W&terben : Forest decline in West Germany inventory allows no inference on the status of the forest ecosystem.
Bernhard Ulrich Insrimre ofsoil Science and Foresr Nutrition Georg-August University D-34W Giiningen Federal Republic of Germany
The awareness of Mldrterben (“forest t s .First, in the middeath”) has two m 1960s the Institute of Soil Science and Forest Nutrition at Georg-August University in Giiaingen began measuring the input-output balance of dissolved materials in two forest ecosystems in the Solling mountains in the Federal Republic of Germany (FRG). These investigations led to the conclusion that the forests were subjected to heavy inputs of acidity due to their filtering effect on gaseous ( S a ) and particulate air pollutants (especially cloud water) (1,2). Second, the silver fir started to decline in the mid-1970s (3, 4). The hypothesis that a general forest decline would happen within the coming years or decades caused the development of integrated and ecosystem-oriented research programs in the FRG beginning in 1983. Summaries of the results achieved have been published by Roberts et al. (5), Schulze et al. (6), and ULrich (7). (See Reference 7 for a detailed presentation of the conclusions given in the following and for literature citations.) In October 1989 an “International Congress on Forest Decline Research Stale of Knowledge and Perspectives” took place in F&dtichshafen, FRG (Congress proceedings will be available in the spring of 1990 and can be ordered from Kernforschungszent”, D-7500 Karlsruhe). Components of the forest ecosystem are the primary producers (green plants able to photosynthesize: trees, shrubs, ground vegetation); the secondary producers (heterotrophic organisms that 4-36
Environ. Sci. Technol., Vol. 24, NO.4, 1990
Cause-effect relationships There have been two initial hypotheses about ubldsterben: first, that soil acidification causes root damage, which results in physiological disturbances including nutrient and water stress; the nutrient stress may be masked by the input of nutrients via deposition of air pollutants. Second, the leaf loss and leaf discoloring are due to direct effects of gaseous air pollutants (mainly SO2, NO,, ozone) and acid mist. These hypotheses led to research on soil acidification and acid stress on mts on the one hand and direct effects of air pollutants on leaves on the other hand. The ecosystem orientation of the research p r o g k s made it possible for I the two research lines to be connected more or less efficiently in case studies. The problem of ubhldsterben is much 5 older than our awareness of it. There is E strong evidence that the natural soilborne stress on tree mts and nutrient i and water uptake has been strongly in! creased by acid deposition. The symp& toms visible in the tree crown (leaf loss 2 6 and discoloration) can be interpreted as k the result of a complex interaction 9 among the following: Crown thinning of Norway spruce adverse changes in the soil (resulting use organic matter as their energy consumption in changes in ofrooting assimilates pattern,in high the
--
L 3
source, mainly soil organisms like decomposers); and soil components (soil solution and the mobilizable material pool in the solid soil phase). All these comwnents are influenced bv the dewsition of air pollutants to a Garying ‘degree. The forest damage inventory considers as damage symptoms only leaf loss and leaf discoloring of trees. Thus it judges only the vitality staNs of tree canopies. By neglecting the status of the m t system of the tree, it allows no conclusion about the vitality of the tree as a whole. By omitting th; other components of the ecosystem, the damage
mt system, aggravation of nutrient and water uptake); direct effects of air pollutants on leaves; Deriodic climatic stress (esoeciallv karm or dry years, frost, Gught);’ the activity of weakness parasites; and, as a change operating against nutrient stress and favoring tree growth, the increasing nutrient input (especially nitrogen) by deposition. This complex interaction allows very differing effects on trees-damage symptoms due to tnxic effects as wellis increases in growth due to increased ni-
0013-936X19010924.0435$02.5010
1990 American Chemical Society
trogen deposition-depending on site conditions, forest history, c l i i t e , and rate of deposition of air pollutants. The role of air pollution in this interaction can be stress-increasing as well as stressdecreasing (by increasing nitrogen supply). In neither case is its role specific. The acidification of the r h m sphere-resulting in soil-borne root stress, decreasing nutrient and water uptake, and decreasing drought tolerance-is part of the constellation leading to the natural aging of trees. This natural aging process is greatly accelerated by acid deposition and leads to early senescence of trees: the symptom of leaf loss. The leaching of cationic nutrients as part of the soil acidification caused by acid deposition increases the natural lack of nutrients in soils originating from rocks that are poor in acidneutralizing minerals. On the other hand, nitrogen input by deposition can overcome the natural problem of limited nitrogen supply-on acid soils and thus lead temporarily to an increase in forest increment. In Table l , nitrogen budgets are presented for a series of case studies in which the input by deposition and the output with seepage water of materials (ions) has been measured over several years. In many of the case studies the nitrogen deposited from air pollutants accumu-
lates in the ecosystem to a degree that equals or exceeds the accumulation in the forest increment.
The development of emissions From the existing data on the development of emissions it is possible to calculate the amount of acidity emitted annually per area and the total emitted since the beginning of industrialization. In Figure 1 the data are presented for the area of the FRG.In this area until now 370 kmol acid equivalents (Hi) per ha have been emitted. By the end of the 1970s the annual emission of acidity amounted to 4.2 kmol H+/ha from SO2 and 2.8 from NO,; this gives a total emission density of 7 kmol H+lha. The annual acidic input into forests (cf. Table l) varied between 17% and 90%of the emission density. If, as a first a p proach, for the whole time span the same variation in the percentage of acid deposition is assumed that we have today, the range of variation of the cumulative acid deposition amounts to 60340 keqlha. Because the eastern neighboring countries especially have much higher emissions, the values given represent minimal values.
Acid load, acid buffering Even if up to 80% of the protons deposited as strong acids can be buffered
in the forest canopy, for example by cation exchange in leaves, the soil finaUy has to carry the acid load. The production of acidity that leads to soil acidification is a natural process in forest ecosystems. The data base available (11) allows the generalization presented in Table 2. In the top part of the table, the acid load for a rotation period of a European beech (Fagus sylvan”) or Norway spruce (Picea abies) stand is compiled. The causes of internal ecosystem acidification are mainly the accumulation of a cation excess in biomass and the export of this biomass from the ecosystem (bioremoval) as well as possible leaching losses of nitrate, which stems from HNOl formed by mineralization of organic bound nitrogen in soil. Leaching of nitrate is naturally a case bound to special conditions, but it becomes of greater importance due to high rates of input of plantavailable nitrogen by deposition (cf. inputhutput balance of nitrogen in Table l). The total acid load varies from 100 to more than 400 kmol H+/ha. Acid deposition amounts to more than 60%of the total acid load. In the bottom part of Table 2, the range of acid buffering in soil is given. The only process that consumes protons and releases Ca, Mg, and K ions without changes in the acid-base status Environ. Sci. Technol., Vol. 24, No. 4, 1990 437
of the soil is the weathering of silicates. The rate of this process depends on the types and amounts of silicates present and only to a minor degree on the pH of the soil. For common forest soils derived from sedimentary rocks and granite, the proton consumptionby silicate weathering is roughly equivalent to the proton production by bioremoval. This means that any additional acid load leads to a decrease in exchange able Ca and Mg. The comparison b e tween the cumulative acid deposition of 60 to >340 h o l H + h and the buffer capacities by cation exchange (Table 2) shows that acid deposition should have led to low base saturation in most sandy soils and in many loamy sods. This is in accordancewith data on the actual material balances of forest ecosystems. In 'Igble 1 the input-output budgets of acidity (Ma d o n s ) and of m b d e conservative anions (sulfate and nitrate) are given for a series of case studies ranging from North Germany to the. Bavarian forest in the south. Ecosystems with soils in the carbonate and cation exchange buffer range accumulate the deposited acidity almost quantitatively in the soil. High leaching losses of Mg and Ca indicate the deaease in lxse saturation. In soils in the carbonate buffer range, the output of Ca is greatly increasedby the dissolution of " a l e s by carbonic acid. In ecosystems with soils in the aluminum buffer range where base saturation approaches zero, the acid balance (Ma cations) varies around zero. The acids deposited, that is, prc430 Envlron.Scl.Technol.,Vol. 24, No. 4, 1880
tons and ammonium ions, release almost equivalent amounts of d u " ions (cation acids), which are leached. This means that the acidity deposited is not neutralized, but changed from stronger acids into weaker acids. Because these.weakeracidp aremainly AI ions, this means that the risk of duminum stress in the deeper rooting zone incream proportionally with the deposition of acidity. Positive or ne@ve deviations from zero are accompanied by similar deviations of the mobile anions sulfate and nim.This indicates either b w of sulfuric acid by aluminum oxides and of deposited nitric acid by plant uptake as acid-consuming processes, or dissolution of aluminum sulfates and net n i a i f i d o n as acidproducing processes.
Soil addificatlmand add strefs From the temporal development of the cumulative acid emission (see Figure 1). the development of acidification of the (rooted) subsoils can be deduced. Around 1930 one quarter of the cumulative acid emission had been r e l d , murid 1950 one half, and in the mid1960s two thirds. With regard to the reaction of forest ecosystems to the deposition of acidity, three phases can be distinguished: F'hase I, in which the base saturation in mil is decreasing toward zero; Phase 11, in which acid stress in the subsoil changes the depth gradient of the h e root system toward a superficial rooting; and F'hase m,in which the superficially rooting trees suffer more and more under sitespc ci6c smsmrs.
FIGURE 1
Annual emission rates of SO, and NO, and cumulative emission of acidity caused by SO, and NO;'
.
4Wr
%theareaof Wesf Germany since 1850.
Many forests in central Europe have been heavily used over centuries and millennia for shifting crops, for harvesting of any kind of wood and often also the litter, and for grazing. These forests were in bad shape, with strongly acidified topsoils and very limited nitrogen supply, at the beginning of industrialization. Many of the spruce and pine forests were founded as plantations after more or less long-lasting periods of heath vegetation. The subsoils, however, as the main rooting zones for trees, have usually been still at medium base saturation (12). Modern forest management, the restriction on the harvest of stem wood, and the increasing input of nitrogen from air pollution improved tree nutrition and resulted in increased forest growth. This development continues in many forest stands as long as the distribution and vitality of the root system allow the use of the nitrogen deposited. In these ecosystems, most of the sulfuric acid deposited was accumulated at higher base saturation in the subsoil by formation of aluminum hydmxo sulfates (or sorption of sulfate by exchange of OH- from AI hydmxo compounds) during Phase I. To a lesser degree this process can still go on (see Table 1 and discussion above). This de-
layed the decrease of base saturation and lengthened Phase I. A smaller fraction of the sulfuric acid deposited caused the leaching of exchangeable Ca and Mg, leading to a decrease in base saturation. In the loamy soils of the subalpine mountains this phase lasted until the 1960s (13). with the exception of mountain ridges exposed to high cumulative acid deposition. Phase II starts when the base saturation in the subsoil also has reached low levels (soil in aluminum buffer range). According to the available data this seems now to be the case for 6040% of the forest soils. A representative inventory of the chemical state of forest soils in the FRG is in preparation. The storage of plant-available cationic nutrients (Ca, Mg, K) in these soils is too low to cover the needs of the next forest generation. We have to assume that 6080% of the forest area has, due to acid deposition, lost its plant-available cationic nutrient storages to such an extent that the maturation and productivity of the next forest generation is not assured. The material balances of these cations in forest ecosystems (see Table 1) show that especially in case of Mg, but also for Ca, the requirement of an aggrading stand cannot be covered from deposition: The Mg budgets are
all negative or zero (Mg leaching exceeds Mg deposition). With respect to Ca the difference between deposition and leaching covers, approximately,the demand of forest increment only in one case study (No. 6). The fate of the existing older stands of timber trees depends greatly on internal ecosystem processes: the couplmg of nutrient uptake and nutrient mineralization from leaf and root litter. Nitrification as the final oxidation process of organic bound nitrogen means the formation of HNO,. In soils in the aluminum buffer range this strong acid cannot be neutralized any more, except by nitrate uptake. Nitrification pulses following warm or dry years, which exceed the rate of nitrate uptake, can cause high aluminum concentrations in the subsoil, which can damage h e roots. The tree reacts by forming new fine mts in the upper soil, where aluminum is complexed by soil organic matter (7). This means, however, that the area of the conductive tissue, which connects the sites of water uptake with those of transpiration, is decreased by taking coarse roots in the subsoil out of function. The water stress hypothesis of crown thinning postulates that chronic water stress resulting in leaf loss should be a consequence of such a development. There are many data and observations in favor of this hypothesis, but it has still to be further tested and fully quantified. If photosynthesis remains at a high enough rate to allow incremental growth in the functioning coarse roots, the Norway spruce would be able to recover by forming new, regenerated shoots if the water supply to the crown increases again (8). If during this development special nutrient ratios in the soil solution reach unphysiological ranges (e.g., MglAl), in which uptake (e.g., of Mg) is d e creased or inhibited, nutrient deficiency symptoms such as yellowing of needles may become part of the damage syndrome. The development in Phase I1 is strongly dependent on the input of plant-available nitrogen into the ecosystem. The superficial rooting on acid soils leads to the accumulation of organic matter rich in Al and Fe in the humus layer on top of the mineral soil. This means that nitrogen is taken out of the nutrient cycle and accumulated in the humus layer. Without nitrogen input this would lead to stunted growth. In central Europe the nitrogen input into forest ecosystems from air pollutants exceeds the amount accumulated in the forest increment (cf. Table 1). This enables the trees to continue growth until some other adverse effectbecomes limiting (14). Environ.Sci.Techncl., Vcl.24, No.4, 1990 439
In Phase III, the superficial rooting the ecosystem, 0 = output from the N. The critical emission densities will increases trees' susceptibility to other ecosystem. A and B may change with be 1.4 and 0.7 kmol N, respectively. stresses like wind throw, potassium decontinuing load. 0 may change with a These limits would not exclude the fact ficiency with low frost hardiness, root change in the state of the ecosystem. that uncropped nature reserves are subdamage by frost, drought, fungi, and From the time dependency of A, B, and jected to changes in species composiinsect attack. Die-back should occur 0 it follows that the critical load may tion due to increasing nitrogen supply. according to the hypothesis if, due to be timedependent. In the following With respect to acidity, I, = 0.05 leaf loss, the amount of photosynthates discussion, a short-term time perspec- kmol H+.ha-l.yearl @H of rainwater is too small to maintain the fine root tive means the next 10-20 years, 5.0), I, = 1 to > 8. The sources are mass necessary for water and nutrient whereas the long-term perspective S a (ED in the FRG 4.2 kmol H+. uptake, or to maintain the water-con- means the the final state to be achieved. k l - y e a r l ) and NO, (ED: 2.8). A is With respect to nitrogen, I. < 0.35 represented by proton-consuming procductive area by incremental growth. Especially in the southern part of kmoi N.IL-~.Yw~, r , = 1.5-6 kmoi esses connected with the accumulation Germany, mountain forests exist where N. The sources are NO,, (emission of nitrate as organic nitrogen and of human interferencehas been much less. density [ED] in F R G 2.8 kmol N-ha-'. sulfate as aluminum sulfate by exThere the state of soils and of the for- year') and NH, (ED 1.6 kmol N). In change of OH-. As already mentioned, ests reflects more closely the natural the ecosystem there exists a considera- the capacity to accumulate organic development. In these ecosystems, acid ble accumulation capacity (A) in the bound nitrogen is already exhausted. deposition can initiate nitrate losses form of organic matter, but for many This is also true for the accumulation of with the seepage water. This adds to the forest ecosystems in central Europe this sulfate (cf. Table 1: negative budgets of acid load, diminishes the accumulation capacity is already exhausted. This is mobile anions). In contrast, ecosystems exist where of sulfuric acid as aluminum-boundSUI-indicated by the leaching of nitrate fate, and thus accelerates the leaching from the soil with the seepage water the leaching of sulfate exceeds deposiof exchangeable Ca and Mg and the (cf. Table 1: negative nitrogen budg- tion. This indicates the dissolution of decrease in base saNration. During ets). No buffer reaction exists, so B is AI-sulfates, which is connected with an Phase I, however, nitrogen nutrition zero. There are several types of output, equivalent proton production. The bufand thus growth is excellent. When en- however. A small amount, roughly fering of acidity by cation exchange is tering Phase II and suffering the first equal to I,, may be denihifiedand leave c o ~ e c t e dwith an equivalent loss of time h m acid stress in the subsoil, the ecosystem in gaseous form. In or- cationic nutrients from the soil profile. these deep-rooting mixed forests can der to maintain water quality the leach- The aim, however, should be to mainsuddenly pass from very vigorous fast- ing of nitrate should approach zero. tain the base saturation. Therefore the cation exchange buffer growing stands into decline. The de- Also, a biomass expoa from the emcline can be followed by vigorous re- system represents an output. In forests represents no buffer capacity that generation because the humus layer on the output due to timber production is should be used. B is therefore equal to top of the mineral soil still represents an around 1 (0.7-1.4) kmol N-ha-1. the rate of alkali and earth alkali cation year'. In uncropped M N reserves, ~ release from silicate minerals during excellent seed bed. This temporal development makes it however, the biomass output is zero. weathering. As already discussed, In order to avoid nitrate leaching however, in managed forest ecosystems understandable why, after a long period of increasing forest growth and slowly from managed forests, the short-term this buffer reaction balances the acid expanding damage on most exposed input should not exceed 1.8 kmol N. load because of timber harvesting. The mountain ridges or forest edges, leaf hx'.yearl and in the long term, 1 kmol output 0 of acidity corresponds to the loss as a decline symptom became a p parent on a large scale from the 1970s on. This happened first in the most heavily loaded regions of central EulThe state of WaldsterbenCaUSe-effect research rope like the Ore Mountains and in the 1980s in many parts of Europe. The fact that the magnesium balance seems In many forest ecosystems considerable soil acidification has taken to be negative in almost all forest ecoplace because of acid deposition. systems subjected to acid deposition (see Table 1: leaching losses exceed input by deposition) makes it understandable that Mg is the nutrient whose deficiency first causes symptoms such as needle yellowing to appear on a large scale. Critical loads of acidity, nitrogen As a basis for the assessment of criti-
cal loads of acidity and nitrogen, the following balance equation is used:
.r + I,
=
A(O + q t )
+ qn
where I,, = naNral input (background), I , = input due to anthropogenic activity, A = rate of accumulation in the ecosystem in a good-natured (i.e., nondangerous) form, B = buffer rate (conversion into good-natured products) in 440 Environ. Sci. Technol., Vol. 24. No. 4, 1990
Early leaf discoloring the action of pathoge Air pollutants cause t needles. The tolerance of trees tants and acid mist decrease
reduction of emissions In the Federal Republic of
fraction of organic bound nitrogen expo& from the ecosystem with timber, which results from nitrate uptake. For the short-term consideration,the input of acidity that exceeds uptake of deposited nitrate in harvested timber should be redud to 0.5 km01 H+. b l - y e a r l . Such rates of acid 1 4 , which comespond to the rate of proton consumption by silicate weathering, should reduce the acid load of the rhizosphere (soil close to the roots) enough to keep the acid stress for the mts at a toleaable level. In the longterm, an input Of 0.1-0.2 km01 H+ha-'.year' should be b u f f e r e d in addition to the internal ecosystem load, either by silicate Weathering OT by lime ing, without causing injury during a r tation period. The critical deposition rates given for acidity and nitrogen are. in agreement with the conclusions of international workshops (15, 16).
not be part of the economic evaluation. Therefore economy cannot define the goals. The delinition of goals has to be based on other forms of reasoning. It seems that the evolution of the emsphere followed the principle of minimizing eutropy production in structuring and organizing ecosystems. We should accept this principle as a guideline for StnICNring and organizing man-made systems that are imbedded in the ecosphere and thus are special kinds of ecosystems.
sion densities of 1, long-term emission densities of 0.7la1101 Hf-kl.year'. For the FRG the results are. shown in (4) Schiilt, P. Forsrwirs. Centmlbl. 1988, Table 3. The data show that a rapid 100,286-87. (5) Roberts, T. M.: Skeffington, R. A,; reduction of emissions by €0-7676 is Blanck, L. W. Forestv 1989, 62. 179necessary (reference year 1982). The .'1' measures taken by the government of (6) Schulze, E. D.; Lange. 0. L.; Orcn, R., Us.; Ecol. S N d 1989, 77, 475, the FRG are expected to decrease S@ (7) Ulrich, 8. In Acidic F'recipitation. Vol2: emissions by 66% before 1995 (referBiological and Ecological Effects: ence year 1982). Even if all European Adriano, D. C.; Johnson, A. H..Eds.; countries accept the necessity to reduce Springer: Berlin, 1989; pp. 189-272. (8) Grubcr, E F&ro 1988,181,205-42. emissions to such an extent, it would (9) Gruber. P. Bet Forschungszcnf,: W o take one to two decades to achiewe this b s w . A 1987.26. 214. goal. By following the change in depo- (IO) Rofoff, A. S&&m Forsrl. Fak. Univ. h t f h g e n 1988, 93; 258. sition in selected forest ecosystems and H.; Warfvinge, €! 0. In Nilsimproving our understanding of trans- (11) Sverdnrp, son. 1.: Oremfelt, €! Nordisk Ministerrod boundary air pollution and cause-effect Mi!MRappon1988, I S , 81-130. relationships in ecosystems, it will be (12) Ulrich, B.: Meyer. H. Be,: FomckungsMl&koy. 8. 1987.6, 133. possible to redefine the critical loads (13) zenf,: Ulrich, B.; Meycr, H.; JSnich, K.; Biitand emission densities early enough to mer, 0 .Forst u. Holr 1989,44, 251-53. prevent a possible overreaction in long- (14) Schulze. E. D. Science 1989.244. 7 7 6 term emission reduction. 83. (15) Nilsson, I., M. Nordisk Ministerrod
collflusioos During the last decade we have beNefessary reduftion of emissiom come aware of what we are. doing by The delinition of the critical load in producing and distributing wastes such terms of critical emission density al- as air pollutants. Even though our lows the calculation of the percentage knowledge of long-term adverse effects decrease in emission required in order is incomplete, it is enough to conclude to not exceed the critical deposition that drastic reductions in emissions are rates, assuming homogemems distribu- nesessary to guarantee an environmental puality that ~ O W manlrind S to c ~ n tion of emission and deposition. For the calculation of the necessary time its development. During the last reduction of emissions the following century we have made the error of two assumptions are made. Fist, the overestimating the ability of naNre to n m i deposited is not leached, but an digest the waste of the industrial sociequivalent amount is exported from the ety. It is a relatively small error, b e ecosystem with timber. This means that cause we have developed, or have the the acidity prcducedby the emission of potential to develop, the technologiesto NO, need not be taken into account solve these problems. We.are. not conwhen calculating the critical proton fronted with unsolvable problem; our load. Second, at present around 0.5 fate is in our hands. kmol H+.ha-'.yearl are buffered in the In addition we have to devise strateatmosphere mainly by reaction with gies that help to avoid such undesirable soil dust, releasing Ca and Mg ions. It developents in the f , , ~ .b n o m i c to is assumed that the rate of this pmcess c o ~ d m t i o n s are. an important remains unchanged. This means that tinding the optimal way of achieving a the critical emission density of acidity Specific goal. AS 10% as the long-term adverse could be increased by 0.5 b l €I+ . effects of our activities remain ha-'.year', allowing short-term emis- uncertain or even unexplored, they can-
MiljdRapport 1 9 8 6 , I I . 232. (16) Nilsson, J.; Greenfelt, P. Nordisk MinistermdMiljoRappon 1988, I S , 418.
B e " d U&ich is a fullpmfesor at the Instinue of Soil Scimce and Forest Nunition at G6m'ngm University. He received a Ph.D. from the Georg-August University in mgen, His research spec^ are in soil chemistry, soil solution-soil phose i , ~ ~ ~ ~" rii do bolanee ~ , ofecosystems. and ecosystem theory Environ. Xi.Technol.,Voi. 24, NO.4, 1690 Ul
