42 FOREST REPORT 2005
2.1 Air pollutants
In the year 2000 75 000 tons of nitrogen were deposited through the air on Switzerland’ s surface.
More than 90% of the total Swiss forest area receives too much nitrogen.
Deposition of air pollutants is higher in the for- est than in open country and highest close to major sources of emission: traffic, inhabited areas, and intensive agriculture.
In humans ozone irritates the mucous mem- branes and restricts the functions of the lungs;
in plants it attacks the cell walls and destroys individual cells in leaves.
Negative impact of air pollution
Whenpollutantsare releasedinto theair, theydo not stayin theat- mosphere. They find their way into our ecosystems as gases,as aerosols(tiniest particles floating in theair),or in rainwater.Indo- ing so theysometimes travelcon- siderable distances and are de- positedfar from their sources of emission in forests or other natu- ral areas. Substances containing nitrogen act as fertilisers in soil and water, and lead to acidified soils,just as sulphur does (>2.2 Soil).
In 1979 Switzerland signed the UNECE (United Nations Economic Commission for Eu- rope) “Convention on Long- range, TransboundaryAir Pollu- tion”. The data collected in the framework of this convention show that sulphur wreaks far lessdamage today than it didin the 70sand80s.Todayit ispre- dominantlynitrogen that creates problems.Nitrogen is largelyre- sponsible for theacidification ef- fects ofairpollution.Inaddition, it over-fertilizes(eutrophies)for- est soil.About 65% of thedepos- ited nitrogen gets into the soil in the form of reducednitrogen compounds suchasammoniaor ammonium, which are both ef- fective fertilizers. In areas of in- tensive farming, the percentage of reduced nitrogencompounds climbs to80 percent!
A total of 75 000 tons of ni- trogen were deposited through the air on Switzerland’s surface in the year 2000.In comparison to adjacent open country, the deposits in the forest are usual- lyhigher,because trees filter the pollutants out of theair.Theyare highest close to major sources of emission suchas traffic,inhabit- ed areas, and intensive agricul- ture.
FURTHERINFORMATION Swiss Federal Research Institute WSL 8903 Birmensdorf
Research Dept. FOREST Section Forest ecosystems and ecological risks (0) 44/ 739 25 95
Swiss Agency for the Environment, Forests and Landscape SAEFL 3003 Bern;
Forest Agency
Section Forest utilization and timber industry
(0) 31/ 324 77 78
43 2 HEALTH AND VITALITY
A total of 75 000 tons of nitrogen were deposited through the air on Switzerland’s surface in the year 2000. In comparison to adjacent open country, the deposits in the forest are usually higher, because trees filter the pollutants out of the air.
2.1.1
Nitrogen deposition
Total depositions of oxidized andreducedN components(inkilograms ofnitrogenperhectare and year)in the year 2000. Depicted at aspatialresolutionof onesquarekilometre.
kgNper ha and year
■≤5
■5–10
■10–15
■15–20
■20–30
■30–40
■>40 2.1.2
Acidifying depositions
Excessivecritical loadsin the year 2000. Acidifyingdepositions exceeding thecritical loadsareshown in light green. Data in acid equivalents perhectare and year under(<0)or over the critical load.
area[%]
<0 0–200 201–400 401–700 701–1000 1001–1500 >1500 70
60 50 40 30 20 10 0
Critical Loads
At what levelsdoes nitrogen start to harm sensitive ecosystems such as forests,raisedbogs,or species- rich dry grasslands?In order to find this out, experts from all over Europedraw upon theval- uesknownascritical loads spec- ifiedin theUNECE Convention.
As farasweknow today,if these critical values are not exceed- ed, the substancesdo not havea negative effect on the functions andstructures of ecosystems.Ex- ceeding thesevalues for longpe- riods, on the other hand, puts ecosystemsat risk.Asingle inci- dent when thevaluesare exceed- ed maynot,however,be direct- lyresponsible for damage toan ecosystem.To draw such a con- clusion,observations overalong period of time are required, to- getherwithcarefulanalysis using dynamicmodels toassess the ef-
[Acid equivalents per hectare and year]
44 FOREST REPORT 2005
Ozone
Since the 1980s the increasing concentrations of tropospheric ozone have regularly made the headlines,especiallyin thewarm- er half of theyear,when,depend- ing on theweather,ozonevalues climb.While in humans ozone ir- ritates mucous membranes and restricts lungcapacity, in plants it attacks the cell wallsand de- stroys individual cells in leaves.
Depending on the sensitivity of the plant species and the ozone concentration,visible leaf or nee- dledamageappears: the leaves changecolour earlier inautumn and drop sooner, their photo- synthetic capacitydecreases, the plant grows more slowlyand the supply of assimilates from the leaves to the roots isdisrupted.
The harmful effects of ozone are difficult to prove because it leaves behind no chemical res- idue that can be analyzed or measured.Visible leaf or needle damage is therefore the onlyef- fect that expertscan easilydetect and characterise. From these ef- fects theycandetermine the lev- els of ozone associated with dif- ferent kinds ofdamage.The first visible ozonedamagewasdetect- edon grapevines inCaliforniain 1958.In the meantime scientists have foundout much moreabout how thisdamagedevelops.How- ever, up till nowonlyafewstud- ies havedescribedandcharacter- isedozone symptoms indetail.
Researchersat theSwissFed- eralResearch InstituteWSLand at Pennsylvania State Univer- sity in the USA are trying to fill in this gap.Since1995 theyhave been operating a research facil- ity in Ticino where they exam- ine the effects of ozone on forest vegetation under natural envi- ronmental conditions. Among other studies, the researchersare developing a database in which theydocument the kinds of vis- ible damage induced by ozone.
Those interested will find pic- fects of pollution on an ecosys-
tem.The models shouldbeable to take intoaccount the intensi- tyof thepollutionaswellas how long the ecosystem hasbeen sub- ject to it.
In 2000 34% ofSwiss forests were exposed to excessive dep- osition from air pollution with acidifying effects. Compared to the end of the 1980s when this figurewas higher than 60%, the situation has improvedin recent years. On the other hand, we still have to be very concerned about nitrogen:more than 90%
2.1.3
Nitrogen depositions in forest ecosystems
Frequency distributionofnitrogen inSwiss forest areas. Areas where nitrogen depositionsexceed thecritical loadsareshown in light grey.
area[%]
Exceeding the critical load [Kilograms of nitrogen per hectare and year]
45 40 35 30 25 20 15 10 5
0 < 0 0–5 5–10 10–20 20–40 >40
2.1.4
Nitrogen depositions on the NFI sites
Estimated andmodelled nitrogen depositions on theNFI sites forlong-term forest ecosystem research. Theredbands mark thecritical levels fornitrogen depositions.
Nitrogen deposition
[Kilograms of nitrogen per hectare and year]
40 35 30 25 20 15 10 5 0
Bettlachstock Neunkirch
Jussy Lausanne
Othmarsingen Vordem
wald Alptal
Beatenberg Schänis
Celerina Nationalpark
Novaggio Chironico
■Estimates based on deposition meas- urements
■Modelled deposition values –
–Critical loads for nitrogen (mass bal- ance)
of our forest areasare getting too much nitrogen.Critical loadval- ues tend tobe exceededmost in theSwissCentralPlateau,on the fringes of the Alps, and in the southern regions ofTicino.InTi- cino much of the high nitrogen pollutioncanbe traced topollut- ants from northernItaly.
45 2 HEALTH AND VITALITY
tures of more than80 plant spe- cies showing ozone damage at www.ozone.wsl.ch. In addition, experts havebeenable toprove that there is a clear connection between higher ozoneconcentra- tion levels, theappearance ofvis- ible symptomsandreducedpho- tosynthetic activity.
In spite of this evidence it is difficult to define a biologically relevant critical level for ozone.
Plant species,age,exposure,soil moisture, temperature, photo- syntheticallyactive radiationand relativeair humidityare all fac- tors that influence the ozone up- take ofplants.
2.1.5a
Black poplar
Visibleozone damageon ablack poplarleaf.
2.1.5b
Sycamore
Leaf cellsdestroyedby ozone.
2.1.6
Photosynthetic activity of ash trees
Photosyntheticactivity ofash(Fraxinusexcelsior L.)inrelation to symptomdevelopment (red) and different ozoneconcentration levels(grey).
Photosynthesis [mmol m-2s-1CO2] %of leaf area with visible symptoms
–
–Photosynthesis in filtered air =approx. 50%of the ozone concentration of the environmental air
––Photsynthesis in unfiltered air (=approx. 100%ozone concentration of the sur- rounding air)
–
–Leaf area with visible symptoms
17.June 24.June 1.July 8.July 15.July 22.July 29.July 5. Aug. 12. Aug. 19. Aug.
140 120 100 80 60 40 20 0
35 30 25 20 15 10 5 0
