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Interannual variability in the global land carbon sink is strongly related to variations in tropical temperature and rainfall. This association suggests an important role for moisture-driven fluctuations in tropical vegetation productivity, but empirical evidence to quantify the responsible ecological processes is missing. Such evidence can be obtained from tree-ring data that quantify variability in a major vegetation productivity component: woody biomass growth. Here we compile a pantropical tree-ring network to show that annual woody biomass growth increases primarily with dry-season precipitation and decreases with dry-season maximum temperature. The strength of these dry-season climate responses varies among sites, as reflected in four robust and distinct climate response groups of tropical tree growth derived from clustering. Using cluster and regression analyses, we find that dry-season climate responses are amplified in regions that are drier, hotter and more climatically variable. These amplification patterns suggest that projected global warming will probably aggravate drought-induced declines in annual tropical vegetation productivity. Our study reveals a previously underappreciated role of dry-season climate variability in driving the dynamics of tropical vegetation productivity and consequently in influencing the land carbon sink.

Background and Aims The onset of spring growth and vessel formation were examined within three deciduous woody plant species, Acer rubrum, Populus balsamifera ssp. trichocarpa and Quercus rubra. We were broadly interested in the lag between the onset of girth expansion and the formation of mature and hydraulically conductive vessels within the new xylem. Methods Dendrometers were installed on 20 trees (6–7 per species), and expansion of both bole and distal stems was monitored throughout the growing season in a common garden. For each species, four to six distal stems were harvested every other week for anatomical examination of vessel formation. Additionally, for Populus and Quercus, hydraulic conductivity measurements and active xylem staining were completed on all stem samples. Key Results For all three species, the timing of girth expansion was similar. Expansion of distal branches occurred 12–37 d earlier than that of the bole. Vessel formation initiated several weeks prior to leaf-out, but no new earlywood vessels were mature at the time of bud break for Acer and Populus and only a few were present in Quercus. Initial stem girth expansion occurred 2 to >6 weeks before the maturation of the first current-year vessels, and there was an additional delay of up to 4 weeks before mature vessels became hydraulically functional. Hydraulic conductivity was strongly correlated with the number and diameter of stained vessels. Conclusions Bud break and leaf expansion relied predominantly on water supplied by vessels formed during prior seasons. Early-season activity is likely affected by the function of older xylem vessels and the environmental factors that influence their structure and function. Understanding the functional lifespan of vessels and the varying contributions of new and older vessels to conductivity are critical to understanding of the phenology and vascular function of long-lived woody plants in response to changing climates.

Understanding the process of wood formation and its dynamics over the growing season is fundamental to interpret the isotopic signature of tree rings. Indeed, the isotopic signal recorded in wood does not only depend on the conditions influencing carbon, water, and nitrogen uptake in the leaves and roots, but also on how these elements are translocated to the stem and incorporated into the developing xylem. Depending on environmental conditions, tree developmental stage, and physiological status, wood formation dynamics can vary greatly and produce tree-ring structures carrying specific isotopic signatures. In this chapter, we present the physiological processes involved in wood formation, along with their relationships with anatomical, developmental, and environmental factors, to understand when and how photosynthetic assimilates are progressively incorporated into the forming xylem, creating the final isotopic signature of a tree ring. First, we review current knowledge on the structure and functions of wood. Then we describe the xylogenesis process (how and when the new xylem cells produced by the cambium develop through successive differentiation phases), and its relationships with physiological, developmental, and environmental factors. Finally, we explain the kinetics of xylemcell differentiation and show why the knowledge recently acquired in this field allows us to better understand the isotopic signals in tree rings.

Tree-ring anatomy, microdensity and isotope records provide valuable intra-annual information. However, extracting signals at that scale is challenged by the complexity of xylogenesis, where two major processes – cell enlargement and wall thickening – occur at different times and rates. We characterized the space-for-time association in the tree rings of three conifer species by examining the duration, overlapping, inter-tree synchronicity and interannual stability during cell enlargement and wall thickening across regular tree-ring sectors (portions of equal tangential width). The number of cells and cell differentiation rates determined the duration of sector formation, which augmented more rapidly throughout the ring for wall thickening than for enlargement. Increasing the number of sectors above c. 15 had a limited effect on improving time resolution because consecutive sector formation overlapped greatly in time, especially in narrow rings and during wall thickening. Increasing the number of sectors also resulted in lower synchronicity and stability of intermediate-sector enlargement, whereas all sectors showed high synchronicity and stability during wall thickening. Increasing the number of sectors had a stronger effect on enhancing time-series resolution for enlargement- than for wall-thickening-related traits, which would nevertheless produce more reliable intra-annual chronologies as a result of the more similar calendars across trees and years in wall thickening The authors thank: H. Cuny for collecting, preparing and observing wood formation and anatomy samples; SILVATECH (Silvatech, INRAE, 2018. Structural and functional analysis of tree and wood facility, doi: 10.15454/1.5572400113627854E12) from UMR 1434 SILVA; 1136 IAM, 1138 BEF and 4370 EA LERMAB from the research center INRAE Grand-Est Nancy for its contribution to wood sample collection (E. Cornu, E. Farré, C. Freyburger, P. Gelhaye, and A. Mercanti) and preparation (M. Harroué); and the editor and two referees for their suggestions. This work was supported by a grant overseen by the French National Research Agency (ANR) as part of the ‘Investissements d'Avenir’ program (ANR-11-LABX-0002-01, Laboratory of Excellence ARBRE), and by the program ‘Soutien aux jeunes chercheurs’ (18_GE5_018) funded by the Conseil Régional Grand Est (France). GPL benefited from a Xunta de Galicia–Fulbright fellowship (ED481B2019/108) and is grateful to Xunta de Galicia for the recognition of the Research Competitive Reference Group GI-1809 BIOAPLIC (ED431C 2019/07). LFU was a recipient of an AgreenSkills+ fellowship funded by the EU’s Seventh Framework Program under grant agreement no. FP7-609398 (AgreenSkills+ contract) SI

In this study, we present the first tree-ring chronology for the tropical tree species Copaifera lucens and its climatic signal in southeastern Brazil. Tree-ring width series were compared with local climate indices using a drought index (Standardized Precipitation Evapotranspiration Index —SPEI), in monthly, bi-monthly and four-monthly scales. We also calculated negative pointer years over the time-span of the tree-ring width. The radial growth of C. lucens showed a positive correlation with the SPEI of the current summer and autumn in all the three analyzed time scales, while the negative pointer years matched with drier years. The species was highly sensitive to very low summer precipitation, which may lead to a 49% reduction in growth. We conclude that the long-living C. lucens has a great potential for dendrochronological studies as it shows a marked climatic signal. Our study also reinforces the importance of rainfall in regulating radial growth in tropical forests and sheds light on the local climate influence on tree growth in recent decades.

A quantitative method was tested to describe crown phenophases in relation to water content and to secondary growth in ring-porous species, based on the hypothesis that new shoots require hydrated tissues to maintain the necessary turgor for extension, leading to a reduction in dry matter content (DMC). We collected a three-year-old branch from 11 Quercus pyrenaica Willd. trees at 10-day intervals to estimate DMC of newly developing buds, leaves, and twigs, and processed two opposite stem microcores for xylogenesis. Branch phenophases and shoot length were recorded in the field. The DMC of all organs decreased during crown development, with a minimum in early June, followed by a gradual increase up to initial values in late September. The shoot extension period concurred with the lowest DMC, but also with the beginning of earlywood maturation in the main stem, suggesting a high tissue hydration only when earlywood vessels become functional to fulfill enough water requirements for shoot and leaf extension. These results confirm the usefulness of DMC to accurately quantify the phenology of primary growth from bud swelling up to full leaf extension, as a complement to qualitative methods. This variation in DMC appears to be linked to secondary growth as a result of earlywood vessel development.

The impact of climate on xylem structure and function has been profusely studied for a variety of species in the last decades, but the ecological role of ring porosity under increasing levels of environmental stress has been scarcely assessed. In this study, we analyse the timing of earlywood vessels occlusions by tyloses in two ring-porous species with contrasting ecological strategies ( Quercus robur and Q. pyrenaica ) along a seasonal drought gradient, and relate it to variations in earlywood vessel diameter and radial growth obtained from tree-ring series. The number of trees showing tyloses increased in summer as a result of more frequent cavitation events under drier conditions, and was more reduced for the more drought-tolerant Q. pyrenaica , which had a higher hydraulic diameter ( D h ) but lower latewood increments. D h values decreased towards the wettest sites, and were negatively related to warm and rainy conditions in winter. Our results showed that large earlywood vessel diameters are not necessarily accompanied by high rates of tyloses formation or limited growth in summer. We hypothesize that trees in seasonal environments can take advantage from large earlywood vessels, because benefits from a more efficient hydraulic system during favourable periods are higher than the risk of xylem impairment in summer.

International audience; Key message This study presents a novel histologic approach to quantify the intra-annual dynamics of carbon sequestration in forming wood. This innovative approach, based on repeated measurements of xylem apparent density, is more direct, and more accurate than the previously published cellular-based approach. Moreover, this new approach, which was tested here on softwoods, is also applicable to hardwoods without any modification. Context Forest ecosystems are key players of the terrestrial carbon cycle. Indeed, wood represents the principal carbon pool of terrestrial biomass, accumulated in trees through cambial activity. Aims Here, we present a novel, simple, and fast approach to accurately estimate the intra-annual dynamics of aboveground woody biomass production based on image analysis of forming xylem sections. Methods During the 2015 growing season, we weekly collected wood samples (microcores) containing the forming xylem on seven Norway spruces (Picea abies (L.) Karst), grown in Hesse forest (North-East France). The microcores were prepared to allow the observation of the forming tissues with an optical microscope. Xylem apparent density and radial increment were then measured directly on images of the histological sections. In order to compare our “histologic approach” with the previously published “cellular approach,” we also counted the number of tracheids in each differentiation zones, and measured the tracheid dimensions all along the last-formed tree ring. Results The two approaches yielded comparable meaningful results, describing xylem size increase and aboveground woody biomass production as bell-shaped curves culminating in May and June respectively. However, the histologic approach provided a shorter time lag between xylem size increase and biomass production than the cellular one. Conclusion Better quantification of the shift between stem growth in size and in biomass will require addressing the knowledge gap regarding lignin deposition kinetics. Nevertheless, our novel histologic approach is simpler and more direct than the cellular one, and may open the way to a first quantification of intra-annual dynamics of woody biomass production in angiosperms, where the cellular approach is hardly applicable.

Summary Environmental conditions and the structure of the dormant cambium are assumed to affect seasonal patterns of cambial activity, hence controlling allocation of non-structural carbohydrates (NSC) to growth. However, seasonal dynamics of xylogenesis, and their connections with NSC content and dormant cambium size, have been rarely assessed along an environmental gradient. We monitored xylogenesis and leaf phenology during 2012 and 2013, and NSC in 2012, for the drought-sensitive Quercus robur and the drought-tolerant Quercus pyrenaica along a water-availability gradient in the north-western Iberian Peninsula, and analysed dependencies of xylem production and phenology on the number of cells in dormant cambium. Study oak species showed comparable seasonal fluctuations in cambial activity and NSC content, despite Q. pyrenaica had a shorter growing season and a lower wood production than Q. robur. A sharp drop in spring NSC levels at all study sites evidenced that stored carbohydrates were crucial for earlywood formation. Under drier conditions, both species extended the growing period in spring and autumn, but reduced and even stopped xylogenesis in summer, showing an enhanced NSC accumulation before dormancy. A higher number of cells in dormant cambium of large dominant oaks accounted for their wider xylem increments and longer active periods. Our study demonstrates that xylogenesis is modulated by predisposing effects of dormant cambium size on xylem production and growing season length. Moreover, the high plasticity of cambial activity in deciduous oaks would confer resistance against recurrent summer drought through the improvement of the NSC status. A lay summary is available for this article.

Tree-ring growth of Pinus canariensis at high elevation on Tenerife, Canary Islands, is thought to be principally controlled by drought-induced stress. Aspect of mountain slopes on this oceanic island strongly influences local water input, with trade winds maintaining humid weather conditions on windward slopes, while leeward slopes remain much more arid. We tested whether P. canariensis tree-ring growth is controlled by local water availability and the large-scale climatic pattern of the North Atlantic Oscillation (NAO) along its elevation range on windward and leeward slopes on Tenerife. Wood cores were taken from 6 stands on windward and 6 on leeward slopes at elevations of 1120 to 1930 m. We found outstanding differences between windward and leeward slopes with regard to tree growth responses to local climate and NAO. Responsiveness to winter temperature was greater in windward stands, while tree growth in leeward stands was mainly controlled by total annual rainfall, which was strongly linked to NAO variation. Tree-ring growth was dependent on water input throughout the whole leeward slope, but elevation gradients were not relevant for tree growth dynamics. The unique exception was the lowest windward stand, located within the most humid environment on Tenerife, which experienced detrimental effects both of rainfall in late winter-spring and heat in summer. We conclude that slope aspect and exposure to trade winds are mostly driving P. canariensis growth. Prospects of growth of this species should consider that NAO-driven drought stress may affect populations on leeward slopes, where drier conditions could become limiting in the future., The staff of Corona Forest Natural Park of Tenerife facilitated access to field sites. V.R. benefited from a visiting fellowship to the University of La Laguna, funded by Consellería de Innovación e Industria, Xunta de Galicia, and research contracts by INIA-Xunta de Galicia and CSIC, partially funded by the European Social Fund. This research was partially funded by Consellería de Innovación e Industria, Xunta de Galicia (PGIDIT06PXIB502262PR), and INIA, Spanish Ministry of Science and Innovation (RTA2006- 00117).

Non-structural carbohydrates (NSC) play a central role in the construction and maintenance of a tree's vascular system, but feedbacks between the NSC status of trees and wood formation are not fully understood. We aimed to evaluate multiple dependencies among wood anatomy, winter NSC, and phenology for coexisting temperate (Quercus robur) and sub-Mediterranean (Q. pyrenaica) oaks along a water-availability gradient in the NW Iberian Peninsula. Sapwood NSC concentrations were quantified at three sites in December 2012 (N = 240). Leaf phenology and wood anatomy were surveyed in 2013. Structural equation modelling was used to analyse the interplay among hydraulic diameter (Dh), winter NSC, budburst date, and earlywood vessel production (EVP), while the effect of Dh and EVP on latewood width was assessed by using a mixed-effects model. NSC and wood production increased under drier conditions for both species. Q. robur showed a narrower Dh and lower soluble sugar (SS) concentration (3.88–5.08 % dry matter) than Q. pyrenaica (4.06–5.57 % dry matter), but Q. robur exhibited larger EVP and wider latewood (1403 µm) than Q. pyrenaica (667 µm). Stem diameter and Dh had a positive effect on SS concentrations, which were related to an earlier leaf flushing in both species. Sapwood sugar content appeared to limit EVP exclusively in Q. pyrenaica. In turn, Dh and EVP were found to be key predictors of latewood growth. Our results confirm that sapwood SS concentrations are involved in modulating growth resumption and xylem production in spring. Q. pyrenaica exhibited a tighter control of carbohydrate allocation to wood formation than Q. robur, which would play a role in protecting against environmental stress in the sub-Mediterranean area. This work was supported by the Spanish Ministry of Economy and Competitiveness (research projects BFU-21451 and CGL2012-34209) with European Regional Development Fund, and by Xunta de Galicia (research projects 10MDS291009PR and GRC2015/008). Gonzalo Pérez-de-Lis received a PhD FPU-ME grant(grantnumberAP2010-4911)fundedbytheSpanishMinistry of Education. The work was partially inspired within the FPS COST Action FP1106 – STReESS and Ecometas net CGL201453840REDT SI

This study addresses relationships between leaf phenology, xylogenesis, and functional xylem anatomy in two ring-porous oak species, the temperate Quercus robur and the sub-Mediterranean Q. pyrenaica. Earlywood vessel (EV) formation and leaf phenology were monitored in 2012 and 2013. Ten individuals per species were sampled at each of three sites located in NW Iberian Peninsula. EV areas measured on microcore sections were used to calculate the hydraulic tree diameter (Dh ), in order to model relationships to phenology. Thermal requirements were evaluated using growing degree days (GDD). A species-specific timing of growth resumption was found. The onset of EV formation and budburst were associated to a particular GDD in each species. The onset and duration of EV enlargement affected Dh (and EV size) in Q. robur, but hardly in Q. pyrenaica. The relationship between the timings of EV formation and xylem structure appears to be stronger for the temperate oak, whose larger vessels may result from thermal-induced earlier resumption. In contrast, the sub-Mediterranean oak would maintain a more conservative hydraulic architecture under warming conditions.

•Context The suitability of thinning to prevent forest growth decline as a result of global warming has not been tested extensively in Macaronesian Canary pine (Pinus canariensis Sweet ex Spreng.). • Aims This study aimed to answer the following questions: (1) are tree growth patterns modified by the aspect and thinning intensity? (2) Is sensitivity to climate modulated by thinning? (3) Do trees of different crown classes respond differentially to thinning intensity and climate? •Methods We used tree-ring series from dominant, codominant, and overtopped trees to study the effects of thinning intensity on basal area increments (BAI) and climate sensitivity on windward (wet) and leeward (dry) slopes on Tenerife, Canary Islands. Three replicated blocks of control, light thinning, and heavy thinning stands were set on each slope in 1988, and cores were extracted in 2007. • Results Heavy thinning induced growth release and increased BAI, mainly on dominant and codominant trees, whereas light thinning effects were negligible; their impacts were more intense on the windward side. Temperature sensitivity was hardly affected by thinning on leeward, where climate control was stronger. On windward, thinning enhanced the influence of summer temperatures. Upper crown classes were more sensitive overall, but overtopped trees responded better in summer. Thinning intensity and aspect greatly influence growth on Canary pine afforestations, but individual responses are highly dependent on crown classes. In addition, thinning may be less effective to modify growth conditions on leeward slopes, at least if not intense. © INRA and Springer Science+Business Media B.V. 2011., This research was partially funded by Consellería de Innovación e Industria, Xunta de Galicia (PGIDIT06PXIB502262PR) and INIA, Spanish Ministry of Science and Innovation (RTA2006-00117).

Little is known concerning the effects of wildfires on tree radial growth and their climatic response under contrasting regimes of fog water inputs on oceanic islands. On Tenerife, Canary Islands, windward slopes are humid with high-fog frequency due to influence of wet trade winds, while climate on leeward slopes is more arid. We used tree-ring records of Pinus canariensis Sweet ex Spreng. to quantify the effects of a fire of known date on radial growth and determine the main limiting climatic factors for growth. Radial growth patterns and their responsiveness to fire severity and climatic variation differed between windward and leeward slopes. Surface fire did not significantly impact growth, while crown fire caused short-term growth reduction, and even cessation, more pronounced on the windward slope. Growth rates, tree-ring common signal, and climate sensitivity were smaller on the windward slope, with cold winters, and summer water stress limiting growth. On the leeward slope, climate explained a greater amount of growth variation mainly due to negative effects of high October-December sea-level pressures causing dry winter conditions. Contrasting growth dynamics on both slopes may result from diverging physiological effects of water inputs and reduced radiation caused by fog drip. Our findings suggest that dating growth suppressions and absent rings are useful to date past high-severity crown fires in P. canariensis forests, in addition to ordinary fire scars dating indicative of low-severity surface fires. © 2011 Springer-Verlag., This research was partially funded by Consellería de Innovación e Industria, Xunta de Galicia (PGIDIT06PXIB502262PR) and INIA, Spanish Ministry of Science and Innovation (RTA2006-00117).