1. 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

2. 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

3. 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), International network of monitoring plots  monitoring of forest condition: ±6000 Level I plots  clarify cause-effect relationships: ±500 Level II plots

4. 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

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

6. 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

7. 5. Results

8. 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

9. 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

10. 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., 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.

11. 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)

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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.

19. 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.

20. 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.

21. 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; …

22. 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

23. Thank you for your attention!