E L S E V I E R Ecological Modelling 83 (1995) 283-293

EI:OLO61I:UL mOOELLIn6

Modelling critical loads for the Solling spruce site

Wim de Vries a, Maximilian Posch b,,, Tonu Oja c, Harmke van Oene d, Hans Kros a, Per Warfvinge e, Paul A. Arp f

a DLO Winand Staring Centre for Integrated Land, Soil and Water Research, P.O. Box 125, NL-6700 AC Wageningen, Netherlands b Water and Environment Research Institute, P.O. Box 250, FIN-O0101 Helsinki, Finland

c Institute of Geography, University o f Tartu. EE-2400 Tartu, Estonia d Dept. o f Ecology and Environmental Research, Swedish University o f Agricultural Sciences, P.O. Box 7072, S-75007 Uppsala, Sweden

e Dept. o f Chemical Engineering II, Lund University, P.O. Box 124 Chemical Center, S-22100 Lund, Sweden f Dept. o f Forest Resources, University of New Brunswick, Fredericton N.B., E3B 6C2 Canada

Accepted 7 January 1994

Abstract

After an introduction to the historical development of the critical load concept, methods for calculating critical loads are presented. Besides empirical relationships three types of models are identified and evaluated: steady-state soil models, dynamic soil models and integrated forest-soil models. Critical load assessments are carried out with examples from each of the three types of models, using data from the Soiling spruce site. The resulting critical loads of sulphur obtained with the dynamic and integrated models are close (within 30%) to the steady-state model calculations. For nitrogen, the spread in the model results is wider due to the very different criteria (soil solution chemistry vs. forest vitality) used for deriving the critical loads. This agreement lends confidence to the simplified critical load calculations carried out on a European scale. However, much work remains to be done, both in the modelling efforts and the data collection, before truly integrated forest-soil models can be used for assessing critical loads on a large regional scale.

Keywords: Acidification; Critical loads; Forest ecosystems; Model comparison; Nitrogen; Soil

I. Introduction

The idea of trying to estimate how much pollu- tion an ecosystem can stand in the long run, and to use this 'critical load' for devising emission reduction policies, gained prominence in the mid 1980s. The first workshop dealing with critical loads was organized by the Nordic Council of

* Corresponding author. Present address: RIVM/MTV, P.O. Box 1, NL-3720 BA Bilthoven, Netherlands.

Ministers in 1986 in Sundvollen (Norway). The workshop report (Nilsson, 1986) provided defini- tions for critical loads and first estimates for critical loads of nitrogen (N) and sulphur (S) for forest soils, groundwaters and surface waters. As a follow-up, the first workshop on critical loads under the auspices of the United Nations Eco- nomic Commission for Europe ( U N / E C E ) was held in 1988 at Skokloster (Sweden), again spon- sored and organized by the Nordic Council of Ministers. At the workshop the many definitions

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284 IV.. de Vries et aL / Ecological Modelling 83 (1995) 283-293

for critical loads were evaluated and it was de- cided to use the following definition agreed by the U N / E C E Working Group on Nitrogen Ox- ides (Nilsson and Grennfelt, 1988): "A quantita- tive estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environ- ment do not occur according to present knowl- edge".

Another objective of that workshop was to establish methods for mapping the geographical areas where the deposition of S and N exceeds the critical load. Consequently, in the same year a 'Task Force on Mapping' was initiated under the U N / E C E Convention on Long Range Trans- boundary Air Pollution (LRTAP). In a workshop held in Bad Harzburg (Germany) in 1989 the methods for mapping critical levels and loads in the ECE countries were documented in a draft manual (ECE, 1990; Sverdrup et al., 1990). The definitions for critical loads as well as methods for calculating them were largely based on the results of the Skokloster workshop. In 1990 a 'Coordination Center for Effects' (CCE) was es- tablished in Bilthoven (The Netherlands) to syn- thesize the different national critical load calcula- tions and to produce standardized European crit- ical load maps which are eventually used by the ECE in the negotiations on emission reduction protocols. Originally, most of the effort has been directed to produce critical load maps of S (Het- telingh et al., 1991), but in the near future N critical load maps will be required as well. In anticipation of that need a workshop was held in L6keberg (Sweden) which dealt with both the acidifying and nutrient aspects of N, and the workshop report (Grennfelt and Th6rnel6f, 1992)

will serve as a background document for future activities in the field.

Soils, especially forest soils, are the most com- mon 'sensitive element of the environment' for which critical loads have been mapped by the European countries. A literature review by Roberts et al. (1989) suggests that direct effects of gaseous air pollutants (502, NOx, 0 3) are of minor importance for the decline in vitality of European forests. Soil-mediated effects of N and S deposition (N accumulation and acidification) are likely to be more important. They concluded that spruce decline in Central Europe mainly results from foliar Mg deficiency due to (i) an increased Mg demand induced by an increased growth in response to elevated N inputs, and (ii) inhibition of Mg uptake caused by ammonium accumulation and aluminum mobilization (Ro- berts et al., 1989). Other effects include vegeta- tion changes due to increased N availability and increased susceptibility of conifers to frost and diseases related to high N contents in needles. These effects are also induced by foliar uptake of gaseous NH 3. In this context a critical annual NH3-1evel of 8/xg m -3 has been derived which is generally exceeded in areas with intensive animal husbandry (De Vries, 1993).

The major aim of this paper is to compare critical loads for an intensively monitored spruce site in Soiling (Germany) derived with steady-state soil models (SMB and PROFILE) and integrated forest-soil models (NAP and ForSVA). Further- more impacts of critical N and S loads on soil solution chemistry are evaluated with a dynamic soil model (NUCSAM). The data from the Solling site (Bredemeier et al., 1995; Tiktak et al., 1995) have been used in a recent workshop (Van

Table 1 Chemical criteria used in the critical load calculations

Criterium Critical value Unit Effect above/be low value

NO 3 concentration 0.1 tool e m -3 Vegetat ion changes AI concentration 0.2 a mol c m - 3 Damage to root system A1/BC ratio 1 b mol m o l - ~ Inhibition of BC uptake

a This AI concentration is generally related to a pH value near 4.0. b This A1/BC ratio is generally related to a base saturation near 5%.

W. de Vries et al. / Ecological Modelling 83 (1995) 283-293 285

Grinsven et al., 1995) to compare a number of fo res t - so i l -a tmosphere models (Tiktak and Van Grinsven, 1995).

2. Methods for calculating critical loads

Methods for calculating critical loads have been discussed in a number of workshops and pre- sented in several reports and publications (e.g., ECE, 1990; Sverdrup et al., 1990; De Vries, 1991; Hettelingh et al., 1991). Critical loads for ecosys- tems can be derived in various ways, either by empirical methods or by models with different degrees of complexity. In the case of forest ecosystems not only direct effects play a (poorly understood) role, but also soil-mediated effects, and to date mostly soil chemical criteria (e.g. a critical AI concentration, A l / C a ratio or N H J K ratio) have been used to derive critical loads with simple steady-state models (Table 1). The largest uncertainty in these calculations is the relation between the critical chemical values and the harmful effects (De Vries, 1993).

Empirical critical load values can be derived by correlating field or laboratory observations on adverse effects with deposition values. These data are also of importance for testing models for calculating critical loads. Different critical load values are obtained depending on the criterium set, i.e. on the 'significant harmful effect ' to be avoided. Empirical critical loads could only be derived for nitrogen (Table 2). Regarding acidity no empirical data on critical loads are available, since there is no statistically significant relation- ship between acidic deposition levels and forest vitality characteristics (needle loss and needle colour).

Steady-state soil models calculate deposition levels which avoid the violation of soil chemical criteria (critical loads) in a steady-state situation. Therefore effects with a finite time scale, such as cation exchange and sulphate adsorption, are not included. The standard model is the so-called Simple Mass Balance (SMB) model, which is cur- rently widely used to produce maps of critical loads of S and N on a European scale. Based on the acidity balance in the soil one obtains for the critical load of S and N, CL(S + N ) (here and in the sequel the units are mol c ha -1 yr -1 for critical loads and related quantities):

CL ( S + N ) = BCtd + BCwe - BCup + Nup + Nde

+ mira ,crit + Acle,crit ( 1 )

where BC stands for base cations (non-marine in case of deposition) and the subscripts td, we, up, de, im and le refer to total deposition, weather- ing, uptake, denitrification, immobilization and leaching, respectively. As can be seen from this equation, the acceptable level of incoming acid- ity, due to S and N deposition, is determined by the net input of base cations, the net removal of nitrogen and an acceptable leaching rate of acid- ity, ACle,cri t. Since base cation deposition is not an ecosystem property, a critical load of potential acidity, CL(Acoot) , is commonly defined by sub- tracting BCtd from CL(S + N):

CL( ACpot) = CL( S + N ) - BCto (2)

From the N balance a critical load for N, CL(N) , can be obtained:

C L ( N ) =Nup+Nde+Nim,cri t+Nle,cri t (3)

where Nle,cri t is the allowed leaching of N at critical load. Finally, from Eq. 1 and Eq. 2 a

Table 2 Empirical critical loads of N for forests and their derivation (see De Vries, 1993)

Effect Criterium Critical load (tool ha- l yr- x) Derivation

Vegetation changes Species diversity 700-1400 Correlation with N deposition Increased susceptibility to drought, pests, etc. N content < 1.8% a 3000 Correlation with N deposition Nutrient imbalances (most sensitive systems) NH 4/K ratio < 5 800 N addition experiments

a Relates to coniferous tree species.

286 W. de Vries et al. / Ecological Modelling 83 (1995) 283-293

critical load for S, CL(S), can be derived as:

CL( S) = BCtd + BCwe - BCup + A C l e , c r i t - N le , c r i t

(4)

The underlying simplifying assumptions and details of the derivation of Eqs. 1-4 can be found in De Vries (1991). Other examples of steady-state soil models are MACAL (De Vries, 1991) and PRO- FILE (Sverdrup and Warfvinge, 1990).

Dynamic soil models have been developed over the last decade in order to simulate the effects of acid deposition on soil solution chemistry. To date, these models have hardly been used to calculate critical loads because of their finite time horizc, ns. However, dynamic soil models are use- ful for gaining insight into the ion dynamics of forest soils and for assessing so-called target loads. In this context, a target load is defined as a deposition level at which a specified (chemical) criterium is reached within a given time period. Such a target load can be either lower or higher than 1:he critical load, depending whether the criteri,am is presently violated or not (Warfvinge et al., 1992a).

Dynamic soil models applied at the Soiling spruce site are listed in Tiktak and Van Grinsven (1995), together with an indication of the type of processes included in their formulation. Gener- ally these models emphasize geochemical pro- cesses, whereas hydrological and biochemical processes are modelled in a very simplified man-

ner, related to element uptake and nutrient cy- cling. One can distinguish between one-layer models excluding the impact of nutrient cycling (e.g. SMART, De Vries et al., 1989; and MAClC, Cosby et al., 1985) and multi-layer models includ- ing nutrient cycling processes (e.g. NUCSAM, Groenenberg et al., 1995; and WATERS'rOF, Wes- selink and Mulder, 1995). The latter models allow to simulate vertical gradients of soil variables (such as Al concentration and At /Ca ratio) which is important, since most fine roots, responsible for nutrient uptake, occur in the upper soil layers.

Integrated forest-soil models are the most am- bitious effort to model the complete ecosystem, i.e. a tree (forest stand) including soil hydrology, soil chemistry and tree growth as well as its reaction to external stress factors. In the context of critical loads integrated forest-soil models are most useful for testing different hypotheses on the cause of 'forest dieback', such as nutrient (Mg) deficiency and stress due to elevated con- centrations of sulphur dioxide and ozone. In this way these models can help deriving critical loads and levels which are directly related to the harm- ful effects, thus avoiding the (uncertain) link via soil chemical criteria.

All integrated models applied at the Soiling site include a description of the biochemistry and tree physiology and almost all of them can simu- late the reaction to different stress factors (Tiktak and Van Grinsven, 1995). However, the suitability of these models to determine critical loads de- pends on the validity of the assumptions, with

T a b l e 3

P r o c e s s e s i n c l u d e d in t he m o d e l s u s e d to assess c r i t ica l l oads

M o d e l T y p e P r o c e s s e s i n c l u d e d

H y d r o l o g y Soil c h e m i s t r y N u t r i e n t cycl ing T r e e phys io logy T r e e s t ress

SMB s t e a d y - s t a t e 0 0 - - -

PROFILE s t e a d y - s t a t e 0 + 0 - - NUCSAM d y n a m i c + + + - -

NAP i n t e g r a t e d 0 + + 0 +

F o r S V A i n t e g r a t e d + + + + +

+ : p r o c e s s i nc luded ; - : p r o c e s s no t i nc luded ; O: (very) s imple d e s c r i p t i o n o f key p r o c e s s e s i nc luded .

W. de Vries et al. / Ecological Modelling 83 (1995) 283-293 287

Table 4 Coefficients of the linear relationship between sulphate and base cation depositions, derived by linear regression from 1974-90 annual deposition values at the Soiling site (Xtd = a" SO4.td + b, X = Ca, Mg, K, BC; all values in tool e ha -1 yr -~) Base cation a b r e

Ca 0.152 91 0.47 Mg 0.048 27 0.45 K 0.027 63 0.43 BC (sea-salt corrected) 0.230 152 0.43

which the various processes are described (espe- cially those related to tree physiology and tree stress) and the definition of harmful effects.

3. Critical loads for the Soiling spruce site

In this section results are presented for critical loads for the Soiling spruce site, assessed with five models: SMB, PROFILE, NUGSAM, NAP and ForSVA. These five models cover the whole range of models described in the previous section (see Table 3). The calculations with the five models have been carried out with two levels of future deposition of base cations (Ca, Mg and K): (i) BC deposition stays at present levels, and (ii) BC deposition decreases linearly with the SO 4 depo- sition (see Table 4).

For the critical load calculations with steady- state soil models (SMB and PROFILE) the chemical criteria given in Table 1 have been used.

3.1. S M B model

The values of the critical loads calculated by the SMB model as well as the parameters entering their calculation (see Eqs. 2-4) are presented in Table 5. The input data for these calculations are taken from Tiktak et al. (1995). The sea-salt corrected deposition of BC (calculated from the deposition of Ca, Mg, K, Na and C1 according to Sverdrup et al., 1990) is an estimation at critical loads for S, using the relationship between BC and SO 4 given in Table 4. The uptake of N and BC refers to the net uptake in stems and branches. Note that the N uptake at the Soiling site is

influenced by the high present N deposition. The denitrification is assumed as 10% of the net N input (deposition minus uptake). The weathering rate is directly taken from Tiktak et al. (1995). The leaching terms are calculated by multiplying a precipitation excess of 424 mm yr-~ (average of 1969-85) with a critical N and acidity concentra- tion of 0.10 and 0.27 mol c m -3, respectively. The value of 0.27 is derived from a critical aluminum concentration of 0.20 tool c m -3 and the assump- tion of a gibbsite equilibrium between protons and Ai.

3.2. P R O F I L E model

PROFILE is a multi-layer steady-state model which can also calculate a critical acid load by summation of the various terms in Eq. 1. How- ever, unlike the sMB model, weathering rates are not a model input, but calculated from mineralog- ical analyses of the soil (as in the SAFE model, see J6nsson et al., 1995). Furthermore, the uptake of N and BC is not an input derived from the annual biomass increase of trees, but calculated from the availability of base cations and an aver- age N / B C ratio in trees (nutrient limitation con- cept, Warfvinge et al., 1992b). Due to the influ- ence of pH and BC deposition on several soil processes the model equations have to be solved iteratively to obtain a critical load. In this appli- cation of PROFILE the circulation of elements

Table 5 Critical loads for N, S and potential acidity calculated with the SMa model from the data for the Soiling spruce site

Variable Value Variable Value (molc ha -1 yr-1) (mol c ha -1 yr -1)

Nu~ 560 BCtd 460 (850) a Nde 50 BCwe 500 Nim 0 BCup 320 Nle,cri t 420 ACle,cri t 1130 b

CL(N) 1030 CL(S) 1350 (1740) a

a The values in brackets refer to the case where BC deposi- tion stays at 1990 level; CL(Acoot)= 1920 mol c ha -1 yr -1, independent of BCtd. b Derived with the AI concentration criterium (0.2 molc m-3).

288 W. de Vries et al. / Ecological Modelling 83 (1995) 283-293

2.4-

~- 2.0- =

~ 1 . 6 -

i 1.2.

i 0.8-

0.4-

0.0

subsoil (80-100 cm)

~ ~ _ constant BC deposition -- decreasing BC deposition

35.

3O

25 ¸ .£

20

~ 1 5

7 lO

5

(b) i",/~, subsoil (80-1 O0 cm)

b'l i

1970 1990 2010 2030 2050 2070 2090

Fig. 1. Molar A I / C a ratio and AI concentration in the subsoil (80-100 cm) of the Soiling spruce site simulated with the NUCSAM model. The AI concentrations are flux weighted an- nual averages. The thin horizontal lines indicate the critical values ([AI] = 0.2 molc m -3 and A l / C a = 1, resp.).

through litterfall or canopy exchange is not in- cluded and the net mineralization of N is as- sumed zero.

Values for the critical load of N, S, and poten- tial acidity at present and expected levels of BC deposition (at critical S loads) for two chemical

criteria ([All = 0.2 molc m -3 and Al/BC = 1) are shown in Table 6. A comparison between the values obtained with PROFILE and those for the Soiling spruce site (Table 5) shows that the weathering rates are nearly equal, but the calcu- lated uptake rates are much higher. Apparently the uptake at the Soiling spruce site is not limited by BC availability, but by other constraints (e.g. the rate of photosynthesis). Critical N and S loads obtained with the PROFILE model, using the A1 concentration criterium and assuming the sul- phate-related BC deposition, are about 20% lower than those obtained with the SMB model. For N this is because N leaching is neglected in the PROFILE calculations, although this is partly com- pensated by the higher N uptake, as compared to the SMB value.

In the situation where BC deposition de- creases with a decrease in S deposition, BC weathering slightly increases, but BC uptake, and through that N uptake, clearly decreases. Conse- quently the critical N load is lower in this case compared to the situation where BC deposition remains at present levels. Furthermore, the A1/BC ratio criterium appears to be less strin- gent (higher value for AClo,crit and thus for CL(S) and CL(Acpot)) than the A1 concentration cri- terium.

3.1. NUCSAM model

The model NUCSAM has been used to evaluate the consequences of the critical loads, obtained

Table 6 Soil parameters and critical loads of N, S and potential acidity for two chemical criteria calculated with the PROFILE model

Variable Value (molc ha -1 yr -1)

Sulphate-related BC dep. 1990 BC dep.

AI /BC = 1 [Al] = 0.2 AI /BC = 1 [Al] = 0.2

Nup 790 690 920 920 Nde 130 130 130 130 BCtd 460 410 850 850 BCwe 490 500 440 460 BCup 790 690 920 920 ACle,cri t 1180 900 2870 900 CL(N) 920 820 1050 1050 CL(S) 1340 1120 3240 1290 CL(ACpo t) 1800 1530 3440 1500

W. de Vries et al. / Ecological Modelling 83 (1995) 283-293 289

with the SMB model, by investigating whether the critical A1 concentration and the A l / C a ratio in soil solution are reached within a 100-year pe- riod. Depositions of N and S were decreased linearly between 1990 and 2000 and reached criti- cal loads in the year 2000 (and stayed constant till 2090). The meteorological input consisted of a periodic repetition of the 1976-89 observations till 2090.

The A1 concentration in the topsoil reached the critical value of 0.2 mol c m -3 soon after 2000, whereas in the subsoil it stayed slightly above this value till 2090, due to Al mobilization from the adsorption complex (equilibrium not yet reached). The differences between a constant and a decreasing base cation deposition were small (see Fig. la). The behaviour of the A I / C a ratio in the topsoil was similar to that of the A1 concen- tration, and the value dropped below one soon after the year 2000. In the subsoil, however, the reaction of the A1/Ca ratio to the decreasing deposition scenarios was completely different (Fig. lb). At constant BC deposition it gradually increased up to 2015, followed by a gradual de- crease. In the scenario with decreasing BC depo- sition the A1/Ca ratio even increased dramati-

cally between 1990 and 2025 followed by a strong decrease. This behaviour of the A I / C a ratio is determined by the Ca concentration; first it de- creased dramatically due to the re-charge of the adsorption complex, but after 2025 the Ca con- centration started to increase, but a steady state was not yet reached by 2090 and the critical value of A l / C a = 1 was still exceeded.

3.2. N A P model

Critical load calculations with the NAP model (see also Van Oene and Agren, 1995) have been carried out with the main criterium that 100 years of forest growth is not limited by Mg. This cri- terium results in a critical load curve that sepa- rates N-and Mg-limited growth (see Fig. 2). Time periods different from 100 years would result in different curves. The area of Mg-limited growth represents those combinations of S and N deposi- tion that result in a forest growth that becomes Mg limited in less than 100 years. In addition we required (1) an optimal growth rate (induced by high N deposition) and (2) an optimal Mg content in needles (induced by high S deposition) without violating the criterium of an A1 concentration

I

7

0

0

z

2400 ((a) constant BC deposition [

2000

1600

1200

800

400

0 0 400 800 1200 1600 2000

S deposition (molcha-lyr -1)

0 400 800 1200 1600 2000 S deposition (molcha-lyr -1)

Fig. 2. Curve separating N-and Mg-limited growth as simulated by the NAP model for (a) constant (1990) BC deposition, and (b) decreasing BC deposition. The critical load values for S and N are obtained where the curves coincide with the critical AI concentration of 0.2 tool c m -3.

290 W. de Vries et al. / Ecological Modelling 83 (1995) 283-293

below 0.2 tool c m -3 (ml concentration increases with S deposition). Critical load values were ob- tained at the point, where a (critical load) curve separating N-and Mg-limited growth reaches the critical Al concentration (Fig. 2). Along and un- der the critical load curve all N deposited is taken up by the trees. Lower S depositions decreased the Mg availability, since less Mg was desorbed from the exchange complex (in the long run, Mg availability would decrease due to the depletion of exchangeable Mg, but this did not happen during the simulation period). This was aggra- vated by the decreasing BC deposition (Fig. 2b), which required a decrease in N deposition as well in order to keep forest growth N limited. Under these conditions, the amount of N that can be taken up by the trees was lower than under constant (1990) BC deposition (Fig. 2a). The pos- sibility of inducing a higher Mg availability by increasing the S load, and thereby a higher N uptake, was limited by the Al concentration cri- terium.

These conditions lead to a critical load of nitrogen of 2070 mol c ha-1 yr-1 and a critical load of sulphur of 1625 mole ha -1 yr -1 for the constant (1990) base cation deposition; and for a decreasing BC deposition CL(N)= 1580 mol~

ha -1 yr -1 and CL(S)= 1560 molc ha -1 yr -x is obtained.

3.1. ForSVA model

In the simulations with the integrated forest- soil model ForSVA (part of the model series ForM-S, see also Oja et al., 1995), a critical S and N load was defined as that S and N deposition rate which would guarantee sustained biomass production over 2-3 sequential tree gap cycles of 290 years each. Long-term 'no reduction' in biomass production was defined when the simu- lated maximum wood biomass reached the same level in each of the three sequential gap cycles. In Table 7, simulation results are shown that depict several critical or near-critical S and N deposition scenarios for sustained biomass production and the related values for the Al concentrations and AI/Ca ratios of the soil percolate solutions. These scenarios (Present, CL1, CL2, SDC, SMB) as- sumed that (1) NH 4 deposition would be propor- tional to S + N deposition, (2) BC deposition would be proportional to S deposition, or would remain constant at 1990 level. Scenarios CL1 and CL2 further assumed, respectively, that it would be possible to reduce S and N depositions in

Table 7 ForSVA-calculated critical deposition rates (in molc ha-1 y r - i ) ensuring sustainable biomass production (scenarios CL1 and CL2) compared with present deposition rates, the SMB critical loads, and the best achievable situation obtained by controlling S deposition at current nitrate loads (SDC). Also shown are the AI concentration and molar A1/Ca ratio in soil percolate solution, both for constant and decreased base cation deposition. Those values change with stand development; shown are ranges under mature stands when numbers are highest

Scenario SO 4 NO 3 NH 4 Sulphate-related 1990 BC dep. (tool c ha- 1 (molc ha- 1 (mol c ha- 1 BC dep. yr - 1) yr- 1) yr- I)

[Al] Al/Ca ta l l Al/Ca (molc m -3) (mol/mol) (molc m -3) (mol/mol)

Present 3730 1410 1470 1.0-1.5 4.7-5.3 1.0-1.5 4.7-5.3 CL1 a 1230 470 490 0.4-0.5 2.7-3.3 0.2-0.3 0.7-1.3 CL2 b 1000 560 450 0.4-0.5 2.7-3.3 0.2-0,3 0.7-1.3 SDC c 200 1410 470 0.6 5.3 0.2 1.3 SMB 1350 500 530 0.4 2.7 0.2-0,3 0.7-1.3

a Sulphate and nitrate deposition is reduced until long-term biomass production is deemed sustainable, keeping the present sulphate:nitrate deposition ratio. b Sulphate deposition is reduced by 75% and nitrate deposition is reduced until long-term biomass production is deemed sustainable. c Nitrate deposition is kept at present level and sulphate deposition is reduced. This scenario ensures 'no loss' in short-term biomass productivity only.

W. de Vries et aL / Ecological Modelling 83 (1995) 283-293 291

proportion of one another, or S deposition by 75% from current levels. Scenario SDC (S depo- sition controlled) assumed that it would be possi- ble to achieve critical loads by reducing S deposi- tion rates while maintaining nitrate deposition at current levels.

In general, the results indicated that sustain- able biomass production would require fairly low N and S load reductions at current BC deposition levels, with soil percolate AI concentrations near 0.2 mol c m -3 and AI/Ca ratios near 1. Only modest decreases in AI concentration and AI/Ca ratios would be obtained when BC deposition decreases in proportion with S deposition. Criti- cal loads for CL1 and CL2 were similar to the SMB critical loads with either BC deposition scenario. Scenario SDC indicated that S loads should be reduced drastically for sustained biomass production at current nitrate deposition rates, at least for the short tenn. Long-term sus- tainable growth would not be achieved without reducing nitrate deposition.

4. Discussion and conclusions

25°° /

l 7 ~d

1500

:~ looo-

500-

;MB 'x

0 ~. o

NAP,2 NAP, 1

x

x x

(SDC) * " - . x

looo l s oo aooo as0o N deposition (molcha- lyr - I )

Fig. 3. Critical loads of S and N for the models discussed in this paper: SMB, PROFILE (decreasing BC deposition; 1: A I / B C = 1, 2: [AI] = 0.2), NAP ([AI] = 0.2; 1:1990 BC deposi- tion, 2: decreasing BC deposition), and ForSVA (for scenarios CL1, CL2 and SDC, resp.; see Table 7). The shaded area indicates those S and N deposition rates for which Std + Ntd < CL(S + N) and Std < BCtd + BCwe - BCup + ACle,cri t and Nt, t < CL(N).

The critical load calculations carried out with the different models are summarized in Fig. 3. Considering the wide range of model complexity -from steady-state (SMB and PROFILE) to inte- grated forest-soil models (NAP and ForSVA) -the results are quite consistent. For critical S loads the results are within a narrow band (_+ 30% of the SMB calculations), but in this case all models used soil-chemical criteria (A1 concentration or AI/BC ratio). Except for the NAP model, also critical N loads lie within a narrow band. With regards to N, one has to keep in mind that very different criteria for computing critical loads have been used: chemical threshold values in the steady-state models, avoiding Mg-deficiency in NAP and long-term sustainable forest growth in ForSVA.

Taking the steady-state calculations as a refer- ence, it has to be noted that not only the values of CL(N) and CL(S) from Table 5 (shown as point labelled SMB in Fig. 3) can be considered critical loads, but every pair of N and S deposi- tion (Nta, Std) on the line separating the white

and gray-shaded area, since they fulfil Std + Ntd < CL(S + N), Ntd < CL(N) and Std < BCtd +

B C w e - B f u p +Acle , c r i t. The last condition makes sure that the deposited sulphur stays below a maximum critical S load, which is allowable at very low N loads where N leaching is negligible (see Eq. 4). Therefore the 'performance' of the models can be judged by their distance from that 'critical load curve'. Note that the shade area in Fig. 3 would extend to the left under the dashed line, if the N leaching criterium were dropped.

One conclusion from the simulations with the dynamic models, including NUCSAM, is that main- taining current rates of atmospheric base cation deposition would be crucial for achieving low AI concentrations and AI/Ca ratios near unity. Oth- erwise, decreasing BC depositions would have to be compensated by liming.

The fair agreement of the critical loads calcu- lated by steady-state and integrated models lends confidence to the simplified critical load calcula- tions carried out on a European scale. However,

292 W. de Vries et al. / Ecological Modelling 83 (1995) 283-293

one has to bear in mind that the models have been only compared with data from one inten- sively monitored site, and it would be premature to consider the steady-state approach as verified. While steady-state models will continue to be used in the near future for mapping critical loads on large regional scales due to their simple calcu- lation schemes and limited data requirements, dynamic models will play an increasingly impor- tant role for assessing target loads or time hori- zons within which damage and/or recovery is to be expected for various emission/deposition sce- narios. Both steady-state and dynamic soil models have the disadvantage that the critical chemical values related to effects, such as the critical A1 concentration and AI/BC ratio, which are highly variable and uncertain. Furthermore, there is no feedback from changes in nutrient cycling in- duced by reduced uptake to soil (solution) chem- istry. Integrated forest soil models are important tools for assessing critical ecosystem loads, but the majority of these models are still lacking a generally agreed description of, e.g., tree physio- logical processes (reflecting the currently limited state of knowledge). In addition, an agreement on how to define an 'undisturbed' or 'healthy' tree (forest stand), e.g. by (long-term) sustainable growth or no base cation deficiency, has not yet been reached. This is especially true with respect to nitrogen, where critical loads could (and should) be derived for different effects, such as vegetation changes and forest growth. In addi- tion, regarding forest growth, one should derive optimal rather than critical N loads. Summarizing it can be said, that currently available dynamic and integrated models should be used more to check the steady-state critical load calculations, but further research is needed to improve these models and their data bases before they can be used for assessing critical loads on a large re- gional scale.

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