RESEARCH
Effects of nitrogen deposition and climate extremes on European forests: combining stable isotopes in tree rings and ecosystem fluxes (NEXTRES)
SUMMARY OF THE PROJECT
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 (Ca), 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 (Ndep) 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 fertilisation 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, Figure 1) in Europe, and four of the most widespread species in European forests: Fagus sylvatica, Quercus spp., 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.
Distribution of proposed sites along a temperature and nitrogen deposition in Europe. The light yellow box indicates the critical loads for nitrogen deposition as reported in Bobbink, R., Loran, C. & Tomassen, H. Review and Revision of Empirical Critical Loads and Dose–Response Relationships (German Environment Agency, 2022).
RESEARCH UNITS
PI: Rossella Guerrieri (UNIBO) - responsible of the tree level information at multidecadal and intra-annual scale trough dendroecological analyses.
co-PI: Giorgio Matteucci (CNR-IBE) - responsible for the climate and deposition data as well as the ecosystem flux data Postdocs: coming soon! FUNDING
European Union – Next Generation EU PRIN2022_GUERRIERI: “NEXTRES - Effects of nitrogen deposition and climate extremes on European forests: combining stable isotopes in tree rings and ecosystem fluxes”, codice proposta 202299J927 - CUP J53D23002640006. 
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SPECIFIC OBJECTIVES AND QUESTIONS
Obj1. To examine the coupling between tree and ecosystem metrics over the recent decades across European forests.
Key open questions are: (Q1.1) Is the increase in WUE (both at tree and ecosystem scale) consistent across the large environmental gradient? Which are the physiological mechanisms underpinning changes in WUE? (Q1.2) Are tree and ecosystem metrics significantly correlated along a large environmental gradient? Is the strength of the relationship (slope) affected by global change factors? (Q1.3) Can we detect changes in N dynamics (retention vs. saturation) at the two levels of Ndep? (Q1.4) How do climate extremes and atmospheric Ndep interact and affect the sensitivity of tree and ecosystem to increasing Ca? Obj2. To elucidate physiological strategies adopted by different species in response to climate extremes along the latitudinal gradient and how they determine the ecosystem response. Key questions are: (Q2.1) Can we observe a divergence (among species) and convergence across sites (for a given species), in the physiological mechanisms underpinning tree responses to climate extremes? (Q2.2) Are tree and ecosystem response decoupled during an extreme events? (Q2.3) How quickly tree species recover from the disturbance and how this is reflected in carbon and water fluxes at the ecosystem scale? (Q2.4) Within each species, is there a difference in the recovery capacity along the gradient? Obj3. To evaluate the role of nitrogen deposition in driving tree and ecosystem responses to climate extremes. Key questions: (Q3.1) Can we detect different differences in N dynamics at the contrasting levels of Ndep (i.e., sites above and below the critical loads for N) during an extreme event? (Q3.2) To what extent is atmospheric deposition mediating sensitivity to drought events and which are the underpinning mechanisms? |