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 d 13 C 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 d 13 C. 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.

We investigated the variability of tree-ring width, wood density and 13 C/ 12 C in beech tree rings (Fagus sylvatica L.), and analyzed the influence of climatic variables and carbohydrate storage on these parameters. Wood cores were taken from dominant beech trees in three stands in Germany and Italy. We used densitometry to obtain density profiles of tree rings and laser-ablation-combustion-GC-IRMS to estimate carbon isotope composition (δ 13 C) of wood. The sensitivity of ring width, wood density and δ 13 C to climatic variables differed; with tree-ring width responding to environmental conditions (temperature or precipitation) during the first half of a growing season and maximum density correlated with temperatures in the second part of a growing season (July-September). δ 13 C variations indicate re-allocation and storage processes and effects of drought during the main growing season. About 20% of inter-annual variation of tree-ring width was explained by the tree-ring width of the previous year. This was confirmed by δ 13 C of wood which showed a contribution of stored carbohydrates to growth in spring and a storage effect that competes with growth in autumn. Only mid-season δ 13 C of wood was related to concurrent assimilation and climate. The comparison of seasonal changes in tree-ring maximum wood density and isotope composition revealed that an increasing seasonal water deficit changes the relationship between density and 13 C composition from a negative relation in years with optimal moisture to a positive relationship in years with strong water deficit. The climate signal, however, Communicated by U. Lüttge is over-ridden by effects of stand density and crown structure (e.g., by forest management). There was an unexpected high variability in mid season δ 13 C values of wood between individual trees ( − 31 to − 24‰) which was attributed to competition between dominant trees as indicated by crown area, and microclimatological variations within the canopy. Maximum wood density showed less variation (930-990 g cm −3 ). The relationship between seasonal changes in tree-ring structure and 13 C composition can be used to study carbon storage and re-allocation, which is important for improving models of tree-ring growth and carbon isotope fractionation. About 20-30% of the tree-ring is affected by storage processes. The effects of storage on tree-ring width and the effects of forest structure put an additional uncertainty on using tree rings of broad leaved trees for climate reconstruction.

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 13 C-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 δ 18O 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.

We present a new, rapid method for high-resolution online determination of delta13C in tree rings, combining laser ablation (LA), combustion (C), gas chromatography (GC) and isotope ratio mass spectrometry (IRMS) (LA-C-GC-IRMS). Sample material was extracted every 6 min with a UV-laser from a tree core, leaving 40-microm-wide holes. Ablated wood dust was combusted to CO2 at 700 degrees C, separated from other gases on a GC column and injected into an isotope ratio mass spectrometer after removal of water vapor. The measurements were calibrated against an internal and an external standard. The tree core remained intact and could be used for subsequent dendrochronological and dendrochemical analyses. Cores from two Scots pine trees (Pinus sylvestris spp. sibirica Lebed.) from central Siberia were sampled. Inter- and intra-annual patterns of delta13C in whole-wood and lignin-extracted cores were indistinguishable apart from a constant offset, suggesting that lignin extraction is unnecessary for our method. Comparison with the conventional method (microtome slicing, elemental analysis and IRMS) indicated high accuracy of the LA-C-GC-IRMS measurements. Patterns of delta13C along three parallel ablation lines on the same core showed high congruence. A conservative estimate of the precision was +/- 0.24 per thousand. Isotopic patterns of the two Scots pine trees were broadly similar, indicating a signal related to the forest stand's climate history. The maximum variation in delta13C over 22 years was about 5 per thousand, ranging from -27 to -22.3 per thousand. The most obvious pattern was a sharp decline in delta13C during latewood formation and a rapid increase with spring early growth. We conclude that the LA-C-GC-IRMS method will be useful in elucidating short-term climate effects on the delta13C signal in tree rings.

Intra-annual density fluctuations (IADFs) are anomalies of tree rings where wood density is abruptly altered after sudden changes in environmental conditions. Their characterisation can provide information about the relationship between environmental factors and eco-physiological processes during tree growth. This paper reports about the variability of anatomical traits and stable carbon isotopic composition along tree rings as resulting from the application of two different methodological approaches: (a) the separation of each ring into different regions (earlywood, latewood and IADF) and the comparison of anatomical and isotopic parameters measured in those specific sectors and (b) the analysis of such features in continuum along ring width. Moreover, different parameters of vessels (i.e. ecd-equivalent circle diameter, elongation, sphericity and convexity of vessel lumen) were considered to identify those more appropriate for the representation of intra-annual anatomical variations. The analysis was conducted on Arbutus unedo L. growing on the Elba Island (Italy); tree rings of this species form IADFs with features clearly responsive to the environmental conditions experienced during plant growth. Results showed that the first approach, although more suitable to obtain data for subsequent statistical comparisons and for the calculation of correlations with environmental parameters, suffers from elements of subjectivity due to the size and position of the selected tree-ring regions. The in continuum method allows a clearer identification of the variation of tree-ring properties along ring width. Regarding anatomical parameters, shape indexes were not suitable indicators of intra-annual variability. The overall analysis suggested that using both methodologies in synergy helps to gain complete information and avoid misleading interpretations of IADFs in tree rings.