Millennium-Scale Crossdating and Inter-Annual Climate Sensitivities of Standing California Redwoods

Extremely decay-resistant wood and fire-resistant bark allow California’s redwoods to accumulate millennia of annual growth rings that can be useful in biological research. Whereas tree rings of Sequoiadendron giganteum (SEGI) helped formalize the study of dendrochronology and the principle of crossdating, those of Sequoia sempervirens (SESE) have proven much more difficult to decipher, greatly limiting dendroclimatic and other investigations of this species. We overcame these problems by climbing standing trees and coring trunks at multiple heights in 14 old-growth forest locations across California. Overall, we sampled 1,466 series with 483,712 annual rings from 120 trees and were able to crossdate 83% of SESE compared to 99% of SEGI rings. Standard and residual tree-ring chronologies spanning up to 1,685 years for SESE and 1,538 years for SEGI were created for each location to evaluate crossdating and to examine correlations between annual growth and climate. We used monthly values of temperature, precipitation, and drought severity as well as summer cloudiness to quantify potential drivers of inter-annual growth variation over century-long time series at each location. SESE chronologies exhibited a latitudinal gradient of climate sensitivities, contrasting cooler northern rainforests and warmer, drier southern forests. Radial growth increased with decreasing summer cloudiness in northern rainforests and a central SESE location. The strongest dendroclimatic relationship occurred in our southernmost SESE location, where radial growth correlated negatively with dry summer conditions and exhibited responses to historic fires. SEGI chronologies showed negative correlations with June temperature and positive correlations with previous October precipitation. More work is needed to understand quantitative relationships between SEGI radial growth and moisture availability, particularly snowmelt. Tree-ring chronologies developed here for both redwood species have numerous scientific applications, including determination of tree ages, accurate dating of fire-return intervals, archaeology, analyses of stable isotopes, long-term climate reconstructions, and quantifying rates of carbon sequestration.     

Frequency of partial and missing rings in Acer saccharum in relation to canopy position and growth rate

Locally absent growth rings are known to occur in trees during periods of environmental stress, but little evidence has been available on their frequency in moist temperate forests or on their potential for causing problems in dendroecological studies. In this study, 95 stem disks of Acer saccharum cut from trees representing a wide range of size and age were examined for ring anomalies using three techniques – inspection of disks for partial rings, crossdating of each tree against regional master chronologies, and use of stand history evidence. The number of ring anomalies in Acer saccharum was inversely related to growth rate and vigor. Mean percentage of ring anomalies was 1.3% in dominant canopy trees, 4.6% in trees of the intermediate crown class, and 16.2% in overtopped trees. All of the overtopped trees had partial or missing rings, with a mean of 10 per tree and a range of 2 to 20. Use of crossdating to correct for ring anomalies appeared to be successful in 88% of the canopy trees. However, crossdating results were inconclusive in 32% of the overtopped trees because of low ring width variability, multiple missing rings, and short duration of trouble-free segments. Because overtopped trees are of limited value in reconstructing disturbance history and often cannot be crossdated reliably, we suggest that they normally be excluded from disturbance chronologies.     

High-fidelity representation of climate variations by Amburana cearensis tree-ring chronologies across a tropical forest transition in South America

Numerous ring-width chronologies from different species have recently been developed in diverse tropical forests across South America. However, the temporal and spatial climate signals in these tropical chronologies is less well known. In this work, annual growth rings of Amburana cearensis, a widely distributed tropical tree species, were employed to estimate temporal and spatial patterns of climate variability in the transition from the dry Chiquitano (16–17°S) to the humid Guarayos-southern Amazon (14–15°S) forests. Four well-replicated chronologies (16–21 trees, 22–28 radii) of A. cearensis were compared with temperature and precipitation records available in the region. The interannual variations in all four A. cearensis tree-ring chronologies are positively correlated with precipitation and negatively with temperature during the late dry-early wet season, the classic moisture response seen widely in trees from dry tropical and temperate forests worldwide. However, the chronologies from the dry Chiquitano forests of southern Bolivia reflect the regional reduction in precipitation during recent decades, while the chronologies from the tropical lowland moist forests in the north capture the recent increase in precipitation in the southern Amazon basin. These results indicate that A. cearensis tree growth is not only sensitive to the moisture balance of the growing season, it can also record subtle differences in regional precipitation trends across the dry to humid forest transition. Comparisons with previously developed Centrolobium microchaete chronologies in the region reveal a substantial common signal between chronologies in similar environments, suggesting that regional differences in climate are a major drivers of tree growth along the precipitation gradient. The difficulty of finding A. cearensis trees over 150-years old is the main limitation involved in the paleoclimate application of this species. The expansion of monocultures and intensive cattle ranching in the South American tropics are contributing to the loss of these old growth A. cearensis trees and the valuable records of climate variability and climate change that they contain.     

Dendrochronological potential of the Azorean endemic gymnosperm Juniperus brevifolia (Seub.) Antoine

Tree-ring inter-annual pattern variation is crucial in dendrochronology, allowing the identification of possible limiting factors on growth. Thus, trees exposed to subtropical or tropical climates without a marked seasonality may show a low degree of interannual variation, impeding a straightforward dendroclimatological approach. Meanwhile, subtropical regions, and areas in transitional climates such as the Azores archipelago, are widely unexplored in terms of dendroclimatology, providing opportunities to work with endemic trees, including the dominant Azorean tree Juniperus brevifolia (Seub.) Antoine. To evaluate the dendrochronological potential of J. brevifolia, we analyzed tree-ring patterns, crossdating capabilities, and correlation with climate parameters. We sampled 48 individual trees from two natural populations (São Miguel and Terceira islands) using an increment borer. Besides, a Trephor tool was used to obtain wood microcores for micro-anatomical analysis. Although the transition between early and latewood was evident, partially indistinct ring boundaries and wedging rings were present in some cases, affecting the crossdating process, but not impeding the establishment of reliable ring-width chronologies. Following detrending, master chronologies were built and correlated with monthly temperature and precipitation data using the treeclim R package. The climate-growth relationships indicated negative correlations with late summer temperature in both populations. Considering our results and the importance of J. brevifolia as a dominant tree in the Azores natural forests, we conclude that it shows an acceptable potential for dendrochronological research. Thus, this study provides baseline information to help fill the knowledge gap regarding the climate-growth relationship of Azorean trees.     

Ancient juniper trees growing on cliffs: toward a long Mediterranean tree-ring chronology

Juniperus phoenicea is a tree that can grow on vertical cliff faces in dry and warm Mediterranean climate conditions. These trees are adapted to extreme growing conditions where the main constraints are verticality, compact hard limestone, and low water supply. They respond to these constraints via various specific features and high longevity. The objective of this study is to confirm whether or not their tree-rings are annual in order to specify growth strategies and accurately date these trees. Trunk morphology, anatomical wood anomalies and radial growth were analyzed on 53 trees in the Ardèche canyon. Crossdating of the ring widths using traditional dendrochronological techniques was unsuccessful, so radiocarbon dating of tree pith was used to assess tree age, and wiggle-match dating was used to test for differences between number of rings counted and radiocarbon dates. Radiocarbon dates span the period 2520–685 BP. Minimal difference between radiocarbon dates and ring counts was apparently small—missing rings occur, but not in large numbers. Tree-ring formation is annual and radial growth is low, which creates stunted old trees. Such old living trees are uncommon in the Mediterranean basin, especially at low elevation. They can provide long tree-ring chronologies back to 792–524 cal BC. Results from the radiocarbon dating indicate that accurate annual dating of these rings may be possible by crossdating. J. phoenicea growing on cliffs offer a valuable model to better understand cliff population ecology and the functional responses of trees that can live in harsh environmental conditions.     

RingdateR: A statistical and graphical tool for crossdating

Crossdating is the defining technique of dendrochronology, ensuring that all measurements in a dataset are annually resolved and absolutely dated. This level of accuracy allows for the development of high-resolution environmental reconstructions of climate, disturbance, and productivity not only in trees, but also in other ring-forming organisms including fish, corals, and bivalves. However, crossdating is a laborious process and can be a significant bottleneck in the development of new chronologies, especially when attempting to find matches among undated, dead-collected material. Several software packages have been developed to aid in crossdating, yet efficiently identifying matches among large numbers of dead-collected samples of unknown antiquity remains a gap in functionality. To address this issue we present RingdateR, a new stand-alone and web-based application for statistical and graphical crossdating that shares many key features with established crossdating applications, but has been optimised for crossdating large collections of dead-collected material. The workflow allows users to load undated measurement time series in common file types (e.g. pos, lps, csv, and xlsx) to be matched against one another (i.e. pairwise analyses) or against an existing chronology. RingdateR provides graphical tools to help identify false or missed rings remaining in the matched measurement time series and to evaluate the impact of the newly crossdated measurement time series on chronology statistics. Whilst the decision as to whether a sample is correctly crossdated ultimately lies with careful visual inspection and discretion of the investigator, guidance from RingdateR can greatly expedite the process, especially when building chronologies that involve dead-collected samples.     

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.     

How do Araucaria angustifolia trees grow in overstocked stands?

The objective of this study was to evaluate growth along the stem of Araucaria angustifolia (Bertol.) Kuntze trees competing in overstocked stands, in order to identify periods when growth and trunk shape are differentiated during the trees' lifespan. The research was carried out in a planted forest of Araucaria angustifolia established in 1946 in the Açungui National Forest in Campo Largo, Paraná, Brazil, when these trees were 65 years old. One thinning was recorded, at some time between 1970 and 1980. Forty-six trees were selected and divided into three development classes (DC) at 65 years of age; these classes considered diameter at 1.30 m (breast height, dbh) with a range of 20 cm (from 10 cm to 70 cm). In addition to dbh, total tree height, and crown height and diameter were measured in the field. From each tree, 14 disks were removed to analyze growth rings and confirm the age of the stand. Some trees in the smallest DC (10 ≤ dbh < 30 cm) were the product of natural regeneration (younger trees that grew after the initial planting). In 63% of the trees, at least one growth ring was missing at breast height. Missing rings at breast height were more common in trees with smaller dbh and crown diameters. The need for more growing space was observed at different periods during the studied lifespan of the trees from three DCs. It resulted in changes in stems shape from conical to cylindrical. Different growth patterns could be observed during the lives of some trees as they outgrew their competition.     

Recent growth increase in endemic Juglans boliviana from the tropical Andes

The spatial coverage of tree-ring chronologies in tropical South America is low compared to the extratropics, particularly in remote regions. Tree-ring dating from such tropical sites is limited by the generally weak temperature seasonality, complex coloration, and indistinct anatomical morphology in some tree species. As a result, there is a need to complement traditional methods of dendrochronology with innovative and independent approaches. Here, we supplement traditional tree-ring methods via the use of radiocarbon analyses to detect partial missing rings and/or false rings, and wood anatomical techniques to precisely delineate tree-ring boundaries. In so doing we present and confirm the annual periodicity of the first tree-ring width (TRW) chronology spanning from 1814 to 2017 for Juglans boliviana (‘nogal’), a tree species growing in a mid-elevation tropical moist forest in northern Bolivia. We collected 25 core samples and 4 cross-sections from living and recently harvested canopy-dominant trees, respectively. The sampled trees were growing in the Madidi National Park and had a mean age of 115 years old, with certain trees growing for over 200 years. Comparison of (residual and standard) TRW chronologies to monthly climate variables shows significant negative relationships to prior year May-August maximum temperatures (r = −0.54, p < 0.05) and positive relationships to dry season May-October precipitation (r = 0.60, p < 0.05) before the current year growing season. Additionally, the radial growth of Juglans boliviana shows a significant positive trend since 1979. Our findings describe a new and promising tree species for dendrochronology due to its longevity and highlight interdisciplinary techniques that can be used to expand the current tree-ring network in Bolivia and the greater South American tropics.     

Climatic Signals in Tree Rings of Heritiera fomes Buch.-Ham. in the Sundarbans,Bangladesh

Mangroves occur along the coastlines throughout the tropics and sub-tropics, supporting a wide variety of resources and services. In order to understand the responses of future climate change on this ecosystem, we need to know how mangrove species have responded to climate changes in the recent past. This study aims at exploring the climatic influences on the radial growth of Heritiera fomes from a local to global scale. A total of 40 stem discs were collected at breast height position from two different zones with contrasting salinity in the Sundarbans, Bangladesh. All specimens showed distinct tree rings and most of the trees (70%) could be visually and statistically crossdated. Successful crossdating enabled the development of two zone-specific chronologies. The mean radial increment was significantly higher at low salinity (eastern) zone compared to higher salinity (western) zone. The two zone-specific chronologies synchronized significantly, allowing for the construction of a regional chronology. The annual and monsoon precipitation mainly influence the tree growth of H. fomes. The growth response to local precipitation is similar in both zones except June and November in the western zone, while the significant influence is lacking. The large-scale climatic drivers such as sea surface temperature (SST) of equatorial Pacific and Indian Ocean as well as the El Niño-Southern Oscillation (ENSO) revealed no teleconnection with tree growth. The tree rings of this species are thus an indicator for monsoon precipitation variations in Bangladesh. The wider distribution of this species from the South to South East Asian coast presents an outstanding opportunity for developing a large-scale tree-ring network of mangroves.     

Dendrochronological analysis of Sequoia sempervirens in an interior old-growth forest

Dendrochronological studies of large and old Sequoia sempervirens are limited by access and complex crossdating, but core sampling at regular height intervals along the main trunks of five standing trees allowed for reconstruction of growth, height, and age while providing within-tree replication for crossdating. We developed a crossdated ring-width chronology (1453–2015) for redwoods growing in an easternmost old-growth forest in the Napa Range of California, determined aboveground tree attributes, investigated the inter-annual climate-growth relationships since the late 19th century, and documented long-term growth trends. Age, height, f-DBH (functional diameter at breast height), and aboveground biomass of these co-dominant trees ranged from 241 to 783 years, 45.7 to 61.5 m, 117.0 to 226.9 cm, and 9.34 to 33.62 Mg, respectively. Bootstrapped correlation and response function analysis showed radial growth positively related to May through August Palmer Drought Severity Index (PDSI) and negatively related to maximum June temperature (r ≥ │0.47│, P < 0.0001), explaining 33.3% of ring-width variation. Bootstrapped correlations over a moving 40-year window indicated strengthening relationships with PDSI and minimum temperature. The long-term growth trend, reflected by the size-detrended metric of residual wood volume increment (RWVI), varied over time and showed an average one-year decrease of 13.3% for 20th and 21st century droughts. A fire detected in August 1931 corresponded with a one-year decrease in RWVI of 43.1% followed by >100% increase within five years. Growth dynamics for redwoods in this interior forest provide a point of comparison for redwoods previously studied in old-growth forests along the latitudinal gradient, highlighting range-wide trends and site-specific differences in responses to climate and fire.     

Crossdating Juniperus procera from North Gondar, Ethiopia

The application of dendrochronology in (sub)tropical regions has been limited by the difficulty in finding trees with distinct annual rings that can be crossdated. Here, we report successful crossdating of Juniperus procera trees from North Gondar, Ethiopia. The trees form annual rings in response to a unimodal rainfall regime. The selection of mesic locations ensured that the trees did not respond to intra-seasonal weather anomalies. Crossdating was achieved by comparison of the wood anatomy directly on the surface of the core samples and purpose-adapted skeleton plotting. Wood-anatomical anomalies, such as false and indistinct rings, were regarded as potentially replicated features and used in crossdating. COFECHA yielded site-specific mean series inter-correlations between 0.52 and 0.59. AMS radiocarbon dating during the bomb era indicated that dating uncertainty is ±1 year.     

Crossdating and analysis of eucalypt tree rings exhibiting terminal and reverse latewood

We investigated crossdating and climate sensitivity in tree-ring series from Eucalyptus delegatensis Baker, R.T. and E. obliqua L'Herit. We first visually crossdated the measured ring width series and then independently verified this crossdating using Xmatch and cross-correlation significance tests. Crossdating was verified in 28 of the 32 study trees. Crossdating success differed between E. delegatensis and E. obliqua. In E. delegatensis crossdating success appears to be related to tree dominance and elevation. In E. obliqua radial azimuth appears to affect crossdating success. We developed two chronologies for each of the species studied. The first of these chronologies was based on all visually crossdated radii and the other on radii for which crossdating had been independently verified. Signal strength was higher in the verified chronologies. Correlation analysis between the verified chronologies and climate data revealed no significant correlation between precipitation and ring width for either species. E. obliqua ring width was significantly correlated with mean minimum and maximum air temperature and vapour pressure deficit during summer of the growing season. The E. delegatensis chronology was significantly correlated with air temperature and frequency of frost during the preceding winter. Ring width in both species was significantly correlated with air temperature during the preceding summer. Potential physiological explanations for these results are discussed. Further study is required to verify the results of climatological analysis and to explore the causes of variation in signal strength within and between trees.     

Evaluating the potential for dendrochronological analysis of live oak (Quercus virginiana Mill.) from the urban and rural environment—An explorative study

Live oak (Quercus virginiana Mill.) is a long lived, broadleaf, evergreen species native to the lower Coastal Plain of the southern and southeastern United States and is frequently planted in the urban environment. However, live oak has received limited attention for its potential for dendrochronological analyses possibly because its growth-ring boundaries may be less defined due to the absence of a distinct dormancy period. The objective of this explorative study was to determine whether live oak rings in both urban and historic trees could be accurately identified and measured for dendrochronological applications. One hundred urban live oaks were assessed as well as four cross-sectional specimens from historic sites in the southeastern United States. Age of all cored live oak ranged from 7 to 29 (average = 16.8 rings), while cross-sections were taken from trees with an average age of 175 years. A chronology was created from cores that had the highest interseries correlations (n = 27), resulting in an average interseries correlation of 0.654. The analysis of live oak cross sections revealed that the rings on live oak are very faint and barely discernible, delimited mainly by a faint row of earlywood vessels with no or barely visible terminal parenchyma. Rings on these sections were erratic (i.e., fading out along the circumference) and non-concentric (i.e., a particular ring could be both wide and narrow at certain points along the circumference) so that any attempt to measure the rings on opposing radii would have produced meaningless results. However, based on the results of this study, we conclude that the dendrochronological analysis of live oak is possible with careful sample preparation and analysis and that results would be reliable, even for urban trees. Results of this study suggest that urban tree-ring series with higher interseries correlations were more vigorous than those with lower values, suggesting that urban environmental stresses related to individual sites may decrease the usefulness of crossdating techniques for evaluating annual rings on younger, urban trees.     

Evaluating the annual nature of juvenile rings in Bolivian tropical rainforest trees

Knowledge on juvenile tree growth is crucial to understand how trees reach the canopy in tropical forests. However, long-term data on juvenile tree growth are usually unavailable. Annual tree rings provide growth information for the entire life of trees and their analysis has become more popular in tropical forest regions over the past decades. Nonetheless, tree ring studies mainly deal with adult rings as the annual character of juvenile rings has been questioned. We evaluated whether juvenile tree rings can be used for three Bolivian rainforest species. First, we characterized the rings of juvenile and adult trees anatomically. We then evaluated the annual nature of tree rings by a combination of three indirect methods: evaluation of synchronous growth patterns in the tree- ring series, ¹⁴C bomb peak dating and correlations with rainfall. Our results indicate that rings of juvenile and adult trees are defined by similar ring-boundary elements. We built juvenile tree-ring chronologies and verified the ring age of several samples using ¹⁴C bomb peak dating. We found that ring width was correlated with rainfall in all species, but in different ways. In all, the chronology, rainfall correlations and ¹⁴C dating suggest that rings in our study species are formed annually.     

Dynamics of composition and structure in an old Sequoia sempervirens forest

Abstract. Dynamics of a Sequoia sempervirens forest in northern California were studied with long‐term plot data (1.44 ha) and recent transect data. The study was conducted in an old stand (> 1100 yr) on alluvial flats. Over three decades (1972–2001), changes in the composition and structure of the tree stratum were minor. Sequoia maintained a broad distribution of stem diameters throughout the period. Annual rates of Sequoia mortality (0.0029) and ingrowth (0.0029) were low, reflecting the great longevity of Sequoia and the slow canopy turnover of the study forest. Transect data also indicated a low frequency of canopy gap disturbance (≤ 0.4% of total land area per yr), but gap size was potentially large (> 0.1 ha) and the fraction of area in gaps (ca. 20%) was similar to other temperate forests. Regeneration quadrats sampled along transects, in gap centers, and on logs revealed that Sequoia regeneration is elevated at gap edges. The longevity of Sequoia and its response to gap disturbances ensure that it will remain a dominant species in the study forest.     

Ages and long-term growth patterns of four threatened Vietnamese tree species

Conservation of threatened tree species requires basic information on growth rates and ages. This information is lacking for many species, but can be obtained relatively easily from tree ring analysis. We studied four threatened Vietnamese species: three conifers from high-elevation forests (Calocedrus macrolepis, Dacrydium elatum and Pinus kwangtungensis) and one broad-leaved species from lowland forest (Annamocarya sinensis). We collected increment cores from remnant populations in protected areas and measured ring width. We built chronologies and found significant correlations with rainfall (all species) and temperature (two species), indicating that rings were formed annually. Diameter-age trajectories showed that species reached reproductive size at 60–80 years. Maximum observed ages were 128–229 years. Some species showed large variation in long-term growth rates among individuals, which was partially explained by strong persistence of growth differences. We also assessed whether growth rates changed over time. For certain size categories in some species, we found higher recent growth rates of juvenile trees compared to those in the distant past. This shift requires a cautious interpretation, but is consistent with a CO₂ fertilization effect. For other size categories, we found contrasting results: extant large trees had higher growth rates as small juveniles compared to extant small trees. Such correlations, which we found for all species, suggest a ‘juvenile selection effect’: the preferential survival of fast-growing juveniles to the canopy. Information on ages, historical growth rates and juvenile selection effect is relevant for the planning of conservation actions.     

Climate response of cork growth in the Mediterranean oak (Quercus suber L.) woodlands of southwestern Portugal

In the Mediterranean climate regions, drought events are expected to affect the growth of forests ecosystems by changing trees growth rates and eventually inducing shifts in their growth patterns. Cork oak (Quercus suber L.) is a strictly western Mediterranean tree species periodically harvested for its bark, the cork. So far, cork oak has received limited attention for dendroclimatological studies due to its typical faint and erratic tree wood rings. Moreover, its distinct cork rings chronologies have been completely neglected. In this study we introduce an approach using cork ring chronologies dated back 9–10 years for climate response. Despite enhancing interannual variability and increasing statistical response to short-term climatic variability, still poorly understood, this study will possibly allow infer long-term climate response. We analyzed the cork ring chronologies of 55 cork samples collected in mature (under exploitation) trees in three distinct locations in southwestern Portugal. Cork growth recorded a high climate signal, with highly significant and coherent responses to the yearly climate-related sources of variation. We successfully assessed trends of cork growth via correlation analysis including selected climate variables among mean monthly temperature, monthly precipitation and, on an annual basis, eight precipitation indices. The high mean sensitivities and inter-series correlations found for cork ring chronologies combined with the significant variance explained by climate variables suggest that climate is likely one dominant signal that affects cork growth, but local environmental stresses can decisively affect this (climate) signal. Assuming cork growth as a proxy for cork oak growth, it seems conceivable that despite the trees being highly resistant to drought stress, cork oak woodlands in southwestern Portugal would have to face lesser growth in a global warming scenario.