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Key message Climate-warming related replacement of beech by oak forests in the course of natural forest succession or silvicultural decisions may considerably reduce ecosystem carbon storage of central European woodlands. Context Climate warming may change the carbon (C) storage in forest biomass and soil through future shifts in tree species composition. With a projected warming by 2–3 K over the twenty-first century, silvicultural adaptation measures and natural succession might lead to the replacement of European beech forests by thermophilic oak forests in drought- and heat-affected regions of central and south-eastern Europe, but the consequences for ecosystem C storage of this species shift are not clear. Aims To quantify the change in C storage in biomass and soil with a shift from beech (Fagus sylvatica) to oak forest (Quercus petraea, Q. frainetto, Q. cerris), we measured the aboveground biomass (AGC) and soil C pools (SOC). Methods AGC pools and SOC stocks to − 100 cm depth were calculated from forest inventory and volume-related SOC content data for beech, mixed beech-oak and oak forests in three transects in the natural beech-oak ecotone of western Romania, where beech occurs at its heat- and drought-induced distribution limit. Results From the cooler, more humid beech forests to the warmer, more xeric oak forests, which are 1–2 K warmer, AGC and SOC pools decreased by about 22% (40 Mg C ha−1) and 20% (17 Mg C ha−1), respectively. The likely main drivers are indirect temperature effects acting through tree species and management in the case of AGC, but direct temperature effects for SOC. Conclusion If drought- and heat-affected beech forests in Central Europe are replaced by thermophilic oak forests in future, this will lead to carbon losses of ~ 50–60 Mg ha−1, thus reducing ecosystem carbon storage substantially.

Aim Climatic changes affect the growth dynamics of temperate trees, but these effects might differ between co‐occurring ring‐ and diffuse‐porous species as well as between mesic and xeric rear‐edge populations. We explore whether recent climate warming has increased the climate sensitivity and within‐stand synchrony of growth in these groups differently. Location The natural beech–oak ecotone in Western Romania at the dry margin of beech occurrence. Taxa Three ring‐porous oak species (Quercus petraea, Q. frainetto and Q. cerris), and diffuse‐porous European beech (Fagus sylvatica) and silver linden (Tilia tomentosa). Methods We correlated tree‐ring records with monthly and seasonal climate data (period 1940–2017). Regional growth synchrony was assessed through the analysis of inter‐series correlation of growth within populations and among populations using mixed models. Results In all five species including two south‐east European oak taxa and silver linden, water availability in summer was the most important climatic determinant of radial growth. This factor has gained in importance since the onset of rapid warming after 1980, while the impact of other climate factors in spring and summer has decreased. Within‐population growth synchrony as a measure of overall climatic stress has increased, or remained stable, since 1980 in beech and silver linden, but has decreased in the oak species, matching declining growth trends in beech and linden and increasing (or stable) trends in the oaks. Main Conclusions The patterns of growth synchrony provide valuable information on tree species' drought susceptibility in efforts to select suitable tree species for climate change‐adapted forestry. The climate vulnerability of beech is higher than that of the more drought‐resistant oak species due to its marked summer‐drought sensitivity of growth.

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