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.     

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.     

Annual Growth Ring Patterns in Brachystegia spiciformis Reveal Influence of Precipitation on Tree Growth1

The availability of exactly dated tree‐ring chronologies is limited in tropical regions. However, these chronologies could contribute widely to studies of the influence of natural and human‐induced factors on tropical forests. We examine the potential for building a chronology based on three sites in the miombo woodland of western Zambia. Brachystegia spiciformis Benth., a dominant species from this vegetation type, is used. Response of the chronology to several climatic factors is examined. All specimens showed very clear growth rings, and cross‐dating between radii of a tree was successful for all trees. Site chronologies could be constructed after cross‐dating of growth ring series of individual trees. The mean growth ring curves of the three sites were significantly similar, allowing for the construction of a regional chronology. Correlation function analysis between the tree‐ring chronology and regional climatic variables revealed that climate at the core of the rainy season, in December and January, has an explicit influence on tree growth. Where precipitation and relative humidity in these months influence tree growth positively, temperature correlates in a negative way. Some 20 percent of the variance in the B. spiciformis tree‐ring chronology is accounted for by wet season rainfall. The successful cross‐dating and correlation between a tree‐ring chronology and climate demonstrated in this study indicate annual ring formation in B. spiciformis trees and sensitivity to climatic conditions.     

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.     

The use of X-ray densitometry to evaluate the wood density profile of Tectona grandis trees growing in fast-growth plantations

Tectona grandis (teak) is an important commercial tree species that is widely used in tropical dendrochronology due to the formation of climate-sensitive annual growth rings. However, young trees growing in plantation conditions exhibit poor ring visibility during the first years of growth, limiting the dendrochronology application. In the present study, we use x-ray densitometry to determine the wood density profile between and within annual rings and at the sapwood-heartwood boundary in trees from fast-growth plantations. The resulting wood density profiles (WDP) can be categorized as uniform, stable growth, unstable growth, and false. The annual ring boundaries were indistinct in trees less than 8 years old. In mature trees, the annual ring boundaries are more defined. In relation to the sapwood-heartwood boundary, the WDP showed a decrease in the wood density; however, this decrease is influenced by the annual ring boundary when the two boundaries coincide. The identification of annual rings in trees growing in fast-growth plantations should be combined with X-ray densitometry and visual identification if wood density data are necessary for deriving other analysis, as climate change, from annual ring.     

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.     

Biases in RCS tree ring chronologies due to sampling heights of trees

The Regional Curve Standardization (RCS) is one of the most employed standardization methods to remove biological signals in long tree ring chronologies. The approach assumes that an overall age-related growth trend typify all tree ring series to be included in a standardized tree ring chronology. Although several potential problems of the method have been examined, the influence of varying the sampling height along tree stems has not been evaluated. Considering that age-related growth trends may vary with stem height, biases may arise when combining samples from unknown or variable sampling heights, a frequent situation with subfossil logs. In this study we perform a detailed stem analysis of 15 lakeshore black spruce (Picea mariana Mill. B.S.P.) trees in the taiga of eastern Canada to describe how the age-related growth trend varies with stem height and evaluate associated biases in RCS chronologies built from living and subfossil trees. Results show that the age-related growth trends vary markedly and systematically along stems, potentially generating large methodological biases in RCS chronologies, especially near the recent chronology end. These biases may lead to erroneous reconstructions of recent climatic trends and cause false divergence between tree ring and climate series. We have developed a correction procedure that appears efficient in removing these biases from chronologies built with the lakeshore trees and associated subfossil logs.     

Overcoming crossdating challenges to generate ring-width chronologies for Sequoia sempervirens across its native range

We utilize a large dendrochronological dataset for Sequoia sempervirens of 1.29 million rings from 5952 series for 278 trees to generate location and regional ring-width chronologies and to investigate constraints on crossdating. Increment cores were collected at regular height intervals along the trunk via free-hanging ropes with some terrestrial deadwood sampled. Inter-annual chronologies span 86–1687 yr for 47 locations in primary and secondary forests across the native Sequoia range. Shared signals based on declining strength of paired correlations with distance, strong correlations within regions, and groupings of principal components guide chronology creation for northern (1032 yr, 108 trees) and southern (865 yr, 52 trees) regions and four sub-regions. The suite of chronologies provides location-specific references as well as more robust regional indices that reflect distinct signals along a climatically-integrative latitudinal gradient. Secondary forests date to near completion, and we use a subset of data to assess crossdating for trees in primary forests. Among tree and landscape attributes, old age is the most important predictor of undated and missing rings, and undated rings align with periods of reduced biomass production distributed across centuries-long lifespans. Crossdating metrics vary by height above the ground with breast height (BH) and 10 m samples having more undated rings than upper samples. At 10 m, samples have the most missing rings. Buttressing affects the lower trunk as BH series have higher ring widths and lower interseries correlations compared to those up to 70 m. Whole-trunk sampling maximizes success for crossdating old, structurally-complex Sequoia and enables dating of crown damage and assessment of biomass dynamics related to environmental change. Reference chronologies, regional assessments, and crossdating insights collectively guide future study of Sequoia for applications including climatology, fire ecology, and seismology.     

Alpine dwarf shrubs show high proportions of nonfunctional xylem: Visualization and quantification of species-specific patterns

Xylem conductive capacity is a key determinant of plant hydraulic function and intimately linked to photosynthesis and productivity, but can be impeded by temporary or permanent conduit dysfunctions. Here we show that persistent xylem dysfunctions in unstressed plants are frequent in Alpine dwarf shrubs and occur in various but species-specific cross-sectional patterns. Combined synchrotron micro-computed tomography (micro-CT) imaging, xylem staining, and flow measurements in saturated samples of six widespread Ericaceae species evidence a high proportion (19%–50%) of hydraulically nonfunctional xylem areas in the absence of drought stress, with regular distribution of dysfunctions between or within growth rings. Dysfunctions were only partly reversible and reduced the specific hydraulic conductivity to 1.38 to 3.57 ×10^?4 m² s^?1 MPa^?1. Decommission of inner growth rings was clearly related to stem age and a higher vulnerability to cavitation of older rings, while the high proportion of nonfunctional conduits in each annual ring needs further investigations. The lower the xylem fraction contributing to the transport function, the higher was the hydraulic efficiency of conducting xylem areas. Improved understanding of the functional lifespan of xylem elements and the prevalence and nature of dysfunctions is critical to correctly assess structure-function relationships and whole-plant hydraulic strategies.     

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.     

Anomalous ring identification in two Australian subtropical Araucariaceae species permits annual ring dating and growth-climate relationship development

Almost all Australian tropical and subtropical regions lack annually-resolved long-term (multi-decadal to centennial scale) instrumental climate records. Reconstructing climate in these regions requires the use of sparse climate proxy records such as tree rings. Tree rings often archive annually-resolved centennial-scale climate information. However, many tropical and subtropical species have short life-spans, the timbers are poorly preserved, and there is a belief that the proxy records of these species are often compromised by ring anomalies. Additionally, for many species the relationship between climate (e.g. temperature and/or rainfall) and tree growth has not been established. These factors have led to tree-ring data being underutilized in the Australian subtropics. Trees in the Araucariaceae family, a common family in northern and eastern Australia, are both longer lived than many species in the Australian subtropics, present growth rings that are annual in nature, and their growth is known to vary with climate. In this study we examine two subtropical Araucariaceae species, Araucaria cunninghamii and Araucaria bidwillii, and quantify the relationship between their radial growth and climate variability. Ring anomalies including false, faint, locally absent, and pinching rings, are found to be present in these species, however, bomb-pulse radiocarbon dating of A. cunninghamii samples together with a whole tree approach helped to identify annual growth patterns despite such anomalous ring boundaries. Additionally, to determine which climate variables most influence growth in these species, dendrometers were installed at two locations in subtropical Southeast Queensland, Australia. We found that rainfall variability drives annual ring growth, while temperature constrains the onset and conclusion of the growth season each year. Our results demonstrate that through the use of A. cunninghamii and A. bidwillii trees which demonstrate annual growth in relation to climate variables there is potential to develop centennial scale climate reconstructions from the Australian subtropics. We provide recommendations on how to best identify ring anomalies in these species to help in the future development of long-term chronologies and climate reconstructions.     

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.     

Effect of climate on tree growth in the Pampa biome of Southeastern South America: First tree-ring chronologies from Uruguay

Tree-ring research in the highland tropics and subtropics represents a major frontier for understanding climate-growth relationships. Nonetheless, there are many lowland regions – including the South American Pampa biome – with scarce tree ring data. We present the first two tree-ring chronologies for Scutia buxifolia in subtropical Southeastern South America (SESA), using 54 series from 29 trees in two sites in northern and southern Uruguay. We cross-dated annual rings and compared tree growth from 1950 to 2012 with regional climate variability, including rainfall, temperature and the Palmer Drought Severity Index – PDSI, the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). Overall, ring width variability was highly responsive to climate signals linked to water availability. For example, tree growth was positively correlated with accumulated rainfall in the summer-fall prior to ring formation for both chronologies. Summer climate conditions were key for tree growth, as shown by a negative effect of hot summer temperatures and a positive correlation with PDSI in late austral summer. The El Niño phase in late spring/early summer favored an increase in rainfall and annual tree growth, while the La Niña phase was associated with less rainfall and reduced tree growth. Extratropical climate factors such as SAM had an equally relevant effect on tree growth, whereby the positive phase of SAM had a negative effect over radial growth. These findings demonstrate the potential for dendroclimatic research and climate reconstruction in a region with scarce tree-ring data. They also improve the understanding of how climate variability may affect woody growth in native forests – an extremely limited ecosystem in the Pampa biome.     

Dendrochronology and dendroclimatology of Ceiba speciosa (A. St.-Hil.) Ravenna (Malvaceae) exposed to urban pollution in Rio de Janeiro city,Brazil

Nowadays, the biological monitoring through the growth rings has received increasing attention from ecologists and toxicologists. Structural analysis of these rings allows the incorporation of a time component in the study of plant responses to environmental variation. This allows also to evaluate long time series from the woody plants. In this paper, we assessed the dendrochronological characteristics of Ceiba speciosa growing in forest environment and under urbanization impact. Stem samples were obtained with Pressler probe into trees growing the campus of the Oswaldo Cruz Foundation, adjacent to one of the main urban thoroughfares of the city of Rio de Janeiro (Avenida Brasil), and at Tinguá Biological Reserve, an important remnant of Atlantic Forest. The samples were processed and analyzed following usual dendrochronological methods, with COFECHA and ARSTAN softwares. A negative exponential curve was used for standardization of the series. The residual chronologies were correlated with precipitation and temperature indexes obtained from NOAA weather database. Growth rings are distinct and annual, marked by bands of marginal parenchyma, thick-walled and radially flattened fibres in latewood and distended rays in earlywood. In both sites, the intercorrelation between the trees was above 0.40. Ages ranged from 11 to 41 years in the urban site and from 27 to 64 years in the forest site. In urban area, mean annual increment and cumulative average growth rates were 6 mm/year and 142.62 mm, respectively. At the forest site, these rates were 4 mm/year and 173.07 mm, respectively. The comparison between cumulative radial increment of the two sites revealed that trees of the urban site had higher increment rates beginning at the start of their development and consequently, they showed similar diameters despite lower ages. Correlation analysis between the chronologies and climatic factors revealed a positive association between growth and hot and rainy periods for both study sites. However, there is an immediate response of urban trees in relation to the rains and, a late response of forest trees to the same factor. The dry and hot climate, typical of urban environments, and the absence of natural water reserves in urban soil, may explain this more immediate response of urban tree growth to rainfall and temperature indexes. Our results revealed that Ceiba speciosa is a plastic and stress-tolerant species that is able to survive and adapt to polluted urban conditions. These features, along with its wide natural distribution and frequent planting for city landscaping, make this species an important biomarker for environmental monitoring studies.     

Dendrochronological assessment of chestnut (Castanea sativa Mill.) for dating purposes in Central Italy

Chestnut is the most common species used for building construction in Central Italy, but the built chronologies are still floating. This is due to a lack of long master chronologies built for chestnut in these regions and because of the lack of agreement with the very few up to date chestnut curves published for other areas. Chestnut appears to be a suitable species for performing dendrochronological analysis because it forms annual rings, and is a ring-porous species. Nevertheless it shows a very fast growth rate, which results in the presence of only a low number of annual rings even in large beams. To further the understanding of the dendrochronological characteristics that are important for dating purposes, this paper discusses an investigation based an a dendrochronological assessment of living chestnut trees.

In total 89 trees from five stands were sampled in Central Italy. The analysis was performed by testing the quality of single master chronologies and comparing the dendrochronological behaviour of trees of the same stand, the crossdating of chestnut chronologies of different provenances, and the interspecific synchronization with oak, beech and silver fir. These trees are the most widespread species in Central Italy and for which reference chronologies have been recently developed - useful for dating purposes. Chestnut chronologies show rather mean sensitivity values. The mean values of synchronization, according to the mean Gleichläufigkeit (GLK) value, mean correlation value (RBT) and expressed population signal (EPS), among the single curves of the same site are not high. Nevertheless, the number of pointer intervals calculated in the site chronologies can be considered rather high in some periods, and the coincidence of maximum and minimum ring width allows the individual curves to be fitted into the mean site chronology. The results of intraspecific synchronization among chestnut site chronologies are not encouraging. Nevertheless, the degree of visual matching among the chronologies play an important role. The best interspecific synchronization was obtained with oak. No notable correlation was found with beech and silver fir.     

Comparative analysis of annual rings of perennial forbs in the Loess Plateau,China

Although recent studies have demonstrated that annual growth rings are present among perennial forbs species at high northern latitudes, little is known about whether there are demarcated growth rings of perennial forbs in the Loess Plateau of China where plant growth is strongly limited by dry climate conditions and severe soil erosion. In this study, we collected the main roots of 11 perennial forbs species along the precipitation gradient in the Loess Plateau, and analyzed the growth rings in the secondary root xylem. We found that ten species showed distinct annual growth rings, and the anatomical patterns, including vessel size and density, varied considerably among different families. Our results suggest, for forbs species in the Loess Plateau, that vessel diameter in the root xylem was strongly correlated with growth rate of the forb’s roots. Ring widths of the forbs showed a significant declining trend, reflecting the deteriorating signal of growth condition with age. In comparison to other families, forb species of Fabaceae usually have the evidently larger vessels that link directly to higher hydraulic capacity and growth rate. In terms of annual ring width patterns, this study provides an applicable approach to detecting effects of limited climatic conditions and life history strategies on herbaceous vegetation in the Loess Plateau.     

Tree ring responses to climate variability of xerophytic thickets from South Soalara,Madagascar

Xerophytic thickets occur along the southwestern part of Madagascar. Although providing a wide variety of resources and services to the local population, this particular vegetation is subjected to deforestation. This study focuses on linking dendroclimatology and dendroecology by examining the spatial and temporal variability of the ecological growth conditions. Information from tree rings was retrieved, on one hand, to identify the problem of the limiting effects of past climate on growth and, on the other hand, to show how local environment takes part in the growth pattern of South Soalara species, in the southwestern part of Madagascar. Methods and principles of dendrochronology were applied on nine species belonging to seven botanical families. A total of 42 stem discs from 3 to 5 trees per species were collected at 30 cm height. All discs exhibited visible tree rings, but anatomical distinctness varied between species. This study highlighted the annual formation of tree rings through successful crossdating techniques. Then, from the nine constructed chronologies, species were grouped into three clusters. Analysis between precipitation and radial growth showed that the response to climate occurred mainly in rainy season. Large-scale climatic drivers such as sea surface temperature (SST¹) of ENSO² regions revealed a teleconnection with tree growth in cluster 1. Mean radial increments were computed from the measured tree ring width and varied from 0.66 to 1.98 mm year⁻¹, showing that those species are slow-growing. All species were recorded as having a certain dendrochronological potential, which was ranked as useful for Rhizogum madagascariense and Terminalia gracilipes, poor for Gyrocarpus americanus and problematic for the other species. It is recommended to increase the number of wood samples and to provide more knowledge on the characteristics of the species in order to improve the quality of the chronology and the climatic signal on tree rings.     

Long Tree-Ring Chronologies Provide Evidence of Recent Tree Growth Decrease in a Central African Tropical Forest

It is still unclear whether the exponential rise of atmospheric CO₂ concentration has produced a fertilization effect on tropical forests, thus incrementing their growth rate, in the last two centuries. As many factors affect tree growth patterns, short -term studies might be influenced by the confounding effect of several interacting environmental variables on plant growth. Long-term analyses of tree growth can elucidate long-term trends of plant growth response to dominant drivers. The study of annual rings, applied to long tree-ring chronologies in tropical forest trees enables such analysis. Long-term tree-ring chronologies of three widespread African species were measured in Central Africa to analyze the growth of trees over the last two centuries. Growth trends were correlated to changes in global atmospheric CO₂ concentration and local variations in the main climatic drivers, temperature and rainfall. Our results provided no evidence for a fertilization effect of CO₂ on tree growth. On the contrary, an overall growth decline was observed for all three species in the last century, which appears to be significantly correlated to the increase in local temperature. These findings provide additional support to the global observations of a slowing down of C sequestration in the trunks of forest trees in recent decades. Data indicate that the CO₂ increase alone has not been sufficient to obtain a tree growth increase in tropical trees. The effect of other changing environmental factors, like temperature, may have overridden the fertilization effect of CO₂.     

Variation in stem radial growth of the Australian conifer, Callitris columellaris, across the world’s driest and least fertile vegetated continent

Climate change could alter the biogeography of many tree species. However, there have been few studies of tree growth across climatic gradients at a continental scale. Callitris columellaris is a widespread conifer that spans many climates and landscape positions across Australia. Our aim was to determine how stem radial growth of C. columellaris varies with tree size and with the biogeographic factors of rainfall, temperature, soil fertility and inter-tree competition. We sampled cores from trees at 85 sites in biomes ranging from tropical savanna to arid desert and temperate forest, and measured widths of the 100 outermost growth rings. We analysed ring width in relation to changes in tree age and diameter, and also evaluated the influence of the biogeographic factors on the width of the ten most recently formed rings. The average width of outermost rings varied only slightly with stem diameter, because the decrease in ring width with age and diameter within trees is offset by an increase with diameter among trees. Our analyses thus explain the weak, inconsistent relationships often observed between stem diameter and growth rate amongst trees. The most important biogeographic factors were the climatic ones: across Australia, ring width increased with both mean annual rainfall and mean annual temperature. These relationships were largely driven by continental scale differences between the tropical and the southern (arid plus temperate) sites, while relationships within climate zones were comparatively weak. Ring width decreased with intense inter-tree competition but showed little correlation with available soil nitrogen or phosphorus.