Adjustment Capacity of Maritime Pine Cambial Activity in Drought-Prone Environments

Intra-annual density fluctuations (IADFs) are anatomical features formed in response to changes in the environmental conditions within the growing season. These anatomical features are commonly observed in Mediterranean pines, being more frequent in younger and wider tree rings. However, the process behind IADF formation is still unknown. Weekly monitoring of cambial activity and wood formation would fill this void. Although studies describing cambial activity and wood formation have become frequent, this knowledge is still fragmentary in the Mediterranean region. Here we present data from the monitoring of cambial activity and wood formation in two diameter classes of maritime pine (Pinus pinaster Ait.), over two years, in order to test: (i) whether the differences in stem diameter in an even-aged stand were due to timings and/or rates of xylogenesis; (ii) if IADFs were more common in large trees; and (iii) if their formation is triggered by cambial resumption after the summer drought. Larger trees showed higher rates of cell production and longer growing seasons, due to an earlier start and later end of xylogenesis. When a drier winter occurs, larger trees were more affected, probably limiting xylogenesis in the summer months. In both diameter classes a latewood IADF was formed in 2012 in response to late-September precipitation, confirming that the timing of the precipitation event after the summer drought is crucial in determining the resumption of cambial activity and whether or not an IADF is formed. It was the first time that the formation of a latewood IADF was monitored at a weekly time scale in maritime pine. The capacity of maritime pine to adjust cambial activity to the current environmental conditions represents a valuable strategy under the future climate change conditions.     

High-resolution climatic analysis of wood anatomical features in Corsican pine from Corsica (France) using latewood tracheid profiles

Key message

We propose a new methodology to identify intra-annual density fluctuations in latewood using cell features and relative radial position within the latewood of pine trees growing on Corsica, France. Climatic forcing of latewood wood anatomical anomalies was analyzed.

Abstract

We analyzed latewood anatomical features from Corsican pine (Pinus nigra ssp. laricio) of high-elevation sites in Corsica (France) derived from digital images of the wood surface. Latewood of each ring during the period 1950–2008 was partitioned into ten equal parts P1–P10. Mean values of the cell parameters cell lumen area (CLA), radial cell width (RCW), radial cell wall thickness (CWT), and modeled latewood density (MLD) were calculated for P1–P10. The cellular profiles for each cell parameter were subjected to principal component analyses. It was possible to quantify macroscopically visible variations of wood anatomy like intra-annual density fluctuations (IADFs) by latewood profiles of different cell parameters. A combination of cell parameter characteristics including their relative radial position within latewood provides a quantification of the cell anatomical variations in an IADF. Individual cell parameter chronologies and principal components of cell parameter profiles were correlated with climate data to determine the climatic forcing on latewood formation. Average cell parameter profiles and deviations from the long-term means are able to describe “normal” and “anomalous” environmental conditions during latewood formation. Cell feature anomalies throughout the latewood during individual years allow the reconstruction of past weather conditions with a high temporal resolution.     

Discrete versus continuous analysis of anatomical and δ13C variability in tree rings with intra-annual density fluctuations

Intra-annual density fluctuations (IADFs) are anomalies of tree rings where wood density is abruptly altered after sudden changes in environmental conditions. Their characterisation can provide information about the relationship between environmental factors and eco-physiological processes during tree growth. This paper reports about the variability of anatomical traits and stable carbon isotopic composition along tree rings as resulting from the application of two different methodological approaches: (a) the separation of each ring into different regions (earlywood, latewood and IADF) and the comparison of anatomical and isotopic parameters measured in those specific sectors and (b) the analysis of such features in continuum along ring width. Moreover, different parameters of vessels (i.e. ecd—equivalent circle diameter, elongation, sphericity and convexity of vessel lumen) were considered to identify those more appropriate for the representation of intra-annual anatomical variations. The analysis was conducted on Arbutus unedo L. growing on the Elba Island (Italy); tree rings of this species form IADFs with features clearly responsive to the environmental conditions experienced during plant growth. Results showed that the first approach, although more suitable to obtain data for subsequent statistical comparisons and for the calculation of correlations with environmental parameters, suffers from elements of subjectivity due to the size and position of the selected tree-ring regions. The in continuum method allows a clearer identification of the variation of tree-ring properties along ring width. Regarding anatomical parameters, shape indexes were not suitable indicators of intra-annual variability. The overall analysis suggested that using both methodologies in synergy helps to gain complete information and avoid misleading interpretations of IADFs in tree rings.     

Living on the edge: Legacy of water availability on Tetraclinis articulata secondary growth under semiarid conditions in Morocco

Tetraclinis articulata is a xerothermic Mediterranean conifer native from NW Africa that grows under semiarid conditions in the Atlas Mountains. This species is particularly well adapted to water scarcity and forms xeric woodlands subjected to recurring drought, aridification and overexploitation. Wood of T. articulata has been considered of limited dendrochronological value due to abundant anatomical anomalies in their growth rings. We studied tree-ring growth and tree-ring features to check its dendrochronological potential and evaluate their climatic signals and drought legacy effects at two semiarid sites with contrasting elevation in Morocco. Tree-ring boundaries were properly identified on wood cores from 60 to 68% of the sampled trees. Intra-annual wood density fluctuations (IADF), microrings, missing rings and rings with undefined limits were most abundant at the low-elevation coastal site than at the mountain site. Microring and missing ring abundance was inversely related to tree-ring growth, independent of cambial age, and mostly dependent on drought occurrence. IADF frequency was independent of tree-ring growth but inversely related to cambial age. A complex response of IADF formation to cool March to June conditions was found at the low-elevation coastal site, while IADF formation in the mountain site was related to May water availability. Undefined ring limits were observed only at the low-elevation site, showing positive relationships with elevated maximum temperatures in previous winter. Previous winter rainfall favored radial growth, but a significant legacy effect of water availability was evident up to three years prior to growth. Tree-ring growth was mostly related to water availability 33 and 26 months prior to growth at low-elevation and mountain sites, respectively. The obtained results reveal a very plastic cambial activity and a strong ability of T. articulata to withstand drought even for long periods.     

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.     

Plastic Response of Tracheids in Pinus pinaster in a Water-Limited Environment: Adjusting Lumen Size instead of Wall Thickness

The formation of wood results from cambial activity and its anatomical properties reflect the variability of environmental conditions during the growing season. Recently, it was found that wood density variations in conifers growing under cold-limited environment result from the adjustment of cell wall thickness (CWT) to temperature. Additionally, it is known that intra-annual density fluctuations (IADFs) are formed in response to precipitation after the summer drought. Although IADFs are frequent in Mediterranean conifers no study has yet been conducted to determine if these structures result from the adjustment of lumen diameter (LD) or CWT to soil water availability. Our main objective is to investigate the intra-ring variation of wood anatomical features (LD and CWT) in Pinus pinaster Ait. growing under a water-limited environment. We compared the tracheidograms of LD and CWT for the years 2010–2013 in P. pinaster growing in the west coast of Portugal. Our results suggest a close association between LD and soil moisture content along the growing season, reinforcing the role of water availability in determining tracheid size. Compared with CWT, LD showed a higher intra- and inter-annual variability suggesting its strong adjustment value to variations in water availability. The formation of a latewood IADF appears to be predisposed by higher rates of cell production in spring and triggered by early autumn precipitation. Our findings reinforce the crucial role of water availability on cambial activity and wood formation in Mediterranean conifers, and emphasize the high plasticity of wood anatomical features under Mediterranean climate.     

Exploring wood anatomy,density and chemistry profiles to understand the tree-ring formation in Amazonian tree species

Long-term analysis of tree growth using annual tree rings is increasingly in demand for tropical tree species. The basis of these studies has traditionally been the anatomical identification of the annual ring boundary. However, the structure of these annual rings has been sparsely explored for complementary physical and chemical wood traits. Here, we explore the relationships among wood density features and chemical elements (S, K, Ca, Mn) involved in the annual tree ring formation of 12 tropical tree species from non-flooded forest in the southern Amazon basin. Transverse wood sections were used for each species to determine: 1) macroscopic distinction (radial growth and wood density), 2) microscopic analyse of vessels, axial and ray parenchyma (anatomy) and 3) X-ray densitometry (physical) and X-ray fluorescence (chemical). For some species, the profiles of wood density, and Ca and Mn content showed intra- and inter-annual patterns that allowed to define and characterize the growth boundary of tree rings. Ca, K and S were mainly distributed in axial parenchyma cells, and around vessels, whereas, Mn was mainly distributed in fibres. Our results showed significant species-specific correlations between tree-ring width, density and concentrations of Ca, K and Mn. The anatomical characterization and the complementary information provided by the density and chemical profiles in some Amazonian species can represent a valuable proxy to improve the definition of annual ring-boundaries and improve the understanding of long-term growth and physiological patterns.     

A new Fiji-plugin for visualizing intra-annual density fluctuations and analyzing intra-annual theoretical volumetric flow rate fluctuations along wood cross-sections

Intra-annual analysis of wood samples by examination of xylem structure offers the possibility to reveal structural adaptations in the xylem of trees to varying climatic conditions with a high temporal resolution. Additionally, it can help closing the gap between annual tree ring width measurements and ecophysiological field studies.Typically, two approaches for intra-annual wood analyses are used: densitometry (x-ray or high frequency measurements) and analyses of microscopic images of cross-sections. While densitometry has recently become the most commonly used method, it requires expensive measurement devices, which are not necessarily part of standard laboratory equipment and accordingly not affordable for a significant number of researchers.We therefore present a new plugin for visualizing intra-annual density fluctuations (IADF) and, more specifically, for analyzing the theoretical volumetric flow rate with an adapted Hagen-Poiseuille law for elliptic conduits per pixel-column. The implemented algorithm represents an alternative for intra-annual analysis of microscopic cross-section images. The plugin has been developed for Fiji, a common package based on the open-source image processing program ImageJ to avoid the use of commercial programs for anatomical analyses.     

Climate control on ring width and intra-annual density fluctuations in <Emphasis Type="Italic">Pinus kesiya</Emphasis> growing in a sub-tropical forest of Manipur,Northeast India

Key message

Growth ring study of Pinus kesiya (khasi pine) growing in sub-tropical forest in Manipur, northeast India was performed to understand climate signatures in ring widths and intra-annual density fluctuations.

Abstract

The growth rings in khasi pine (Pinus kesiya Royle ex Gordon) growing in sub-tropical Reserve Forest in Imphal, Manipur, northeast India were analysed to understand environmental signals present in ring-width series and intra-annual density fluctuations (IADFs). For this the growth ring sequences in increment core samples collected from 28 trees were precisely dated and a ring-width chronology spanning AD 1958–2014 developed. The correlation analyses between ring-width chronology and weather data of Imphal revealed that a cool April–May–June favour tree growth. The wood anatomical features of growth rings revealed the occurrence of IADFs in early- and latewoods. The IADFs in earlywood were found to be associated with reduced precipitation in months from April to July. However, the wetter conditions in late growing season, especially August/September triggered the formation of IADFs in latewood. Our findings endorse that the IADF chronologies of khasi pine could emerge as an important proxy of summer monsoon rainfall in long-term perspective in data scarce region of northeast India.

     

Fire-related features of wood anatomy in a sweet chestnut (Castanea sativa) coppice in southern Switzerland

Explorative wood anatomical analysis was conducted on a Castanea sativa stand in southern Switzerland, where a moderate-intensity surface fire burned in April 1997. Cross-sections were sampled at multiple heights from 20 scarred shoots, 20 apparently intact shoots, along with cores taken from 20 reference trees outside the fire area. Thin sections were prepared from uphill and downhill locations on the circumference for up to 5 years preceding and following the event year. The thin sections were visually observed in order to identify a response to the known fire event preserved in the wood anatomical structure. Anatomical features were observed at the uphill and downhill locations on both scarred and intact (unscarred) cross-sections, and they occurred in a subset of the observed samples. The features observed in both scarred and intact cross-sections were an apparent increase in vessel density and a decrease in lumen area of the second row of earlywood vessels, along with tyloses formation in the first row of earlywood vessels. Furthermore, the scarred region exhibited a zone of delayed cambial death following the fire, and the onset of woundwood was often initiated later in the season or the following year. Using the type of features and their location around the circumference, we inferred that the observed features may have formed in response to local heating of the cambium, and likely formed in response to canopy damage.     

Comparison of methods for the demarcation between earlywood and latewood in tree rings of Norway spruce

The precise demarcation between earlywood and latewood is important for the detailed analysis of intra-annual tree ring features. Different techniques based on visual assessment, wood anatomy analysis and X-ray densitometry have been developed and are currently used for this purpose. Depending on the chosen method, tree species and environmental conditions, the results can significantly vary. Thus, it is important to determine the technique optimal for a particular research. Here, we investigated Norway spruce (Picea abies) tree rings to examine the agreement among the following demarcation methods: (1) direct visual assessment, (2) Mork’s index (anatomical definition of the transition from earlywood to latewood based on cell wall-lumen ratio) and (3) fixed and floating density thresholds applied to intra-ring density profiles. The aim was to modify both the Mork’s criterion and density thresholds on the basis of reference values given by visual identification of earlywood/latewood transition. A total of 231 tree rings were analysed by all methods. Our results showed that the usage of floating threshold (defined for each ring separately based on density profiles) is more reliable in comparison with fixed threshold (the same threshold value used for all tree rings and samples). Statistical analysis revealed the best correspondence between visual identification of earlywood/latewood transition and demarcation based on the standard Mork’s index and the floating density threshold derived as 80 % of maximum latewood density. In terms of Mork’s index calibration, the results showed that to determine latewood cells in Norway spruce trees growing in temperate conditions, it is sufficient to use an index value equal to 0.83. The results are applicable for the studied spruce population growing in a temperate climate. The methodology itself, however, is universal and can help to calibrate criteria for earlywood-latewood demarcation under specific conditions.     

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.     

Temporal annotation of high-resolution intra-annual wood density information of Eucalyptus urophylla and its correlation with hydroclimatic conditions

Three different Eucalyptus urophylla clones grown under two different spacing regimes in an experimental site in the state of São Paulo, Brazil, were analyzed to test effects of clone identity, spacing, cambial age and hydroclimatic conditions on high-resolution intra-annual wood density profiles. Since distinct periodic tree-ring boundaries were not visible on the stem cross-sectional surfaces, finding an alternative method for synchronization of density profiles was crucial for the analysis. The challenge was to generate intra- and inter-tree synchronized density profiles that possess high amplitude variation and low phase variation. Thus, we developed a protocol and workflow of how such high-resolution density profiles can be spatially aligned and temporally annotated to enable correlation analyses between trees and with time series of environmental stimuli. Mean wood density was significantly different between clones, but not between the spacings. Wood density increased significantly with increasing cambial age and decreasing growth rate. Principal component analysis showed that the overall variability in the temporally annotated density profiles is dominated by a highly significant common signal. We found significant negative correlation values for precipitation, indicating that water supply is the main driver of stem growth at the site, and providing evidence for the correctness of the method. The developed workflow can easily be adjusted to the analysis of other intra-annual tree-ring features like anatomical xylem cell traits or isotopic signals in the wood. It has a large potential to be used as a general guideline for the synchronization of intra-annual tree-ring traits, especially when distinct tree-ring boundaries are missing, as it is often the case under tropical climatic conditions. The workflow supports the development of spatially aligned and temporally annotated chronologies under non-annual growth rhythms.     

Climatic signals in tree-ring widths and wood structure of Pinus halepensis in contrasted environmental conditions

Tree-ring widths (RW), earlywood (EW) and latewood (LW) widths, the transition from early to latewood (T) and the occurrence of intra-annual density fluctuations in EW (E-ring) and in LW (L-ring), as well as the presence of resin canals in EW and LW, were analyzed in Aleppo pine (Pinus halepensis Mill.) from three sites in Spain and one in Slovenia to find out if the anatomical characteristics can provide additional seasonal climate–growth information from contrasted environmental conditions. Principal component analysis was applied to elucidate the relationship between the measured parameters and climate. Principal component factor PC1 proved to be related to parameters of EW and the climatic variables of winter-spring; PC2 to parameters of LW and climatic variables of summer–autumn; PC3 to conditions during transitions from humid to dry periods. The three PCs vary between sites and are determined by the climatic conditions during their formation. The study demonstrates that wood anatomical features may provide complementary information to that contained in tree-ring widths. Since such results are obtained on contrasting sites, it is likely that it may be generalized over the wide range of P. halepensis distribution representing a useful proxy for studies on a regional scale.     

Key parameters controlling stiffness variability within trees: a multiscale experimental–numerical approach

Microstructural properties of wood vary considerably within a tree. Knowledge of these properties and a better understanding of their relationship to the macroscopic mechanical performance of wood are crucial to optimize the yield and economic value of forest stocks. This holds particularly for the end-use requirements in engineering applications. In this study the microstructure–stiffness relationships of Scots pine are examined with a focus on the effects of the microstructural variability on the elastic properties of wood at different length scales. For this purpose, we have augmented microstructural data acquired using SilviScan-3? (namely wood density, cell dimensions, earlywood and latewood proportion, microfibril angle) with local measurements of these quantities and of the chemical composition derived from wide-angle X-ray scattering, light microscopy, and thermogravimetric analysis, respectively. The stiffness properties were determined by means of ultrasonic tests at the clear wood scale and by means of nanoindentation at the cell wall scale. In addition, micro-mechanical modeling was applied to assess the causal relations between structural and mechanical properties and to complement the experimental investigations. Typical variability profiles of microstructural and mechanical properties are shown from pith to bark, across a single growth ring and from earlywood to latewood. The clear increase of the longitudinal stiffness as well as the rather constant transverse stiffness from pith to bark could be explained by the variation in microfibril angle and wood density over the entire radial distance. The dependence of local cell wall stiffness on the local microfibril angle was also demonstrated. However, the local properties did not necessarily follow the trends observed at the macroscopic scale and exhibited only a weak relationship with the macroscopic mechanical properties. While the relationship between silvicultural practice and wood microstructure remains to be modeled using statistical techniques, the influence of microstructural properties on the macroscopic mechanical behavior of wood can now be described by a physical model. The knowledge gained by these investigations and the availability of a new micromechanical model, which allows transferring these findings to non-tested material, will be valuable for wood quality assessment and optimization in timber engineering.     

Plant traits and wood fates across the globe: rotted,burned, or consumed?

Wood represents the defining feature of forest systems, and often the carbon in woody debris has a long residence time. Globally, coarse dead wood contains 36–72 Pg C, and understanding what controls the fate of this C is important for predicting C cycle responses to global change. The fate of a piece of wood may include one or more of the following: microbial decomposition, combustion, consumption by insects, and physical degradation. The probability of each fate is a function of both the abiotic environment and the wood traits of the species. The wood produced by different species varies substantially in chemical, micro- and macro-morphological traits; many of these characteristics of living species have 'afterlife' effects on the fate and turnover rate of dead wood. The colonization of dead wood by microbes and their activity depends on a large suite of wood chemical and anatomical traits, as well as whole-plant traits such as stem-diameter distributions. Fire consumption is driven by a slightly narrower range of traits with little dependence on wood anatomy. Wood turnover due to insects mainly depends on wood density and secondary chemistry. Physical degradation is a relatively minor loss pathway for most systems, which depends on wood chemistry and environmental conditions. We conclude that information about the traits of woody plants could be extremely useful for modeling and predicting rates of wood turnover across ecosystems. We demonstrate how this trait-based approach is currently limited by oversimplified treatment of dead wood pools in several leading global C models and by a lack of quantitative empirical data linking woody plant traits with the probability and rate of each turnover pathway. Explicitly including plant traits and woody debris pools in global vegetation climate models would improve predictions of wood turnover and its feedback to climate.     

Microstructure alignment of wood density profiles: an approach to equalize radial differences in growth rate

We studied intra-annual wood density profiles of Douglas-fir tree rings (Pseudotsuga menziesii [Mirb.] Franco) in southwestern Germany. Growth rates are variable around the tree circumference. This leads to differences in wood formation, which can be observed in the shape of the density profiles of the same tree ring measured in different radial directions. Due to this spatial variation in density profiles, we need a reliable method to determine an average profile, which preserves the common characteristics of the data. To this end, we developed a multiple interval-based curve alignment (MICA) procedure. It identifies characteristic points within the profiles such as minima, maxima and inflection points. These reference points are shifted gradually against each other within a proportionally defined baseline interval. Using our progressive alignment approach, we are able to calculate an average profile that represents very well the characteristics of all measured curves of a specific tree ring. We applied the procedure to get year-specific average profiles using various trees. This results in representative mean density profiles that preserve the density variations common to all aligned profiles. Individual noise is reduced, thereby enabling the analysis of the impact of weather variations on wood density.     

Association genetics in Pinus taeda L. I. Wood property traits

Genetic association is a powerful method for dissecting complex adaptive traits due to (i) fine-scale mapping resulting from historical recombination, (ii) wide coverage of phenotypic and genotypic variation within a single experiment, and (iii) the simultaneous discovery of loci and alleles. In this article, genetic association among single nucleotide polymorphisms (58 SNPs) from 20 wood- and drought-related candidate genes and an array of wood property traits with evolutionary and commercial importance, namely, earlywood and latewood specific gravity, percentage of latewood, earlywood microfibril angle, and wood chemistry (lignin and cellulose content), was tested using mixed linear models (MLMs) that account for relatedness among individuals by using a pairwise kinship matrix. Population structure, a common systematic bias in association studies, was assessed using 22 nuclear microsatellites. Different phenotype:genotype associations were found, some of them confirming previous evidence from collocation of QTL and genes in linkage maps (for example, 4cl and percentage of latewood) and two that involve nonsynonymous polymorphisms (cad SNP M28 with earlywood specific gravity and 4cl SNP M7 with percentage of latewood). The strongest genetic association found in this study was between allelic variation in alpha-tubulin, a gene involved in the formation of cortical microtubules, and earlywood microfibril angle. Intragenic LD decays rapidly in conifers; thus SNPs showing genetic association are likely to be located in close proximity to the causative polymorphisms. This first multigene association genetic study in forest trees has shown the feasibility of candidate gene strategies for dissecting complex adaptive traits, provided that genes belonging to key pathways and appropriate statistical tools are used. This approach is of particular utility in species such as conifers, where genomewide strategies are limited by their large genomes.     

xRing—An R package to identify and measure tree-ring features using X-ray microdensity profiles

Climate influences tree-ring density and ring-density variables extracted from X-ray images have been widely used for climate reconstructions. The R package xRing was developed to identify and measure tree rings on X-ray microdensity profiles automatically. This package is available for free and it offers functions to visualize and calibrate X-ray images, to detect tree-ring borders and to identify earlywood-latewood transition using wood density variations at the inter- and the intra-ring scale. The most important functions are calibrateFilm, detectRings, correctRings, detectEwLw, and getDensity. Outputs of these functions are S3 objects, for which specific methods are provided, including plot and print. The non-linear relationship between optical density of the film and wood density is defined by the function calibrateFilm. The function detectRings detects tree rings using wood density profiles as input. This function uses the difference between local maximum and minimum values to identify tree-ring borders automatically. The correctRings function is used to call a Graphical User Interface (GUI) to visualize and to correct tree-ring borders manually. After correcting tree-ring borders, the detectEwLw function is used to compute earlywood and latewood widths by dividing rings according to relative intra-ring density changes. The getDensity function computes for each tree ring the minimum (maximum) density and the mean earlywood, latewood and whole-ring density. Finally, a list with dataframes with tree-ring width and density variables can be obtained using the function getRwls. One of the major advantages of xRing package is that requires little knowledge of R language, but at the same time it can be easily changed or adapted by experienced users.