An atlas of megadroughts in Europe and in the Mediterranean Basin during the Common Era provides insights into climate variability.

The Earth’s carbon and hydrologic cycles are intimately coupled by gas exchange through plant stomata. However, uncertainties in the magnitude and consequences of the physiological responses of plants to elevated CO2 in natural environments hinders modelling of terrestrial water cycling and carbon storage. Here we use annually resolved long-term 13C tree-ring measurements across a European forest network to reconstruct the physiologically driven response of intercellular CO2 (Ci) caused by atmospheric CO2 (Ca) trends. When removing meteorological signals from the 13C measurements, we find that trees across Europe regulated gas exchange so that for one ppmv atmospheric CO2 increase, Ci increased by 0.76 ppmv, most consistent with moderate control towards a constant Ci=Ca ratio. This response corresponds to twentieth-century intrinsic water-use efficiency (iWUE) increases of 14 ±10 and 22 ± 6% at broadleaf and coniferous sites, respectively. An ensemble of process-based global vegetation models shows similar CO2 effects on iWUE trends. Yet, when operating these models with climate drivers reintroduced, despite decreased stomatal opening, 5%increases in European forest transpiration are calculated over the twentieth century.This counterintuitive result arises from lengthened growing seasons, enhanced evaporative demand in a warming climate, and increased leaf area, which together oppose effects of CO2-induced stomatal closure. Our study questions changes to the hydrological cycle, such as reductions in transpiration and air humidity, hypothesized to result from plant responses to anthropogenic emissions

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Tree populations located at the geographical distribution limit of the species may provide valuable information about tree-growth response to changes on climatic conditions. We established nine Pinus nigra, 12 P. sylvestris and 17 P. uncinata tree-ring width chronologies along the eastern and northern Iberian Peninsula, where these species are found at the edge of their natural range. Tree-growth variability was analyzed using principal component analysis (PCA) for the period 1885-1992. Despite the diversity of species, habitats and climatic regimes, a common macroclimatic signal expressed by the first principal component (PC1) was found. Moreover, considering the PC1 scores as a regional chronology, significant relations were established with Spanish meteorological data. The shared variance held by the tree chronologies, the frequency of narrow rings and the interannual growth variability (sensitivity) increased markedly during the studied period. This shows an enhancement of growth synchrony among forests indicating that climate might have become more limiting to growth. Noticeably, an upward abrupt shift in common variability at the end of the first half of the 20th century was detected. On the other hand, moving-interval response functions showed a change in the growth-climate relationships during the same period. The relationship between growth and late summer/autumn temperatures of the year before growth (AugustSeptember, negative correlation, and November, positive correlation) became stronger. Hence, water stress increase during late summer previous to tree growth could be linked to the larger growth synchrony among sites, suggesting that climate was driving the growth pattern changes. This agrees with the upward trend in temperature observed in these months. Moreover, the higher occurrence of extreme years and the sensitivity increase in the second half of the 20th century were in agreement with an increment in precipitation variability during the growing period. Precipitation variability was positively related to tree-growth variability, but negatively to radial growth. In conclusion, a change in tree-growth pattern and in the climatic response of the studied forests was detected since the mid-20th century and linked to an increase in water stress. These temporal trends were in agreement with the observed increase in warmer conditions and in precipitation variability.

-In Mediterranean climates trees may go through two periods of dormancy, resulting in special anatomical features such as false rings and other intra-annual density fluctuations (IADFs). In this paper, ring growth and the presence of IADFs were studied in Pinus pinea L. growing in the coastal and inland regions of Alentejo (southern Portugal). In order to identify the triggering factors associated with the IADFs, a new classification was proposed for the IADFs in P. pinea: Type E (latewoodlike cells within earlywood); Type E+ (transition cells between earlywood and latewood); Type L (earlywoodlike cells within latewood) and Type L+ (earlywoodlike cells between latewood and earlywood of the next tree ring). Response function analyses showed that radial growth of P. pinea was strongly correlated with precipitation in southern Portugal. The climatic response of P. pinea was higher in the inland area where the summer drought is more severe, the winter temperatures are lower and the soils have low water-holding capacity, in comparison with the coastal area. IADFs were frequent in P. pinea and most of the IADFs were observed in latewood. The presence of IADFs was correlated with fluctuations in climate parameters during the growing season. The IADF type E+ was linked to precipitation events early in summer. The IADF type L and L+ were associated with above-average precipitation in early autumn.false tree-ring / Mediterranean climate / Pinus pinea / radial growth / wood anatomy Résumé -Signification climatique de la largeur des cernes et des fluctuations intra annuelles de la densité chez Pinus pinea dans une région méditerranéenne sèche du Portugal. Dans les climats méditerranéens, les arbres peuvent traverser deux périodes de dormance, ce qui a pour consé-quence des caractéristiques anatomiques particulières telles que des faux cernes et des fluctuations intra annuelles de densité (IADFs). Dans cet article, la croissance des cernes et la présence de IADFs ont été étudiées chez Pinus pinea L. poussant dans les régions côtières et intérieures de l'Alentejo (sud-ouest du Portugal). Dans le but d'identifier les facteurs déclenchants associés à l'IADFs, une nouvelle classification a été proposée pour l'IADFs chez Pinus pinea : Type E (cellules ressemblant à du bois final dans le bois initial) ; Type E+ (cellules de transition entre bois initial et bois final) ; Type L (cellules ressemblant à du bois initial dans du bois final) et Type L+ (cellules ressemblant à du bois initial entre bois final et bois initial du prochain cerne). Les analyses des fonctions de réponse ont montré que la croissance radiale de Pinus pinea était fortement corrélée avec les précipita-tions dans le sud-ouest du Portugal. La réponse climatique de Pinus pinea a été plus forte dans la zone intérieure où la sécheresse d'été est plus sévère, les températures hivernales plus basses et où les sols ont une plus faible capacité de rétention de l'eau, comparativement aux zones côtières. IADFs a été fréquent chez Pinus pinea et la majorité d'IADFs ...

Forecasted increase drought frequency and severity may drive worldwide declines in forest productivity. Species-level responses to a drier world are likely to be influenced by their functional traits. Here, we analyse forest resilience to drought using an extensive network of tree-ring width data and satellite imagery. We compiled proxies of forest growth and productivity (TRWi, absolutely dated ring-width indices; NDVI, Normalized Difference Vegetation Index) for 11 tree species and 502 forests in Spain corresponding to Mediterranean, temperate, and continental biomes. Four different components of forest resilience to drought were calculated based on TRWi and NDVI data before, during, and after four major droughts (1986, 1994-1995, 1999, and 2005), and pointed out that TRWi data were more sensitive metrics of forest resilience to drought than NDVI data. Resilience was related to both drought severity and forest composition. Evergreen gymnosperms dominating semi-arid Mediterranean forests showed the lowest resistance to drought, but higher recovery than deciduous angiosperms dominating humid temperate forests. Moreover, semi-arid gymnosperm forests presented a negative temporal trend in the resistance to drought, but this pattern was absent in continental and temperate forests. Although gymnosperms in dry Mediterranean forests showed a faster recovery after drought, their recovery potential could be constrained if droughts become more frequent. Conversely, angiosperms and gymnosperms inhabiting temperate and continental sites might have problems to recover after more intense droughts since they resist drought but are less able to recover afterwards.

We investigated the tree growth and physiological response of five pine forest stands in relation to changes in atmospheric CO 2 concentration (c a ) and climate in the Iberian Peninsula using annually resolved width and d 13 C treering chronologies since AD 1600. 13 C discrimination (D % c i /c a ), leaf intercellular CO 2 concentration (c i ) and intrinsic water-use efficiency (iWUE) were inferred from d 13 C values. The most pronounced changes were observed during the second half of the 20th century, and differed between stands. Three sites kept a constant c i /c a ratio, leading to significant c i and iWUE increases (active response to c a ); whereas a significant increase in c i /c a resulted in the lowest iWUE increase of all stands at a relict Pinus uncinata forest site (passive response to c a ). A significant decrease in c i /c a led to the greatest iWUE improvement at the northwestern site. We tested the climatic signal strength registered in the d 13 C series after removing the low-frequency trends due to the physiological responses to increasing c a . We found stronger correlations with temperature during the growing season, demonstrating that the physiological response to c a changes modulated d 13 C and masked the climate signal. Since 1970 higher d 13 C values revealed iWUE improvements at all the sites exceeding values expected by an active response to the c a increase alone. These patterns were related to upward trends in temperatures, indicating that other factors are reinforcing stomatal closure in these forests. Narrower rings during the second half of the 20th century than in previous centuries were observed at four sites and after 1970 at all sites, providing no evidence for a possible CO 2 'fertilization' effect on growth. The iWUE improvements found for all the forests, reflecting both a c a rise and warmer conditions, seem to be insufficient to compensate for the negative effects of the increasing water limitation on growth.

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