Tree ring anatomy indices of Pinus tabuliformis revealed the shifted dominant climate factor influencing potential hydraulic function in western Qinling Mountains

Trees can adjust xylem anatomical structure related with potential hydraulic functions to cope with climate variability. We therefore need a better understanding of how climate variability constrains wood anatomy and tree radial growth. Pinus tabuliformis dominates natural forests and plantations over the western Qinling Mountains, which is one of the ecologically vulnerable areas in China. Here, we investigated the response of P. tabuliformis tree-ring anatomical structure to climate variability by applying wood anatomy analysis, and evaluated the influences of anatomical traits on potential hydraulic functions and the climate significance of intra-annual density fluctuations (IADFs). We found that with the increasing temperature from spring to summer, the negative effect of temperature on the formation and enlargement of earlywood and transition-wood tracheids was gradually enhanced. However, spring precipitation not only had a direct and positive influence on the formation of earlywood, but also had a delaying impact on the transition-wood cell enlargement. Besides, the smaller earlywood tracheid size of P. tabuliformis could be a substantially characteristic reflecting spring drought. The contribution of lumen diameter on conduit wall reinforcement was dominated in earlywood, while the contribution of cell wall thickness was greater than that of lumen diameter in latewood. The different contributions of anatomical traits on conduit wall reinforcement would further affect the response of potential hydraulic function to climate. IADFs of P. tabuliformis could be a potential indicator to reflect the abnormal summer precipitation events in the western Qinling Mountains. IADFs with strong and weak intensity indicated years with high and low rates of change in mid-summer precipitation, respectively. Future warmer and drier climate in the western Qinling Mountains will likely result in the production of smaller tracheids to ensure hydraulic safety, which means the stronger drought resistant of P. tabuliformis in the future. In this study, we linked the xylem anatomy and potential hydraulics functions with intra-seasonal climate variability in the context of climate warming and drying, and proposed some xylem anatomical indices reflecting potential drought events.     

Wood anatomy and carbon‐isotope discrimination support long‐term hydraulic deterioration as a major cause of drought‐induced dieback

Hydraulic impairment due to xylem embolism and carbon starvation are the two proposed mechanisms explaining drought‐induced forest dieback and tree death. Here, we evaluate the relative role played by these two mechanisms in the long‐term by quantifying wood‐anatomical traits (tracheid size and area of parenchyma rays) and estimating the intrinsic water‐use efficiency (iWUE) from carbon isotopic discrimination. We selected silver fir and Scots pine stands in NE Spain with ongoing dieback processes and compared trees showing contrasting vigour (declining vs nondeclining trees). In both species earlywood tracheids in declining trees showed smaller lumen area with thicker cell wall, inducing a lower theoretical hydraulic conductivity. Parenchyma ray area was similar between the two vigour classes. Wet spring and summer conditions promoted the formation of larger lumen areas, particularly in the case of nondeclining trees. Declining silver firs presented a lower iWUE than conspecific nondeclining trees, but the reverse pattern was observed in Scots pine. The described patterns in wood anatomical traits and iWUE are coherent with a long‐lasting deterioration of the hydraulic system in declining trees prior to their dieback. Retrospective quantifications of lumen area permit to forecast dieback in declining trees 2–5 decades before growth decline started. Wood anatomical traits provide a robust tool to reconstruct the long‐term capacity of trees to withstand drought‐induced dieback.     

A phenology-based approach to the analysis of conifers intra-annual xylem anatomy in water-limited environments

Xylem structure and cambial phenology (i.e. onset and cessation of cambial cell division) of conifers growing under severe water-limitations can change dramatically in relation to moisture availability. In hyperarid environments, analytical tools commonly used to investigate intra-annual variability of xylem anatomy (i.e. tracheidograms), may fail to capture the complexity of tree phenological responses to environmental conditions. This greatly limits our ability to accurately date the onset of intra-annual density variations, including the transition between earlywood and latewood. I present a new approach for developing phenological tracheidograms (“pheno-tracheidograms”) calibrated to account for the seasonal variations in cell division rates. Pheno-tracheidograms were developed for a population of Pinus ponderosa in the Mojave Desert (Nevada, USA) during the period 2015–2016 in order 1) to determine the onset date of latewood formation and 2) to investigate relationships between environmental conditions, lumen diameter, and cell wall thickness targeting specific climatic windows for each tracheid. Pheno-tracheidograms were standardized at the tree-level, showing more flexibility compared to tracheidograms standardized according to a pre-determined number of cells. By displaying cellular parameters with respect to the date of formation of the tracheid to which each measurement is associated, pheno-tracheidograms allowed to determine the beginning of latewood formation with daily resolution. Lumen diameter was significantly correlated with the onset date of cellular enlargement, while cell wall thickness showed a weaker relationship with the beginning of secondary wall deposition. Soil moisture positively affected the duration of cell enlargement and tracheid lumen diameter, particularly in the earlywood, while cell wall thickness was not significantly influenced by environmental conditions. Pheno-tracheidograms represent an empirical, yet effective way to date intra-annual xylem structures and to investigate high-resolution climate-anatomy relationships in conifer species from arid environments characterized by high phenological plasticity.     

Effects of prolonged drought on the anatomy of sun and shade needles in young Norway spruce trees

Predicted increases in the frequency and duration of drought are expected to negatively affect tree vitality, but we know little about how water shortage will influence needle anatomy and thereby the trees’ photosynthetic and hydraulic capacity. In this study, we evaluated anatomical changes in sun and shade needles of 20‐year‐old Norway spruce trees exposed to artificial drought stress. Canopy position was found to be important for needle structure, as sun needles had significantly higher values than shade needles for all anatomical traits (i.e., cross‐sectional needle area, number of tracheids in needle, needle hydraulic conductivity, and tracheid lumen area), except proportion of xylem area per cross‐sectional needle area. In sun needles, drought reduced all trait values by 10–40%, whereas in shade needles, only tracheid maximum diameter was reduced by drought. Due to the relatively weaker response of shade needles than sun needles in drought‐stressed trees, the difference between the two needle types was reduced by 25% in the drought‐stressed trees compared to the control trees. The observed changes in needle anatomy provide new understanding of how Norway spruce adapts to drought stress and may improve predictions of how forests will respond to global climate change.     

Intra-annual wood anatomical features of high-elevation conifers in the Great Basin,USA

Mountain conifers in the Great Basin of North America have provided some of the longest, continuous, and annually resolved paleoclimate records. Climate-growth relationships at the cellular level, which help understand wood formation processes that underlie dendroclimatic reconstructions, are at present largely unexplored in the Great Basin. We analyzed 42 trees located in the Snake Range (eastern Nevada, USA) at three sites along an elevation gradient. Sampled species included white fir (Abies concolor), Douglas fir (Pseudotsuga menziesii), limber pine (Pinus flexilis), bristlecone pine (Pinus longaeva), and Engelmann spruce (Picea engelmannii). Wood anatomical features were quantified for two consecutive years, 2011 and 2012. Lumen area, cell wall thickness, lumen diameter, and wall-to-cell ratio were measured for the total ring as well as for earlywood and latewood. Mean standardized tracheidograms highlighted differences between 2011 and 2012, in particular concerning lumen area and wall-to-cell ratio. Most annual variation was due to earlywood, rather than latewood. Anatomical parameters of limber pine, the only species that could be tested at both the montane and subalpine sites, varied with elevation. Principal component analysis showed that the main axis of variability was related to dimensional parameters (e.g. lumen area), which reflected differences in water availability.     

The effects of tracheid dimensions on variations in maximum density of Picea glehnii and relationships to climatic factors

An investigation was made of the effects of tracheid dimensions on variations in the maximum density of Picea glehnii Mast., which were associated with climatic changes. Radial cell diameter and the thickness of the tangential cell walls of the last-formed cells in 90 annual rings of nine trees with different annual ring widths were analyzed by image analysis. Correlations between maximum density and tracheid dimensions indicated that changes in maximum density were due mainly to changes in cell wall thickness of the last-formed cells in annual rings and were not due to changes in radial cell diameter. The effects of climatic factors on tracheid dimensions were examined by application of dendroclimatological techniques. A chronology of cell wall thickness that represented common signals among trees was established. Simple correlation and response function analyses of the chronology revealed that cell wall thickness was influenced positively by summer temperature and negatively by precipitation in August, and these responses were similar to those of maximum density. The study demonstrated that variations in maximum density were due to variations in the cell wall thickness of the last-formed cells, which varied depending on the weather in summer. Received: 8 February 1999 / Accepted: 7 October 1999     

华北低山丘陵区常用树种木质部解剖特征及水其力学抗旱性

木本植物木质部解剖特征与水分运输和干旱适应策略密切相关,但目前对华北低山丘陵区常用树种这方面的研究仍然不足。为研究这一地区植物木质部解剖特征与抗旱性的关系,研究以抗旱树种和非抗旱树种各5种为研究对象,通过测定与木质部横截面导管、薄壁组织相关的大量解剖学性状和非结构性碳浓度,比较两类树种木质部解剖特征的差异和解剖性状间的关联,以探究这些树种水力学的干旱适应策略差异。结果显示:1)10个树种的16个木质部性状均有较大变异性;2)两类树种间的平均导管直径和导管密度无显著差异,但抗旱树种导管壁厚度、最大导管直径、旁管薄壁组织比例和轴向薄壁组织比例以及非结构性碳(NSC)浓度显著大于非抗旱树种;3)抗旱树种的导管壁厚度与平均导管直径、最大导管直径和潜在最大导水率均呈显著正相关关系,最大导管直径与潜在最大导水率呈显著正相关关系,但非抗旱树种不存在这些关系。本研究抗旱树种同时具有较大的最大导管直径和较厚的导管壁,在保证较高的水分运输效率的同时又具备一定的抗栓塞能力,较多的旁管薄壁组织和NSC也为抗旱树种提供了更大的木质部水储存和栓塞修复能力。     

Bayesian analysis of Douglas-fir hydraulic architecture at multiple scales

We used a Bayesian hierarchical model to analyze the variation in xylem anatomy, hydraulic properties, and the relationship between anatomy and properties within Douglas-fir trees. The hierarchical scales in our study included fertilization treatments (fertilized and unfertilized), trees within the treatments, and positions within the trees. We measured tracheid diameter, tracheid length, percent latewood, number of pits per cell, density, and specific conductivity (K _s) on seven positions in each of 16 fertilized and 16 unfertilized trees: the trunk at cambial age 52 (breast height), 25, and 5; a branch at cambial age 20 and 7; and a root at cambial age 42 and 22. Vulnerability to embolism was also measured on the oldest trunk, branch, and root positions. For any measurement, there was little variation between treatments, however, there was great variation among positions. Tracheid diameter, tracheid length, number of pits per cell, K _s, and vulnerability to embolism decreased vertically from the roots to the branches. Correlations were evident between some positions for tracheid diameter, percent earlywood, pits per cell, and vulnerability to embolism, mostly in the fertilized treatment. We found evidence for large-scale relationships (among all observations from all trees) between density and tracheid diameter, K _s and diameter, vulnerability and diameter, K _s and pits per cell, and vulnerability and pits per cell. At a smaller scale of within position, however, usually only the branches and roots maintained the relationship.     

Xylem traits of peatland Scots pines reveal a complex climatic signal: A study in the Eastern Italian Alps

Long-term climate reconstructions are frequently based on tree-ring high-resolution proxies extracted from subfossil peatland trees. Peatlands are peculiar ecosystems characterized by high moisture in the upper soil part which creates a harsh living environment for trees. The climate mostly indirectly influences tree growth determining seasonal variations in the water table level. Within this framework, the aim of this study was to investigate climate responses of trees (Pinus sylvestris L.) growing inside and outside a Southern Alpine peat bog, by using tree-ring and wood anatomical traits (e.g. tracheid number and dimension, cell-wall thickness). Our results showed differences in the xylem structure and climate signal recorded by peatland and mineral soil trees. Peatland trees were characterized by narrow rings and tracheids with thinner cell wall. Summer temperature and precipitation were the major drivers of xylem formation in peatland trees. At intra-annual level wood anatomical traits revealed a complex within-ring signal during the growing season. The multi-parameters approach together with the high-resolution gained by using tree-ring sectors allowed us to obtain new detailed information on the xylem development of peatland trees and climate drivers that influenced it.     

Drought-induced increase in water-use efficiency reduces secondary tree growth and tracheid wall thickness in a Mediterranean conifer

In order to understand the impact of drought and intrinsic water-use efficiency (iWUE) on tree growth, we evaluated the relative importance of direct and indirect effects of water availability on secondary growth and xylem anatomy of Juniperus thurifera, a Mediterranean anisohydric conifer. Dendrochronological techniques, quantitative xylem anatomy, and ¹³C/¹²C isotopic ratio were combined to develop standardized chronologies for iWUE, BAI (basal area increment), and anatomical variables on a 40-year-long annually resolved series for 20 trees. We tested the relationship between iWUE and secondary growth at short-term (annual) and long-term (decadal) temporal scales to evaluate whether gains in iWUE may lead to increases in secondary growth. We obtained a positive long-term correlation between iWUE and BAI, simultaneously with a negative short-term correlation between them. Furthermore, BAI and iWUE were correlated with anatomical traits related to carbon sink or storage (tracheid wall thickness and ray parenchyma amount), but no significant correlation with conductive traits (tracheid lumen) was found. Water availability during the growing season significantly modulated tree growth at the xylem level, where growth rates and wood anatomical traits were affected by June precipitation. Our results are consistent with a drought-induced limitation of tree growth response to rising CO₂, despite the trend of rising iWUE being maintained. We also remark the usefulness of exploring this relationship at different temporal scales to fully understand the actual links between iWUE and secondary growth dynamics.     

Climatic impacts and drought control of radial growth and seasonal wood formation in Pinus halepensis

Short- and long-term growth responses to drought and climatic influences still remain poorly understood. In this study, we investigated the impact of climatic drivers (temperature, precipitation) and drought, using the Standardized Precipitation Index (SPI) calculated at different time scales (1–48?months), on earlywood (EW) and latewood (LW) widths in Pinus halepensis. Nine forests subjected to dry summer conditions were sampled in Mediterranean semi-arid areas from north-eastern Spain. In addition, we explored the seasonal dynamics of cambial activity and wood formation in relation to short-term climate variability. We found two peaks of tracheid cell production corresponding to EW (May–June) and LW (mid-July–August) growth phases, associated with a sharp decrease in enlarging cells in early July in response to low water availability. In the period of analysis (1970–2005), EW growth was positively correlated with precipitation in previous December and current January, April, May and June, while it was negatively correlated with temperature in June and July. LW was correlated positively with minimum temperatures in January. Probably this was an indirect relationship as a consequence of increased EW width at higher January temperatures. Drought affected more negatively EW than LW formation as evidenced the higher SPI-EW correlation (r?=?0.72) than the SPI-LW one (r?=?0.54). The strongest EW response to drought was observed in July, whereas the highest LW response to drought occurred in September; and this seasonal pattern matched the phases of lowest EW and LW tracheid production. Under a future reduction of winter and spring precipitation, the studied forests may show a decrease in tracheid cell production, causing a decline of radial growth, a reduction in hydraulic conductivity and, indirectly, a hampered carbon uptake in such semi-arid woodlands.     

ROXAS – A new tool to build centuries-long tracheid-lumen chronologies in conifers

Measurements of tree-ring features (ring width, density, isotopes concentration, etc.) are well-established proxies of environmental variability and, in particular, climate fluctuation at local, regional and continental scales. In recent years, tree-ring anatomical structure (conduit size, density, cell wall thickness, ray abundance, etc.) has been recognized as a novel source of valuable ecological information. However, despite the high potential interest, these kinds of investigations have been significantly constrained by the methodological limitations and time-consuming procedures of data collection.In this paper, we present ROXAS: an image analysis tool specifically designed to automatically recognize and measure conduit lumen area and calculate reliable statistics in a reasonable amount of time. With ROXAS, many of the aforementioned limitations in analyzing tree-ring anatomical structure can potentially be overcome. While ROXAS was previously used exclusively for angiosperm analysis, we demonstrate in this paper for the first time how it can also be used to analyze an entire sample of a conifer wood.The use of ROXAS for the analysis of conifer anatomy is exemplified by a 120-year long Pinus leucodermis sample including about 75,000 tracheid cells. The results of ROXAS fully automatic tracheid detection are compared to the results obtained after using in-built manual editing capabilities. While both approaches proved to be efficient, the quality of the fully automatic tracheid detection is found to be generally sufficient for most research applications.     

RAPTOR: Row and position tracheid organizer in R

Mechanistic understanding of tree-ring formation and its modelling requires a cellular-based and spatially organized characterization of a tree ring, moving from whole rings, to intra-annual growth zones and individual cells. A tracheidogram is a radial profile of conifer anatomical features, such as lumen area and cell wall thickness, of sequentially- and positionally-ranked tracheids. However, its construction is tedious and time-consuming since image-analysis-based measurements do not recognize the position of cells within a radial file, and present-day tracheidograms must be constructed manually.Here we present the R-package RAPTOR that complements tracheid anatomical data obtained from quantitative wood anatomy software (e.g., ROXAS, WinCELL, ImageJ), with the specific positional information necessary for the automated construction of tracheidograms. The package includes functions to read and visualize tracheid anatomical data, and uses local search algorithms to ascribe a ranked position to each tracheid in identified radial files. The package also provides functions to ensure that tracheids are adequately aligned for identifying the first tracheid in each radial file, and obtaining the correct ranking of tracheids along each radial file. Additional functions allow automating the analyses for multiple samples and rings (batch mode) and exporting data and plots for quality control.RAPTOR allows tracheidogram users to take advantage of the latest generation of cell anatomical measuring systems. With this R-package we aim to facilitate the construction of more robust and versatile tracheidograms for the benefit of the research community.     

Siberian spruce tree ring anatomy: imprint of development processes and their high-temporal environmental regulation

Wood anatomy was offered as spatiotemporal proxy record for tracheid differentiation kinetics due to its advantages in terms of much longer cover period and less demanding measurements. In this study, external and internal regulation of earlywood-to-latewood transition and properties of latewood of Picea obovata Ledeb were considered. The values and interrelations between cell number, tree ring width, maximal and mean radial cell diameter, maximal cell wall thickness and position of the transition to thick-walled tracheids were investigated within site and along the altitudinal gradient. Correlations with moving 21-day climatic series were used to estimate high-resolutional external influences. Relationships between tree ring traits are spatially stable and close within one stage of differentiation and between cells production and expansion. Relationships between sites differ in upper and lower parts of the gradient. Most of traits respond to the primary limiting factors near summer solstice; however, maximal cell wall thickness responds positively to the temperatures at the + 10 °C threshold. Altitudinal anatomical patterns revealed interaction of intrinsic and external factors in the regulation of tracheid differentiation. Timing of climatic response highlighted role of photoperiod as a trigger in the earlywood-to-latewood transition, and crucial role of the growth season ending for latewood development.     

Wood anatomical changes in roots of European ash (Fraxinus excelsior L.) after exposure

Global changes have distinct influence on fluvial processes in torrents causing erosion on slopes and riverbanks even in forested areas. Continuous as well as discontinuous erosion of riverbanks covered by trees frequently results in destabilisation and finally cause the trees affected to tip over. These uprooted trees may lead to a blockage of the river and, in the case of a collapse of the resulting dam, resulting in severe flooding or even debris-flow surges. Dating the time of root exposure along riverbanks allows the reconstruction of erosion dynamics. In the previous studies, roots of coniferous trees have shown anatomical changes (cell size reduction in earlywood) after exposure. This anatomical feature in the xylem of exposed roots has helped to determine the time of exposure at an annual resolution. In the current study, this methodology is now applied for the first time to European ash (Fraxinus excelsior L.) to determine its specific reactions to root exposure.

First results show different reactions in vessel and fibre dimensions. Fibres show a distinct decrease in lumen area due to exposure, whereas vessel lumen area displays more variability ranging from 50% size reduction to no changes. It is discussed that vessel size characteristics are also related to ecophysiological and climatic patterns.     

Growth reactions of Pinus sylvestris L. and Quercus pubescens Willd. to drought years at a xeric site in Valais, Switzerland

In Valais, an inner-Alpine dry valley in Switzerland, low-elevation Scots pine (Pinus sylvestris L.) forests are changing. While pine shows high mortality rates, deciduous species, in particular pubescent oak (Quercus pubescens Willd.), are becoming more abundant. We hypothesise that increasing drought and the species-specific drought tolerance are key factors in these processes. In this study, the growth reaction to drought years of pine and oak growing at a xeric site in Valais was analysed using dendrochronological and wood anatomical methods. Congruent with theoretical expectations, the tree-ring widths of both species, the mean lumen area of earlywood vessels in oak and the number of tracheids in a radial row in pine decreased in response to dry conditions. However, both species also showed reactions deviating from those known from mesic sites: In oak, the mean lumen area of latewood vessels increased in drought years. In pine, in the driest year of the period (1976), the mean radial diameter increased in latewood and decreased only slightly in earlywood. These results emphasises that the process of wood formation and cell functionality at xeric sites is not completely understood yet. Both species seem to have difficulties to adapt the size of their water-conducting cells to strongly reduced water availability in drought years. Additionally, the cell number is strongly reduced. Thus it remains unclear if both species can maintain sufficient water transport under increasingly dry conditions.     

Canopy status modulates formation of wood rays in scots pine under hemiboreal conditions

Non-structural carbohydrates (NSC) reserves are crucial for trees to cope with weather extremes, thus to ensure their survival and ecological plasticity. The NSC reserves can depend on social status, suggesting uneven plasticity of trees at the stand level. In stemwood of Scots pine (Pinus sylvestris L.), which is a widespread and important species, NSC reserves are stored in parenchyma in wood rays (WR). The quantity of WRs is adjusted intra-annually, allowing retrospective analysis of factors affecting their formation. Accordingly, the differences in WR quantity in stemwood of dominant and intermediate (canopy trees with reduced and narrow crowns) maturing Scots pine were assessed by quantitative wood anatomy. Tangential cuts from the outermost 30 tree-rings were analysed. The relative ray area was intermediate, i.e. covering about 5% of the tangential cut, yet expressed high individuality among the trees. The size and amount of WR mainly differed between the earlywood and latewood; WRs in latewood were higher although narrower in comparison to earlywood, yet their total amount was higher in earlywood. Canopy status had only a slight effect, as quantity and height of WR tended to be higher for the intermediate trees, particularly in earlywood. The size and quantity of WR expressed inter-annual variation, which was mainly related to the meteorological conditions prior to the formation of the tree-ring (previous summer and autumn) indicating legacy effects of climatic factors on NSC and susceptibility of trees to cumulative effects of weather extremes. However, the climatic signals in the inter-annual variation of WR were weaker than observed before, likely due to location of the studied stand in the mid-part of the species range. Nevertheless, the observed differences in mean values and inter-annual variation of WR suggested a within-species diversity of carbon allocation patterns, supporting adaptability of the species.     

云南哀牢山6种常绿阔叶树木质部解剖特征的轴向和径向变化

West、Brown和Enquist提出的树木水分传导的分形网络模型(简称WBE模型)认为,树木连续分枝之间的导管或管胞直径按照一定的比率均匀变细,其总的水力阻力与水分传导的路径长度无关,从而使不同部位叶片获得基本相当的水分供应。该模型对树木高生长的水力限制假说提出了置疑。为了验证WBE模型中树木导管或管胞均匀变细的假说,该文研究了云南哀牢山中湿性常绿阔叶林中6种常绿阔叶树, 腾冲栲(Castanopsis wattii)、景东石砾(Lithocarpus chintungensis)、木果石砾(L. xylocarpus)、长尾青冈(Cyclobalanopsis stewardiana)、滇木荷(Schima noronhae)和舟柄茶(Hartia sinensis)木质部解剖特征随树高和年龄的变化。对这6个树种共14株样木进行了不同高度树干圆盘和边材生长轮取样,样木的高度为15~25 m,按照常规木材解剖的处理和分析方法,在显微镜下测定木材切片的导管直径和密度等特征。结果表明:在14株样木中,有4株树木导管直径随树木高度增加呈线性减小, 1株没有明显变化,其它9株树木导管直径在树冠以下的树干部分变化幅度较小或没有明显变化,而从树冠基部往上直到树木顶端导管直径显著减小。同一植株随着高度的增加,导管密度增加并且在树冠内增加更显著。有三分之一的树木导管占边材面积的比例随树高增加没有明显变化,其余树木导管占边材面积比在树冠以上有所减小。多数树木理论比导率在树冠以下没有明显变化而在树冠基部往上显著降低。在从髓芯开始往外的20~40个年轮范围内导管直径增加显著,但大部分植株导管直径在40个年轮后趋于稳定。不同高度圆盘导管直径随形成层发育时间的变化呈相似的趋势,并且相同发育年龄的导管直径没有明显差异。该文的研究结果说明,导管直径的轴向和径向变化一定程度上补偿了水分运输阻力随树木个体增大而增加的缺陷,但是6种常绿阔叶树树干的导管基本不按一定比率均匀变细,不支持WBE模型。     

Radial-growth and wood-anatomical changes in overaged Quercus pyrenaica coppice stands: functional responses in a new Mediterranean landscape

Recent land-use changes in intensively managed forests such as Mediterranean coppice stands might profoundly alter their structure and function. We assessed how the abandonment of traditional management practices in coppice stands, which consisted of short cutting-cycles (10–15 years), has caused overaging (stems are usually much older than when they were coppiced) and altered their wood anatomy and hydraulic architecture. We studied the recent changes of wood anatomy, radial growth, and hydraulic architecture in two stands of Quercus pyrenaica, a transitional Mediterranean oak with ring-porous wood forming coppice stands in W–NW Spain. We selected a xeric and a mesic site because of their contrasting climates and disturbance histories. The xeric site experienced an intense defoliation after the severe 1993–1994 summer drought. The mesic site was thinned in late 1994. We studied the temporal variability in width, vessel number and diameter, and predicted the hydraulic conductivities (K _h) of earlywood and latewood. In the mesic site, we estimated the vulnerability to xylem cavitation of earlywood vessels. Overaging caused a steep decline in latewood production at a cambial age of 14 years., which was close to the customary cutting cycle of Q. pyrenaica. The diameter distribution of vessels was bimodal, and latewood vessels only accounted for 4% of the K _h. Overaging, acting as a predisposing factor in the decline episode, was observed at the xeric site, where most trees did not produce latewood in 1993–1995. At the mesic site, thinned trees formed wider tree-rings, more latewood and multiseriate tree-rings than overaged trees. The growth enhancement remained 8 years after thinning. Most of the hydraulic conductivity in earlywood was lost in a narrow range of potentials, between −2.5 and −3.5 MPa. We have shown how hydraulic conductivity and radial growth are closely related in Q. pyrenaica and how aging modulates this relationship.     

A synoptic view on intra-annual density fluctuations in Abies alba

Tree growth, as an archive of environmental conditions, has proven to provide valuable proxies for climate reconstructions. Density measurements were successfully used in the field of dendroclimatology in the past, with recent efforts to obtain sub-annual predictors from density profiles. Particular attention is laid on anomalies in the density profile named intra-annual density fluctuations (IADFs). Density fluctuations in earlywood with higher than ordinary density are assumed to be triggered by drought events in spring or early summer. How they are formed, linked to chemical and anatomical characteristics and their functionality is not well understood, although necessary to interpret growth-climate relationships of this trait. An explorative assessment of radial profiles from silver fir, showing an IADF in earlywood of 2005, included cell wall and lumen area, cellulose to lignin ratio, microfibril angle, tracheid length as well as size and shape of pits, beside density. The resulting synoptic view of these traits depicted deviations at the IADF position, indicating that multiple phases of wood formation are affected by a triggering event. These anatomical and chemical adaptions result in a hydraulically safer tissue with improved mechanical strength. The presented approach provides first insights on multiple involved wood traits and leads towards a more comprehensive understanding of IADF formation.