starch, nonstructural soluble sugars and water content in the stem was conducted from February to November 2006 in Populus × canadensis Moench ‘I-214’ and Populus deltoides Marsh. ‘Dvina’. Anatomical analyses of wood formation were performed by measuring the width of the zones with differentiating and mature xylem. At the end of the growing period, wood density was assessed by microdensity analyses. Xylem differentiation at the top of the tree started at the beginning of April for both genotypes and proceeded down the stem at about 0.5 m day⁻¹, occurring almost at the same time as leaf opening. The rate of growth and wood density was superior in Dvina, but this higher productivity could not be explained by differences in the number of cambial initials and the duration of xylogenesis. However, the most productive poplar genotype showed higher glucose, fructose and sucrose content in the outer wood. The nonstructural soluble sugars available in the cambial zone followed the intra-annual pattern of xylem formation, with a higher concentration when the growth rate was maximum. The accumulations of nonstructural soluble sugars at a certain time during stem growth corresponded with a higher carbon availability to the actively growing meristems in the stem.

Background and Aims Wood formation in trees represents a carbon sink that can be modified in the case of stress. The way carbon metabolism constrains growth during stress periods (high temperature and water deficit) is now under debate. In this study, the amounts of non-structural carbohydrates (NSCs) for xylogenesis in black spruce, Picea mariana, saplings were assessed under high temperature and drought in order to determine the role of sugar mobilization for osmotic purposes and its consequences for secondary growth. Methods Four-year-old saplings of black spruce in a greenhouse were subjected to different thermal conditions with respect to the outside air temperature (T0) in 2010 (2 and 5 °C higher than T0) and 2011 (6 °C warmer than T0 during the day or night) with a dry period of about 1 month in June of each year. Wood formation together with starch, NSCs and leaf parameters (water potential and photosynthesis) were monitored from May to September. Key Results With the exception of raffinose, the amounts of soluble sugars were not modified in the cambium even if gas exchange and photosynthesis were greatly reduced during drought. Raffinose increased more than pinitol under a pre-dawn water potential of less than –1 Mpa, presumably because this compound is better suited than polyol for replacing water and capturing free radicals, and its degradation into simple sugar is easier. Warming decreased the starch storage in the xylem as well the available hexose pool in the cambium and the xylem, probably because of an increase in respiration. Conclusions Radial stem growth was reduced during drought due to the mobilization of NSCs for osmotic purposes and due to the lack of cell turgor. Thus plant water status during wood formation can influence the NSCs available for growth in the cambium and xylem.

Different irrigation effects on stem radius variation (ΔR) and maximum daily shrinkage (MDS) in Populus deltoides ‘Dvina’ and Populus×canadensis ‘I-214’ were studied to assess differences in drought tolerance between clones. One-year-old trees growing in concrete tanks were submitted to two irrigation regimes (natural rainfall and irrigation) from 24 June to 10 August, and ΔR was monitored by automatic point dendrometers. Independently of the irrigation regime, ‘Dvina’ showed a higher stem radial increment than ‘I-214’. In both clones, the first response to changed soil water content was a significant increase in MDS, whilst ΔR decreased about 20 d later when pre-dawn leaf water potential (Ψpd) dropped below –0.4 MPa. However, they displayed different strategies to overcome drought. ‘Dvina’ maintained a positive ΔR for longer than ‘I-214’, which had lower leaf Ψpd and greater leaf abscission at the end of the drought period. After irrigation resumed, ‘Dvina’ showed a higher capacity to restore stem growth. ‘I-214’ was probably unable to recover secondary growth because of higher leaf abscission during drought stress and the production of newly expanded leaves during recovery. It is concluded that the larger radial growth of ‘Dvina’ derived from a better water use (carbon uptake versus water loss) than ‘I-214’ under limited water availability.