Jet stream position explains regional anomalies in European beech forest productivity and tree growth.

The mechanistic pathways connecting ocean-atmosphere variability and terrestrial productivity are well-established theoretically, but remain challenging to quantify empirically. Such quantification will greatly improve the assessment and prediction of changes in terrestrial carbon sequestration in response to dynamically induced climatic extremes. The jet stream latitude (JSL) over the North Atlantic-European domain provides a synthetic and robust physical framework that integrates climate variability not accounted for by atmospheric circulation patterns alone. Surface climate impacts of north-south summer JSL displacements are not uniform across Europe, but rather create a northwestern-southeastern dipole in forest productivity and radial-growth anomalies. Summer JSL variability over the eastern North Atlantic-European domain (5-40E) exerts the strongest impact on European beech, inducing anomalies of up to 30% in modelled gross primary productivity and 50% in radial tree growth. The net effects of JSL movements on terrestrial carbon fluxes depend on forest density, carbon stocks, and productivity imbalances across biogeographic regions.

Climate-change-driven growth decline of European beech forests.

The growth of past, present, and future forests was, is and will be affected by climate variability. This multifaceted relationship has been assessed in several regional studies, but spatially resolved, large-scale analyses are largely missing so far. Here we estimate recent changes in growth of 5800 beech trees (Fagus sylvatica L.) from 324 sites, representing the full geographic and climatic range of species. Future growth trends were predicted considering state-of-the-art climate scenarios. The validated models indicate growth declines across large region of the distribution in recent decades, and project severe future growth declines ranging from -20% to more than -50% by 2090, depending on the region and climate change scenario (i.e. CMIP6 SSP1-2.6 and SSP5-8.5). Forecasted forest productivity losses are most striking towards the southern distribution limit of Fagus sylvatica, in regions where persisting atmospheric high-pressure systems are expected to increase drought severity. The projected 21st century growth changes across Europe indicate serious ecological and economic consequences that require immediate forest adaptation.

Legacies of past forest management determine current responses to severe drought events of conifer species in the Romanian Carpathians.

Worldwide increases in droughts- and heat-waves-associated tree mortality events are destabilizing the future of many forests and the ecosystem services they provide. Along with climate, understanding the impact of the legacies of past forest management is key to better explain current responses of different tree species to climate change. We studied tree mortality events that peaked in 2012 affecting one native (silver fir; growing within its natural distribution range) and two introduced (black pine and Scots; growing outside their natural distribution range) conifer species from the Romanian Carpathians. The three conifers were compared in terms of mortality events, growth trends, growth resilience to severe drought events, climate-growth relationships, and regeneration patterns. The mortality rates of the three species were found to be associated with severe drought events. Nevertheless, the native silver fir seems to undergo a self-thinning process, while the future of the remaining living black pine and Scots pine trees is uncertain as they register significant negative growth trends. Overall, the native silver fir showed a higher resilience to severe drought events than the two introduced pine species. Furthermore, and unlike the native silver fir, black pine and Scots pine species do not successfully regenerate. A high diversity of native broadleaf species sprouts and develops instead under them suggesting that we might be witnessing a process of ecological succession, with broadleaves recovering their habitats. As native species seem to perform better in terms of resilience and regeneration than introduced species, the overall effect of the black pine and Scots pine mortality might be compensated. Legacies of past forest management should be taken into account in order to better understand current responses of different tree species to ongoing climate change.