In het kader van het vlina-project werden gemodelleerde deposities van stikstof en zwavel geconfronteerd met kritische depositieniveaus (kritische lasten) door toe-passing van de Simple Mass balance. Deze kritische las-ten geven een depositieniveau aan waarvoor een vooropgesteld beschermingscriterium voor een receptor van het bosecosysteem net gehaald wordt. Afhankelijk van het vooropgestelde beschermingscriterium of norm wordt een andere kritische last verkregen. In dit thema-nummer werden het aantal beschermingscriteria uitge-breid tot die m.b.t.:
• verzuring: voorkomen van verdere bodemverzuring en Al-toxiciteit bij haarwortels
• vermesting: verhinderen van nitraatcontanimatie in ondiep grondwater, verandering in kruidsamenstel-ling, verstoring voedingsbalans
Bij de verzuringseffecten blijkt de kritische last voor het voorkomen van bodemverzuring ongeveer dubbel zo streng (lees gevoelig) dan deze gerelateerd aan de haarwortelsterfte (Al-toxiciteit). Beperkte emissiereduc-ties die toereikend zijn om de effecten van de verzurende depositie op haarwortelsterfte te voorkomen, zijn aldus onvoldoende voor het behoud van buffercapaciteit (voorkomen verdere bodemverzuring). Daarnaast blijkt loofbos gevoeliger te zijn voor wortelschade dan naald-bos door de hogere opnamecapaciteit van basische kationen. Op basis van de huidige deposities is de kriti-sche last voor het criterium behoud van buffercapaciteit in alle receptorpunten in Vlaanderen overschreden; voor het criterium haarwortelschade wordt een over-schrijding geconstateerd in resp. 86 % (loofhout) en 95 % (naaldhout) van de receptorpunten.
Bij de vermestingseffecten worden, naast de verschillen in criterium, grote verschillen waargenomen tussen naaldhout en loofhout. De grotere gevoeligheid inzake vermestingseffecten bij naaldbomen wordt veroorzaakt door een geringe stikstofvastlegging, hogere verdamping van het geïntercepteerde regenwater en het voorkomen van naaldbos in Vlaanderen op goed gedraineerde voe-dingsarme zandgronden. Op basis van de huidige depo-sities wordt de kritische last voor het criterium behoud van de samenstelling van bosflora in alle receptor-punten overschreden, bij het criterium voor het beschermen van ondiep grondwater (25 ppm NO3) wordt een overschrijding geconstateerd in 99 % van de receptorpunten onder naaldhout en in 53 % onder loof-hout. Voor het criterium kritische stikstofgehalten in de bladeren worden de laagste overschrijdingen waargeno-men, nl. in 81 % van de receptorpunten onder naald-hout en slecht in 34 % van de gevallen onder loofnaald-hout. De strengere kritische last onder naaldhout leidt ook tot
meer overschrijdingen van de kritische lasten zodat maatregelen tot emissiereducties vooral eerst in loofbos tot opheffen van de effecten zal leiden. Gerichte omvor-ming van naaldbos tot loofbos in Vlaamse bosgebieden kan bijkomend, als effectgerichte maatregel, een belang-rijke bijdrage vormen om vermestingseffecten onge-daan te maken.
Voor de strengste eisen, zoals het behoud van de soorten-samenstelling in de kruidlaag en de buffercapaciteit moet voor een verbetering op korte termijn gezorgd worden door emissiereducties. Om een vermindering van de buffercapaciteit van bosbodems tegen te gaan, zijn drastische reducties van 2000 Zeq ha-1jaar-1
noodzakelijk. Gezien de dominantie van stikstof in de verzurende depositie kunnen deze het best door stik-stofreducties gerealiseerd worden. Daarnaast kan bos-beheer het bodemherstel op langere termijn bevorderen door de inbreng van bodemverplegende pionierboom-soorten (boswilg, berk, trilpopulier) of linde afhankelijk van de grootte van de verjongingsgroepen.
H o o f d s t u k 5 S u m m a r y
Forest condition monitoring in Belgium-Flanders start-ed in 1987 according to the guidelines of the un/ece icp Forests and the eu Scheme on the protection of Forests against Air Pollution. Atmospheric deposition is moni-tored since 1992 in 6 selected forest areas. Two perma-nent plots (0.25 ha) are located in coniferous (pines) and 4 in broad-leaved (beech, oak, ash) stands.
Total atmospheric deposition can be separated into wet deposition and dry deposition. Wet deposition consists of particles and gases which are extracted from the atmosphere by rain or snow. Dry deposition represents deposition due to turbulent exchange between atmos-phere and underlying surfaces. Because of the high aero-dynamic roughness of forests, the latter is strongly enhanced above forest, which results in elevated deposi-tions compared to low vegetation types. The precipita-tion reaches the forest floor as througfall and stemflow. At the level II sites throughfall and stemflow collectors have been installed to sample this deposition load to the forest floor. The chemical composition from this deposi-tion differs, however, from total deposideposi-tion. On the one hand, part of dry deposition can be taken up (e.g. ammo-nium) resulting in an underestimate of total deposition. On the other hand, it can overestimate total deposition because canopy leaching or exchange processes (e.g. base cations) enriche the incident precipitation. This study shows that ammonium and sulphate are the pre-vailing ions in the atmospheric deposition in Flanders. The prevalence of ammonium is responsible for the slight acidity of the precipitation. Using the model of Ulrich, canopy leaching of base cations is estimated to range from 0.6 till 3.1 kmolcha-1year-1in stands grow-ing on richer soil types. In the stands located on poor sandy soils the canopy leaching of cations is negligible (0.25 kmolcha-1year-1). Canopy leaching is mainly attributed by the canopy uptake of ammonium. On average, ammonium uptake in the stands during the monitoring period varies from 0.6 till 1.6 kmolcha-1 year-1(resp. 8 till 22 kg N ha-1year-1).
Total nitrogen deposition at the investigated sites varies between 34 and 46 kg N ha-1year-1, whereas sulphur deposition ranges between 18 and 32 kg S ha-1year-1. Total deposition of acidifying compounds varies between 3.8 and 5.5 kmolcha-1year-1and belongs to the highest classes in Europe. Taking into account that the atmospheric deposition is partly neutralised by base cations (12 till 24%), total potential acidifying deposi-tion ranges from 2.7 till 5.0 kmolcha-1year-1over the
period 1992-2002. Trend analysis of throughfall deposi-tion reveals a decrease in total nitrogen deposideposi-tion by 0.9 to 3.4 kg N ha-1year-1, which is mainly due to declin-ing ammonium deposition rates. Sulphur declines at an annual rate of 1.2 till 2.7 kg S ha-1and the potential acid-ifying deposition at an average rate of 0.23 kmolcha-1. The decrease in acidifying deposition is, however, partly offset by a concomitant decrease in base cations. Monitoring of gaseous air pollutants at the measuring tower site in Brasschaat reveals a declining trend for SO2 and a steady increase of ozone concentrations. The latter is especially due to increased background levels. For NOxno clear tendency can be discerned.
Annual average fluxes of SO2, calculated by the gradient method, follows declining trends between 1997 and 2002. This can be explained by the drop in SO2levels and raised canopy resistances (Rc).
Ozone deposition during the period 2000-2002 ranges between 64 and 103 kg ha-1yearly. At least 25 % of ozone deposition can be attributed to non-stomatal pathways. Analysis of ozone fluxes reveals that ozone concentrations and fluxes are decoupled. It is found that meteorological factors (rainfall, dew) might better account for stomatal and cuticular uptake of ozone, as ozone levels do. Rainfall or dew events can enhance ozone quenching at wet surfaces or improve the water supply which subsequently stimulates stomatal open-ing.
An average net ammonia flux of -90 ng m-2s-1(23.6 kg N ha-1year-1) is measured with corresponding concentra-tion ([NH3]) and deposition velocity of 4.1 ÷ 6.5 µg m-3
and 3.0 ÷ 4.6 cm s-1, respectively. Subdivision into cate-gories of day/nighttime, wind sector and canopy wet-ness helps explain fluxes and concentrations.
Variability in fluxes largely depends on canopy wetness and wind sector. Fluxes (F) are largest during dry condi-tions but when compared to the maximum flux permit-ted by turbulent transfer (Fmax), deposition efficiency (F/Fmax) is greater during wet events. The low F/Fmax ratios and substantial canopy resistance (Rc) are all indicative of the reduced capability for leaf surface to retain ammonia, especially at high ammonia levels. When all deposition fluxes of nitrogen compounds are totalised, the total dry deposition of nitrogen amounts to ca. 35 kg N ha-1year-1. This implies that contribution of dry deposition is more important than the wet depositon.
Critical loads (cl) are estimated for more than 1400 receptors supporting forest vegetation in northern Belgium using simple mass balance method. Necessary data are derived from an historical soil database, recent data from forest surveys, meteo data, level i and ii plots and regional studies concerning elemental sequestra-tion in woody biomass. Deposisequestra-tion estimates are per-formed with the ops-model, which has been validated with N and S deposition measured at 6 level ii plots over the period 1993-1998.
Magnitude of the critical loads depends on the criteria used for acceptable leaching of acidity and nitrogen. For eutrophying nitrogen, critical loads are related to pro-tection of shallow ground water for nitrate leaching (25 and 50 ppm NO3), maintaining balance nutrition (20 kg NO3leaching) or to conservation of floral compo-sition (1.4 kg NO3leaching). For acidifying nitrogen and sulphur, critical loads are related to criteria preventing root damage from Al toxicity (mol Ca/Al = 1) and further acidification of the soil profile (anc = 0).
The study reveals that demands to conserve species composition and prevent further soil acidification are the most stringent, followed by requirements to prevent nitrate leaching to contaminate ground water, misbal-ancing nutritional status and impairing root systems. Deciduous forest ecosystems are less vulnerable to nitrate leaching than coniferous sites. Calculations indicated that 53 % of deciduous plots received atmos-pheric nitrogen deposition in excess of their cl com-pared to 99% for coniferous sites. It is concluded that critical loads related to nitrate contamination of shallow ground water are less exceeded at deciduous sites. This is attributable to:
• higher critical loads due to
1. better dilution of nitrates in the soil water because of lower canopy evaporation;
2. higher immobilisation of N in woody biomass; 3. higher denitrification rates due to presence on
richer and wetter soils.
• higher depositions at coniferous sites due to more efficient canopy scavenging (> 20 %) and location nearby animal farms.
Emission reduction strategies must especially be aimed at reducing ammonia emissions being the main cause of acidification and eutrophication in Flemish forests.
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