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

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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Aim The aim was to decipher Europe‐wide spatio‐temporal patterns of forest growth dynamics and their associations with carbon isotope fractionation processes inferred from tree rings as modulated by climate warming. Location Europe and North Africa (30‒70° N, 10° W‒35° E). Time period 1901‒2003. Major taxa studied Temperate and Euro‐Siberian trees. Methods We characterize changes in the relationship between tree growth and carbon isotope fractionation over the 20th century using a European network consisting of 20 site chronologies. Using indexed tree‐ring widths (TRWi), we assess shifts in the temporal coherence of radial growth across sites (synchrony) for five forest ecosystems (Atlantic, boreal, cold continental, Mediterranean and temperate). We also examine whether TRWi shows variable coupling with leaf‐level gas exchange, inferred from indexed carbon isotope discrimination of tree‐ring cellulose (Δ13Ci). Results We find spatial autocorrelation for TRWi and Δ13Ci extending over a maximum of 1,000 km among forest stands. However, growth synchrony is not uniform across Europe, but increases along a latitudinal gradient concurrent with decreasing temperature and evapotranspiration. Latitudinal relationships between TRWi and Δ13Ci (changing from negative to positive southwards) point to drought impairing carbon uptake via stomatal regulation for water saving occurring at forests below 60° N in continental Europe. An increase in forest growth synchrony over the 20th century together with increasingly positive relationships between TRWi and Δ13Ci indicate intensifying impacts of drought on tree performance. These effects are noticeable in drought‐prone biomes (Mediterranean, temperate and cold continental). Main conclusions At the turn of this century, convergence in growth synchrony across European forest ecosystems is coupled with coordinated warming‐induced effects of drought on leaf physiology and tree growth spreading northwards. Such a tendency towards exacerbated moisture‐sensitive growth and physiology could override positive effects of enhanced leaf intercellular CO2 concentrations, possibly resulting in Europe‐wide declines of forest carbon gain in the coming decades.

This paper examines tree‐ring width and δ13C chronologies from a network of five Iberian pine forests to determine their sensitivity to climate variability under different site conditions. Interseries comparisons revealed better and more homogenous agreement among δ13C records than among tree‐ring width series of the different sites. This suggests that δ13C ratios may preferentially record large‐scale climatic signals, whereas ring‐width variations may reflect more local factors. A negative relationship was found between ring‐width and δ13C. As inferred from response function analyses, ring‐width and δ13C showed significant relationships with climate. The analyses of different sites and species revealed unshared tree‐ring width responses to summer temperature and precipitation, whereas all δ13C series were highly sensitive to current year summer precipitation and, to a lesser extent, to current summer temperature. A strong summer precipitation signal seems to dominate the δ13C of trees growing under Mediterranean climate, even when the mean climatic site conditions do not indicate distinct summer drought. Therefore, δ13C values reflect precipitation variability during the summer season better than tree‐ring widths. This demonstrates that δ13C from tree‐rings can be a very useful tool for climatic reconstruction in the Mediterranean region, especially when climate‐growth relationships are weak.

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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, δ 13 C and δ 18 O). 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.

Although solar radiation at the surface plays a determinant role in carbon discrimination in tree rings, stable carbon isotope chronologies (δ13C) have often been interpreted as a temperature proxy due to the co-variability of temperature and surface solar radiation. Furthermore, even when surface solar radiation is assumed to be the main driver of 13C discrimination in tree rings, δ13C records have been calibrated against sunshine duration or cloud cover series for which longer observational records exists. In this study, we use different instrumental and satellite data over northeast Spain (southern Europe) to identify the main driver of tree-ring 13C discrimination in this region. Special attention is paid to periods in which the co-variability of those climate variables may have been weaker, such as years after large volcanic eruptions. The analysis identified surface solar radiation as the main driver of tree-ring δ13C changes in this region, although the influence of other climatic factors may not be negligible. Accordingly, we suggest that a reconstruction of SSR over the last 600 years is possible. The relation between multidecadal variations of an independent temperature reconstruction and surface solar radiation in this region shows no clear sign, and warmer (colder) periods may be accompanied by both higher and lower surface solar radiation. However, our reconstructed records of surface solar radiation reveals a sunnier Little Ice Age in agreement with other δ13C tree-ring series used to reconstruct sunshine duration in central and northern Europe