The Solling Norway Spruce site

E(OLOfiI(flL mOOELLInfl ELSEVIER

Ecological Modelling 83 (1995) 7-15

The Soiling Norway Spruce site Michael Bredemeier

a, A a l d r i k T i k t a k b , . , K e e s v a n H e e r d e n

b

a Research Centrum Forest Ecosystems, University of G6ttingen, Habichtsweg 55, D-37075 G6ttingen, Germany b National Institute of Public Health and Environmental Protection (~VM), P.O. Box 1, 3720 BA Bilthoven, Netherlands

Accepted 19 November 1993

Abstract

The Soiling F1 site is a typical Norway Spruce (Picea abies) plantation forest on acid soil, with a well-developed mor humus layer, low soil biological activity and sparse ground vegetation. Inputs, outputs and internal transfers of chemical constituents have been measured continuously for more than twenty years, and were complemented by plant physiological, hydrological, micrometeorological and soil biological measuring programmes in that time. A comprehensive dataset was compiled from many sources to apply a variety of models to the Soiling spruce site dataset in the framework of the workshop "Comparison of Forest-Soil-Atmosphere Models" at Leusden in the Netherlands. This paper gives background information on the site, such as land use history, climate, soil and deposition regime, and on the dataset (important trends and limits of accuracy). Keywords: Acidification; Atmosphere; Dataset; Forest ecosystems; Monitoring; Nitrogen; Soil; Spruce

I. Introduction

In the framework of the workshop "Comparison of F o r e s t - S o i l - A t m o s p h e r e Models" (Van Grinsven et al., 1995), a dataset was compiled from many sources to apply a variety of integrated models to the Solling F1 Norway Spruce site. This p a p e r gives background information on this dataset and on the monitoring site, in order to place the dataset in a wider context of general regional characteristics, land use history and particular features pertaining to this site and the m e a s u r e m e n t s pursued there. In an accompany-

* Corresponding author.

ing p a p e r (Tiktak et al., 1995), the dataset itself and the deposition scenario are presented. The Soiling experimental forest is the longest running intensive forest ecosystem study in Europe. The results of the element flux studies, compiled e.g. by Ulrich et al. (1979), have substantially contributed to the current knowledge of the biogeochemistry of forest systems and the notion of acidification and forest decline. The Leusden workshop and the dataset compiled for it make the results of the Solling project accessible to the modelling community in a more comprehensive form than ever before. We hope and are quite positive that both the modellers and the scientists doing field measurements will benefit from this activity.

0304-3800/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-3800(95)00079-8

8

M. Bredemeier et aL /Ecological Modelling 83 (1995) 7-15

2. General regional characteristics The Soiling is a mountain range in the northern centre of Germany of intermediate elevations

covering the sub-montane and the montane zone (250-500 m a.s.l.). It is located between the basins of the river Weser to the west and the river Leine to the east and consists of a sandstone massive Honnov~"

J Ff~,r~iK

J

Franklurt a.l~L

Oosset

U$[or Fig. 1. Situation of the research area and of the experimental plot on the Soiling plateau, about 55 km northwest of GSttingen. B refers to beech (Fagus sylvatica) stands, F refers to Norway Spruce (Pouchy abies) stands, W is meadow and A is arable land. All data for the workshop are collected for the F1 Norway Spruce stand.

M. Bredemeier et aL /Ecological Modelling 83 (1995) 7-15

from the triassic geological formation of New Red Sandstone. It is a relatively remote, rural area of rather low population density and with a high percentage of forest cover. There are no major industrial facilities in the Soiling itself. Due to its location on the northern fringe of the German highlands and the relatively high elevations, however, it is actually very exposed to long-range transported air pollutants, which is reflected in high deposition rates. The sources of this atmospheric load are for the most part the R h e i n / R u h r industrial region to the west and the industrial centres of eastern Germany and the Czech Republic to the southeast. Fogs and low clouds are quite frequent and contribute to the atmospheric deposition load. The monitoring site is situated on a plateau at a height of 505 m above sea level (Fig. 1). The geographical position is 9° 30'E, 51°40'N.

3. Forest and land use history

The beginnings of forest management in Germany date back to only about 200 years ago. Earlier, exploitation and devastation of forest areas was a common feature, and as a consequence grass- and heathland covered large areas that are under forest again today. This is also the case for the Soiling mountains. Historical chronicles (cited in Ellenberg et al., 1986) report that at the end of the 16th century some 15 000 pigs were put out to pasture in the forest together with sheep, cattle and deer, and that they would prevent regeneration of the forest and cause over-aging of the old trees. Around 1740 the higher elevations of the Soiling were covered by open birch forest, frequently on swampy terrain with a lot of tree-less areas therein. The devastation continued throughout the 18th and 19th century due to the demand for wood products of the regional mines, ironworks, and glass and porcelain factories and came to the point of acute wood deficiency in the region. In the middle of the nineteenth century the drainage of the bogs in the high Soiling was started with newly developed technical equipment, which was

9

completed 40 years afterwards. The drained bog areas were frequently planted with spruce, and controlled management of these forests began. In the area of the F1 plot, tne rights of use of the community of Sievershausen were redeemed in 1887 and 1889. In a map of 1880, which unfortunately is not very precise, the F1 area is indicated as grassland (or "extensive pasture"). The documents of the forest district of Dassel, within which the F1 is located, allow the conclusion that the site was planted in 1888 with 4-year-old spruce saplings, planted in bundles. The age of the trees today (1993) is 109 years, according to this information. The land use history of the particular F1 site prior to the spruce planting cannot be reconstructed with full certainty. This, in turn, makes reconstruction of the initial nutrient and acid/base status of the soil difficult and somewhat uncertain, a fact that should be duly considered when modelling from specified start scenarios.

4. Climate and weather

The climate is a "mountain climate" with an average annual temperature of 6 - 7 ° C (12-13°C during the growing season). Average annual precipitation is just above 1000 mm a-1, with precip-

20 temperat.__ure(°C_.__~_)

2.0

6.8

3.6

r a i n f j (mm) 120

3.8

10.5

9.2

15

90

,.~ ~4 [-].

t

60

30

(3' ~ *5

~

(,¢) temperature

[]

(>) ratnfall

Numbers above bars indicate days with fog J

F

M

A

M

J

J

A

S

O

N

D

Fig. 2. Long-term climatological characteristics of the weather station "Silberborn", which is situated near the F1 Norway Spruce stand (see Fig. 1).

10

M. Bredemeier et aL / Ecological Modelling 83 (1995) 7-15

itation distributed quite evenly throughout the year (70-110 mm mean monthly rainfall, see Fig. 2). 15-20% of the annual precipitation is in the form of snow. In Fig. 2, some general climatological information from the neighbouring weather station in the village Silberborn is given for the period 1931-1960. Snow is very frequent and in some winters a snow cover persists from late autumn to April. After intermittent melting periods or rain falling on the snow, the snow pack on the tree crowns gets heavy; as a consequence, many spruces on the F1 have a broken top. The highest average precipitation amounts are in the summer months (Fig. 2), often in thunderstorms. This aspect of the site hydrology entails that at the time of highest potential evapotranspiration ample amounts of water are supplied. Therefore severe drought should hardly ever occur.

5. Deposition regime and trends The Soiling spruce site was integrated into a study of deposition patterns on the European scale (Hauhs et al., 1989). It turned out to have one of the highest input rates of S of all the sites in that network. As already mentioned above, the Selling seems to be particularly affected by longrange transported air pollutants, due to its geographical situation and orographical exposition.

100

(kg ha" a ~) --o--- S flux in net throughfall S flux in bulk precipitation

80

60

40 20

1969

1973

1977

1981

1985

1989

year

Fig. 3. Trend in m e a n annual sulphur flux in bulk precipitation and net throughfall at the F1 Norway Spruce stand.

15

(kg ha~ a ~) nitrate-N flux in net throughfall

10

1973

1975

1977

1979

1981

1983

1985

1987

1989

1991

Fig. 4. Trend in m e a n annual nitrate-N flux in net throughfall at the F1 Norway Spruce stand.

A fairly clear trend of decreasing sulphate fluxes in throughfall can be observed, however, for the past ten years (Fig. 3). The trend line indicates a decrease of ca. 30 kg ha a -1 or roughly 40% of S deposition from the highest values in the mid-seventies. A similar trend seems to occur in the bulk precipitation. Due to air pollution control legislation in the former West Germany, SO 2 emissions have decreased by about 60% in that area (UBA, 1992), which is reflected in the throughfall fluxes of sulphate at the Soiling spruce site. This observation, in turn, encourages the fidelity in throughfall measurements as a measure of total atmospheric deposition. The large-scale Integrated Forest Study (IFS) carried out in the USA found the same relationship and arrived at the same conclusion (Johnson and Lindberg, 1992). For the input of nitrogen, such a clear trend is lacking. If there is any trend in the throughfall nitrate fluxes at all, it is still slightly increasing (Fig. 4). Nitrogen oxide and ammonia emissions as the precursors for atmospherically deposited nitrate could not be as effectively reduced to date as the sulphur emissions (UBA, 1992). Foggy days are particularly frequent in autumn and winter (Fig. 2). Under high pressure conditions in winter the prevailing wind direction is from the Southeast, and air masses from the industrial centres of eastern Germany and the Czech Republic are transported to the Soiling. Iii

M. Bredemeier et al. ~Ecological Modelling 83 (1995) 7-15 Table 1 Profile description of the soil at the Soiling F1 NorwaySpruce stand Horizon Depth Symbol Texture no. [cm] 1 5.0-4.5 L litter 2 4.5-2 F fermentation 3 2-0 H humus 4 0-8 AEh loam 5 8-30 Bw loam 6 30-60 Bwg loam 7 60-85 2Bw loamy silt 8 85-140 2Bw2 loamy silt

such periods, peak concentrations of SO 2 are observed at the site. The fluxes of sulphate in throughfall also exhibit a distinct "winter seasonality", due to higher atmospheric concentrations and deposition rates (particularly with fog) in the wintertime (Matzner, 1988).

6. The soil characteristics 6.1. Profile description The soil is a Spodic dystric Cambisol (FAO). A profile description by horizons is listed in Table 1, the mineral composition of the soil is given in Table 2 (after Matzner, 1988). The profile has developed in a solifluction of periglacial deposited loess (clay content 17%) with the weathered sandstone parent material (clay content 23%). All grain size fractions are present in this mixture, although the silt fraction which stems from the loess is dominating.

Stone content 4% 8% 30% 35% 35%

11

Rooting intensity strong strong strong moderate little very little very little

A mor humus layer of ca. 5 cm thickness is covering the soil. Roots are strongly concentrated in that humus layer and the uppermost mineral soil, where organic compounds are present and capable of complexing aluminum ions. Below ca. 10 cm mineral soil depth, rooting becomes scarce, either due to adverse effects of aluminum in deeper mineral soil or due to the soil hydrological characteristics (described in the section below) or a combination of both. Stone content is appreciable in the deeper horizons (unweathered residual sandstones in the solifluction layer). The stone content has to be considered when calculating total available element stores from exchangeable cations per unit soil weight.

6.2. Hydrological characteristics The loamy texture and the relatively high clay content provide a good water storage capacity of the F1 soil. There is another important hydrologi-

Table 2 Mineralogical analysis of the total soil and of the clay fraction (Matzner, 1988) at the Soiling F1 NorwaySpruce stand Depth (cm) Fraction Quartz Feldspars lllite/ Kaolinite micas 7-20 70-100 > 110

total clay fraction total clay fraction total clay fraction

62 3 33 3 63 4

12 0 1 0 2 0

24 83 60 82 31 83

4 14 6 15 4 13

12

M. Bredemeier et al. / Ecological Modelling 83 (1995) 7-15 hydraulic conductivity (crn d 1) 10-3 10.2 10-1 10o 101 102 10 ~ I

I-I I I I I

30 E

c-

j--

60

_1

t I j

"0

90

I

120 J t-

I

150

- -1 kPa . . . . 3 kPa ........ 10 kPa

Fig. 5. Hydraulic conductivity as a function of depth. The three lines indicate the conductivityat -1 kPa, - 3 kPa and - 10 kPa. cal feature of the site, however: the hydraulic conductivity of the soil layers decreases strongly with depth (Fig. 5). Resulting from the low saturated hydraulic conductivity of the deeper soil layers, under wet conditions water cannot pass as quickly downward through the profile as it infiltrates the soil surface, and this may lead to transient ponding of the soil. The profile morphology reflects these conditions of stagnant water by small bleaching zones and manganese concretions in the Bwg-horizon (30-60 cm depth). So there is certainly an influence of ponding, although it is probably moderate, and only transient. It is difficult to judge to what extent the anoxic conditions under ponding contribute to the shallow rooting of the spruce trees. 6,3. Chemical characteristics

The soil of the Solling spruce site is severely acidified. I n p u t - o u t p u t budget calculations indicate that a substantial part of the acidification has been caused by atmospheric deposition (Bredemeier, 1987; Bredemeier et al,, 1990), although it cannot be stated with certainty from which acid/base status the soil started when the spruce was planted (see above, site history). The entire mineral soil is in the aluminum

buffer range (according to Ulrich, 1983), the uppermost mineral soil has even passed into the A I / F e buffer range, where reactive AI compounds are already exhausted and iron dissolution/exchange reactions take part in the neutralization of H ÷. The repeated soil inventories showed an interesting trend of exchangeable H ÷ in the 0-10 cm soil depth (Fig. 6). From zero in the first inventory of 1968 to the last one in 1991, exchangeable hydrogen ion store has constantly increased. Exchangeable Ca 2÷, by contrast, has decreased to half of the initial store in the same period of time, and seems to keep a minimum value since the end of the seventies. The data in Fig. 6 represent mean values of different numbers of samples with appreciable standard deviations. The trend of H + is nevertheless significant. The composition of CEC in the mineral soil below the A horizon is very uniform, and this uniformity extends to very deep layers. Generally, A1 covers 95% of the exchange complex, base saturation is about 5% and hydrogen is almost absent. The low value of base saturation seems to be a minimum value, which has also been found in a total number of 2500 samples collected at various sites in Germany. This minimum value is probably an equilibrium value resulting from current atmospheric base cation deposition (Prenzel and Schulte-Bisping, 1991). Under such condi-

25

H" (kmol c ha ~)

Ca ~" (kmol c ha ~) -4_

6

(<-) exchangeable H* store _ •

2O

5 4

15 3 10 2

5

0

1968

1

19'72

19'z6

~980

19'84

1~88

0

1~2

Fig. 6. Trends in exchangeablehydrogenand calcium stores in the 0-10 cm mineral soil layer at the Soiling F1 Norway Spruce stand.

13

M. B r e d e m e i e r et aL / E c o l o g i c a l M o d e l l i n g 8 3 ( 1 9 9 5 ) 7 - 1 5

100

organicN store (kg ha1)

organicmatterstore (Mg ha~)

2100 1900

90 80

! ! / i i i i" ii

70 t

°

t

a

~

/

1700 1500

/

60

1300

50

1100

40 1965

1970

1975 year

1980

900 1985

Fig. 7. Trend of total organic matter and nitrogen stores in

the organic (L, F and H) soil layers at the Soiling F1 Norway Spruce stand. The line between the last two data points is dashed, because the 1983 data point is somewhat questionable (see text). tions, the nutrition of the stand is only maintained by input of these elements via throughfall. The total Mg amount in the spruce biomass, for instance, is now about twice as high as the remaining soil store. Given that, it is evident that the soil alone could not provide the Mg for the growth of a following forest generation.

6. 4. Trend of humus layer stores Inventories of the element stores in the humus layer were made approximately every five years between 1968 and 1983. Both the total organic matter in the O-horizon and the nitrogen store roughly doubled in that time span (Fig. 7). This very strong trend is caused by the dense canopy (shaded, cool humus surface), sparse ground vegetation and, for the most part, the lack of soil biological activity. The average annual input of N to the site is 31 kg ha-1 via throughfall (Matzner, 1988), to which about 9 kg might add via direct assimilation of NH~- in the canopy (Eilers et al., 1992). Compared to that, the storage rate in the increment of the forest stand is only about 10 kg ha a -1, and is decreasing as the stand passes into the senescence phase. The last data point in Fig. 7 (year 1983), however, is somewhat questionable, since they indicate an increase in organic dry matter of

the humus layer of 9.4 t ha-1 a-1 while litterfall is slightly more than 3 t ha-1 a-1. It has to be noted that the statistical variation to the mean values in Fig. 7 is more than 30%. The data in Fig. 7 indicate that over the last 20 years the humus layer has accumulated almost seven times as much N (1000 kg ha -1) as the spruce stand. This means that the high nitrogen inputs to that forest ecosystem and the litterfall N flux are primarily transferred to and bound in the humus layer. This, however, is not a stable compartment, i.e., upon opening of the canopy by clearcutting or natural decay the organic matter will be mineralized and much of the organic N eventually be transformed to nitrate. Net nitrification is an ecosystem-internal source of strong mineral acidity (Bredemeier et al., 1990), and may then considerably contribute to soil acidification. The total potential acidity related to the nitrification of 2000 kg ha-1 of organic N is more than 140 kmol H +. When modelling the transition to the next forest generation on the F1, this extra internal acid source should be considered.

7. Trend of tree development

7.1. Stem growth The stem growth rate (average of 11.2 m 3 ha-1 a-1) did not change significantly since 1967. Con-

0.06

Mg2. concentration(%) --'o~1968 --~1983

0.05 0.04

o__....____._-------~

0.03 0.02 0.01 0.00

0

1

2 leaf age (a)

3

4

Fig. 8. Relation between Mg concentration in leaves and leaf age at the Soiling F1 Norway Spruce stand as measured in 1968 and 1983.

14

M. Bredemeieret aL /Ecological Modelling 83 (1995) 7-15

60

CI" flux in humus lysimeters (.kg ha~ a"~) y=O.577x+11.6 r2= 0.31

50 O

O

o

40 0

30

0

0°°

o

20 10

°o

i

10

2'0

4'0

i

so

60

CI flux in throughfall (kg ha "t a ~)

Fig. 9. Correlation of annual chloride fluxesin throughfalland underneath the organic soil layers at the Soiling F1 Norway Spruce stand. sidering the age of the stand (83 years in 1967) a decreasing growth rate could be expected. The steady growth rate can be explained by the fact that nitrogen deposition increased. Most sites exposed to a low nitrogen deposition are nitrogen limited. Increasing the nitrogen deposition causes an increased growth rate. 7. 2. L e a f nutrient concentrations

rors in the data. It is quite clear that there are a lot of sources for errors and imprecision in such a comprehensive monitoring programme as the one on which the actual Soiling dataset is based. While it is not possible to discuss every aspect of that in detail in this paper, we show two exampies, and hope to stimulate a critical awareness of the dataset users in this way. Errors and biases in the data may result, e.g., from the error of chemical analysis or lacking representativeness of spatial a n d / o r temporal distribution of sampling points. While the analytical inaccuracy is generally considered small (single analytical value's error < 10%), representativeness of sampling must be considered a general problem, which can distort the results. An example is given in Fig. 9. It shows the annual fluxes of chloride in throughfall versus the ones in the humus lysimeters (i.e., ceramic suction plates installed underneath the humus layer). Theoretically, these values should match, since CI- can be regarded as an inert "tracer", it

2500

-

-

( SO~ ) [p.molc L-'] Yearly average

2000 1500 1000 500

Notable is the low Mg concentration ir{ the needles. There is no significant trend of the nutrient concentration in the needles, but the Mg concentration in fallen needles seems to be decreased. This can be explained by an increased reallocation of Mg from older needles to current year needles (Fig. 8).

0 • 1000

( AI3. ) [pmolc L"1] Year y average

r

4o01

.

/....,..

\.

.

.

8. S o m e r e m a r k s on data precision a n d p o s s i b l e pitfalls

In the effort to reconstruct measured time series of data by models, it is important to assess which precision the measured values have, and also to look carefully for possible systematic er-

Fig. 10. Time courses of aluminum and sulphur-S concentrations in humus lysimeters at the Soiling F1 Norway Spruce stand. Improper installation caused high and stronglyvarying concentrations in the first six years of monitoring; an example of an obviousbug in the dataset.

M. Bredemeier et al. ~Ecological Modelling 83 (1995) 7-)5

should not undergo substantial chemical or biological ir, teractions on the passage through the organic layer. Fig. 9, however, shows that there is only a weak correlation (r 2 = 0.31) between these two independent measurements of C1- input to the soil. The scatter can, for the most part, be attributed to spatial variation of fluxes with an insufficient number of replicate sampling points to resolve this variability. Fig. 10 gives an example of a typical bug in a time series of data. The aluminum and sulphate concentrations in the humus lysimeter samples are quite high and show high dynamic variations in the first years of sampling (1973-1978), then suddenly turn to a much smaller and more stable level for the rest of the monitoring period shown. The project logbook reports that humus lysimeters were changed and newly installed at the end of 1978. The first plates must have been improperly installed, i.e., with some mineral soil above them; since organic layer material is poor in A1, the high AI concentrations of the first measuring phase could otherwise not be explained. It is clear that it would be useless to apply any process model to such a time series of chemical parameters, it is an obvious artefact. There may be more artifacts in the data, however, which are not as easy to discover. So we encourage again an alert and critical attitude of the people using the dataset. References Bredemeier, M., 1987. Stoffbilanzen, interne Protonenproduktion und Gesamts~iurebelastung des Bodens in verschiedenen Wald6kosystemen Norddeutschlands. Ber. der

15

Forschungsz, Wald6kosysteme 33, Univ. G6ttingen, 183 pP. Bredemeier, M., Matzner, E. and Ulrich, B., 1990. Internal and external proton load to forest soils in Northern Germany. J. Environ. Qual., 19: 469-477. Eilers, G., Brumme, R. and Matzner, E., 1992. Above-ground N-uptake from wet deposition by Norway Spruce (Picea abies Karst.). For. Ecol. Manage., 51: 239-249. Ellenberg, H., Mayer, R. and Schauermann, J. (Editors), 1986. Okosystemforschung - Ergebnisse des Solling-Projekts. Ulmer Verlag, Stuttgart, 507 pp. Hauhs, M., Rost-Siebert, K., Raben, G., Paces, T. and Vigerust, B., 1989. Summary of European data. In: J.L. Malanchuck and J. Nilsson (Editors), The Role of Nitrogen in the Acidification of Soils and Surface Waters. UN ECE and Nordic Council of Ministers, Solna, Sweden. Johnson, D.W. and Lindberg, S.E. (Editors), 1992. Atmospheric Deposition and Forest Nutrient Cycling: a Synthesis of the Integrated Forest Study. Springer, New York, 707 pp. Matzner, E., 1988. Der Stoffhaushalt zweier Wald6kosysteme im Soiling. Ber. der Forschungsz. Wald6kosysteme 40, Univ. G6ttingen. Prenzel, J. and Schulte-Bisping, H., 1991. Ionenbinding in deutschen Waldb6den - Eine Auswertung von 2500 Boden u n t e r s u c h u n g e n aus 25 J a h r e n . Berichte des Forschungszentrums Wald6kosysteme, Reihe B, Bd. 29. Tiktak, A., Bredemeier, M. and van Heerden, K., 1995. The Soiling dataset: Site characteristics, monitoring data and deposition scenario. Ecol. Model., 83: 17-34. UBA, 1992. Daten zur Umwelt. E. Schmidt Verlag, Berlin, 675 pp. Ulrich, B., 1983. Interaction of forest canopies with atmospheric constituents. In: B. Ulrich and J. Pankrath (Editors), Effects of Accumulation of Air Pollutants in Forest Ecosystems. D. Reidel Publ., Dordrecht, pp. 33-45. Ulrich, B., Mayer, R. and Khanna, P.K., 1979. Die Deposition von Luftverunreinigungen und ihre Auswirkungen in Wald6kosystemen im Solling. Sauerl~inder Verlag, Frankfurt, 210 pp. Van Grinsven, H.J.M., Driscoll, C.T. and Tiktak, A., 1995. Workshop on Comparison of Forest-Soil-Atmosphere Models: Preface. Ecol. Model., 83: 1-6.