Waldsterben: forest decline in West Germany

W&terben : Forest decline in West Germany Bernhard Ulrich Insrimre ofsoil Science and Foresr Nutrition Georg-August University D-34W Giiningen Federal Republic of Germany The awareness of Mldrterben (“forest death”) has two mts. First, in the mid- 1960s the Institute of Soil Science and Forest Nutrition at Georg-August Uni- versity 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 in- vestigations led to the conclusion that the forests were subjected to heavy in- puts of acidity due to their filtering ef- fect 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 re- search 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 presen- tation of the conclusions given in the following and for literature citations.) In October 1989 an “International Con- inventory allows no inference on the status of the forest ecosystem. Cause-effect relationships There have been two initial hypothe- ses about ubldsterben: first, that soil acidification causes root damage, which results in physiological distur- bances 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 -- I research p r o g k s made it possible for 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 soil- 3 borne 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 L use organic matter as their energy consumption in changes in of rooting assimilates pattern, in high the source, mainly soil organisms like de- mt system, aggravation of nutrient composers); and soil components (soil and water uptake); solution and the mobilizable material direct effects of air pollutants on gress on Forest Decline Research Stale pool in the solid soil phase). All these of Knowledge and Perspectives” took comwnents are influenced bv the dew- Deriodic climatic stress (esoeciallv leaves; place in F&dtichshafen, FRG (Con- gress 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, sition of air pollutants to a Garying ‘de- gree. The forest damage inventory con- siders 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 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 ground vegetation); the secondary pro- ducers (heterotrophic organisms that as a whole. By omitting th; other com- ponents of the ecosystem, the damage symptoms due to tnxic effects as wellis increases in growth due to increased ni- 4-36 Environ. Sci. Technol., Vol. 24, NO. 4, 1990 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 nitro- gen supply). In neither case is its role specific. The acidification of the rhm- sphere-resulting in soil-borne root stress, decreasing nutrient and water uptake, and decreasing drought toler- ance-is part of the constellation lead- ing to the natural aging of trees. This natural aging process is greatly acceler- ated 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 origi- nating from rocks that are poor in acid- neutralizing 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 Ta- ble 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 devel- opment 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. Ta- ble 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 to- day, the range of variation of the cumu- lative acid deposition amounts to 60- 340 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 de- posited as strong acids can be buffered in the forest canopy, for example by cation exchange in leaves, the soil fi- naUy has to carry the acid load. The production of acidity that leads to soil acidification is a natural process in for- est 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 eco- system acidification are mainly the ac- cumulation of a cation excess in bio- mass and the export of this biomass from the ecosystem (bioremoval) as well as possible leaching losses of ni- trate, which stems from HNOl formed by mineralization of organic bound ni- trogen in soil. Leaching of nitrate is naturally a case bound to special condi- tions, but it becomes of greater impor- tance due to high rates of input of plant- available nitrogen by deposition (cf. inputhutput balance of nitrogen in Ta- ble 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 pro- tons 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 consumption by sili- cate 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 accordance with data on the actual material balances of forest eco- systems. In 'Igble 1 the input-output budgets of acidity (Ma d o n s ) and of mbde conservative anions (sulfate and nitrate) are given for a series of case studies ranging from North Germany to the. Bavarian forest in the south. Eco- systems with soils in the carbonate and cation exchange buffer range accumu- late the deposited acidity almost quanti- tatively 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 increased by the dissolution of " a l e s by car- bonic 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, prc- tons and ammonium ions, release al- most 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. Be- cause these. weakeracidp aremainly AI ions, this means that the risk of dumi- num stress in the deeper rooting zone incream proportionally with the depo- sition of acidity. Positive or ne@ve deviations from zero are accompanied by similar deviations of the mobile an- ions sulfate and nim. This indicates Soil addificatlm and add strefs From the temporal development of the cumulative acid emission (see Fig- ure 1). the development of acidification of the (rooted) subsoils can be deduced. Around 1930 one quarter of the cumu- lative acid emission had been r e l d , murid 1950 one half, and in the mid- 1960s 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 to- ward zero; Phase 11, in which acid either bw of sulfuric acid by alumi- num oxides and of deposited nitric acid by plant uptake as acid-consuming processes, or dissolution of aluminum sulfates and net niaif idon as 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 producing processes. ci6c smsmrs. 430 Envlron.Scl.Technol.,Vol. 24, No. 4, 1880 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 har- vesting 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 ni- trogen supply, at the beginning of in- dustrialization. Many of the spruce and pine forests were founded as planta- tions 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 nutri- tion and resulted in increased forest growth. This development continues in many forest stands as long as the distri- bution and vitality of the root system allow the use of the nitrogen deposited. In these ecosystems, most of the sul- furic acid deposited was accumulated at higher base saturation in the subsoil by formation of aluminum hydmxo sul- fates (or sorption of sulfate by ex- change of OH- from AI hydmxo com- pounds) 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 frac- tion 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 cu- mulative acid deposition. Phase II starts when the base satura- tion 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 in- ventory of the chemical state of forest soils in the FRG is in preparation. The storage of plant-available cationic nutri- ents (Ca, Mg, K) in these soils is too low to cover the needs of the next forest generation. We have to assume that 60- 80% of the forest area has, due to acid deposition, lost its plant-available cat- ionic nutrient storages to such an extent that the maturation and productivity of the next forest generation is not as- sured. 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 ex- ceeds 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 in- ternal ecosystem processes: the coup- lmg of nutrient uptake and nutrient mineralization from leaf and root litter. Nitrification as the final oxidation proc- ess 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 alu- minum 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 func- tion. The water stress hypothesis of crown thinning postulates that chronic water stress resulting in leaf loss should be a consequence of such a develop- ment. There are many data and obser- vations 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 syn- drome. The development in Phase I1 is strongly dependent on the input of plant-available nitrogen into the ecosys- tem. The superficial rooting on acid soils leads to the accumulation of or- ganic 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 en- ables the trees to continue growth until some other adverse effect becomes lim- iting (14). Environ.Sci.Techncl., Vcl.24, No.4, 1990 439 In Phase III, the superficial rooting increases trees' susceptibility to other stresses like wind throw, potassium de- ficiency with low frost hardiness, root damage by frost, drought, fungi, and insect attack. Die-back should occur according to the hypothesis if, due to leaf loss, the amount of photosynthates is too small to maintain the fine root mass necessary for water and nutrient uptake, or to maintain the water-con- ductive area by incremental growth. Especially in the southern part of Germany, mountain forests exist where human interference has been much less. There the state of soils and of the for- ests reflects more closely the natural development. In these ecosystems, acid deposition can initiate nitrate losses with the seepage water. This adds to the acid load, diminishes the accumulation of sulfuric acid as aluminum-bound SUI- fate, and thus accelerates the leaching of exchangeable Ca and Mg and the decrease in base saNration. During Phase I, however, nitrogen nutrition and thus growth is excellent. When en- tering Phase II and suffering the first time h m acid stress in the subsoil, these deep-rooting mixed forests can suddenly pass from very vigorous fast- growing stands into decline. The de- cline can be followed by vigorous re- generation because the humus layer on top of the mineral soil still represents an excellent seed bed. This temporal development makes it understandable why, after a long period of increasing forest growth and slowly expanding damage on most exposed mountain ridges or forest edges, leaf 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 Eu- rope like the Ore Mountains and in the 1980s in many parts of Europe. The fact that the magnesium balance seems to be negative in almost all forest eco- systems subjected to acid deposition (see Table 1: leaching losses exceed in- put by deposition) makes it understand- able that Mg is the nutrient whose defi- ciency 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 ) + q n where I,, = naNral input (background), I , = input due to anthropogenic activ- ity, A = rate of accumulation in the ecosystem in a good-natured (i.e., non- dangerous) form, B = buffer rate (con- version into good-natured products) in the ecosystem, 0 = output from the ecosystem. A and B may change with continuing load. 0 may change with a change in the state of the ecosystem. From the time dependency of A, B, and 0 it follows that the critical load may be timedependent. In the following discussion, a short-term time perspec- tive means the next 10-20 years, whereas the long-term perspective means the the final state to be achieved. With respect to nitrogen, I. < 0.35 N. The sources are NO,, (emission density [ED] in F R G 2.8 kmol N-ha-'. year') and NH, (ED 1.6 kmol N). In the ecosystem there exists a considera- ble accumulation capacity (A) in the form of organic matter, but for many forest ecosystems in central Europe this capacity is already exhausted. This is indicated by the leaching of nitrate from the soil with the seepage water (cf. Table 1: negative nitrogen budg- ets). No buffer reaction exists, so B is zero. There are several types of output, however. A small amount, roughly equal to I,, may be denihified and leave the ecosystem in gaseous form. In or- der to maintain water quality the leach- ing of nitrate should approach zero. Also, a biomass expoa from the em- system represents an output. In forests the output due to timber production is around 1 (0.7-1.4) kmol N-ha-1. year'. In uncropped M N ~ reserves, however, the biomass output is zero. In order to avoid nitrate leaching from managed forests, the short-term input should not exceed 1.8 kmol N. hx'.yearl and in the long term, 1 kmol kmoi N.IL-~.Yw~, r , = 1.5-6 kmoi N. The critical emission densities will be 1.4 and 0.7 kmol N, respectively. These limits would not exclude the fact that uncropped nature reserves are sub- jected to changes in species composi- tion due to increasing nitrogen supply. With respect to acidity, I, = 0.05 kmol H+ .ha-l.yearl @H of rainwater 5.0), I, = 1 to > 8. The sources are S a (ED in the FRG 4.2 kmol H+. kl -year l ) and NO, (ED: 2.8). A is represented by proton-consuming proc- esses connected with the accumulation of nitrate as organic nitrogen and of sulfate as aluminum sulfate by ex- change of OH-. As already mentioned, the capacity to accumulate organic bound nitrogen is already exhausted. This is also true for the accumulation of sulfate (cf. Table 1: negative budgets of mobile anions). In contrast, ecosystems exist where the leaching of sulfate exceeds deposi- tion. This indicates the dissolution of AI-sulfates, which is connected with an equivalent proton production. The buf- fering of acidity by cation exchange is co~ec ted with an equivalent loss of cationic nutrients from the soil profile. The aim, however, should be to main- tain the base saturation. Therefore the cation exchange buffer represents no buffer capacity that should be used. B is therefore equal to the rate of alkali and earth alkali cation release from silicate minerals during weathering. As already discussed, however, in managed forest ecosystems this buffer reaction balances the acid load because of timber harvesting. The output 0 of acidity corresponds to the lThe state of WaldsterbenCaUSe-effect research In many forest ecosystems considerable soil acidification has taken place because of acid deposition. Early leaf discoloring the action of pathoge Air pollutants cause t needles. The tolerance of trees tants and acid mist decrease 440 Environ. Sci. Technol., Vol. 24. No. 4, 1990 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 em- sphere followed the principle of mini- mizing eutropy production in structur- ing and organizing ecosystems. We should accept this principle as a guide- line for StnICNring and organizing man-made systems that are imbedded in the ecosphere and thus are special kinds of ecosystems. reduction of emissions In the Federal Republic of fraction of organic bound nitrogen ex- po& 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 b l -yea 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 rhi- zosphere (soil close to the roots) enough to keep the acid stress for the mts at a toleaable level. In the long- tion to the internal ecosystem load, ing, without causing injury during a re tation period. The critical deposition rates given for acidity and nitrogen are. in agreement with the conclusions of international workshops (15, 16). Nefessary reduftion of emissiom The delinition of the critical load in terms of critical emission density al- lows the calculation of the percentage decrease in emission required in order to not exceed the critical deposition rates, assuming homogemems distribu- tion of emission and deposition. For the calculation of the necessary reduction of emissions the following two assumptions are made. Fist, the nim deposited is not leached, but an equivalent amount is exported from the ecosystem with timber. This means that the acidity prcduced by the emission of NO, need not be taken into account when calculating the critical proton load. Second, at present around 0.5 kmol H+ .ha-'.yearl are buffered in the atmosphere mainly by reaction with soil dust, releasing Ca and Mg ions. It is assumed that the rate of this pmcess remains unchanged. This means that the critical emission density of acidity ha-'.year', allowing short-term emis- should be redud to 0.5 km01 H+. term, an input Of 0.1-0.2 km01 H+- ha-'.year' should be b u f f e r e d in addi- either by silicate Weathering OT by lim- could be increased by 0.5 b l €I+. sion densities of 1, long-term emission densities of 0.7 la1101 Hf -kl.year'. For the FRG the results are. shown in Table 3. The data show that a rapid reduction of emissions by €0-7676 is necessary (reference year 1982). The measures taken by the government of the FRG are expected to decrease S@ emissions by 66% before 1995 (refer- ence year 1982). Even if all European countries accept the necessity to reduce emissions to such an extent, it would take one to two decades to achiewe this goal. By following the change in depo- sition in selected forest ecosystems and improving our understanding of trans- boundary air pollution and cause-effect relationships in ecosystems, it will be possible to redefine the critical loads and emission densities early enough to prevent a possible overreaction in long- (4) Schiilt, P. Forsrwirs. Centmlbl. 1988, 100,286-87. (5) Roberts, T. M.: Skeffington, R. A,; Blanck, L. W. Forestv 1989, 62. 179- '1'. (6) Schulze, E. D.; Lange. 0. L.; Orcn, R., Us.; Ecol. S N d 1989, 77, 475, (7) Ulrich, 8. In Acidic F'recipitation. Vol2: Biological and Ecological Effects: Adriano, D. C.; Johnson, A. H.. Eds.; Springer: Berlin, 1989; pp. 189-272. (8) Grubcr, E F&ro 1988,181,205-42. (9) Gruber. P. Bet Forschungszcnf,: W o - b s w . A 1987.26. 214. (IO) Rofoff, A. S&&m Forsrl. Fak. Univ. (11) Sverdnrp, H.; Warfvinge, €! 0. In Nils- son. 1.: Oremfelt, €! Nordisk Ministerrod Mi!MRappon 1988, I S , 81-130. (12) Ulrich, B.: Meyer. H. Be,: Fomckungs- zenf,: Ml&koy. 8. 1987.6, 133. (13) Ulrich, B.; Meycr, H.; JSnich, K.; Biit- mer, 0. Forst u. Holr 1989,44, 251-53. (14) Schulze. E. D. Science 1989.244. 7 7 6 h t f h g e n 1988, 93; 258. term emission reduction. 83. (15) Nilsson, I . , M. Nordisk Ministerrod MiljdRapport 1 9 8 6 , I I . 232. (16) Nilsson, J.; Greenfelt, P. Nordisk Minis- termdMiljoRappon 1988, I S , 418. collflusioos During the last decade we have be- come aware of what we are. doing by producing and distributing wastes such as air pollutants. Even though our knowledge of long-term adverse effects is incomplete, it is enough to conclude that drastic reductions in emissions are nesessary to guarantee an environmen- tal puality that ~ O W S manlrind to c ~ n - time its development. During the last century we have made the error of overestimating the ability of naNre to digest the waste of the industrial soci- ety. It is a relatively small error, b e cause we have developed, or have the potential to develop, the technologies to solve these problems. We. are. not con- fronted with unsolvable problem; our fate is in our hands. In addition we have to devise strate- gies that help to avoid such undesirable developents in the f , , ~ . b n o m i c co~dmtions are. an important to tinding the optimal way of achieving a Specific goal. AS 10% as the long-term adverse effects of our activities remain uncertain or even unexplored, they can- B e " d U&ich is a full pmfesor at the Instinue of Soil Scimce and Forest Nuni- tion 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 , ~ ~ ~ ~ r i o ~ , " i d bolanee ofeco- systems. and ecosystem theory Environ. Xi.Technol.,Voi. 24, NO. 4, 1690 Ul