Gonzalo Pérez-De-Lis

Summary: The study of seasonal xylem hydraulics has predominantly focused on embolism‐induced losses, whereas growth‐driven increases in hydraulic capacity have received little attention. We assessed the intra‐annual dynamics of xylem formation and gain of conductivity in the current‐year ring of three species with contrasting tree‐ring structure, sessile oak ( Quercus petraea , ring‐porous), European beech ( Fagus sylvatica , diffuse‐porous) and Norway spruce ( Picea abies , conifer), using micro‐cores collected over 3 yr in north‐eastern France. Ring conductivity loss (indicated by tylose‐occluded vessels) was also examined in both angiosperms. Due to the differences in water‐transport efficiency and cavitation vulnerability between earlywood and latewood vessels, oak presented distinct intra‐annual dynamics of current‐ring basal area increment (BAI) and the gain and loss of conductive area (CA), theoretical hydraulic conductivity ( K _h ) and specific hydraulic conductivity ( K _s ). Conversely, the intra‐annual gain in BAI, CA and conductivity was largely coordinated in beech and spruce. Our results imply that the proportional gain in ring hydraulic capacity could potentially be estimated as a delayed BAI curve in diffuse‐porous and conifer species, but not in ring‐porous species; and the comparison of native conductivity measurements within or across species should account for their timing within the growing season, particularly in ring‐porous species.

Increasing drought pressure under anthropogenic climate change may jeopardize the potential of tropical forests to capture carbon in woody biomass and act as a long-term carbon dioxide sink. To evaluate this risk, we assessed drought impacts in 483 tree-ring chronologies from across the tropics and found an overall modest stem growth decline (2.5% with a 95% confidence interval of 2.2 to 2.7%) during the 10% driest years since 1930. Stem growth declines exceeded 10% in 25% of cases and were larger at hotter and drier sites and for gymnosperms compared with angiosperms. Growth declines generally did not outlast drought years and were partially mitigated by growth stimulation in wet years. Thus, pantropical forest carbon sequestration through stem growth has hitherto shown drought resilience that may, however, diminish under future climate change.

Despite lignin being a key component of wood, the dynamics of tracheid lignification are generally overlooked in xylogenesis studies, which hampers our understanding of environmental drivers and blurs the interpretation of isotopic and anatomical signals stored in tree rings. Here, we analyzed cell wall formation in silver fir (Abies alba Mill.) tracheids to determine if cell wall lignification lags behind secondary wall deposition. For this purpose, we applied a multimodal imaging approach combining transmitted light microscopy (TLM), confocal laser scanning microscopy (CLSM), and confocal Raman microspectroscopy (RMS) on anatomical sections of wood microcores collected in northeast France on 11 dates during the 2010 growing season. Wood autofluorescence after laser excitation at 405 and 488 nm associated with the RMS scattering of lignin and cellulose, respectively, which allowed identification of lignifying cells (cells showing lignified and nonlignified wall fractions at the same time) in CLSM images. The number of lignifying cells in CLSM images mirrored the number of wall-thickening birefringent cells in polarized TLM images, revealing highly synchronized kinetics for wall thickening and lignification (similar timings and durations at the cell level). CLSM images and RMS chemical maps revealed a substantial incorporation of lignin into the wall at early stages of secondary wall deposition. Our results show that most of the cellulose and lignin contained in the cell wall undergo concurrent periods of deposition. This suggests a strong synchronization between cellulose and lignin-related features in conifer tree-ring records, as they originated over highly overlapped time frames.

Summary: The kinetics of wood formation in angiosperms are largely unknown because their complex xylem anatomy precludes using the radial position of vessels and fibers to infer their time of differentiation. We analyzed xylogenesis in ring‐porous ash (Fraxinus angustifolia) and diffuse‐porous beech (Fagus sylvatica) over 1 yr and proposed a novel procedure to assess the period of vessel and fiber enlargement using a referential radial file (RRF). Our approach captured the dynamics of wood formation and provided a robust estimation of the kinetics of vessel and fiber enlargement. In beech, fibers and vessels had a similar duration of enlargement, decreasing from 14 to 5 d between April and July. In ash, wide vessels formed in April enlarged at a rate of 27 × 10³ μm² d⁻¹, requiring half the time of contemporary fibers (6 vs 12 d), and less time than the narrower vessels (14 d) formed in May. These findings reveal distinct cell‐type‐dependent mechanisms for differentiation in diffuse‐porous and ring‐porous trees, enhancing our understanding of angiosperm wood cell kinetics. Our approach presents an effective method for investigating angiosperm wood formation and provides a more accurate representation of vessel and fiber morphogenesis in wood formation models.

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. 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. 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. 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.

Summary: 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 wood anatomy of Copaifera lucens Dwyer was studied with an emphasis on its growth ring boundaries. Growth rings are visible to the naked eye and demarcated by marginal parenchyma bands and, sometimes, by thick-walled fibers in the latewood. Secretory canals are associated with marginal parenchyma bands, but not all marginal parenchyma bands are associated with canals. Paratracheal parenchyma is vasicentric to lozenge-aliform. Rays are 1–4-seriate, heterocellular and non-storied. Vessels are visible to the naked eye, diffuse, predominantly solitary, some in multiples, sometimes filled with gums. Crystals present. Wood anatomical characteristics of C. lucens are in agreement with those previously reported for other species of Copaifera. In addition to what had already been described for C. lucens, we observed gelatinous fibers, and some bifurcate fibers, and extremely rare clustered vessels. The growth ring boundaries are well-defined in mature wood but less distinctive near the pith. There are also partial and confluent (wedging) rings, which are difficult to classify by anatomy only, but which represent false rings and complicate tree-ring analysis in this species.