Description: Aging in trees implies a progressive reduction in the growth rate, related to a shortening of the growing period and changes in the photosynthetic capability and efficiency. These changes may continue during the old-growth life stages following the juvenile phase and are reflected in tree-ring properties such as growth increment, density or stable isotopes. We studied possible climate age effects in time series of several tree-ring parameters (ring width, wood density and stable carbon and oxygen isotopes) of mature individuals from two age groups of Pinus uncinata and P. nigra at two locations in Spain. The aim was to test whether age differences in trees in the old-growth life stages could lead to diverging climate responses. The results show some differences in response to climate between age groups at a monthly level, but most of these divergences are not significant for seasonal climate variables. Regardless of the age group, the main limiting climate factors constrained tree growth equally. Although our findings do not support the idea of an age-dependent response to climate that may lead to inaccurate climate reconstructions, further studies using tree-ring density and stable isotope series are urgently needed to verify the current results.

Description: δ13C and δ18O of tree rings contain time integrated information about the environmental conditions weighted by seasonal growth dynamics and are well established as sources of palaeoclimatic and ecophysiological data. Annually resolved isotope chronologies are frequently produced by pooling dated growth rings from several trees prior to the isotopic analyses. This procedure has the advantage of saving time and resources, but precludes from defining the isotopic error or statistical uncertainty related to the inter-tree variability. Up to now only a few studies have compared isotope series from pooled tree rings with isotopic measurements from individual trees. We tested whether or not the δ13C and the δ18O chronologies derived from pooled and from individual tree rings display significant differences at two locations from the Iberian Peninsula to assess advantages and constraints of both methodologies. The comparisons along the period 1900–2003 reveal a good agreement between pooled chronologies and the two mean master series which were created by averaging raw individual values (Mean) or by generating a mass calibrated mean (MassC). In most of the cases, pooled chronologies show high synchronicity with averaged individual samples at interannual scale but some differences also show up especially when comparing δ18O decadal to multi-decadal variations. Moreover, differences in the first order autocorrelation among individuals may be obscured by pooling strategies. The lack of replication of pooled chronologies prevents detection of a bias due to a higher mass contribution of one sample but uncertainties associated with the analytical process itself, as sample inhomogeneity, seems to account for the observed differences.

Description: Detecting and extracting the dominant climatic signal from tree-ring records derived from Mediterranean areas remains challenging because the relation between climate and tree-growth are usually characterized by a complex interplay of temperature and precipitation signals, with high spatial and temporal variability. Although several studies have established climate−growth relationships in old forests on the Iberian Peninsula (IP), a reliable calibration level between tree-ring data and the instrumental records making possible the inference of past climate has not yet been established, mainly due to low correlation coefficients (i.e. r ≤ 0.4) and/or instability over time of the climate−growth relationships. We tested for spatial significance and temporal stability of climatic signals in a collection of tree-ring proxies at the Cazorla Range (NCZ), located in the southeast of the IP. The aim was to identify suitable proxies for further use in climate reconstructions. The tree-ring variables under investigation included tree-ring width (TRW), latewood width (LWw), maximum latewood density (MXD) and stable carbon (δ13C) and oxygen (δ18O) isotopes. Our results show how the strength and temporal stability of the relationship between tree-ring proxies and selected seasonal climate variables largely depend on the climate data used. Moreover, imprecise identification of the climate signal may lead to erroneous evaluations of temporal stability. Overall, from the set of proxies measured at NCZ, TRW is suitable to reconstruct summer to autumn temperature while δ13C can potentially be used as a proxy for summer precipitation reconstructions. The calibration−verification trials using both regression and scaling techniques revealed how scaling retains more inter-annual variability but decreases the values of the reduction of error (RE).

Description: We present the first European network of tree ring delta(13)C and delta(18)O, containing 23 sites from Finland to Morocco. Common climate signals are found over broad climatic-ecological ranges. In temperate regions we find positive correlations with summer maximum temperatures and negative correlations with summer precipitation and Palmer Drought Severity Indices (PDSI) with no obvious speciesspecific differences. Regional delta(13)C and delta(18)O chronologies share high common variance in year-to-year variations. Long-term variations, however, exhibit differences that may reflect spatial variability in environmental forcings, age trends and/or plant physiological responses to increasing atmospheric CO(2) concentration. Rotated principal component analysis (RPCA) and climate field correlations enable the identification of four sub-regions in the delta(18)O network - northern and eastern Central Europe, Scandinavia and the western Mediterranean. Regional patterns in the delta(13)C network are less clear and are timescale dependent. Our results indicate that future reconstruction efforts should concentrate on delta(18)O data in the identified European regions.

Description: This dendroclimatological research is based on two close pine forests (Pinus sylvestris and Pinus uncinata) located at the Northern Iberian System (Spain), and three tree-ring variables (ring widths, δ13C and δ18O). The climate-tree growth system was assessed at local and regional scales using three climate datasets. Calibration of tree-ring records with climate showed a diversity of information recorded in the different variables, such as a general response to temperature and precipitation of current growing period, and an important contribution of previous year conditions understood as the use of food reserves. The analysis of the stability of climate-tree growth relationships throughout the twentieth century showed a shift of those climatic variables to which trees responded and results suggested an enhancement of reserve use on current tree growth. The results obtained in this research made clear a physiological adaptation of trees to changing climate. The results provided hints that the recent warming coupled to slight precipitation decay are forcing growth of studied trees to a higher stress status and to a higher climate-growth synchronisation. These instabilities also have implications on future dendroclimatic reconstructions performed with trees growing under changing environments.

Description: Past temperature variations are usually inferred from proxy data or estimated using general circulation models. Comparisons between climate estimations derived from proxy records and from model simulations help to better understand mechanisms driving climate variations, and also offer the possibility to identify deficiencies in both approaches. This paper presents regional temperature reconstructions based on tree-ring maximum density series in the Pyrenees, and compares them with the output of global simulations for this region and with regional climate model simulations conducted for the target region. An ensemble of 24 reconstructions of May-to-September regional mean temperature was derived from 22 maximum density tree-ring site chronologies distributed over the larger Pyrenees area. Four different tree-ring series standardization procedures were applied, combining two detrending methods: 300-yr spline and the regional curve standardization (RCS). Additionally, different methodological variants for the regional chronology were generated by using three different aggregation methods. Calibration verification trials were performed in split periods and using two methods: regression and a simple variance matching. The resulting set of temperature reconstructions was compared with climate simulations performed with global (ECHO-G) and regional (MM5) climate models. The 24 variants of May-to-September temperature reconstructions reveal a generally coherent pattern of inter-annual to multi-centennial temperature variations in the Pyrenees region for the last 750 yr. However, some reconstructions display a marked positive trend for the entire length of the reconstruction, pointing out that the application of the RCS method to a suboptimal set of samples may lead to unreliable results. Climate model simulations agree with the tree-ring based reconstructions at multi-decadal time scales, suggesting solar variability and volcanism as the main factors controlling preindustrial mean temperature variations in the Pyrenees. Nevertheless, the comparison also highlights differences with the reconstructions, mainly in the amplitude of past temperature variations and in the 20th century trends. Neither proxy-based reconstructions nor model simulations are able to perfectly track the temperature variations of the instrumental record, suggesting that both approximations still need further improvements.