Visible injury responses

Environmental Pollution 68 (1990) 355-366

Visible Injury Responses U. A r n d t , ~ N. Billen," G. Seufert, a* W. L u d w i g , b K. B o r k h a r t ~ & B. O h n e s o r g e ~ a Institut ffir Landeskultur und Pflanzen6kologie, b Landesanstalt ffir Bienenkunde, c Institut fiir Phytomedizin, Universit~it Hohenheim, D-7000 Stuttgart 70 Hohenheim, Germany

A BSTRA CT During a five year experiment on the causes of forest disease, symptoms of visible injury and pest infestations in trees treated with various air pollutants in open-top chambers were observed. Though the long-term experiment was originally not intended to include such investigations, insect infestation and some discoloration of the trees (Beech, Fagus sylvatica; Fir, Abies alba; Spruce, Picea abies) could not be avoided. Abundance and size of some of the insects were measured after two years and at the end of the experiment. Because it was unknown when the first infestation in the chambers occurred, quantitative investigations of the population size provided little information. Visible injury on leaves and needles was infrequent in general. When it occurred, it appeared to be caused by at least three stress factors. However, three different types of symptoms on beech and fir could be attributed mainly to air pollutants. These symptoms consisted of two types of foliar necrosis and browning in beech and needle tip chlorosis in fir. These symptoms have been observed under certain conditions in the German forests. The spruce clone used, however, developed no injuries which could be connected definitely to treatment effects.

INTRODUCTION A five year experiment using open-top chambers and realistic pollutant concentrations was deliberately designed not to produce externally visible s y m p t o m s o f foliar injury. W i t h o u t knowing the sensitivity of y o u n g beech t r e e s (Fagus sylvatica L.), fir (Abies alba Mill.), and spruce (Picea abies L. * Present address: CCR--Ispra Environment Institute TP 050, 1-21020, Ispra Varese, Italy. 355 Environ. Pollut. 0269-7491/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

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Karst.) to 502, 03 and simulated acid precipitation (AP), it was thought that longer periods of exposure to these pollutants would be necessary to cause visible injury than was possible in the experiments. However, this assumption proved to be wrong. After 20 weeks, the beech trees in the chambers exposed to SO 2 + AP and SO2 + O3 + AP showed symptoms of injury. The SO 2 concentrations were subsequently lowered (Seufert et al., this volume). It was also clear that a certain visual monitoring of the plants would be necessary during the experiment and that the gradual visible changes needed to be described and discussed. To accomplish this, methods for quantifying visible injury, derived from ecotoxicological research and practical emission control technology, were used. Externally visible symptoms of injury are often the first clues of the damaging effects of air pollutants. Therefore, special techniques for assessing visible injury were developed previously (Guderian et al., 1960). In 1969, the first color pictorial atlas on sulphur dioxide damage to plants was published (van Haut & Stratmann, 1970). Practically at the same time a corresponding work was published in the US, which dealt with the symptomatology of various pollutant effects and, for the first time, integrated the concept of bioindicators with the visible injury (Jacobson & Hill, 1970). Injury symptoms on plants are not always due to effects of air pollutants. Edaphic, atmospheric and biotic factors can also cause visible symptoms of injury. In order to allow a differential diagnosis of damage, the knowledge of deficiency symptoms caused by unfavorable location and visible alterations caused by biotic influences are equally important. This is the primary reason why various pictorial atlases of recent times have dealt with the problem of identifying the cause of injury in a comparative manner (Malhotra & Blauel, 1980). These publications primarily show acute effects of air pollution and are thus mainly useful with emitter-related damage examinations. The symptomatology of forest disease in West Germany is distinctly different from that observed in studies of acute effects of air pollutants. The predicate 'forest diseases of a new kind' ('novel forest diseases') has one of its causes here. Therefore, a special documentation on the appearance of the vegetation makes sense, specially for the comparison of acute and chronic effects. Due to the long-term nature of the disease development, the cause of the symptoms can be hardly considered as a single factor. Therefore, Hartmann et al. (1988) speak of 'different, complex working causes' and 'Kronenverlichtung' is an intergrating and essential criterion for the forest disease inventory. Besides the 'pictorial atlas of forest diseases' published by these authors, there are other publications available today that represent the conditions in the US (Skelly et al., 1987) or central Europe (CEC, 1984). Comparative visible injury symptoms in the two continents have been

Visible injury responses

357

described through a field excursion of German and American scientists in 1984 (KFA-Jiilich, 1987). With the pictorial material and with the analysis of data obtained at Hohenheim it is possible to discuss the symptoms observed in the chambers. During the five year experiment, the young trees were quite variable in appearance. This was attributed to the following reasons: (1) (2) (3) (4) (5) (6)

growth in spatially limited chambers; undesired, mechanical effects caused by the investigators; season; infestation with pests; experimental manipulation such as artificial dry periods; and treatment with pollutants.

This relatively large number of influencing factors, and possibly additional unrecognized ones, may resemble a field situation (Arndt & Seufert, this volume), but the resulting interactions can be confounding. Because this renders an interpretation of the observations considerably more difficult, only those symptoms which have been traced back with high probability to the air pollutant treatments will be discussed in detail in the following sections.

ENTOMOLOGICAL FINDINGS During the first years of the experiment an infestation by different pests was observed, and was controlled manually by continual removal of the insects by a large number of student helpers. Larvae of Ladybug (Coccinella septempunctata) and Bloom-fly (Chrysopa vulgaris) were also actively introduced into the chambers but were ineffectual. Systematic entomological examinations in the chambers occurred long after infestation was noted and, therefore, the exact time of the first infestation could not be determined. Thus, it is difficult to make accurate statements about the influence of the various pollutant treatments on the population density of certain insects. An entomological survey in 1986 showed that by the third year of the experiment there was considerable infestation on beech by the 'Buchenblattlaus' (Phyllaphisfagi L.). Interestingly, the chambers polluted with 03 + AP and SO2 + 03 4-AP had a significantly lower infestation than the other treatment chambers (Fig. 1). A similar pattern was observed for the infestation of the fir with the 'Tannentrieblaus' (Dreyfusia sp.), the 'WeiBtannentrieblaus' (Mindarus abietinus Kch.) and the 'Kiefernadelschildlaus' (Nucalaspis abietis Schrk.). In this survey, the ozone treatment chamber had fewer pests.

358

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Arndt

et al.

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No pathogenic organisms Mindarus abietinus Nuculaspis abietis

Infestation with significant damage

P r o p o r t i o n of infested beech leaves and fir needles with visually recognizable pathogenic organisms = ambient air.

More intense examinations were undertaken in 1986, especially on the small 'Fichtenquirlschildlaus' (Physokermes hemicryphus Dalm.), and details were presented by Ludwig (1989). The effect criteria used in the study were relatively independent of the time of first infestation. Table 1 shows the body areas of the imagoes, calculated as length x width of the insects, and Table 2 shows the number of the eggs and larvae found in the incubator bubbles. The results indicated that the insects in all three chambers with gaseous air pollution were significantly larger than in the chamber treated with TABLE 1 Body Area Imagoes o f P h y s o k e r m e s h e m i e r y p h u s Dalm. (mm 1) on Spruce

AP AP AP AP AP

OTC-version

Size (ram 2)

n

+s

n

Chi-Sq.

Signif.

(pH 4'0) (pH 5"0, 6"0) + 03 + SO/ + 0 3 + SOz

3"7 6"4 5"1 6"2 6'6

54 20 26 50 26

1"3 1"4 1"5 1"9 1'8

74 80 104 80

26"9 14"6 36"3 34"9

*** *** *** ***

* p = 0"05; ** p = 0"01; *** p = 0"001.

Visible injury responses

TABLE

359

2

Number of the Eggs and Larvae in Incubator Bubbles of Physokermes hemicryphus Dalm. on Spruce

AP AP AP AP AP

0 TC-version

Number o f eggs~larvae

n

+s

n

Chi-Sq.

SigniJi

(pH 4"0) (pH 5"0, 6"0) + 03 + SO2 + 0 3 + SO2

155 194 101 197 181

19 15 16 36 16

68 63 51 64 72

32 35 55 35

3-9 5"3 5-t 0"9

0"05* 0'02* 0"02* 0-3

* p =

0-05;

** p =

0-01;

***

p = 0.001.

TABLE

3

Relative Abundance of Different Homoptera in the Chambers ( - = Absent; + = Single Occurrence; + + = Medium to Strong Occurrence; + + + = Strong to Very Strong Occurrence) Control Control 0 3 + AP S02 + AP AP p H 4"0 AP p H 5"0 Beech Phyllaphis fagi (Callaphidae) Spruce Physokermes hemicryphus (Cocc., Lecaniidae) Physokermes piceae (Cocc., Lecaniidae) Nucalapsis abietis (Cocc., Diaspidae) Cinara pilicornis (Lachnidae) Fir Nucalaspis abietis (Cocc., Diaspidae) Cinara petinatae (Lachnidae) Dreyfusia nfisslini (Adelgidae) Mindarus abietinus (Adelgidae)

+++

+++

0 3 q- S O 2

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++

++

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_

++

--

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++

+++

+

+++

+

+

_

_

+ + +

+

++

+

++

+

+++

+ AP

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U. Arndt et al.

simulated acid precipitation (AP). Considering the results it looked as if the polluting gases had a positive impact on the growth of the insects but not on the number of eggs and larvae in all the treatments except the 0 3 + AP. In this treatment, the numbers were reduced. During the last vegetation period of the experiment, a general survey of the infestation of pests was carried out. Homoptera and their relative abundance in the different chambers are shown in Table 3. The initial low infestation of beech in the chambers with gaseous pollutants in 1986 was much greater in the final evaluation (Table 3). Physokermes hemicryphus abundance also increased in the spruce. Infestation generally increased during the last three years of the experiment, particularly in the beech. Experiments on beech were consequently omitted in the last year of the experiment. The trees, however, were left in the chamber to minimize disturbance to the conifers. In conclusion, a quantitative examination of the interactions of emissions, forest trees and forest insects can unobjectionably be carried out only in specially designed experiments. Still the empirical results presented here show the important influence of insects in the expression of visible injuries.

S Y M P T O M A T O L O G I C A L OBSERVATIONS In summer 1986 systematic measurements of the growth changes (Billen et al., this volume) and observations on the symptomatology of beech, fir and spruce were carried out in the Hohenheim experiment. Due to the numerous factors affecting foliar appearance and due to the deliberately realistic pollutant concentrations that were used not to cause acute effects, no specific cause by visible injuries was expected. However, the influence of pollution, biotic pathogens and possible nutritional deficiency on visible injury was determined (Fig. 2). During January to February 1985, after a long frost period with temperatures as low as - 25°C, clear injury symptoms on fir were observed in the chambers treated with SO z + A P and SO 2 + 0 3 + AP (Fig. 3). The development of tip necrosis on the individual trees was quite uniform. It occurred exclusively on the last annual set of needles, but did not lead to general abscission even though severely injured needles fell off later. After the concentrations were lowered from about 100~tg SOz/m 3 as a weekly mean value to about 40~tg/m 3 (Seufert & Arndt, 1988), normal sprouting followed in spring 1985. The occurrence of such symptoms as a reaction to the combined SO 2 and frost treatment was first observed by Wentzel (1956). This has also been observed in other locations polluted with SO2 (Wentzel, 1965; Keller, 1978;

Visible injury responses

361

VISIBLE INJURIES OCCUR

yes ~

prooffumigationeffects by microscopical, analytical, and other evaluation

check on pests and I fungi Y-~" I

I n° [ check nutrition status ] Y - ~

;no

@ Fig. 2. Proceedingprocesswith the symptomatologiceffectsanalysis(Billen,1988). Ranft et al., 1979; Michael et al., 1982). The air pollution apparently inhibits the development of frost resistance. As to how far the alteration of membrane permeability is of importance in this context has not been clarified completely. The development of visible injuries corresponds to an acute SO2 effect, similar to that observed during the summer (van Haut & Stratmann, 1970) and should not to be mistaken for the necroses that occur with dryness or potassium deficiency (Hartmann et al., 1988). Of the three species of trees observed, beech had the least ability to grow under the given experimental conditions. It showed increasing infestation of pests and various distinctly visible symptoms. Despite the previously mentioned counter measures the experiments had to be stopped in the fifth year. In 1986, the following types of alterations could be distinguished on beech leaves: (1) color shifts; (2) deformations; and (3) special visible injuries.

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Neither the color shifts nor the leaf deformations could unequivocally be attributed to one of the pollution treatments. The 'saw tooth pattern' of the beech leaf mentioned by Prinz et al. (1985) to be a typical visible ozone injury s y m p t o m has been observed in different treatments in Hohenheim, but was interpreted as a natural variation o f the leaves (Svoboda, 1972; Roloff, 1986). Quite often but predominantly in the chambers polluted with SO/, leaves developed a corrugated appearance and looked like the leaves o f ' H a i n b u c h e ' (Carpinus betulus) (Roloff, 1986). However, it was not considered to be a pollution effect. O f the 1745 leaves observed overall, special visible injuries were distinguished by five s y m p t o m types: (1)

(2)

(3)

(4)

Predominantly in the chambers polluted with 0 3 + AP, but also in the others, small white spots occurred in groups and rows on the adaxial side o f the leaves. This symptom, declared to be a typical visible s y m p t o m for ozone effects by Fliickiger et al. (1984), can probably be traced to the 'Buchenzirpe' (Typhlocyba cruenta H.S.) in the H o h e n h e i m experiment (Bucher & Landolt, 1985). Intercostal chlorosis oriented towards the leaf margins predominantly on older leaves was also observed in all the chambers and thus cannot be attributed to a specific pollutant effect. Corresponding with the s y m p t o m and a low potassium supply, a mineral deficiency is the likely cause of the s y m p t o m (Bergmann, 1986). A virus infection could be excluded. (F. Nienhaus, the Bonn University). Large necroses, oriented around the main vein of the leaf and towards the base, were observed in a number o f treatments. Butin (1983) describes this visible s y m p t o m as 'Blattbr~iune' of the beech triggered by the fungus Apiognomonia errabunda Rob., H6hn. Exclusively in the chambers polluted with 0 3 + AP, small brown to purple punctuous necroses were observed that were caused by collapsed palisade parenchyma cells. This points to an injury by ozone (Hill et al., 1961). The visible injuries occurred more often in the lower parts o f the tree.

Fig. 3. Frost injuries on the annual needle set of 1984 on fir in the chambers treated with SO2 + AP and SO2 + 03 + AP; normal sprouting followed after SO2 concentration was lowered. Fig. 4. Purple-brown, punctuous necroses on beech leaves from open-top chambers polluted with AP + 0 3 -~- S O 2. Fig. 5. The same leaves as in Figure 4 but with partially covered small leaves being presented completely here. Fig. 6. Chlorotic needles of the needle set of 1984 in the chambers polluted with AP + O 3 + SO2 at the time of the final evaluation.

Fig. 3

Fig. 4

Fig. 5

Fig. 6

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363

(5) Distinct reddish-brown, punctuous necroses similar to those mentioned under (4) were observed in the chambers polluted with SO2 + 03 + AP and the palisade parenchyma was injured, also. The symptoms occurred almost exclusively on leaves exposed to full sunlight (Fig. 4). Shaded portions of leaves did not show any symptoms (Fig. 5). The symptoms were apparently not due to weather damage during the season (Hartmann et al., 1988), but rather to the effects of ozone and sulphur dioxide + ozone. They resembled very much the visible injuries indicated by Bucher and Landolt (1985). The pollution combination 0 3 + SO 2 + NO2, used by the working team of Guderian (Klumpp et al., 1988) leads also to the same symptoms, but with the necroses appearing larger and darker. Thus, the visible injury can be attributed to an ozone-dominated effect on beech (Figs 4 and 5), even though a certain variability was unmistakable. A premature abscission of beech leaves, as sometimes observed in the field or artificially induced in poplars in fumigation experiments with ozone (Keller, 1986), could not be observed here. Compared to beech, the conifers generally exhibited less visible injury. A systematic survey in 1986, however, revealed various alterations in color and special visible symptoms (Billen, 1988). The fir showed, among other things, chlorotic spots and zones on the needles that were similar to those from other fumigation experiments (Guderian et al., 1985). But, it should also be stated that the symptoms were not restricted to the pollution treatments and that the nutrient supply was low (Sch~itzle et al., this volume). In this case, an unknown complex of factors contributed to the observed symptoms. The yellow coloring on the adaxial side of the spruce needles, often observed in the field and usually related to nutrient deficiency, intensive radiation and also ozone pollution (Prinz, 1983; Prinz et aL, 1984; Z6ttl, 1985; Rehfuess, 1987), was not observed in the Hohenheim Study. On the contrary, there was an incidence of Rhizosphaera kalkhoffi with visible injury symptoms similar to those observed in the forests (Seufert & Arndt, 1986). Various other injury symptoms also appeared on the fir needles (Billen, 1988). Of those, only the chlorotic needle tips shall be described here because they developed exclusively in the pollution treatments. The visible injuries already existing in 1986 in the annual needle sets of 1983 and 1984 became increasingly distinct until the final evaluation (Fig. 6). Such a symptom has been described as a chronic SO2 injury (van Haut & Stratmann, 1970) but has not been observed previously under the influence of A P + O 3. Light green needle tips can also be due to potassium or magnesium deficiency (Buchner, 1985; Bergmann, 1986). However, the nutrient supply in the unpolluted chambers (visible injuries did not occur)

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was lower than in the polluted ones (Schfitzle et al., this volume). According to Evers and Hildebrandt of the Forstliche Versuchsanstalt BadenWfirttemberg (pers. comm.) the analyzed contents alone should not lead to externally visible deficiency symptoms. For that reason, a combination effect of pollutant and poor nutrient conditions is thought to be the cause of injury. A 'photochemical bleaching' as suggested by Lichtenthaler and Buschmann (1983) or purely an ozone effect (Cowling et al., 1987) as the cause of this symptom is unlikely.

FINAL CONSIDERATION The interpretation of the symptomatological appearances has shown that the Hohenheim experimental conception could, due to its nature, only represent one type of geographic location. From the beginning, it was inadequate to answer all the questions concerning the dying forests. Pest infestation also confounded the experiment, and in some cases caused visible injury. This infestation obscured the treatment effects. While insect infestation studies were not planned initially, such an evaluation became part of the overall Hohenheim experiment as it progressed. It became evident that sucking insects in some cases were inciting injury in the sense of Manion (1981), as they led to externally visible symptoms. However, insect infestation was at sufficient level not to prevent other experiments, except for the beech during the last year of the experiment. Thus, the infestation can be considered 'field-like' and acceptable in the sense of the experimental set-up. Certain visible injuries as, for example, those produced at high elevations could not be reproduced here due to the chamber construction and the geographic setting (the necessary radiation was absent). Also, the use of a spruce clone though unknown in regard to its sensitivity to pollutants, was not expected to exhibit visible symptoms of injury and was used to keep the genetic variability among the individuals low. In fact, spruce did not show any external reactions. The visible injuries described for beech and fir have to be seen as valuable but rather accidental information for the aforementioned reasons. Yet it complements the results from the nutrition budget and the gas exchange appropriately.

REFERENCES Bergmann, W. (1986). Farbatlas--Erniihrungsst6rungen bei Kulturpflanzen. VEB Gustav Fischer-Verlag, Jena, 306 pp.

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Billen, N. (1988). Zuwachs und Symptomatologie yon Jungbfiumen nach definierter Immissionsbelastung in Open-top Kammern. Unver6ffentlichte Diplomarbeit aus dem lnstitut fiir Landeskultur und Pflanzen6kologie der Universitfit Hohenheim, 118 pp. Bucher, J. & Landolt, W. (1985). Zur Diagnose yon Ozonsymptomen auf Waldb~iume. Schweiz. Z. Forstwes., 136, 863-5. Buchner, A. (1985). Nadelverf~irbungen, die auf N~ihrstoffmangel beruhen. Forst. u. Holzwirt, 40, 297-85. Butin, H. (1983). Krankheiten der Wald- und Parkbdume. Georg Thieme-Verlag, Stuttgart, 172 pp. CEC (Commission of the European Communities) (1984). Diagnosis and classification of new types of damage affecting forests. Special edition of A FZ, 39, 20 pp. Cowling, E., Krahl-Urban, B. & Schimansky, Chr. (1987). Wissenschaftliche Hypothesen zur Erkl~irung der Ursachen. In Waldschdden. Dokumentation zur Ursachenforschung, ed. KFA-Jfilich, pp. 120-5. Fliickiger, W., Braun, S. & Fliickiger-Keller, H. (1984). Untersuchungen iiber Waldsch~iden in der Nordwest-Schweiz. Sehweiz. Z. Forstw., 135, 389-444. Guderian, R., van Haut, H. & Stratmann, H. (1960). Probleme der Erfassung und Beurteilung von Wirkungen gasf6rmiger Luftverunreinigungen auf die Vegetation. Ztsch. Pflanzenkrankh. und Pflanzenschutz, 67, 257-64. Guderian, R., Kiippers, K. & Six, R. (1985). Wirkungen von Ozon, Schwefeldioxid und Stickstoffdioxid auf Fichte und Pappel bei unterschiedlicher Versorgung mit Magnesium und Kalzium sowie auf die Blattflechte Hypogymnia physodes. VDI-Berichte, 560, 657-701. Hartmann, G., Nienhaus, F. & Butin, H. (1988). Farbatlas Waldschdden--Diagnose yon Baumkrankheiten. Ulmer-Verlag, Stuttgart, 256 pp. Hill, A. C., Pack, M. R., Treshow, M., Downs, R. F. & Transtrum, L. G. (1961). Plant injury induced by ozone. Phytopath., 51, 356-63. Jacobson, J. S. & Hill, A. C. (eds) (1970). Recognition of air pollution injury to vegetation. A pictorial atlas. APCA, Pittsburgh. Keller, Th. (1978). Frostsch~iden als Folge einer 'latenten' Immissionsschfidigung Staub-Reinhalt. Luft, 38, 24-6. Keller, Th. (1986). Ozon bewirkt vorzeitigen Blattfall. AFZ, 41, 73. KFA-J/ilich (Hrsg) (1987). Waldsch~iden. Dokumentation zur Ursachenforschung Jfilich, Germany, 137 p. Klumpp, A., Klumpp, G. & Guderian, R. (1988). Wuchsleistung und ~iuSere Sch~idigungsmerkmale bei Buche nach Einwirkung yon Ozon, Schwefeldioxid und Stickstoffdioxid. AFZ, 43, 731-4. Lichtenthaler, H. K. & Buschmann, C. (1983). Das Waldsterben, Verlauf, Ursachen und Konsequenzen. Fridericiana, 33, 39-66. Ludwig, W. (1989). Untersuchungen zur Populationsdynamik der kleinen Fichtenquirlschildaus (Physokermes hemicryphus DALM.) unter besonderer Beriicksichtigung der Fichtenerkrankung. Dissertation, Univ. Hohenheim. Malhotra, S. S. & Blauel, R. A. (1980). Diagnosis of air pollutant and natural stress symptoms on forest vegetation in Western Canada. Information Report NORX-228, Can. For. Service, 84 pp. Manion, P. D. (1981). Tree Disease Concepts. Prentice-Hall, Inc., Inglewood Cliffs, NY, 399 pp.

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Michael, G., Feiler, S., Ranft, H. & Tesche, M. (1982). Der Einflul3 von Schwefeldioxid und Frost auf Fichten (Picea abies L. Karst.). Flora, Jena, 172, 317-26. Prinz, B. (1983). Gedanken zum Stand der Diskussion fiber die Ursachen der Waldsch~iden in der Bundesrepublik Deutschland. Forst-u. Holzwirt, 38, 460-6. Prinz, B., Krause, G. H. M. & Jung, K. D. (I 984). Neuere Untersuchungen der LIS zu den neuartigen Waldsch~iden. Diisseldorfer Geobot. Kolloq. H., 1, 25-36. Prinz, B., Krause, G. H. M. & Jung, K. D. (1985). Untersuchungen der LIS Essen zur Problematik der Waldsch~iden. In Waldschiiden--Theorie und Praxis auf der Suche nach Antworten, ed. G. Von Kortzfleisch. Oldenbourg-Verlag, Miinchen, pp. 143-94. Ranft, H., Bellmann, CH., Feiler, S., Michael, G. & Tesche, M. (1979). Untersuchungen zum Zusammenwirken von Immissions- und FrosteinfluB im Fichtenrauchschadgebiet. Beitr. fd. Forstwirtschaft, 4, 160-5. Rehfuess, K. E. (1987). Sch~den an Fichten--Untersuchungsstandort Luchsplatzl. In Waldschdden. Dokumentation zur Ursachenforschung, ed. KFA-Jiilich, p. 72-5. Roloff, A. (1986). Morphologie der Kronenentwicklung von Fagus sylvatica L. (Rotbuche) unter besonderer Berficksichtigung m6glicherweise neuartiger Ver~nderungen. Ber. Forschungszentrums Wald6kosysteme/ Waldsterben, G6ttingen, 18, 265 pp. Seufert, G. & Arndt, U. (1986). Beobachtungen in definiert belasteten Model6kosystemen mit jungen Waldfiumen. AFZ, 41, 545-9. Seufert, G. & Arndt, U. (1988). Experiments on canopy/soil leaching effects of air pollutants in model ecosystems with forest trees. Geo. Journal, 17(2), 261 70. Skelly, J. M., Davis, D. D., Merrill, W., Cameron, E. A., Brown, H. D., Drummond, D. B. & Dochinger, L. S. (Eds)(1987). Diagnosing injury to Eastern forest trees. Publ. of Nat. Acid Precip. Ass. Program, Forest Response Program and Veg. Survey Res. Coop., 122 pp. Svoboda, A. M. (1972). Die Variabilit~it der Buchenbl~itter. Ceckoslovenska Akad. Ved, Studie 2, Prag. van Haut, H. & Stratmann, H. (1970). Farbtafelatlas iiber SchwefeldioxidWirkungen an Pflanzen. Verlag Giradet, Essen, 206 pp. Wentzel, K. F. (1956). Winterfrost 1956 und Rauchsch/iden. AFZ, 11, 541-3. Wentzel, K. F. (1965). Die Winterfrostsch~iden 1962-63 in Koniferenkulturen des Ruhrgebietes und ihre vermutlichen Ursachen. Forstarchiv, 36, 41-59. Z6ttl, H.W. (1985). Waldsch~iden und N~ihrelementversorgung. Diisseldorfer Geobot. Kolloq. H., 2, 3141.