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.     

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.     

The importance of early summer temperature and date of snow melt for tree growth in the Siberian Subarctic

Wood material for at least 12 larch trees at six sites [Larix sibirica Ldb, Larix gmelinii (Rupr.) Rupr, Larix cajanderi Mayr] near the northern timberline in Siberia was analyzed to investigate influence of climatic factor changes on tree-ring growth at high latitudes. Tree-ring cell size, maximum latewood density and ring width measured by means of image analysis and X-ray radiodensitometry and calculated latewood cell-wall thickness were used. Correlation analysis of tree-ring structure parameter chronologies with temperatures averaged over periods of 5 days (pentad) shows that early summer temperature (mean for 5-6 pentads, depending on the region, starting from the middle of June) and date of snow melt are the most important factors that define seasonal growth and tree-ring structure. Analysis of instrumental climatic data indicates that a positive trend of early summer temperature was combined with winter precipitation (October-April) increase and this combination leads to later snow melt. Based of the results of tree-ring growth modelling, it was shown that later snow melt (hence, delayed initiation of cambial activity and, as a result, decrease of wood production) explains the changes in the relationship between tree ring width and summer temperature dynamics observed after the 1960s for a large area of the Siberian Subarctic. The understanding of the role of winter precipitation in controlling ring growth, through its effect on the timing of cambial activation, suggests the possibility of using ring structure parameters to create reconstructions of past winter precipitation variations.     

Using X-ray CT based tree-ring width data for tree growth trend analysis

Changes in the environment influence the growth of tree species in Europe. Understanding the drivers of these growth changes is important to predict further growth and adapt forest management. To disentangle the different drivers of growth changes, it is common practice to apply mixed modeling techniques to tree-ring width series. Mixed modeling requires precise, replicated and well cross-dated tree-ring width series. The goal of this study was to compare a recently developed ring width measuring method based on X-ray Computed Tomography images (CT scan) with the standard LINTAB measuring method and to examine whether the same growth trends are detected with both methods using common beech (Fagus sylvatica) and sessile oak trees (Quercus petraea) as a case study. Although the CT scan method has a lower resolution than LINTAB measurements, it is of interest since it measures wood density in addition to ring width and it is less laborious in comparison to standard ring width measuring methods. No significant differences in ring width were found between the two measuring methods. The small non-significant difference between the two methods could largely be explained by the drying of cores needed for CT scanning. The same growth trends were detected with both methods: for common beech and sessile oak in Southern Belgium. These findings suggest that ring widths measured on CT scan images can be used as input for long-term modeling of tree growth changes for the targeted tree species.     

Intra-annual variability of anatomical structure and δ<Superscript>13</Superscript>C values within tree rings of spruce and pine in alpine,temperate and boreal Europe

Tree-ring width, wood density, anatomical structure and ¹³C/¹²C ratios expressed as δ¹³C-values of whole wood of Picea abies were investigated for trees growing in closed canopy forest stands. Samples were collected from the alpine Renon site in North Italy, the lowland Hainich site in Central Germany and the boreal Flakaliden site in North Sweden. In addition, Pinus cembra was studied at the alpine site and Pinus sylvestris at the boreal site. The density profiles of tree rings were measured using the DENDRO-2003 densitometer, δ¹³C was measured using high-resolution laser-ablation-combustion-gas chromatography-infra-red mass spectrometry and anatomical characteristics of tree rings (tracheid diameter, cell-wall thickness, cell-wall area and cell-lumen area) were measured using an image analyzer. Based on long-term statistics, climatic variables, such as temperature, precipitation, solar radiation and vapor pressure deficit, explained <20% of the variation in tree-ring width and wood density over consecutive years, while 29–58% of the variation in tree-ring width were explained by autocorrelation between tree rings. An intensive study of tree rings between 1999 and 2003 revealed that tree ring width and δ¹³C-values of whole wood were significantly correlated with length of the growing season, net radiation and vapor pressure deficit. The δ¹³C-values were not correlated with precipitation or temperature. A highly significant correlation was also found between δ¹³C of the early wood of one year and the late wood of the previous year, indicating a carry-over effect of the growing conditions of the previous season on current wood production. This latter effect may explain the high autocorrelation of long-term tree-ring statistics. The pattern, however, was complex, showing stepwise decreases as well as stepwise increases in the δ¹³C between late wood and early wood. The results are interpreted in the context of the biochemistry of wood formation and its linkage to storage products. It is clear that the relations between δ¹³C and tree-ring width and climate are multi-factorial in seasonal climates.     

Dendrochronology of maritime pine in the middle of the Atlantic Ocean

The Azores Archipelago, located in the North Atlantic Ridge, experiences heavy rainfall and mild temperatures with weak seasonal differences due to oceanic influence. To our knowledge, there have been no dendrochronological studies in the Azores. The aim of this study is to explore the dendrochronological potential of Pinus pinaster Ait. growing in this archipelago and to determine what limiting factor is regulating tree growth. To do so, we have sampled adult maritime pine trees growing in a plantation, in the Pico island of the Azores.Tree ring boundaries were not always easily distinguished, suggesting that in some years cambial activity did not stop during winter. Despite this, it was possible to successfully crossdate the tree-ring series and to establish a tree-ring width chronology with a strong common signal. Climatic correlations revealed a positive response to spring precipitation but no temperature signal in the tree-ring width chronology. Tree-ring width was also negatively correlated with the North Atlantic Oscillation (NAO) and the sea level pressure (SLP) in May − June.Intra-annual density fluctuations (IADFs), which are anatomical features formed in response to variations in environmental conditions during the growing season, were present in 85% of the tree rings. IADFs were identified based on its position within the ring: type E⁺, characterized as a transition wood from early- to latewood; type L, the most frequent, characterized as earlywood-like cells within latewood; and type L⁺, characterized as earlywood-like cells between latewood and earlywood of the next tree ring. Each IADF type presented a unique climatic signal: type E⁺ was positively correlated with early summer precipitation and early spring temperature; type L was positively correlated with early autumn precipitation and temperature; and type L⁺ was positively correlated with late autumn precipitation.In conclusion, the tree-ring width chronology established for maritime pine growing in the Pico Island of Azores contains a clear climatic signal for spring precipitation, whereas IADFs frequency correlated better with precipitation later in the growing season. For this reason, we suggest that IADFs should be included in future dendrochronological studies in the Macaronesia Biogeographical region since they can improve the climatic signal present in tree-ring width chronologies.     

MtreeRing: An R package with graphical user interface for automatic measurement of tree ring widths using image processing techniques

Accurate measurements of ring-width series are essential for dendrochronological analyses. We present an R package MtreeRing for ring-width measurements on scanned digital images. A morphological alternate sequential filter is used for noise reduction in the original image. Ring boundaries are determined by the steepest negative slopes in the light reflectance of latewood-earlywood transitions. To automatically identify tree rings, the package provides three alternative methods (watershed-based segmentation, Canny edge detector, and a linear detection algorithm), each with advantages and disadvantages and suited to different wood anatomical features. The user can also manually mark tree rings on species with complex anatomical structures. The arcs of inner-rings and angles of successive inclined ring boundaries are used to correct ring-width series. Differences in ring-width measurements between MtreeRing and WinDENDRO in a given coniferous species (Larix gmelinii) were assessed, and no significant difference between programs was found. Furthermore, the package provides an R-based web application which was developed using the Shiny framework. This beginner-friendly application allows viewing and interacting with tree ring images. It requires no programming experience and can run on either a local computer or a remote server.     

Dendroclimatic potential of dendroanatomy in temperature-sensitive Pinus sylvestris

The most frequently and successfully used tree-ring parameters for the study of temperature variations are ring width and maximum latewood density (MXD). MXD is preferred over ring width due to a more prominent association with temperature. In this study we explore the dendroclimate potential of dendroanatomy based on the first truly well replicated dataset. Twenty-nine mature living Pinus sylvestris trees were sampled in North-eastern Finland at the cool and moist boreal forest zone, close to the latitudinal tree line, where ring width, X-ray MXD as well as the blue intensity counterpart MXBI were compared with dendroanatomical parameters. Maximum radial cell wall thickness as well as anatomical MXD and latewood density appeared to be the most promising parameters for temperature reconstruction. In fact, these parameters compare favorably to MXD derived from X-ray techniques as well as MXBI, in terms of shared variation and temperature correlations across frequencies and over time. The reasons for these results are thought to be the unprecedentedly high measurement resolution of the anatomical technique, which provide the optimal resolution – the cell – whereas X-ray techniques have a slightly lower resolution and BI techniques even lower. While the results of this study are encouraging, further tests on longer and multigenerational chronologies are required to more generally and fully assess the dendroclimate potential of anatomical parameters.     

Cross-sectional interpolation of annual rings within a 3D root model

Ring width of a given year can be highly variable throughout the cross section of a stem. This is especially true for roots. Therefore, the entire circumference of tree rings is often needed for studies focusing on specific reactions of individual trees on certain environmental conditions. Also, ring reconstructions are of interest for biomass calculations estimated by the cross-sectional area. The aim of the study is thus to reconstruct tree rings of cross sections within a 3D root-surface model, which will be the basis for an upcoming 3D root-development model. A FARO ScanArm was used for the acquisition of the 3D root structure (Technologies Inc., 2010). Afterwards ring-width data was measured along 4 radii per cross section and the resulting ring boundaries were integrated into the 3D root model. A weighted interpolation algorithm was used to reconstruct entire ring-width profiles of the cross sections. The algorithm considered the ring-width variations of the adjacent radii as well as the outer shape of the cross section. Hence, the intention was to estimate ring width around the root circumference using ring widths measured along 4 radii and the surface dimensions of roots. Interpolated ring-width data was compared to the measured tree-ring data as a control for the developed interpolation algorithm. Comparisons between modelled and empirical values showed a mean absolute error of about 0.06 mm deviation, and with a few exceptions the growth patterns could be accurately simulated. This has permitted additional radii measurements to be replaced by model interpolations.     

The potential of intra-annual density information for crossdating of short tree-ring series

The crossdating of tree-ring series is typically based on tree-ring width sequences, which is a crude abstraction of the growth signal stored in tree rings. In contrast, intra-annual wood density data allows a much more detailed comparison of wood growth processes and new measurement techniques scale well to measure large amounts of samples. Thus, chronologies of intra-annual densitometric curves can be built. Here, we investigate to what extent intra-annual wood density information can improve crossdating. We evaluate different approaches on a data set of Norway spruce trees (Picea abies) and compare the results to standard methods that are based on ring width or maximum density. Our results show that intra-annual densitometric data indeed increases crossdating success rate notably for short tree ring series that cover less than 25 years.     

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.     

Evaluation of X-ray densitometry to identify tree-ring boundaries of two deciduous species from semi-arid forests in Brazil

The presence of visible annual rings in semi-arid tropical trees may allow the application of dendrochronological methods. However, variation in water availability may cause the formation of narrow, irregular ill- or non-defined annual rings hindering the correct dating of tree-ring series. We aimed to evaluate X-ray densitometry as a method to identify tree rings of two deciduous tree species from the Caatinga forest, a semi-arid region in the northeast of Brazil, and compare with two other methods commonly used in dendrochronology, the sliding-stage micrometer and image analysis. Xylem was observed macro- and microscopically and wood anatomical features were assessed in Aspidosperma pyrifolium and Poincianella pyramidalis trees. In both species, tree-ring boundaries were identified considering intra-annual density patterns and wood anatomical features. No significant differences in tree-ring widths were found among methods. X-ray densitometry measurements showed a positive correlation with the measurements obtained with image analysis and sliding-stage micrometer in A. pyrifolium and P. pyramidalis, revealing the high reliability of the methods used. However, inter-correlation of tree-ring width series showed differences in the accuracy of crossdating across measuring methods. The maximum, mean and minimum density values were species-dependent, with mean wood density of A. pyrifolium lower than P. pyramidalis. Our results highlight X-ray densitometry as an important and complementary tool to identify tree-rings boundaries in semi-arid tree species, especially in A. pyrifolium. Along with other measuring methods, it may provide higher accuracy in dendrochronological studies in semi-arid or subtropical environments.     

利用树木年轮重建赣南地区1890年以来 2-3月份温度的变化

采用树木年轮气候学方法,利用江西赣南地区马尾松(Pinus massoniana)的年轮宽度资料,分析了马尾松径向生长与气候要素变化的相关及响应关系,结果表明赣南地区马尾松径向生长与当年2—3月份的平均温度相关性最高。在响应分析的基础上,重建了江西赣南地区1890年以来2—3月份温度的变化历史。重建序列显示在过去119a中研究区存在3个较为明显的冷期(1892—1906年、1918—1922年、1944—1957年)和3个明显的暖期(1909—1917年、1959—1968年和1998—2008年)。     

Assessing bias in diameter at breast height estimated from tree rings and its effects on basal area increment and biomass

Above-ground forest productivity can be reliably estimated from tree-ring width measurements. In doing so, annual growth is linked to the tree’s basal area increment (BAI), which is the change in cross-sectional area associated with each annual ring. When BAI is estimated from ring-width series, a value for the diameter of the tree is required. This diameter is ideally measured in the field, but can also be estimated as the sum of the annual ring widths. Tree biomass can also be estimated directly from the diameter estimates derived from tree-rings. Summing the ring widths, however, typically underestimates the tree’s true diameter. To evaluate this potential bias in diameter, we compared field-measured diameter and diameter estimated from the sum of the ring widths using tree-ring chronologies for seven common species in the eastern United States. We then evaluated the impacts of using the biased diameter estimates on derived BAI and biomass values. To simulate field-sampling error (i.e., failure to reach the pith when obtaining a core sample), we re-calculated BAI and biomass after removing a portion of the innermost rings from each tree. Comparisons of these various methods quantify the substantial and consistent underestimations in forest productivity estimates. To reduce the bias in diameter when using ring widths, we developed a regression model to adjust the diameter using core samples. This model is predicated on having some field-measured diameter values available at a site to calibrate and validate the model, but it can then be used to produce estimates at similar sites with similar species where no field-measured diameter values are available. Values of BAI and biomass derived from model-estimated diameter were more accurate at representing absolute growth than values produced by using the sum of the ring widths. Assessing the interannual variations in tree-growth is dependent on having metrics that accurately reflect the area and mass of wood produced. Our results suggest that published estimates of BAI and biomass using the sum of the ring widths to estimate diameter have substantially underestimated these productivity metrics. Our new procedure allows for more reliable estimates of productivity metrics that use diameter-at-breast height derived from tree rings.     

Long-term increase of March temperature has no negative impact on tree rings of European larch (Larix decidua) in lowland Poland

European larch trees grown outside their natural range were intensively studied in terms of their adaptability to increased late winter temperature. A total of 264 cores of larch were taken from 12 sites throughout northern Poland at sites outside of its natural range. Clustering of de-trended chronologies reveal that research sites are grouped geographically. The mean ring widths, however, are similar amongst the four regions. In total, 21?% of the single correlations for all months for tree-ring widths were significant at the 5?% error level. For earlywood and latewood, the figures are 18 and 19?%, respectively. A generally positive precipitation effect was found, however, no significant negative effect was observed. Temperature had mixed effects on ring width. Higher temperatures in late winter and early spring increased ring width in region 2. High summer temperatures in both the previous and the current year, on the other hand, generally had a negative impact on tree-ring width during the growing season. On most of the sites, the observed long-term trend for increasing temperatures in March had no significant negative influence on tree growth. The visual condition of European larch together with the lack of any negative response of tree rings to observed March temperature increase suggest that it may be beneficial to introduce this species into new areas.     

Assessing forest vulnerability to climate warming using a process‐based model of tree growth: bad prospects for rear‐edges

Growth models can be used to assess forest vulnerability to climate warming. If global warming amplifies water deficit in drought‐prone areas, tree populations located at the driest and southernmost distribution limits (rear‐edges) should be particularly threatened. Here, we address these statements by analyzing and projecting growth responses to climate of three major tree species (silver fir, Abies alba; Scots pine, Pinus sylvestris; and mountain pine, Pinus uncinata) in mountainous areas of NE Spain. This region is subjected to Mediterranean continental conditions, it encompasses wide climatic, topographic and environmental gradients, and, more importantly, it includes rear‐edges of the continuous distributions of these tree species. We used tree‐ring width data from a network of 110 forests in combination with the process‐based Vaganov–Shashkin‐Lite growth model and climate–growth analyses to forecast changes in tree growth during the 21st century. Climatic projections were based on four ensembles CO₂ emission scenarios. Warm and dry conditions during the growing season constrain silver fir and Scots pine growth, particularly at the species rear‐edge. By contrast, growth of high‐elevation mountain pine forests is enhanced by climate warming. The emission scenario (RCP 8.5) corresponding to the most pronounced warming (+1.4 to 4.8 °C) forecasted mean growth reductions of ?10.7% and ?16.4% in silver fir and Scots pine, respectively, after 2050. This indicates that rising temperatures could amplify drought stress and thus constrain the growth of silver fir and Scots pine rear‐edge populations growing at xeric sites. Contrastingly, mountain pine growth is expected to increase by +12.5% due to a longer and warmer growing season. The projections of growth reduction in silver fir and Scots pine portend dieback and a contraction of their species distribution areas through potential local extinctions of the most vulnerable driest rear‐edge stands. Our modeling approach provides accessible tools to evaluate forest vulnerability to warmer conditions.     

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.     

Characteristics of a multi-species conifer network of wood properties chronologies from Southern Australia

Progress in quantitative wood anatomy has resulted in a growing number of increasingly understood proxies from the tree-ring archive. Much of this work has been based on tree species in the Northern Hemisphere. Here, we present and examine a relatively dense network of wood property chronologies (wood density, tracheid radial diameter, cell wall thickness and ring width) from several species in Tasmania, southern Australia. We ask how the relationships amongst the different types of chronologies differ within and amongst species. We also consider how each chronology responds to monthly climate. In general terms, and similar to findings in the Northern Hemisphere, relationships between the various wood properties and climate are stronger than those between climate and ring width chronologies. An important exception to this is the highest elevation Lagarostrobos franklinii site. Additionally, strongest response to climate for the wood properties generally occurs for the concurrent growing season compared to the prior growing season for ring width. Relationships amongst the various chronology types differ for the various species, with L. franklinii also showing some variation in these relationships by site (possibly associated with elevation). Results suggest there is considerable value in further exploring the potential for developing anatomical wood chronologies for climate reconstruction from other species for which ring widths do not exhibit a strong climate signal.     

An assessment of growth ring identification in subtropical forests from northwestern Argentina

Most subtropical forests in South America are located in regions with a marked seasonality in precipitation, which may induce the formation of annual bands in woody species. Due to the lack of precise information on tree-ring visibility, we evaluated the wood characteristics of 37 tree species in the subtropical Yungas and Chaco forests from northwestern (NW) Argentina. Anatomical features associated with the delimitation of growth bands were examined to establish the presence of tree rings. Different forest types reflect the precipitation gradients and wood anatomical features vary accordingly. Characteristics of wood structure are closely related to the dominant climatic patterns of each forest, revealing a common pattern of anatomical arrangements in terms of water transport and safety. In the Chaco and transitional forests, ring boundaries are related to marginal parenchyma whereas in montane forests growth ring boundary is mostly associated with the presence of thicker fibers at the end of the ring. The largest proportion of species with clearly marked growth rings occurs in the montane forest type of NW Argentina. Clear growth rings is a requisite for dendrochronological applications, hence the present work represents the first regional attempt to address the potential of subtropical species in South America to be used in dendrochronological studies.