Tree-ring Wood Research Articles

Tree-ring wood density reveals differentiated hydroclimatic interactions in species along a bioclimatic gradient

Climate variations are influencing the growth dynamics of forests, with direct consequences on their biomass productivity. The scientific community, concerned with refining the understanding of tree-climate interactions, has recently used tree-ring density to elucidate ecological mechanisms that may be overlooked by earlywood, latewood, and total tree-ring widths. In this study, we model the sensitivity of intra-annual wood density in tree-rings for 15 species distributed across a broad bioclimatic and biodiverse gradient in the Mexican Republic. Maximum, minimum, latewood, and earlywood density (MaxD, MinD, LWD, and EWD, respectively) were associated with climatic data through correlation analysis and mixed-effects models. Maximum temperature (TMAX) and precipitation (P) stand out as key drivers of temporal density fluctuations, notably influencing MaxD and LWD as indicators of hydroclimatic regime changes. MinD and EWD are less responsive but prove valuable in gauging environmental sensitivity. Abies religiosa from the Southern Semi-Arid Highlands and Picea martinezii from Mediterranean California were the most responsive species to climate variables. These results open new paradigms for densitometry as a proxy for ecological processes that species face in anticipating dieback phenomena, mortality rates, and resilience mechanisms, with implications for productivity and carbon rates in the face of predicted climate variations. However, the wood density-climate relationship is even more complex, and integrative research combining other proxies (e.g., wood anatomy and chemical composition) is recommended to refine wood density variations and understand the causes of the interspecific differences found in this study.

Detecting changes in industrial pollution by analyzing heavy metal concentrations in tree-ring wood from Romanian conifer forests

The impact of air pollution on forests, especially in urban areas, has been increasingly discussed recently. Many pollutants, including heavy metals, are released into the atmosphere from various sources, such as mining, non-ferrous metal processing plants, and fossil fuel combustion. These pollutants can adversely affect not only tree growth but also other species, including humans.This study compared the concentrations of several elements in tree-ring wood from two conifer species (Silver fir, Abies alba; Norway spruce, Picea abies) growing in polluted and unpolluted areas. Two regions in northern Romania (Bicaz and Tarnița) that were subjected to historical pollution changes were selected. Two chemical analyses were used: inductively coupled plasma mass spectrometry (ICP-MS) and X-ray fluorescence spectrometry (XRF).The silver fir trees from the intensively polluted area in the Tarnița region were negatively impacted by industrial pollution: the Mn concentrations were, on average, three times higher in polluted areas than in unpolluted areas (ca. 30 vs. 10 mg kg−1). This finding was consistent for both ICP-MS and XRF analyses. However, in Norway spruce, this difference was found only in the XRF data, which detected Mn concentrations seven times higher in trees from polluted areas than those from unpolluted areas (ca. 700 vs. 100 mg kg−1). In the Tarnița region, Norway spruce accumulated more heavy metals than silver fir, but the most pronounced differences between polluted and unpolluted areas were found in silver fir.The two analytical methods are commonly used to determine metal concentrations in wood, and they complement each other, with ICP-MS having a low detection limit for some elements and XRF having higher detection limits and better accuracy. Each method has its advantages and disadvantages, and the optimal method depends on many factors, such as the type of heavy metal analyzed, its concentration in wood, sample type, cost, analysis time, and sample preparation.

Planted or Natural Pine Forests, Which One Will Better Recover after Drought? Insights from Tree Growth and Stable C and H Isotopes

Increasing intensity and frequency of droughts are leading to forest dieback, growth decline and tree mortality worldwide. Reducing tree-to-tree competition for water resources is a primary goal for adaptive climate silviculture strategies, particularly in reforested areas with high planting density. Yet, we need better insights into the role of stand type (i.e., natural forests versus plantations) on the resilience of pine forests to droughts across varying time scales. In this study, we combined dendrochronological data and stable C (δ13C) and H (δ2H) isotopes measured in tree-ring wood as well as in specific wood chromatographically isolated compounds to investigate contrasting responses to drought of natural versus planted stands of two representative pine species, i.e., Pinus pinaster and Pinus nigra in southeastern Europe. Natural stands exhibited about two-fold increase in tree-ring growth in average (basal area at 20 years-BAI20) as compared to planted stands. A response function analysis showed contrasting seasonal growth patterns for both species, which were related to monthly mean temperature and precipitation. Both stand type and species variables influenced growth resilience indices. Both pine species revealed contrasting resilience patterns among forest types; whereas planted stands seemed to be less sensitive to yearly droughts as determined by a higher recovery index (CRc) for P. pinaster, the contrary was found in the case of P. nigra. On the other hand, while resistance CRT and resilience CRS indices were higher for planted than natural forests in the case of P. pinaster, little differences were found for P. nigra. Beyond comparisons, carbon stable isotopes shed lights on the role of forest types in dry sites, being δ13C consistently lower in natural than in planted forests for both pine species (p < 0.05). We concluded that planted forest assimilated more carbon as per unit of water used than natural stands in response to droughts. Both δ13C and δ2H isotopic signals were positively correlated for both species for planted forests. However, a lack of correlation was evidenced for natural stands. Consistent with δ13C observations, δ2H concentrations in woody phenolic compounds (guaiacol and oleic acid) revealed contrasting patterns among forest types. This puts forward that δ2H concentrations in woody phenolic compounds (rather than in woody tree ring) accounts for other confounding factors in tree ring formation that can be associated with forest type. Our results highlight the value of stable isotope approaches versus conventional dendrochronological tools in drought studies and call for the consideration of forest type as an endogenous aspect defining the vulnerability of pine forests to climate.

Increasing Wood δ15N in Response to Pig Manure Application

Intensive livestock management impacts forest and trees in different ways. Pig manure is a major source of nitrogen (N) pollution of surface and ground waters in some European regions such as north-eastern Spain, but it is understudied how manure application impacts agroforestry systems. How pig manure affects tree radial growth and the N cycle was assessed by measuring N concentrations in soil, leaves and wood and δ15N in tree-ring wood in two tree species widely planted for agricultural (Prunus dulcis) and reforestation (Pinus halepensis) purposes in the study area. Soil physicochemical characteristics and the biomass and structure of major soil microbial groups were also measured. Trees irrigated with pig effluent (manure application) and control trees not subjected to manure application were compared. Soil N, phosphorus (P) and potassium (K) concentrations of fertilized trees increased, but soil microbiota biomass decreased. Similar growth between fertilized and non-fertilized pine trees was found, but lower growth in fertilized almond trees was observed. Leaf N concentrations decreased but δ15N wood increased in trees subjected to pig manure application. Pig manure application alters the N cycling in the soil and within trees.

Altitude-specific differences in tree-ring δ2H records of wood lignin methoxy in the Qinling mountains, central China

The Qinling Mountains (Qinling Mts.) are characterized as a distinct transition of bioclimatic zones along the north-south geographic boundary in China. Although ongoing global warming may affect the growth of most tree species across elevation gradients, the effect of modern warming on the stable hydrogen isotopes of tree-ring wood is still unknown. In this study, we developed early- and late-wood δ2H chronologies from 1900 to 2018 CE of lignin methoxy groups (δ2HLM) at 700-m intervals from 1700 to 3100 m a.s.l. along the elevation gradient in Mount Taibai (Mt. Taibai), the peak of the Qinling Mts. The δ2HLM values of the earlywood were relatively enriched in 2H compared to the latewood, and an elevation-dependent depleting trend along the altitudinal gradient was observed. Comparing the δ2HLM values with modelled δ2H values of precipitation (δ2HPre) of the sample sites, similar isotopic patterns can be obtained. The reconstructions of δ2HPre values confirm the strong coherence between the isotopic composition of the source water and the methoxy groups in the growing season. The consistency of δ2HLM chronologies between the early- and late-wood at the higher sites was stronger than that at the lower sites. The additional lagged effect and climate response revealed significant seasonal and altitudinal differences. At the timberline site, δ2HLM values of earlywood were mainly related to the January and February temperatures, whereas δ2HLM values of latewood correlated most strongly with temperature from July to September, and both values were controlled by the amount of total precipitation. In addition, the combined new δ2HLM chronologies, based on the lag-1 autocorrelation and the weighted average of the current year earlywood and previous latewood, could be used to merge the corresponding temperature signals from the middle- and low-elevation sites. Altitudinal correlations of elevation offset between the annual δ2HLM chronologies and interpolations of precipitation and temperature suggested that the δ2HLM values at higher elevations better represent regional climatic changes. Therefore, topographic effects such as elevation differences should be considered in large-scale applications of δ2HLM values in future studies.

Learning from the past to improve in the future: tree-ring wood anatomy as retrospective tool to help orchard irrigation management

Learning from the past to improve in the future: tree-ring wood anatomy as retrospective tool to help orchard irrigation management

Long-term nutrient imbalances linked to drought-triggered forest dieback

Drought-induced forest dieback is causing reductions in productivity, increasing tree mortality and impairing terrestrial carbon uptake worldwide. However, the role played by long-term nutrient imbalances during drought-induced dieback is still unknown. To improve our knowledge on the relationships between dieback and nutrient imbalances, we analysed wood anatomical traits (tree-ring width and wood density), soil properties and long-term chemical information in tree-ring wood (1900–2010) by non-destructive Micro X-ray fluorescence (μXRF) and destructive (ICP-OES) techniques. We studied two major European conifers with ongoing drought-induced dieback in mesic (Abies alba, silver fir) and xeric (Pinus sylvestris, Scots pine) sites. In each site we compared coexisting declining (D) and non-declining (ND) trees. We used dendrochronology and generalized additive and linear mixed models to analyse trends in tree-ring nutrients and their relationships with wood traits. The D trees presented lower growth and higher minimum wood density than ND trees, corresponding to a smaller lumen area of earlywood tracheids and thus a lower theoretical hydraulic conductivity. These differences in growth and wood-anatomy were more marked in silver fir than in Scots pine. Moreover, most of the chemical elements showed higher concentrations in D than in ND trees during the last two-five decades (e.g., Mn, K and Mg), while Ca and Na increased in the sapwood of ND trees. The Mn concentrations, and related ratios (Ca:Mn, Mn:Al and P:Mn) showed the highest differences between D and ND trees for both tree species. These findings suggest that a reduced hydraulic conductivity, consistent with hydraulic impairment, is affecting the use of P in D trees, making them more prone to drought-induced damage. The retrospective quantifications of Mn ratios may be used as early-warning signals of impending dieback.

Cyclic variations of gas components in tree-ring chronologies as response to climatic cycles

Vacuum-extracted gas samples from tree-ring wood of stems and large roots of some conifer species (of Tomsk oblast) are studied using a photoacoustic laser gas analysis. The retrieved CO2, (CO2+H2O) chronologies are analyzed by spectral and cross-spectral analysis methods. A quasi-cyclic character of variations of the chronologies has been revealed, which is probably due to changes in climatic conditions and solar activity.

Oak tree-rings record spatial-temporal pollution trends from different sources in Terni (Central Italy).

Monitoring atmospheric pollution in industrial areas near urban center is essential to infer past levels of contamination and to evaluate the impact for environmental health and safety. The main aim of this study was to understand if the chemical composition of tree-ring wood can be used for monitoring spatial-temporal variability of pollutants in Terni, Central Italy, one of the most polluted towns in Italy. Tree cores were taken from 32 downy oaks (Quercus pubescens) located at different distances from several pollutant sources, including a large steel factory. Trace element (Cr, Co, Cu, Pb, Hg, Mo, Ni, Tl, W, U, V, and Zn) index in tree-ring wood was determined using high-resolution laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We hypothesized that the presence of contaminants detected in tree-rings reflected industrial activities over time. The accumulation of contaminants in tree-rings was affected by anthropogenic activities in the period 1958-2009, though signals varied in intensity with the distance of trees from the industrial plant. A stronger limitation of tree growth was observed in the proximity of the industrial plant in comparison with other pollutant sources. Levels of Cr, Ni, Mo, V, U and W increased in tree-ring profiles of trees close to the steel factory, especially during the 80's and 90's, in correspondence to a peak of pollution in this period, as recorded by air quality monitoring stations. Uranium contents in our tree-rings were difficult to explain, while the higher contents of Cu, Hg, Pb, and Tl could be related to the contaminants released from an incinerator located close to the industrial plant. The accumulation of contaminants in tree-rings reflected the historical variation of environmental pollution in the considered urban context.

Tree ring wood density of Scots pine and European beech lower in mixed-species stands compared with monocultures

Mixed species stands are on the advance in Central Europe and many recently published studies have reported that they can overyield monocultures in terms of volume growth. However, as forest research has in the past been focused on monocultures, knowledge of how mixed-species stands and monocultures compare in terms of wood quality remains limited. Based on five triplets of fully stocked monocultures and mixed stands of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.), we analysed whether tree species mixing modifies wood quality and, more precisely, tree ring wood density.From a total of 322 trees we sampled increment cores for the analyses of tree ring width and tree ring wood density using a LIGNOSTATION™. We found that tree ring width of Scots pine was, on average, 14% wider in mixed compared with pure stands. Tree ring width of European beech did not differ between pure and mixed stands. Tree ring wood density was lower in mixed stands compared to pure stands for both Scots pine (−12%) and European beech (−8%). Tree ring wood density and tree ring width were negatively correlated in the case of Scots pine and positively correlated for European beech.When considering tree size and Stand density index, it was found that only tree ring width and mean tree ring wood density of European beech were influenced by stand density. Tree size had a significant effect only on tree ring wood density of European beech. The overall result of larger tree rings of Scots pine in mixed stands and a lower tree ring wood density of both species in mixed stands compared to pure stands was not influenced by stand density or tree size.Based on the measured values of tree ring wood density we conducted estimates of how mixed stands performed in terms of biomass. We found stem biomass to be 8% lower in mixed stands compared to pure stands. Reasons for the revealed differences in tree ring wood density and consequences for, among others, overyielding, carbon storage, and wood quality are discussed.

Variations in gas components and total pressure in stem and root disc wood of conifer species

The content of CO2 and H2O in vacuum-extracted tree-ring wood of stem and root discs of certain conifer species was measured. Annual distributions of these gas components were found to exhibit cyclic behavior. Moreover, distinct cyclicity was also revealed in interannual variations of the total pressure of the vacuum-extracted tree-ring gas samples. It may be safely assumed that diffusion of stem CO2 into the atmosphere will show the same periodic variability. Two hypotheses about the origin of the cyclicity in the treering CO2 distributions are examined.

Forest GPP Calculation Using Sap Flow and Water Use Efficiency Measurements.

This is a protocol to evaluate gross primary productivity (GPP) of a forest stand based on the measurements of tree's sap flow (SF), 13C derived water use efficiency (WUE), and meteorological (met) data. GPP was calculated from WUE and stomatal conductance (gs), the later obtained from SF up-scaled from sampled trees to stand level on a daily time-scale and met data. WUE is obtained from 13C measurements in dated tree-ring wood and/or foliage samples. This protocol is based on the recently published study of Klein et al., 2016 .

Pruning effects on ring width and wood hydrosystem of Prosopis flexuosa DC from arid woodlands

Pruning is a silvicultural practice that could potentially improve the productivity in arid woodlands. However, there is not enough information about the influence of this practice on the tree-ring wood anatomy of desert Prosopis woodlands. We analyzed the effect of pruning trees of different diameter class (small and large, PDDC) and pruning intensity in small trees (heavy and intermediate, PDI). We evaluated tree-ring width, number of vessels, total vessel area, mean vessel size (total vessel area/number of vessels), vessel density (number of vessels/tree-ring area) and ratio of vessel area to tree-ring area in Prosopis flexuosa, a species widely used for firewood and poles in arid and semiarid areas of South America. For PDDC, trees were pruned during the winter of 2003, for which we focused on the period between 1995 and 2010 (8 years before and after pruning, respectively) and for PDI, trees were pruned during the winter of 2004, and we focused on the period between 1997 and 2010 (7 years before and after pruning, respectively). The data was analysed using Mixed Models. We observed that, small trees had a greater response to pruning than large individuals. In this sense, pruning increased radial growth (F2,247=6.08, p≤ 0.01) and number of vessels (F2,247=6.36, p≤0.01), and decreased the mean vessel size (F2,247=6.91, p≤ 0.001) and ratio of vessel area to tree-ring area (F2,247=4.84, p≤ 0.01) in small trees. Moreover, the response depended on pruning intensity, which was more evident in intermediate pruning. The increase in the vessel number and the decrease in mean vessel size observed in small trees denoted that pruning induces the generation of new, small-diameter vessels. This is considered a strategy to protect the water-conducting system in response to pruning. Studies at the anatomical level conducted in the present research revealed in detail how pruning influences wood production and anatomical modifications, indicating the importance of conducting wood anatomical analyses on tree rings in order to refine interpretations of the effect of pruning on stem growth.

Radiocarbon measurements at LAC-UFF: Recent performance

In 2012 a single stage accelerator mass spectrometer from NEC was installed at the Radiocarbon Laboratory of Universidade Federal Fluminense (LAC-UFF), Niterói, Brazil. Here, we present a status report of our facility. We discuss some modifications applied to our combustion protocol in an attempt to reduce our procedural blank, mostly to processed organic samples. Measurements of reference materials indicate low precision and accuracy that are partially related to beam optics through the acceleration tube. We observed that once the beam current intensity increases the measured 13C+/12C+ becomes erratic. Therefore, in order to maintain the AMS-δ13C values within reasonable values, so that fractionation corrections using the spectrometer 13C+/12C+ values does not affect the final 14C results, we are forced to limit the 12C− beam intensity to ⩽30μA. This requirement was confirmed during our accuracy tests, when measuring selected annual tree-rings wood samples from a Parana pine (Araucaria angustifolia) between 1927 and 1997 previously measured at the Keck Carbon Cycle AMS Facility (KCCAMS), at the University of California, Irvine (UCI). At the LAC-UFF tree-ring wood samples were processed and measured in 4 different batches during a period of about 5months. The 14C results were later compared to the high-precision data obtained at KCCAMS/UCI and reached a good agreement. Recently a problem associated with graphitization yield were finally identified and new measurements with secondary standards are promising.

Intra-annual dendroclimatic reconstruction for northern British Columbia, Canada, using wood properties

Analyses of tree-ring wood properties combined with an understanding of tree physiology provided detailed intra-annual insights into historical and seasonal climate phenomena in sub-boreal forests of northern British Columbia. This study investigated historical climate trends in northern British Columbia, Canada, through the use of tree-ring proxies, and established a means of reconstructing intra-annual climate patterns from wood density, fibre properties and tree-ring width data. Specific attention was given to investigating how dendroclimatological analyses of intra-ring wood and fibre properties could be interpreted to improve the strength of proxy climate records. Trees were sampled at six sites in northern British Columbia. Spruce trees were collected from the Smithers area, whilst Douglas-fir trees were sampled at the northern latitudinal extent of their range near Babine and Francois Lakes, and at a precipitation-limited site near Valemount. Wood cores were analysed by Windendro® software with an ITRAX scanning densitometer, and by the SilviScan system located at the Australian Commonwealth Scientific and Research Organization. A mean June temperature proxy record dating to 1805 and a July–August mean temperature proxy record for Smithers extending from 1791 to 2006 were constructed from spruce ring width and maximum density chronologies. Douglas-fir ring width, spruce minimum density, and Douglas-fir maximum cell-wall thickness chronologies were used to reconstruct a May–June precipitation record extending from 1820 to 2006, and a July–August total precipitation record for Fort St. James that extends from 1912 to 2006. The results of the study demonstrate that a combination of multivariate and single-variate analyses provide detailed insights into seasonal radial growth characteristics. Tree physiological responses to climate at different times throughout the growing season, and temperature and precipitation fluctuations over the historical record are discussed.

Tree-ring wood anatomy and stable isotopes show structural and functional adjustments in olive trees under different water availability

Background and Aims Olive tree (Olea europaea L.) is a drought-tolerant tree species cultivated in Mediterranean-type environments. Although it is tolerant to drought, dry conditions decrease its productivity. A thorough analysis of the hydraulic architecture and wood anatomical plasticity, as well as of their physiological significance, is needed to understand how olive trees will adapt to the predicted increase in frequency and severity of drought in the Mediterranean region.

The long way down--are carbon and oxygen isotope signals in the tree ring uncoupled from canopy physiological processes?

The carbon (δ(13)C) and oxygen (δ(18)O) stable isotope composition is widely used to obtain information on the linkages between environmental drivers and tree physiology over various time scales. The tree-ring archive can especially be exploited to reconstruct inter- and intra-annual variation of both climate and physiology. There is, however, a lack of information on the processes potentially affecting δ(13)C and δ(18)O on their way from assimilation in the leaf to the tree ring. As a consequence, the aim of this study was to trace the isotope signals in European beech (Fagus sylvatica L.) from leaf water (δ(18)O) and leaf assimilates (δ(13)C and δ(18)O) to tree-ring wood via phloem-transported compounds over a whole growing season. Phloem and leaf samples for δ(13)C and δ(18)O analyses as well as soil water, xylem water, leaf water and atmospheric water vapour samples for δ(18)O analysis were taken approximately every 2 weeks during the growing season of 2007. The δ(13)C and δ(18)O samples from the tree rings were dated intra-annually by monitoring the tree growth with dendrometers. δ(18)O in the phloem organic matter and tree-ring whole wood was not positively related to leaf water evaporative enrichment and δ(18)O of canopy organic matter pools. This finding implies a partial uncoupling of the tree-ring oxygen isotopic signal from canopy physiology. At the same time, internal carbon storage and remobilization physiology most likely prevented δ(13)C in tree-ring whole wood from being closely related to intra-annual variation in environmental drivers. Taking into account the post-photosynthetic isotope fractionation processes resulting in alterations of δ(13)C and δ(18)O not only in the tree ring but also in phloem carbohydrates, as well as the intra-annual timing of changes in the tree internal physiology, might help to better understand the meaning of the tree-ring isotope signal not only intra- but also inter-annually.

Reconstructing contamination events on riverbanks in southern Québec using dendrochronology and dendrochemical methods

The riverbanks of the Saint-François and Massawippi Rivers are affected by hydrocarbons and heavy metals to varying degrees. The events associated with this riverbank contamination are not documented in the government reports or archives, which is why dendrochemical methods are useful for dating such pollution events. Samples of alluvial soils and trees were taken from the banks of the Saint-François River in Windsor and Richmond as well as the Massawippi River, where the level of hydrocarbon (C 10–C 50) contamination in the banks is low to moderate (300–700 mg/kg of dry soil). Core samples were taken from seven red ash ( Fraxinus pennsylvanica Marsh.) and dendrochemical analyses were done on the tree rings. Dendrochemistry was successfully used to determine that there were heavy metals in the rings, and also to estimate the date of the contamination events by using tree-ring wood. Several contamination events were identified through the presence of lead and other contaminants in the xylem of the trees analyzed at the selected sites. This suggests that various contaminants were transported by the river on several occasions and deposited on floodplains during successive floods. Also, a marked increase of some heavy metals like aluminum and nickel in the tree specimens can be attributed to recent atmospheric pollution. The emissions of metal-bearing particles may have varied widely over the century due to the different types of industrial activities, technological changes and atmospheric emission regulations. All of these anthropogenic changes reflected in the rings of the trees that showed variations in the geochemical profiles. Though several major contamination events can be determined, it is still difficult to identify all of the potential sources of pollution, of which there are many in this vast catchment.

Tree-ring Δ13C reveals the impact of past forest management on water-use efficiency in a Mediterranean oak coppice in Tuscany (Italy)

• The effects of thinning and heavy stand density reduction was investigated in Turkey oak ( Quercus cerris L.) forests of central Italy, to evaluate the physiological responses and the growth status of trees that survived a past coppice cut and thinning to convert the stand to high-forest.• The working hypothesis was that a strong decrease in stand density would cause a decreasing in canopy-intrinsic water-use efficiency (measured as the ratio of CO 2 assimilation to stomatal conductance, A / g ), thus an increase in tree-ring carbon isotopic discrimination (Δ 13 C).• The tree-ring Δ 13 C of the remaining trees (“survivors”) was found to have significantly ( P 13 C at control site which was characterized by high density and competition by trees. An increase in survivors tree-rings Δ 13 C probably indicates an improved water availability, possibly induced by a decrease in competition and in stand density or a decrease in the precipitation intercepted by the canopy (i.e., a stronger increase in g over A since a decrease in A is highly unlikely). A change in foliar nitrogen, foliar Δ 13 C and content in chlorophylls was also recorded seven years after thinning.• Thinnings carried out to convert old abandoned coppices into high-forest stands induce short-term stimulation of Turkey oak growth by increasing light and water availability. We were able to make a detailed reconstruction of the impact of past silvicultural treatment on the stand using a tree-ring wood Δ 13 C time-series.

UV-B absorbing compounds in present-day and fossil pollen, spores, cuticles, seed coats and wood: evaluation of a proxy for solar UV radiation

UV-B absorbing compounds (UACs) in present-day and fossil pollen, spores, cuticles, seed coats and wood have been evaluated as a proxy for past UV. This proxy may not only provide information on variation of stratospheric ozone and solar UV in the period preceding and during the Antarctic ozone hole (1974-present day), but also on the development and variation of the stratospheric ozone layer and solar surface UV during the evolution of life on Earth. Sporopollenin and cutin are highly resistant biopolymers, preserving well in the geological record and contain the phenolic acids p-coumaric (pCA) and ferulic acid (FA). pCA and FA represent a good perspective for a plant-based proxy for past surface UV radiation since they are induced by solar UV-B via the phenylpropanoid pathway (PPP). UV-B absorption by these monomers in the wall of pollen and spores and in cuticles may prevent damage to the cellular metabolism. Increased pCA and FA in pollen of Vicia faba exposed to enhanced UV-B was found in greenhouse experiments. Further correlative evidence comes from UV-absorbing compounds in spores from 1960-2000 comparing exposure of land plants (Lycopodium species) to solar UV before and during ozone depletion and comparing plants from Antarctica (severe ozone depletion), Arctic, and other latitudes with less or negligible ozone depletion. Wood-derived compounds guaiacyl (G), syringyl (S), and p-hydroxyphenyl (P) are produced via the PPP. The proportions of P, G, and S in the lignin differ between various plant groups (e.g. dicotyledons/monocotyledons, gymnosperms/angiosperms). It is hypothesized that this lignin composition and derived physiological and physical properties of lignin (such as tree-ring wood density) has potential as a proxy for palaeo-UV climate. However validation by exposure of trees to enhanced UV is lacking. pCA and FA also form part of cutin polymers and are found in extant and fossil Ginkgo leaf cuticles as shown by thermally-assisted hydrolysis and methylation (THM)-pyrolysis-GC-MS. Potentially, the time scale for reconstruction of ozone column thickness and UV-B based on the UAC UV proxy may be decadal, centennial, millennial and possibly billenial. For further development of the UACs and pCA and FA-based UV proxy, it is necessary to obtain the UV dose-response (content of UACs, pCA and FA in sporopollenin and cutin) relationships for validation, based on outdoor UV radiation manipulations experiments with plants, and comparative analysis of stored plants (herbaria) or fossil material of the same or related plant species.