Xylem Growth Research Articles

Effect of Nitrogen Addition on the Intra-Annual Leaf and Stem Traits and Their Relationships in Two Dominant Species in a Subtropical Forest

Exploring the relationship between leaf and xylem growth and nitrogen (N) deposition on tree growth helps us better understand how N deposition affects tree growth, but relevant studies are still limited. We tested if leaf and xylem growth and their relationship were affected by N deposition across multiple stages of leaf development. We studied two dominant tree species (Castanopsis chinensis and Schima superba) in a subtropical forest in South China, monitoring their leaf and xylem growth traits under canopy N addition of 0 (CK) and 50 kg N ha−1 year−1 (CAN) and compared these traits and their relationships across different treatments, periods, and species. We found that CAN significantly increased C. chinensis’s leaf carbon (C) content, while the same treatment reduced leaf C content in S. superba. Specific leaf area (SLA), specific leaf weight (SLW), leaf N content, and C:N ratio showed more variation between species and within species across developmental stages than in response to different N addition treatments. Nitrogen addition also significantly increased the xylem width in both species, but the weekly xylem increment before July was notably higher in S. superba than in C. chinensis. Moreover, the leaf C content during early leaf development (ELD) following N addition was significantly and negatively correlated with the start date of xylogenesis in C. chinensis but had no significant effect on the onset of xylogenesis in S. superba. Additionally, regardless of species, SLA in ELD was negatively correlated with xylem width at the end of the early growing season (EW) under CK. However, this relationship became insignificant under CAN, suggesting that N addition alleviated the trade-off between SLA in ELD and EW. These results demonstrated that the relationship between leaf and xylem growth is influenced by both the leaf developmental stage and N availability. Traditional analyses of the leaf–xylem relationship, based solely on mature leaf traits, might oversimplify the effects of N deposition on tree growth.

Timing of Drought and Severity of Induced Leaf Desiccation Affect Recovery, Growth and Autumnal Leaf Senescence in Fagus sylvatica L. Saplings

Increased water limitations due to climate change will pose severe challenges to forest ecosystems in Europe. We investigated the response of potted saplings of Fagus sylvatica L., one of the major European tree species, to a spring and a summer water-withholding period with control–control (C-C), control–drought (C-D), drought–control (D-C) and drought–drought (D-D) treatments. We focused on recovery capacity and phenological and growth traits and questioned the extent to which an earlier drought influenced the response to a second drought in the same growing season. To examine the impact of the level of drought stress, a distinction was made between saplings with less or more than half of their leaves desiccated due to the spring drought (D<50 and D>50). The timing of the drought influenced the immediate post-drought response: saplings severely affected by the spring drought (D>50) resprouted, whereas saplings severely affected by the summer drought (C-D and D<50-D) did not. The spring treatment influenced the onset of visual symptoms in the summer drought, with saplings less affected in the spring drought (D<50-D) developing symptoms three days later than the saplings not subjected to drought in the spring (C-D), whereas severely affected saplings (D>50-D) had not yet display symptoms seventeen days after the first visual symptoms in the spring control saplings (C-D). The timing of autumnal leaf senescence displayed the legacies of the spring treatment. The saplings heavily affected by the spring drought showed a slower decrease in relative chlorophyll content and delayed leaf senescence (D>50-C and D>50-D), which may enable the repair of damaged tissues. The saplings that were less affected by the spring drought (D<50-C) showed earlier autumnal leaf senescence, which is likely an acclimation response. Interestingly, a larger diameter increment in autumn for all of the saplings that experienced the summer drought (C-D, D<50-D and D>50-D) may indicate the recovery of hydraulic capacity by new xylem growth. Our results underline the plasticity of young F. sylvatica saplings in response to (repeated) drought.

Differences in xylem and phloem structure in living stumps of Norway spruce (Picea abies (L.) Karst.) and silver fir (Abies alba Mill.)

Summary Living stumps reported in many tree species were the first, circumstantial evidence of root grafting. Although stump overgrowth has been extensively studied in various tree species, anatomical observations have mainly focused on the xylem part. Here, we investigated the xylem and phloem structure in living stumps of two conifer species: Norway spruce (Picea abies (L.) Karst.) and silver fir (Abies alba Mill.). The mechanism of wound closure was similar in the two species, however, there were differences in the structure of wound tissues in wood and bark. Immediately after the injury, a callus consisting of parenchymal cells was formed, followed by wound-wood with wide increments. In addition, the wood structure and morphology of tracheids were changed. Tracheids lost their typical elongated and tapering structure; instead, numerous crooked and forked tracheids were present. Traumatic resin canals were present in both species but were more abundant in P. abies, which also had normal resin canals. While the structure of the xylem growth rings eventually normalized in both species, the width of the annual xylem increments was diminished. In both species, the typical structure of annual phloem increment was lost in the non-collapsed phloem and sclereids were very abundant in the collapsed phloem. In A. alba, the collapsed phloem was rich in swollen parenchyma cells containing blue-stained contents, while in P. abies, numerous large traumatic resin canals were conspicuous. The study showed that similar patterns of vascular tissue restoration exist in A. alba and P. abies in response to tree felling. However, there are certain species-specific peculiarities in the wood and phloem anatomical response, which might affect long-term stump functioning and survival.

PagKNAT2/6b regulates tension wood formation and gravitropism by targeting cytokinin metabolism.

Tension wood is a specialized xylem tissue associated with gravitropism in angiosperm trees. However, few regulators of tension wood formation have been identified. The molecular mechanisms underpinning tension wood formation remain elusive. Here, we report that a Populus KNOTTED-like homeobox gene, PagKNAT2/6b, is involved in tension wood formation and gravity response. Transgenic poplar plants overexpressing PagKNAT2/6b displayed more sensitive gravitropism than controls, as indicated by increased stem curvature. Microscopic examination revealed greater abundance of fibre cells with a gelatinous cell wall layer (G-layer) and asymmetric growth of secondary xylem in PagKNAT2/6b overexpression lines. Conversely, PagKNAT2/6b dominant repression plants exhibited decreased tension wood formation and reduced response to gravity stimulation. Moreover, sensitivity to gravity stimulation showed a negative relationship with development stage. Expression of genes related to growth and senescence was affected in PagKNAT2/6b transgenic plants. More importantly, transcription activation and electrophoretic mobility shift assays suggested that PagKNAT2/6b promotes the expression of cytokinin metabolism genes. Consistently, cytokinin content was increased in PagKNAT2/6b overexpression plants. Therefore, PagKNAT2/6b is involved in gravitropism and tension wood formation, likely via modulation of cytokinin metabolism.

Growth rate determines the superior xylem growth of Siberian spruce over Siberian larch in the southern Altai Mountains, Northwest China

Growth rate determines the superior xylem growth of Siberian spruce over Siberian larch in the southern Altai Mountains, Northwest China

Anatomical Characteristics of the Xylem of Populus euphratica at Different Groundwater Burial Depths in the Lower Tarim River (China) and Its Response to Temperature Extremes

The anatomical characteristics of xylem and their relationship with temperature during the year can be studied at the cellular scale by using micro-coring technology and the wood anatomy method. In this study, we used Populus euphratica Oliv. trees with different groundwater burial depths in the lower Tarim River as the research subjects. Micro-core samples of Populus euphratica were collected near two sampling sites, TY1 and TY2, which have different groundwater burial depths. We analyzed the differences in xylem anatomical characteristics and their relationship with extreme temperatures under these varying groundwater conditions using wood anatomy methods. The results showed that the anatomical parameters at TY1, with a higher groundwater table, were greater than those at TY2, which had a lower groundwater table. Specifically, the conduit density, total conduit area, average conduit area and maximum conduit area of Populus euphratica xylem were significantly and positively correlated with both maximum and minimum temperatures. The principal components of xylem parameters at TY1, with the higher water table, were significantly and positively correlated with both maximum and minimum air temperatures. In contrast, the principal components of xylem parameters at TY2, with the lower water table, were not significantly correlated with either maximum or minimum air temperatures. The sensitivity analysis indicated that the sensitive maximum air temperature for the principal component parameter index change of Populus euphratica xylem was 34.1 °C, and the sensitive minimum air temperature was 16.1 °C. Therefore, different moisture conditions affected the sensitivity of xylem parameter growth to temperature, with the temperature threshold for Populus euphratica xylem growth being between 16.1 °C and 34.1 °C.

Auxins differentially affect growth in Scots pine and Norway spruce in spring and autumn

Trees adapt to changing environmental conditions largely by regulating growth processes, and ongoing climate aridization puts pressure on growth processes during spring and summer. However, trees can be limited in their ability to utilize favourable early-season and late-season conditions for growth, and many of these limitations can be due to shortages in auxin transport and/or signalling. We studied whether leaf spraying with auxins in spring and autumn can aid in the growth of Pinus sylvestris and Picea abies saplings. In spring, spruce demonstrated prominent positive effects of auxin supplementation on the growth of current-year shoot and xylem growth, with increased xylem cell formation, whereas a limited effect was observed in pine. Autumnal auxin treatment had prominent positive effects on the growth of the root system and xylem area in pine, activating xylem cell enlargement and increasing carbon deposition in cell walls. In spruce, the effects of auxin spraying on xylem growth were less prominent, and the effects on root growth were absent compared to those in pine. The metabolome analysis of pine organs demonstrated that the activation of growth processes in the xylem and roots was accompanied by a decrease in nonstructural carbohydrates in the cambial zone but an increase in roots, indicating that auxin influenced the link between growth and carbon balance in an organ-specific manner. The major amino acid content was not influenced by auxin treatment. The observed results indicate that changes in the auxin balance during the autumnal period can activate growth processes to increase plant adaptation to changing climate conditions.

Analysis of genes related to xylem cell wall development based on transcriptomics in Populus alba ‘Berolinensis’ tension wood

Populus alba ‘Berolinensis’ is a fast-growing, high-yielding species with strong biotic and abiotic stress resistance, and widely planted for timber, shelter belts and aesthetic purposes. In this study, molecular development is explored and the important genes regulating xylem formation in P. alba ‘Berolinensis’ under artificial bending treatments was identified. Anatomical investigation indicated that tension wood (TW) was characterized by eccentric growth of xylem and was enriched in cellulose; the degree of lignification was lower than for normal wood (NW) and opposite wood (OW). RNA-Seq-based transcriptome analysis was performed using developing xylem from three wood types (TW, OW and NW). A large number of differentially expressed genes (DEGs) were screened and 4889 counted. In GO and KEGG enrichment results, genes involved in plant hormone signal transduction, phenylpropanoid biosynthesis, and cell wall and secondary cell wall biogenesis play major roles in xylem development under artificial bending. Eight expansin (PalEXP) genes were identified from the RNA-seq data; four were differentially expressed during tension wood formation. Phylogenetic analysis indicated that PalEXLB1 belongs to the EXPB subfamily and that the other PalEXPs are members of the EXPA subfamily. A transcriptional regulatory network construction showed 10 transcription factors located in the first and second layers upstream of EXP, including WRKY, ERF and bHLH. RT‒qPCR analysis in leaves, stems and roots combined with transcriptome analysis suggests that PalEXPA2, PalEXPA4 and PalEXPA15 play significant regulatory roles in cell wall formation during tension wood development. The candidate genes involved in xylem cell wall development during tension wood formation marks an important step toward identifying the molecular regulatory mechanism of xylem development and wood property improvement in P. alba ‘Berolinensis’.

High preseason temperature variability drives convergence of xylem phenology in the Northern Hemisphere conifers

Wood growth is key to understanding the feedback of forest ecosystems to the ongoing climate warming. An increase in spatial synchrony (i.e., coincident changes in distant populations) of spring phenology is one of the most prominent climate responses of forest trees. However, whether temperature variability contributes to an increase in the spatial synchrony of spring phenology and its underlying mechanisms remains largely unknown. Here, we analyzed an extensive dataset of xylem phenology observations of 20 conifer species from 75 sites over the Northern Hemisphere. Along the gradient of increase in temperature variability in the 75 sites, we observed a convergence in the onset of cell enlargement roughly toward the 5th of June, with a convergence in the onset of cell wall thickening toward the summer solstice. The increase in rainfall since the 5th of June is favorable for cell division and expansion, and as the most hours of sunlight are received around the summer solstice, it allows the optimization of carbon assimilation for cell wall thickening. Hence, the convergences can be considered as the result of matching xylem phenological activities to favorable conditions in regions with high temperature variability. Yet, forest trees relying on such consistent seasonal cues for xylem growth could constrain their ability to respond to climate warming, with consequences for the potential growing season length and, ultimately, forest productivity and survival in the future.

Anatomical structure and physiological characteristics of Robinia pseudoacacia of different stand ages

Drought intensity and frequency have been increased as a result of global warming. Exploring the drought resistance mechanism of Robinia pseudoacacia plantations of different stand ages on the Loess Plateau is crucial for understanding the stability of forest productivity in the region. We investigated anatomical traits, hydraulic function, and non-structural carbohydrate content of the xylem, as well as their association, in R. pseudoacacia plantations of different stand ages in a semi-arid region. The results showed that the vessel diameter, total pit membrane area, pit membrane area, vesture area, and vestured overlap of young and middle-aged stands were larger than those of mature stands, and the pit density was significantly lower in mature stands. Hydraulic conductivity was significantly related to vessel diameter, pit membrane area, and vesture area. Hydraulic conductivities of branches in young, middle-aged, and mature stands were 2.30, 2.12, and 0.76 kg·m-1·s-1·MPa-1, respectively, with embolism values of 54.5%, 53.8%, and 45.1%. Hydraulic conductivity was significantly related to soluble sugar and starch contents. The soluble sugar contents of branches in young, middle-aged and mature stands were 4.9%, 4.2%, and 3.8%, respectively. Xylem growth capacity of R. pseudoacacia in mature stand declined, resulting in the formation of small vessels with many small pits, which reduced hydraulic conductivity while maintaining hydraulic safety, resulting in a decrease of non-structural carbohydrates content. This study revealed the drought response mechanism of R. pseudoacacia plantations with different ages, providing a scientific foundation for the management and nurturing of R. pseudoacacia plantations on the Loess Plateau.

The three-dimensional distribution of bordered pits across growth rings of stem segment in Platycladus orientalis (Cupressaceae) seedlings

Summary The dimensions and distribution pattern of bordered pits are critical to the xylem water transport function of trees. However, studies on the three-dimensional distribution pattern of pits across xylem growth rings are still limited. Here, Micro-Computed Tomography (Micro-CT) was used to acquire xylem images of two-year-old Platycladus orientalis seedlings. We analyzed the tracheid lumen diameter of 665 tracheids, pit volume, and pit diameter of 11 810 inter-tracheid pits from three radial profiles. The volume ratio of inter-tracheid pits was about 0.90–0.96%. The average pit diameter was 6.8 μm and the average pit volume was 58.9 μm3, and more than 80% of the pits had a diameter of 5–8 μm and a volume of 20–90 μm3. Both the pit diameter and the pit volume increased gradually in the radial direction from the second-year growth ring to the current-year growth ring, and the pit diameter and volume were significantly higher in earlywood than in latewood. The distribution pattern of the tracheid diameter was similar to that of the pit diameter, and the wider the tracheids, the larger the pits. The distribution of bordered pits in P. orientalis may be a critical trait explaining the xylem embolism spread pattern in this species.

Mediterranean service trees respond less to drought than oaks

Some widely distributed timber species, such as oaks, are vulnerable to low soil water availability and drought. Therefore, selecting broadleaved minor species with lower sensitivity to drought could be an alternative in seasonally dry areas. However, the growth performance of these minor hardwood species is unknown, particularly under warm and dry conditions at the southern limit of their distribution range. We assessed the radial growth response to drought by correlating tree-ring and climate data in coexisting service trees (Sorbus domestica) and two oak species (Quercus faginea, Quercus pyrenaica). Trees were sampled in three Mediterranean sites located in Spain with different precipitation patterns. We used the Vaganov–Shashkin (VS) growth model to infer the main climate constraints of growth. To ascertain whether climate is changing tree phenology, we also simulated changes in xylem onset and cessation timings and compared them with leaf falling dates in service trees. Service trees showed a trend towards advancing leaf fall, but this was not related to xylem growth. Oaks responded more to a drought index than service trees. The strongest responses corresponded to droughts peaking from spring to summer, but oaks tended to respond to longer droughts (9–13 months) than service trees (2–7 months). These different responses are due to the positive responses of all species to high precipitation in the growing season, but the sensitivity of oak growth to warm summer conditions and increased atmospheric water demand. The VS model indicated a more bimodal growth pattern in services trees than in oaks, which could allow the former to recover better after a dry summer. Service trees could be more widely used as a source of valuable timber wood under Mediterranean continental conditions.

Xylem formation of Populus euphratica and its response to water-heat factors in the lower reaches of Tarim River, China.

To deeply understand the effects of water and temperature factors on the xylem formation of Populus euphratica, taking the Yingsu section in the lower reaches of Tarim River as an example, we selected micro-coring samples of P. euphratica around monitoring wells F2 and F10 in the 100 and 1500 m distance from the channel of Tarim River. We used wood anatomy method to analyze the xylem anatomy of P. euphratica and its response to water and temperature factors. The results showed that the changes of the total anatomical vessel area and the vessel number of P. euphratica in the two plots were basically consistent during the whole growing season. The vessel number of xylem conduits of P. euphratica increased slowly with the increases of groundwater depth, while the total conduit area increased firstly and then decreased. The total vessel area, minimum vessel area, average vessel area, and maximum vessel area of P. euphratica xylem increased significantly with the increases of temperature in the growing season. The contribution of groundwater depth and air temperature to P. euphratica xylem varied among different growth stages. In the early growing season, air temperature had the largest contribution to the number and total area of xylem conduits of P. euphratica. During the middle growing season, air temperature and groundwater depth jointly affected the parameters of each conduit. During the later growing season, groundwater depth had the largest contribution to the number and total area of conduits. Results of the sensitivity analysis indicated that the groundwater depth sensitive to xylem vessel number change of P. euphratica was 5.2 m and that to the change in the total conduit area was 5.9 m. The temperature sensitive to total vessel area of P. euphratica xylem was 22.0 ℃, and that to average vessel area was 18.5 ℃. Therefore, the sensitive groundwater depth affecting xylem growth was at the range of 5.2-5.9 m, and the sensitive temperature was at the range of 18.5-22 ℃. This study could provide scientific basis for the restoration and protection of P. euphratica forest in the lower reaches of Tarim River.

Tissue Metabolic Responses to Artificial Bending and Gravitation Stimuli in Betula platyphylla

Betula platyphylla Suk (Asian white birch) is an economically important tree species in the paper-pulping and biofuel industries. To investigate the mechanism of wood formation at the metabolic level, we evaluated metabolic responses associated with tension-wood formation. Four-year-old trees were subjected to artificial bending treatment for 6 weeks. The xylem growth rate of tension wood (TW) was significantly faster than that of opposite wood (OW), and it exhibited a higher cellulose content. Metabolomics analysis was performed on metabolites of TW, OW and normal wood (NW), and 183 metabolites were identified, of which levels of 142 were altered between groups. Metabolites related to fatty-acid and amino-acid metabolism, the glycolytic pathway, and the metabolism of fructose, mannose and starch sucrose were abundant in TW. Glucose 1-phosphoric acid, fructose and mannose associated with tension-wood development were elevated. Levels of xylitol and ribosol (related to the conversion of glucose), coniferol (the main monomer of lignin) and shikimic acid (an intermediate in lignin synthesis) were decreased in TW. These metabolites are likely involved in xylem development.

Cambial phenology and wood formation of Korean pine in response to climate change in Changbai Mountain, Northeast China

Korean pine (Pinus koraiensis Sieb. et Zucc.) is a highly ecological and economic species due to its rarity, varied usage and favorable effect on the environment. To better understand the adaptation strategies of intra-annual radial growth of Korean pine to local environmental regimes, we continuously monitored the cambial activity and wood formation in northeastern China weekly from April to October across the two consecutive years of 2020 and 2021 using Trephor. The results indicated that the warmer spring in 2021 resulted in an earlier onset of cambial activity compared to that of 2020 (DOY 118 vs DOY129), but the cessation of wood formation was similar (DOY 250 vs DOY 248). In both years, similar thresholds were estimated using logistic regression, xylem growth was active when the daily mean air temperature was 11.5 °C. Sensitivity analysis revealed that 86% of xylem growth was attributed to the growth rate and 14% to its duration. The total number of xylem cells was positively correlated with air temperature, soil temperature and relative humidity, and negatively correlated with vapor pressure deficit and soil water content in both years. Temperature is the main factor limiting the growth of Korean pine. Our findings complement a description of cambial phenology and wood formation dynamics of Korean pine at cellular levels and provide a better understanding of the climate-growth relationship, which will enable the improvement of climate-growth models used to predict climate change.

Intra-Annual Growth and Its Response to Climatic Factors of Two Salix Species under Warm Temperate Environment

Monitoring cambial activity and intra-annual growth dynamics is an effective method for identifying tree growth response to climate change. However, there have been few pieces of research on intra-annual wood formation of diffuse-porous species under monsoonal warm temperate environment. Here, we monitored weekly the cambial activity and xylem differentiation of Salix babylonica and Salix matsudana by collecting microcores during the 2018 growing season. Two willow species exhibited similar cambial activity and xylem differentiation processes, of which the onset and cessation of xylem growth was from middle/late March to early/middle November. The onset and cessation of cambial activity were slightly earlier for Salix matsudana (19 March and 12 October) than for Salix babylonica (26 March and 17 October), which peaked on 2 June and 31 May, respectively. Salix babylonica showed wider xylem increment and higher growth rate than that of Salix matsudana, of which the intra-annual xylem width was 8525 ± 1201 µm and 7603 ± 826 µm, respectively, fitted by Gompertz function. Moreover, the maximum growth rate of Salix babylonica and Salix matsudana was 79.75 μm day−1 and 66 μm day−1, respectively, occurring on 4 June (DOY155) and 26 May (DOY146). Both temperature and water availability were important factors influenced the xylem growth for two species, which Salix matsudana had a stronger response to temperature but not to water availability than Salix babylonica. These results suggested that Salix babylonica seem to grow better under moist warm temperate environment due to it being a more conservative response to the climate than Salix babylonica. These observed species-specific differences at the intra-annual scale may help researchers more accurately anticipate the species suitability in temperate forests.

Changes in the Activity of the CLE41/PXY/WOX Signaling Pathway in the Birch Cambial Zone under Different Xylogenesis Patterns.

The balance between cell proliferation and differentiation into other cell types is crucial for meristem indeterminacy, and both growth aspects are under genetic control. The peptide-receptor signaling module regulates the activity of the cambial stem cells and the differentiation of their derivatives, along with cytokinins and auxin. We identified the genes encoding the signaling module CLE41-PXY and the regulator of vascular cambium division WOX4 and studied their expression during the period of cambial growth in the radial row: the conducting phloem/cambial zone and the differentiating xylem in two forms of Betula pendula, silver birch and Karelian birch. We have shown that the expression maximum of the BpCLE41/44a gene precedes the expression maximum of the BpPXY gene. Non-figured Karelian birch plants with straight-grained wood are characterized by a more intensive growth and the high expression of CLE41/44-PXY-WOX4. Figured Karelian birch plants, where the disturbed ratio and spatial orientation of structural elements characterizes the wood, have high levels of BpWOX4 expression and a decrease in xylem growth as well as the formation of xylem with a lower vessel density. The mutual influences of CLE41-PXY signaling and auxin signaling on WOX4 gene activity and the proliferation of cambium stem cells are discussed.

A Phloem-Expressed PECTATE LYASE-LIKE Gene Promotes Cambium and Xylem Development.

The plant vasculature plays essential roles in the transport of water and nutrients and is composed of xylem and phloem, both of which originate from undifferentiated cells found in the cambium. Development of the different vascular tissues is coordinated by hormonal and peptide signals and culminates in extensive cell wall modifications. Pectins are key cell wall components that are modified during cell growth and differentiation, and pectin fragments function as signals in defence and cell wall integrity pathways, although their role as developmental signals remains tentative. Here, we show that the pectin lyase-like gene PLL12 is required for growth of the vascular bundles in the Arabidopsis inflorescence stem. Although PLL12 was expressed primarily in the phloem, it also affected cambium and xylem growth. Surprisingly, PLL12 overexpression induced ectopic cambium and xylem differentiation in the inflorescence apex and inhibited development of the leaf vasculature. Our results raise the possibility that a cell wall-derived signal produced by PLL12 in the phloem regulates cambium and xylem development.

A methodological framework to optimize models predicting critical dates of xylem phenology based on dendrometer data

Dendrometer measurements are a frequently used alternative to the laborious and time consuming microcoring to investigate intra-annual growth dynamics of trees. However, since dendrometer data not only comprise cambial growth, but also hydrological fluctuations, both signals need to be disentangled to derive critical dates of xylem phenology from dendrometer data. For this purpose, various approaches can be found in the literature, however a systematic comparison of the different options is still missing. In this study we present a methodological framework to evaluate the accuracy of different mathematical fittings to derive tree-ring phenology from dendrometer data and apply this approach to a data set comprised of three conifer species where high-resolution band dendrometer measurements and microcore sampling have been done in parallel. Based on our study we provide evidence that the most common approaches to derive onset and cessation of xylem cell enlargement from dendrometer data, i.e. applying absolute and relative thresholds to deterministic models fitted to dendrometer data, caused systematic deviations from the reference of xylogenesis observations, but could be improved by small adaptations. The most precise and unbiased predictions of xylem growth phenology were obtained by fitting the three-parameter Gompertz model to the dendrometer data and applying a relative threshold of 3.5% of annual increment to the model predictions for growth onset and an absolute threshold of 4.5 µm day-1 based on the first model derivative for growth cessation. Our framework enables an improved usage of dendrometer data for the prediction of the onset and cessation of xylem cell enlargement, which are important ecological indicators to quantify the effects environmental changes on forest growth and the terrestrial carbon cycle.

Hydraulic traits are coupled with plant anatomical traits under drought–rewatering cycles in Ginkgo biloba L.

Investigating the responses of plant anatomical traits of trees to drought-rewatering cycles helps us to understand their responses to climate change; however, such work has not been adequately reported. In this study, Ginkgo biloba L. saplings were subjected to moderate, severe, extreme and lethal drought conditions by withholding water according to the percentage loss of hydraulic conductivity (PLC) and rewatering on a regular basis. Samples of phloem, cambium and xylem were collected to quantify their cellular properties including cambium and phloem cell vitality, xylem growth ring width, pit aspiration rates and pit membrane thickness using light microscopy and transmission microscopy. The results showed that the mortality rate of G. biloba saplings reached 90% at approximately P88 (xylem water potential inducing 88% loss of hydraulic conductivity). The onset of cambium and phloem cell mortality might be in accordance with that of xylem embolism. Close negative correlations between xylem water potential and PLC and between xylem water potential and cambium and phloem mortality suggested that xylem hydraulic traits are coupled with anatomical traits under declining xylem water potential. Cambium and phloem cell vitality as well as xylem growth ring width decreased significantly with increasing drought conditions. However, xylem pit membrane thickness, cambial zone width and cambial cell geometry were not affected by the drought-rewatering cycles. The tracheid radial diameter, intertracheid cell wall thickness and tracheid density decreased significantly during both drought conditions and rewatering conditions. In addition to hydraulic traits, cambium and phloem cell vitality can be used as anatomical traits to evaluate the mortality of G. biloba under drought. Future work is proposed to observe the dynamics of pit aspiration rates under drought-rewatering cycles in situ to deepen our understanding of the essential role of bordered pits in the 'air-seeding' mechanism.