NEXTRES

The ability of forests to continue providing important ecosystem services and mitigating climate change depends on their ability to cope with - and adapt to - global change components, such as more frequent climate extremes (specifically drought and heatwaves) and changes in atmospheric pollutants (namely carbon dioxide, CO2, and reactive nitrogen, N, compounds). The 2003 heatwaves cannot be considered as an exceptional event anymore, as prolonged summer droughts and temperatures anomalies are becoming more common and frequent across the whole European continent, due to the human-induced climate change. Frequent droughts and hot extremes are not the only challenge forests need to face. Since the industrial revolution, atmospheric chemistry has been altered, with steadily increases in CO2 concentrations, but also to changes in reactive N in the oxidised and reduced forms. While almost half of the CO2 emitted by fossil fuel combustion remains in the atmosphere (and hence contribute to global warming), reactive N compounds are deposited back to terrestrial and aquatic ecosystems, directly altering the N cycle, but indirectly also the carbon and water cycles. While N deposition could stimulate tree growth in a CO2 richer word, excess N could result in forest dieback, through soil acidification and nutrient imbalances but also by making trees more vulnerable to climate extremes. How do these global change components interact and affect forest carbon, water and N cycling? What are the ecological mechanisms involved? Are those mechanisms synchronized (in space and time) at tree and ecosystem scale? Answering these questions is of paramount importance to reduce the uncertainties on the sustainability of the CO2 fertilization effects on forest carbon sink and hence, to predict future forest function and climate mitigation potential. In order to answer these fundamental questions, NEXTRES will consider 12 forests along a climate and N deposition gradient (from 3 to 42 kg ha-1 yr-1) in Europe, and four of the most widespread species in European forests: Fagus sylvatica, Quercus petrea, Picea abies, Pinus sylvestris. Forests sites will be selected within established networks, namely ICOS and/or Fluxnet (for the ecosystem scale measurements of carbon and water fluxes with eddy covariance technique) and ICP Forests (for atmospheric nitrogen deposition). NEXTRES will complement existing data with dendroecological data (growth, stable carbon, oxygen and nitrogen isotope ratios), allowing to elucidate physiological mechanisms underpinning response to global change drivers from the multi-decadal to the intra-annual resolutions. Expected impacts of NEXTRES include not only the advancement of scientific knowledge and education; progress on the complex topic of forest response to global change drivers is also crucial to support data-based policies towards the ambitious goals set within the Paris Agreement and European Green Deal of a climate neutral Europe by 2050.

Expected results

Goals addressed within NEXTRES fall within the ‘hot’ scientific debate (both at European and international levels) on the ecopysiological mechanisms underpinning forest resilience to global change drivers, including nitrogen deposition as additional variables – so far excluded in the drought-centered theoretical framework. This is first project providing tree ring isotopes coupled with flux data along a large European gradient – an approach that so far has been only presented in the US, in study where the coordinator of the project was involved (Guerrieri et al. 2019 116 (34): 1690916914).

Achieved results

NEXTRES generated unique long-term datasets of tree growth, intrinsic water-use efficiency (the ratio between CO₂ assimilation and stomatal conductance), annual and intra-annual stable carbon isotope composition, annual oxygen and nitrogen isotope records, complementing existing European monitoring networks with tree-level physiological information that was previously unavailable. By integrating these datasets with ICOS and Fluxnet observations, NEXTRES enabled, for the first time, direct comparisons between tree physiological processes and ecosystem carbon and water fluxes across a pan-European forest network.

At the multi-decadal scale, intrinsic water-use efficiency increased across most species and sites, confirming the widespread enhancement of tree water-use efficiency under rising atmospheric CO₂. However, equivalent increases were not consistently observed at the ecosystem scale, where water-use efficiency remained stable or even declined at some sites because changes in gross primary productivity and evapotranspiration did not mirror tree-level responses. These results demonstrate that physiological

adjustments inferred from tree rings cannot be directly extrapolated to ecosystem functioning and highlight the importance of integrating observations across different scales.

To investigate forest responses to climate extremes, NEXTRES produced one of the first pan-European datasets of intra-annual δ¹³C measurements across multiple tree species and climatic regions using laser ablation isotope ratio mass spectrometry. Climate extremes were identified through a consistent analysis of ERA5 Land-derived anomalies in temperature, precipitation and climatic water balance (such as the SPEI), computed relative to the 1991–2020 baseline, evaluated in terms of magnitude, persistence and cross-variable coherence, and used to classify years such as 2018 (hot and dry) and 2019 (hot and wet) as extreme across most sites. Our data suggest that the typical seasonal pattern of intrinsic water-use efficiency was substantially altered during drought years, although responses varied according to species, latitude and the nature of the extreme event. Conifer forests generally exhibited more conservative water-use strategies than broadleaved species, while northern Fagus sylvatica forests showed greater sensitivity to drought, reflected by stronger reductions in growth and larger intra-annual changes in intrinsic water-use efficiency. Conversely, the late spring frost event investigated in the Italian beech forest produced physiological responses opposite to those observed during drought, highlighting that different climate extremes trigger distinct adaptive strategies. Combined analyses of carbon and oxygen isotope ratios demonstrated that stomatal regulation was the dominant mechanism controlling changes in intrinsic water-use efficiency during extreme events. Furthermore, by integrating isotope records with ecosystem flux measurements and wood formation dynamics, NEXTRES showed that intra-annual isotope signals closely reflected seasonal patterns of ecosystem carbon uptake, opening new opportunities for linking tree physiology with ecosystem functioning at unprecedented temporal resolution.

NEXTRES also established the first pan-European framework for assessing the combined effects of atmospheric nitrogen deposition and climate extremes by integrating tree-ring δ¹⁵N measurements with atmospheric deposition data and ecosystem observations. Tree growth generally tended to be higher at sites receiving intermediate to high nitrogen deposition, although this pattern reflected interactions between nitrogen availability and climate. Whereas, no consistent differences in intrinsic water-use efficiency across nitrogen deposition levels were observed among species. Temporal declines in tree-ring δ¹⁵N observed at several historically nitrogen-rich sites suggest changing nitrogen availability through time, potentially associated with reductions in atmospheric nitrogen deposition and increasing climatic constraints on nitrogen uptake. While further data analyses and elaboration are ongoing, the project generated unique datasets and analytical approaches that will enable future quantification of the interacting effects of atmospheric CO₂, nitrogen deposition and climate extremes on forest carbon, water and nitrogen cycling.

Beyond presenting results to scientific community, NEXTRES actively engaged society through a dedicated project website and social media channels, a public science event on forests and global change, and educational activities for children aged 7–10 through the UniJunior programme, where interactive lectures and hands-on teaching materials introduced how tree rings can reveal the impacts of climate change on forests. These activities contributed to increasing public awareness of forest resilience and promoting scientific literacy among younger generations.

D.D. del MUR n. 104 del 02/02/2022

Codice progetto MUR: 202299J927

CUP: J53D23002640006

Coordinatore di progetto: DISTAL

Ruolo UNIBO: PI

Sostegno finanziario UE: 190.000 € (di cui 104.574€ PI DISTAL)

Ambiti di ricerca: Arboricoltura; Selvicoltura; Gruppo Ecologia Forestale

Responsabile Scientifico: Maria Rosa Rossella Guerrieri

Durata: 28/09/2023 - 28/09/2025

Gruppo di ricerca: Maria Rosa Rossella Guerrieri, Marco Montedoro, Giorgio Matteucci, Matteo Rossi, Francesco Mazzenga.