Search

The larch budmoth (Zeiraphera diniana Gn. - LBM) offers a unique example of cyclic fluctuations in insect populations. During regular LBM mass outbreaks, defoliation of European larch (Larix decidua Mill.) subalpine trees results in distinct ring-width reductions in the host trees. Although several observations, reconstructions and models suggest that LBM outbreaks travel from the southwest to the northeast along the Alpine arc, gaps in the underlying data still hamper our mechanistic understanding of the spatio-temporal system dynamics. Evidence of historical LBM outbreaks before 1964 is generally associated with uncertainty and is particularly scarce for the Central Italian Alps. Here, we introduce four new larch ring-width chronologies from Val di Sole in the Central Italian Alps and use time-series analyses and comparisons with non-host trees (Picea abies Karst.) to reconstruct LBM mass outbreaks. We identify distinct fingerprints of 15 spatially-synchronized LBM events that occurred between 1774 and 1964 CE. Our results are important for improving qualitative space-time models to simulate travelling wave dynamics of insect populations, and for correcting ring-width-based summer temperature reconstructions from this part of the Alpine arc.

A first assessment of the main climatic drivers that modulate the tree-ring width (RW) and maximum latewood density (MXD) along the Italian Peninsula and northeastern Sicily was performed using 27 forest sites, which include conifers (RW and MXD) and broadleaves (only RW). Tree-ring data were compared using the correlation analysis of the monthly and seasonal variables of temperature, precipitation and standardized precipitation index (SPI, used to characterize meteorological droughts) against each species-specific site chronology and against the highly sensitive to climate (HSTC) chronologies (based on selected indexed individual series). We find that climate signals in conifer MXD are stronger and more stable over time than those in conifer and broadleaf RW. In particular, conifer MXD variability is directly influenced by the late summer (August, September) temperature and is inversely influenced by the summer precipitation and droughts (SPI at a timescale of 3 months). The MXD sensitivity to August–September (AS) temperature and to summer drought is mainly driven by the latitudinal gradient of summer precipitation amounts, with sites in the northern Apennines showing stronger climate signals than sites in the south. Conifer RW is influenced by the temperature and drought of the previous summer, whereas broadleaf RW is more influenced by summer precipitation and drought of the current growing season. The reconstruction of the late summer temperatures for the Italian Peninsula for the past 300 years, based on the HSTC chronology of conifer MXD, shows a stable model performance that underlines periods of climatic cooling (and likely also wetter conditions) in 1699, 1740, 1814, 1914 and 1938, and follows well the variability of the instrumental record and of other tree-ring-based reconstructions in the region. Considering a 20-year low-pass-filtered series, the reconstructed temperature record consistently deviates

Erica arborea (L) is a widespread Mediterranean species, able to cope with water stress and colonize semiarid environments. The eco-physiological plasticity of this species was evaluated by studying plants growing at two sites with different soil moistures on the island of Elba (Italy), through dendrochronological, wood-anatomical analyses and stable isotopes measurements. Intra-annual density fluctuations (IADFs) were abundant in tree rings, and were identified as the key parameter to understand site-specific plant responses to water stress. Our findings showed that the formation of IADFs is mainly related to the high temperature, precipitation patterns and probably to soil water availability, which differs at the selected study sites. The recorded increase in the 13C-derived intrinsic water use efficiency at the IADFs level was linked to reduced water loss rather than to increasing C assimilation. The variation in vessel size and the different absolute values of δ18O among trees growing at the two study sites underlined possible differences in stomatal control of water loss and possible differences in sources of water uptake. This approach not only helped monitor seasonal environmental differences through tree-ring width, but also added valuable information on E.arborea responses to drought and their ecological implications for Mediterranean vegetation dynamics. © 2013 John Wiley & Sons Ltd.

Monitoring the surface composition of CO2 derived from subsurface reservoirs is an important part of the carbon capture and storage (CCS) chain. Most approaches use geochemical or geophysical instrumental approaches but these have the drawback that no long-term time series are available, which depend on a predefined monitoring location. We test a flexible approach using natural archives based on the measurement of radiogenic 14C concentrations in tree rings to detect geogenic CO2 fluxes derived from natural springs in the Latera Caldera, central Italy. The approach can be used as a preliminary check to evaluate natural CO2 leaks from sites designated for CO2 storage, as well as evaluating the extent of leakage in an unforeseen area. An extensive database of soil gas composition and fluxes is available for that site, permitting direct comparison of the tree ring isotopic composition and point sources of CO2 from the subsurface against the mean atmospheric standard. We sampled oak trees (Quercus cerris and Quercus pubescens) directly at the CO2 source (ON), and at short (50m, NEAR), intermediate (500m, FAR) and long distances (~3000m, CONTROL) from CO2, sources, and measured the radioactive 14C concentration in tree rings at ~10 year intervals from 2012 back to 1976. To accurately date the tree rings we constructed a tree-ring chronology using standard dendrochronological methods. We tested whether variation in 14C concentration in tree rings and ring-width are related to distance of trees from CO2 source, as well as climate factors, i.e. precipitation and temperature. Results show that local point sources of CO2 at the location where the tree grows are effectively recorded by the 14C concentration in the cellulose of this tree. The fossil CO2 signal is sharply delineated since already at short distance from the source (~50m, NEAR) the 14C incorporation is at the detection limit of the tested approach. Tree-ring width of the oaks at Caldera Latera is mainly limited by the amount of precipitation during the growing season, from March to October, while distance to the CO2 point source has no detectable effect on radial growth, likely due to the continuous presence of the enhanced CO2 flux to which the trees adapt their physiology from germination. While the approach is promising and permits data collection at any forested site, a more detailed sampling transect between 0 and 50m from a point source of CO2 is needed to determine the exact detection limit of the signal in the 14C concentration of the tree ring cellulose. © 2013 Elsevier Ltd.

Xylella fastidiosa (Xf) Wells, Raju et al., 1986 is a bacterium that causes plant diseases in the Americas. In Europe, it was first detected on the Salento Peninsula (Italy), where it was found to be associated with the olive quick decline syndrome. Here, we present the results of the first tree-ring study of infected and uninfected olive trees (Olea europaea L.) of two different cultivars, one resistant and one susceptible, to establish the effects induced by the spread of the pathogen inside the tree. Changes in wood anatomical characteristics, such as an increase in the number of vessels and in ring width, were observed in the infected plants of both the cultivars Cellina di Nardò (susceptible to Xf infection) and Leccino (resistant to Xf infection). Thus, whether infection affects the mortality of the tree or not, the tree shows a reaction to it. The presence of occlusions was detected in the wood of both 4-year-old branches and the tree stem core. As expected, the percentage of occluded vessels in the Xf-susceptible cultivar Cellina di Nardò was significantly higher than in the Xf-resistant cultivar Leccino. The δ 18O of the 4-year-old branches was significantly higher in infected trees of both cultivars than in noninfected trees, while no variations in δ 13C were observed. This suggests a reduction in leaf transpiration rates during infection and seems to be related to the occlusions observed in rings of the 4-year-old branches. Such occlusions can determine effects at leaf level that could influence stomatal activity. On the other hand, the significant increase in the number of vessels in infected trees could be related to the tree’s attempt to enhance water conductivity in response to the pathogen-induced vessel occlusions.

Key message: In Abies alba Mill. (silver fir), the frequency of intra-annual density fluctuations (IADFs) increases along the latitudinal transect, from North to South, is higher in pure than in mixed stands, and their formation is linked to spring and/or summer drought conditions. Abstract: Trees respond to climate, recording information in tree rings and their anatomical features, such as IADFs. The IADFs are regions within the tree ring where density changes in response to tree physiological processes and environmental conditions. The objective of this study was to assess the effect of different forest-stand structures and of climate on IADF formation and frequency in populations of silver fir along a latitudinal gradient in Italy (Trentino, Molise and Calabria regions). In doing so, we aimed to compare the frequency of IADFs in pure or mixed stands and to understand the ability of silver fir to cope with environmental fluctuations. Results showed higher frequency of IADFs in pure than mixed stands (in Trentino and Calabria) and in younger trees (namely in the pure stand in Calabria), because of the higher sensitivity to environmental variability. The formation of IADFs in silver fir emerged as a response to cope with drought. Summer precipitation (both pure and mixed stands in Trentino) and early spring/summer (both pure and mixed stands in Calabria) played a key role in the formation of IADF-type E+. The formation of IADF-type L+, on the other hand, was related to temperature/precipitation in late summer and early autumn (both pure and mixed stands in Molise) and to precipitation in summer (pure stand in Calabria). Our findings support the theory that IADFs are an important structural/functional mechanism for responding to climate fluctuations.

Mixed forests of Quercus ilex L. and Pinus pinea L. are widely found throughout the Mediterranean Basin, being representative of two co-existing functional types: evergreen-sclerophyllous drought-resistant species and Mediterranean-adapted drought-avoidant conifers. Their contrasting physiological strategies to cope with water deficit influence all the processes regulating their growth such as wood formation, leading to peculiar tree-ring anatomical features such as intra-annual density fluctuations (IADFs). Intra-annual density fluctuations are abrupt changes in wood anatomical traits within a tree ring, appearing as latewood-like cells within earlywood or earlywood-like cells within latewood, and are frequently found in Mediterranean species as a response to seasonal climate changes. In this study, we characterized the anatomical traits and composition of carbon and oxygen stable isotopes in IADFs occurring in tree rings of Q. ilex and P. pinea trees co-existing at a same site in Southern Italy, in order to link their xylem hydraulic properties with the related physiological mechanisms. The relationships between IADF occurrence and seasonal mean temperature and total precipitation were investigated, with the aim of assessing whether they can be used as indicators of species-specific responses to intra-annual climate fluctuations. Results show that IADF period of formation is during autumn months for both species. The influence of climate on IADF occurrence was found to be an indicator of species-specific response to climate: an increased stomatal conductance associated to the formation of a wood safer against embolism was found in Q. ilex, while a tighter stomatal control associated to a more efficient wood with regard to hydraulic conductivity occurred in P. pinea. Moreover, the assessment of the influence of climate on IADF occurrence indicates that, with rising temperatures, Q. ilex would form fewer IADFs compared with P. pinea. Other study cases are desirable to assess the suggested forecasts and to link the plasticity of the species to form IADFs with their effective adaptive capability to compete for resources, and to explain how it may influence future population development.

The tree-ring stable carbon and oxygen isotope chronologies from two forest sites located in the Forni Glacier forefield (Italy)—one along the glacier stream (GL) and the other toward the valley slope (SL)—were analyzed with the aim of disentangling the precipitation and glacier meltwater inputs in source water δ18O, as reflected by the tree-ring cellulose δ18O. The cellulose δ18O from the GL trees has a negative correlation with winter and summer temperatures, whereas the cellulose δ18O from the SL trees has a positive correlation with precipitation δ18O. The isotopic signature of the source water at the GL site is also influenced by waters of glacial origin, as confirmed by the 18O-depleted glacier meltwater inputs (GMWI_δ18O) estimated by means of an isotope model. The GMWI_δ18O values are consistent with the mean difference measured between the δ18O in the glacier stream and in the precipitation and the winter and summer temperature explains up to 37 percent of the GMWI_δ18O variance. Our results show an increasing influence of glacier meltwater throughout the past decade for the GL site. Our analysis opens new opportunities to reconstruct changes in water regimes of the glacier streams by means of the tree-ring cellulose δ18O.

It is noteworthy that the largest part of global vegetation biomass depends on a thin layer of cells: the vascular cambium. Understanding the wood formation processes and relationships with environmental factors is a crucial and timely research question requiring interdisciplinary efforts, also to upscale the information gained and to evaluate implications for tree growth and forest productivity.We provide an overview of wood formation processes up to tree-ring development, bearing in mind that the combined action of intrinsic factors and environmental drivers determines the anatomical traits of a tree ring formed at a specific time and position within the tree’s architecture. After briefly reviewing intrinsic factors, we focus attention on environmental drivers highlighting how a correct interpretation of environmental signals in tree rings must be grounded in a deep knowledge of xylogenesis and consequent wood anatomical traits. We provide guidelines on novel methods and approaches recently developed to study xylem formation. We refer to existing literature on established techniques for retrospective analyses in tree-ring series of anatomical and isotopic traits, to assess long-term ecophysiological responses to environmental variations, also giving advice on possible bias because of interand within-tree variability.Finally, we highlight that, once the temporal axis of intra-annual tree-ring variability of xylem traits is established by xylogenesis analysis, a multidisciplinary approach linking classical dendro-ecology, wood functional traits (dendro-anatomy) and eco-physiology (here focusing on dendro-isotopes) allows a better interpretation of past environmental events hidden in tree rings, and more reliable forecasts of wood growth in response to climate change.

Intra-annual density fluctuations (IADFs) in tree rings are generally considered structural anomalies caused by deviations from the “normal course” of xylogenesis during the growing season. This definition is based on the bias that, under “normal conditions”, cambial activity stops once a year. Each tree ring can thus be dated to one calendar year, which is one of the principles of dendrochronology. The formation of IADFs can be triggered directly by environmental changes, especially in precipitation and temperature, that affect cambial activity and cell differentiation. It can also be the result of limited photosynthesis, due to defoliation induced by biotic or abiotic constraints. Often indicated with alternative terms, IADFs were first described in the 1930s, and recently reported for many trees and shrubs from different ecosystems throughout the world, particularly for Mediterranean species. Different types of IADFs have been detected; their formation and structural properties depend on many factors including tree genotype, age, size, rooting depth, habitat, soil, climate, photosynthetic activity, and allocation strategies. Whether IADFs affect the adaptive capability of plants remains, however, unclear. We provide an overview of the main anatomical features of IADFs and their occurrence in tree rings from various environments and climatic regimes. We propose a simplified way of classifying them and discuss the hypotheses about their functional role and the factors triggering their formation. To understand the ecological role of IADFs better, we recommend a multidisciplinary approach, involving wood anatomy, dendroecology, and stable isotopes, which has already been applied for Mediterranean species. We conclude by considering that IADFs appear to be the “rule” rather than “anomalies” in some ecosystems where they help plants cope with fluctuating environmental conditions. Moreover, their anatomical structure represents a valuable proxy of past climatic conditions at a sub-seasonal resolution and may be relevant to adapt hydraulic functioning of living trees to changing climatic conditions.

International audience; AbstractKey MessageThe investigation of the relations between climate and intra-annual density fluctuation (IADF) traits (e.g. frequency, width and functional anatomical traits) inQuercus ilextree rings of stands living on two opposite slopes, highlighted the influence of local site conditions on the climate signal contained in tree rings. Moreover it allowed to identify the main site-specific environmental drivers for IADF formation.ContextWood functional traits such as IADFs result from the adaptation of tree physiological processes to seasonal climate variability. They could be used to reconstruct tree response to intra-annual variations in environmental conditions.AimsWe investigated the relationship between IADF features and climate in Quercus ilex L. populations living on opposite slopes, aiming to evaluate the influence of local site conditions on wood plasticity in response to seasonal climate variability.MethodsDominant trees were sampled and dated. IADF frequency and width, non-lumen fraction, vessel size, and frequency were measured. Time series analyses using precipitation and temperature data were performed.ResultsA decrease in IADF frequency over time influenced by summer and autumn temperature was observed at both sites. IADFs were more frequent, wider, with smaller vessels and a higher non-lumen fraction in trees growing at the north slope compared to the south slope site. IADF anatomical parameters of south slope tree rings were mainly influenced by precipitation, while the ones of north-slope tree rings by temperature.ConclusionTemperature-limited IADF occurrence in Q. ilex tree rings, probably leading to an earlier stop in cambial activity, and IADF anatomical traits were influenced by site conditions, suggesting them as indicators of site-specific responses to climate.

The integrated use of tree rings and outer tissues, and lichens, was tested for monitoring how pollutant concentrations vary in space and over time nearby an incinerator in industrial area in Central Italy. Trace elements in thalli of lichen Xanthoria parietina and in leaves, bark, wood of Quercus pubescens, as well as carbon, oxygen and nitrogen isotope ratios in tree rings were analyzed. Some trace elements in the leaves differed significantly between the plots, though this was not the case in lichens and bark. The values of δ13C and δ18O showed the same trend in all plots, while the values of δ15N were higher in the distal plot. The results indicated that trace elements were intercepted and collected by tree bark and leaves, as well as lichens, at low concentrations, and that they hardly entered into tree xylem tissues during the growing season to be stored into the woody tissues. Indeed, the study did not highlight marked changes over time and space, in accumulation of airborne pollutants in the selected biomonitors, most probably due to the low levels of industrial development. Nevertheless, the analysis of tree ring cores in combination with bark and leaves, and lichens might potentially contribute to depict historic impacts of airborne pollutants at pronounced concentrations.

Forest fires may alter the physiological and growth processes of trees by causing stress in trees and modifying the availability of soil nutrient. We investigated if, after a high-severity fire, changes in tree-ring growth can be observed, as well as changes in the nitrogen and carbon isotope composition of tree rings of surviving trees. Two wildfires that occurred in Pinus sylvestris L. stands in Northern Italy, one at the beginning and one at the end of the vegetative season, were chosen as the focus of this study. After the fires, the surviving trees showed growth suppression with very narrow tree rings or locally absent rings. The carbon isotope ratio was more negative in tree rings formed in the 5 years following fire, indicating better water supply in a situation of less competition. The nitrogen isotope ratio followed opposite trends in the two wildfire stands. In trees cored in the stand where the fire happened at the beginning of the vegetative season, there was no change in the nitrogen isotope ratio, whereas in samples collected in the other fire site, higher nitrogen isotope ratios were observed in the tree rings formed after the fire, reflecting changes in the soil nitrogen supply. Modifications in the growth and isotope composition of the fire-stressed trees disappeared from 6 to 10 years after the fire. By studying trees before and after fire, we were able to show that fire affects not only the growth of surviving trees, but also their physiological processes. © 2011 Springer-Verlag.

This study presents new evidence that radiocarbon, combined with dendrochronological and stable isotopes analysis in tree rings and needles, can help to better understand the influence of pollution on trees. Pinus pinea individuals, adjacent to main roads in the urban area of Caserta (South Italy) and exposed to large amounts of traffic exhaust since 1980, were sampled and the time-related trend in the growth residuals was estimated. We found a consistent decrease in the ring width starting from 1980, with a slight increase in δ13C value, which was considered to be a consequence of environmental stress. No clear pattern was identified in δ15N, while an increasing effect of the fossil fuel dilution on the atmospheric bomb-enriched 14C background was detected in tree rings, possibly as a consequence of the increase in traffic exhausts. Our findings suggested that radiocarbon is a very sensitive tool to investigate small-scale (i.e. traffic exhaust at the level crossing) and large-scale (urban area pollution) induced disturbances. © 2009 Elsevier B.V. All rights reserved.

Climate changes induced by the anthropogenic alteration of the atmospheric radiative balance are expected to change the productivity and composition of forest ecosystems. In Europe, the Mediterranean is considered one of the most vulnerable regions according to climatic forecasts and simulations. However, although modifications in the inter-specific competition are envisaged, we still lack a clear understanding of the ability of the Mediterranean vegetation to adapt to climate changes. We investigated how two co-occurring tree species commonly used in afforestation programmes, the native Abies alba Mill. and the nonnative Picea abies L. Karst., adapt to climate change by assessing their growth performance and physiological responses in relation to past climate variability. Growth was addressed by analysing tree-ring width and carbon and oxygen stable isotopes. Statistical relationships between isotopic value and monthly climate data suggest that the two species underwent ecophysiological adaptation to Mediterranean climatic constraints. These adaptations are also expressed in the ring-width data. Based on the carbon isotope ratio reflecting the stomatal response to drought, we found that the precipitation in the first period of the growing season, i.e. early spring, is a major factor influencing the annual growth of A. alba, which although native, proved to be sensitive to drought. P. abies, on the other hand, showed a higher tolerance to summer drought stress. These findings should help define criteria for sustainability and effective forest conservation in the Mediterranean region. © 2008 Elsevier B.V. All rights reserved.

The importance that nitrogen (N) deposition has in driving the carbon (C) sequestration of forests has recently been investigated using both experimental and modeling approaches. Whether increased N deposition has positive or negative effects on such ecosystems depends on the status of the N and the duration of the deposition. By combining delta(13)C, delta(18)O, delta(15)N and dendrochronological approaches, we analyzed the impact of two different sources of NO(x) emissions on two tree species, namely: a broad-leaved species (Quercus cerris) that was located close to an oil refinery in Southern Italy, and a coniferous species (Picea abies) located close to a freeway in Switzerland. Variations in the c(i)/c(a) ratio and the distinction between stomatal and photosynthetic responses to NO(x) emissions in trees were assessed using a conceptual model, which combines delta(13)C and delta(18)O, delta(15)N in leaves, needles and tree rings was found to be a bioindicator of N input from anthropogenic emissions, especially at the oil refinery site. We observed that N fertilization had a stimulatory effect on tree growth near the oil refinery, while the opposite effect was found for trees at the freeway site. Changes in the c(i)/c(a) ratio were mostly related to variations in delta(13)C at the freeway site and. thus, were driven by photosynthesis. At the oil refinery site they were mainly related to stomatal conductance, as assessed using delta(18)O. This study demonstrates that a single method approach does not always provide a complete picture of which physiological traits are more affected by N emissions. The triple isotope approach combined with dendrochronological analyses proved to be a very promising tool for monitoring the ecophysiological responses of trees to long-term N deposition. (C) 2008 Elsevier Ltd. All rights reserved.

Questions:Could mountainpine (Pinus mugo Turra subsp. mugo) located abovethe timberline and at the southern limit of its potential geographic distributionbea promising speciestomonitorclimatic changeinspecificenvironmentssuchas the Mediterranean mountains? Does mountain pine krummholz upwardexpansion result from current climate change? Has the relationship betweenclimate and tree ring growth of mountain pine changed? To what extent doesclimatic variability affect tree ring growth? Could mountain pine upwardexpansion contribute to loss of biodiversity in the undisturbed tree line ecotoneon the Majella massif?Location:MajellaNationalPark,Apennines,Italy.Methods: Dendroclimatological analysis was performed to examine the rela-tionshipbetweentreeringsofmountainpineandclimaticparametersatthetreeline. Climate–growth relationships were analysed using correlation function(CF)analysisandmovingCF(MCF).Results: In the Majella massif, climate variables affected mountain pine treegrowth at the tree line ecotone. Significant correlations were found with springtemperatures and summer precipitation. Essentially, 3 mo played a key role inmountain pine growth: April and May of the current year and October of theprevious year. MCF showed that mountain pine is a sensitive species, in termsoftreeringgrowth.Conclusions: Growing above the limit of tree vegetation, mountain pineproved to be a promising species for monitoring climate- and land use-driveninfluences over tree ring growth at high elevations. Our results show thatmountain pine is particularly sensitive to climate at the beginning and at theendofthegrowingseason.IntroductionIn the central Apennines (Italy), the low altitudinal rangeand marked influence of mediterranean climate set theboundary between alpine and sub-alpine bioclimatic beltsat 2550 m a.s.l. Within the Mediterranean Basin, theApennine range represents a temperate mountain climatic‘island’withinwhichthecentralApenninesformanarchi-pelago of arctic-alpine ‘islands’ (van Gils et al. 2012). TheApennine chain acted as refugia for trees during theW€urmian Late Glacial (Watson 1996), playing an impor-tant role in preservation of endemic high-altitude vegeta

Elevated CO2 increases intrinsic water use efficiency (WUEi) of forests, but the magnitude of this effect and its interaction with climate is still poorly understood. We combined tree ring analysis with isotope measurements at three Free Air CO2 Enrichment (FACE, POP-EUROFACE, in Italy; Duke FACE in North Carolina and ORNL in Tennessee, USA) sites, to cover the entire life of the trees. We used δ13C to assess carbon isotope discrimination and changes in water-use efficiency, while direct CO2 effects on stomatal conductance were explored using δ18O as a proxy. Across all the sites, elevated CO2 increased 13C-derived water-use efficiency on average by 73% for Liquidambar styraciflua, 77% for Pinus taeda and 75% for Populus sp., but through different ecophysiological mechanisms. Our findings provide a robust means of predicting water-use efficiency responses from a variety of tree species exposed to variable environmental conditions over time, and species-specific relationships that can help modelling elevated CO2 and climate impacts on forest productivity, carbon and water balances. © 2012 The Authors. New Phytologist © 2012 New Phytologist Trust.

Summary: Woody species in Mediterranean ecosystems form intra-annual density fluctuations (IADFs) in tree rings in response to changes in environmental conditions, especially water availability. Dendrochronology, quantitative wood anatomy and high-resolution isotopic analysis (using a laser ablation technique) were used to characterize IADFs in Arbutus unedo shrubs grown on two sites with different water availability on the island of Elba (Italy). Our findings show that IADF characterization can provide information about the relationship between environmental factors and tree growth at the seasonal level. At the more xeric site, IADFs mainly located in the early and middle parts of the annual ring, showed a decrease in vessel size and an increase in δ13C as a result of drought deficit. Opposite trends were found at the more mesic site, with IADFs located at the end of the ring and associated with a lower δ13C. Moreover, at the first site, IADFs are induced by drought deficit, while at the second site IADFs are linked with the regrowth in the last part of the growing season triggered by favourable wet conditions. This combined approach is a promising way for dating problematic wood samples and interpreting the phenomena that trigger the formation of IADFs in the Mediterranean environment. © The Authors (2010). Journal compilation © New Phytologist Trust (2010).

Recently a divergence between tree-ring parameters from temperature-limited environments and temperature records has been observed worldwide but comprehensive explanations are still lacking. From a dendroclimatic analysis performed on a high-altitude tree-ring network of Pinus cembra (L.) in the Central Italian Alps we found that site aspect influences non-stationary growth-climate relationships over time. A general increasing divergence between ring width and the summer temperature record (J-A) has been observed especially for chronologies from SW-facing slopes, whereas chronologies from N-facing sites showed stable relationships over time. The monthly analysis revealed that the decrease in sensitivity was mostly accounted for by the changes in the relationships with June temperature (decreasing correlations especially for S- and W-facing site chronologies), whereas trees from N-facing sites showed an increasing sensitivity to July temperatures. Our data suggest that at high altitudes, low temperatures at the beginning of the growing season no longer limit growth. We also found that our temperature-sensitive trees did not linearly respond in radial growth to the extreme heat event of summer 2003, and formed an annual ring of average width, resulting in a strong divergence from the temperature record. Our findings underline the importance of site ecology for tree-ring based climate reconstructions using temperature-sensitive ring-width chronologies, and may help in solving the 'divergence problem'. © Springer Science + Business Media B.V. 2009.

Volcanic eruptions impact the global and the hemispheric climate, but it is still unknown how and to what degree they force the climate system and in particular the global carbon cycle. In this paper, the relationships between individual eruptions (reconstructed for the past using written records), tree primary productivity (estimated using ring widths), photosynthetic rate and stomatal conductance (assessed by carbon and oxygen isotope data) are investigated, to understand the impact of volcanic eruptions on net primary production. Data from a mixed stand of Fagus sylvatica L. and Acer pseudoplatanus L. located in the area of the Vesuvio volcanic complex (Southern Italy) showed a significant decrease in ring width following each eruption. Isotope analyses indicate a change in climatic conditions after such events. Specifically, the lower oxygen isotope ratio in the tree-ring cellulose strongly suggests an increase in relative humidity and a decrease in temperature, with the latter resulting in a strong limitation to tree-ring growth. The carbon isotope ratio was only moderately but not significantly reduced in the years of volcanic eruption, suggesting no major changes in C fixation rates. This work is a case study on the effects of volcanic eruptions resulting in strong climatic changes on the local scale. This is an opportunity to explore the process and causal relationships between climatic changes and the response of the vegetation. Thus, we propose here a realistic model scenario, from which we can extrapolate to global scales and improve our interpretations of results of global studies. © 2007 Blackwell Publishing Ltd.