Long-term Intensive Monitoring in Flemish forests under recovery from acidification Arne Verstraeten, Johan Neirynck, Nathalie Cools, Bruno De Vos, Geert Sioen, Peter Roskams, Gerald Louette, Maurice Hoffmann NAEM 2016 – Lunteren, The Netherlands Parallel 2d: Long-term Ecological Research

I will present Background on European forest monitoring (1985−present) Long-term Intensive forest Monitoring in Flanders (1987−present) Level II core plots in Flanders Objectives of the network Results - Acidifying and eutrophicating deposition - Soil solution Conclusions

1. Background on European forest monitoring (1985−present) Forest dieback in Central Europe in the 1970’s  clear link with air pollution and ‘acid rain’ UNECE Convention on Long-Range Transboundary Air Pollution (CLRTAP), 1979, Geneva  abate acidifying emissions International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests), 1985 International network of monitoring plots  monitoring of forest condition: ±6000 Level I plots  clarify cause-effect relationships: ±500 Level II plots

2. Long-term Intensive forest Monitoring in Flanders (1987−present) Plots were installed in 1987−1991 72 Level I plots crown condition 5 Level II core plots (all part of LTER-site) deposition, soil, soil solution, litterfall, tree mineral nutrition, growth, meteorology, ground vegetation, crown condition, LAI, … 6 Level II additional plots (2 part of LTER-site) soil, growth, ground vegetation, crown condition

3. Level II core plots in Flanders Table 1 Characteristics of the 5 Level II core plots in Flanders.

4. Objectives of the network Evaluate trends in acidifying and eutrophicating deposition in forests in relation to target loads and critical loads Measure to measure (monitoring state and trends) Evaluate ecosystem response and ecosystem status based on trends in soil solution chemistry in relation to critical limits Learn from measuring

5. Results

Acidifying and eutrophicating deposition Trends in deposition below canopy (throughfall + stemflow) DPSIR SO42‒ , NH4+ and ACID (SO42– + NH4+ + NO3–) decreased sharply (1994−2014) NO3‒ and base cations (Ca2+ + K+ + Mg2+) decreased in 3 plots Verstraeten et al. (2012) ATMOS ENVIRON

Acidifying and eutrophicating deposition Trends in deposition below canopy (throughfall + stemflow) DPSIR Table 2 Mann-Kendall trends (1994−2014) for deposition below canopy (kmolc ha–1 y–1) of NH4+, NO3–, SO42–, base cations and ACID, with slope and significance (ns: not significant, *: p < 0.05, **: p < 0.01, ***: p < 0.001).   NH4+ NO3− SO42− Base cations ACID Wijnendale -0.06*** ns -0.05*** -0.12*** Ravels -0.07*** -0.13*** Brasschaat -0.01** -0.02*** Gontrode -0.01*** -0.03** Hoeilaart -0.04*** -0.04** -0.10*** SO42‒ , NH4+ and ACID (SO42– + NH4+ + NO3–) decreased sharply (1994−2014) NO3‒ and base cations (Ca2+ + K+ + Mg2+) decreased in 3 plots Verstraeten et al. (2012) ATMOS ENVIRON

Acidifying and eutrophicating deposition Target loads for total acidifying deposition (incl. canopy exchange) DPSIR 2010-target* (2660 kmolc ha–1 y–1, NEC-directive 2001/81/EG) was reached in all plots 2030-target* (1400 kmolc ha–1 y–1, MINA, VLAREM II) currently was reached only in Hoeilaart *Buysse H, Celis D, Van Avermaet P, et al., 2010. Verzurende depositie en overschrijding kritische lasten. Visionair Scenario Milieuverkenning 2030, studie uitgevoerd in opdracht van de Vlaamse Milieumaatschappij, MIRA, MIRA/2010/04, VMM en VITO.

Acidifying and eutrophicating deposition Critical loads for total inorganic N deposition (incl. canopy exchange) DPSIR N critical load for ground vegetation (714‒1071 kmolc ha–1 y–1) (UNECE, 2004) was reached only in Hoeilaart Still far above the N critical load for corticulous lichens (221 kmolc ha–1 y–1) (Fenn et al., 2008 Environ Pollut)

Soil solution Trends in the concentrations of N and S compounds DPSIR SO42‒ concentrations decreased sharply at the 5 plots NO3‒ concentrations decreased in 4 plots and very suddenly in 2 plots NO3‒ leaching remains very high in Gontrode Verstraeten et al. (2012) ATMOS ENVIRON

Soil solution Trends in the concentrations of N and S compounds DPSIR SO42‒ concentrations decreased sharply at the 5 plots NO3‒ concentrations decreased in 4 plots and very suddenly in 2 plots NO3‒ leaching remains very high in Gontrode Verstraeten et al. (2012) ATMOS ENVIRON

Soil solution Trends in pH DPSIR Soil solution pH increased (2005−2013) Note that pH is very low (<3.5−4.5) in the mineral soil Verstraeten et al., accepted, SCI TOTAL ENVIRON

Soil solution Trends in the concentrations of Dissolved Organic Carbon (DOC) DPSIR DOC concentrations increased in soil solution (2002−2013) Reflects acidification recovery (higher pH, lower Al concentration) (Monteith et al., 2007 NATURE) Verstraeten et al. (2014) ATMOS ENVIRON

Soil solution Trends in the concentrations of Dissolved Organic Nitrogen (DON) DPSIR DON concentrations increased in soil solution and below canopy (2005−2013) Stimulation of biotic activity by climate warming? (Pitman et al., 2010 SCI TOTAL ENVIRON) Verstraeten et al., accepted, SCI TOTAL ENVIRON

Soil solution Trends in Acid Neutralizing Capacity (ANC) ANC = Ca2+ + K+ + Mg2+ + Na+ - Cl− - NH4+ - SO42− - NO3− Trends in Acid Neutralizing Capacity (ANC) DPSIR Soil solution ANC increased, but is still < 0 at most depths in most plots Soil acidification is slowing down, and already reversing in Hoeilaart (O horizon) Verstraeten et al. (2012) ATMOS ENVIRON

Soil solution Trends in the DON:TN ratio DPSIR The DON:TN ratio in deposition and soil solution increased (2005−2014) A larger proportion of DON reflects recovery from N saturation (improving N status) Verstraeten et al., in prep.

Soil solution Relation between DON:TN ratio and inorganic N deposition DPSIR Increasing DON:TN ratio in soil solution is related to decreasing inorganic N (DIN) deposition Verstraeten et al., in prep.

Soil solution Trends in the DOC:NO3− ratio – stoichiometric control DPSIR The DOC:NO3− ratio in soil solution increased (2002−2014) The DOC:NO3− ratio passed the critical inflection point for heterotrophic bacteria in soils (5.22) in 4 plots (Taylor and Townsend, 2010 NATURE) This reflects initial recovery from N saturation Verstraeten et al., in prep.

Conclusions Monitoring Atmospheric depositions of acidifying and eutrophicating compounds decreased sharply in highly acidified Flemish forests (1994–2014); N-depositions are generally approaching the critical loads for ground vegetation but are still far above those for corticulous lichens; Open question: how about N critical loads for ectomycorrhizal fungi; indications of a strong relation (Suz et al., 2015 MOL ECOL) Ecosystem responses Soil solution analysis revealed that the decrease in atmospheric depositions initiated chemical recovery from acidification and N saturation Long-term research relevance These results show that long-term field observations at parallel sites across regions-, Europe-, worldwide, made with a sufficiently high frequency provide a powerful tool to study cause-effect relationships; The time series are extremely valuable information for scientists, forest managers and policy makers Future research Vegetation and ectomycorrhizal fungi response; Ecosystem functioning dependence of soil conditions; …

Acknowledgements I would like to thank: The colleagues from our technical staff and laboratory for the collection and analysis of samples and advice on statistics My PhD promoters from Ghent University: prof. dr. ir. Stefaan De Neve prof. dr. ir. Steven Sleutel

Thank you for your attention!