Occurrence of pollution-sensitive epiphytic lichens in woodlands affected by forest decline: a new hypothesis

Flora (1999) 194, 159-168 http://www. urbanfischer.de/joumals/flora ©by Urban & Fischer Verlag

Occurrence of pollution-sensitive epiphytic lichens in woodlands affected by forest decline: a new hypothesis MARKUS HAUCK and MICHAEL RuNGE Albrecht-von-Haller-Institut fiir Pflanzenwissenschaften, Abteilung Okologie und Okosystemforschung, Universitat Gottingen, Untere Karspiile 2, D-37073 Gottingen, Deutschland Accepted: February 28, 1998

Summary Influences of forest dieback on epiphytic lichen vegetation in montane spruce forests of the Harz Mountains (Northern Germany) are described. The epiphytic lichen vegetation of damaged forest stands consists of more species and a higher number of endangered, loss toxitolerant lichen taxa than that of intact forest stands. Therefore, we tested the hypothesis that concentrations of potentially phytotoxic substances in stem flow are lower in damaged stands than in intact stands, because the higher needle surface of the latter should result in a higher pollutant interception. In accordance with this hypothesis, lower concentrations of the elements S, Zn, AI, and Mn and of hydronium ions were established in the stem flow of a damaged stand compared to an intact stand under identical climatological conditions. This phenomenon could be a precondition of the higher lichen diversity in the damaged stand. Key words: lichens, epiphytes, forest decline, stem flow, element content, coniferous forests

1. Introduction Atmospheric pollutants, mainly sulphur dioxide, nitrogen oxides, and the acids that form after the solution of these pollutants in water, have been identified as the cause of the widespread forest dieback in montane areas. Moreover, these pollutants are known to be responsible for a drastic reduction of lichen diversity in Central Europe. Thus, it seems reasonable to assume that montane forests damaged by air pollution should also be particularly poor in lichen species. But on the contrary, at a number of montane sites a richer epiphytic lichen vegetation was observed in heavily damaged forest stands than in less affected stands. In Germany this phenomenon has been found in the Bavarian Forest (MACHER & STEUBING 1984, 1986), the Alps (KasTNER & LANGE 1986), the Black Forest (BARTHOLMESS 1989, GLIEMEROTH 1990), and the Harz Moutains (Niedersachsisches Umweltrninisterium 1992). This observation caused some confusion in the discussion on the reasons for forest dieback. KasTNER & LANGE (1986) and GLIEMEROTH (1990), for example, concluded that the missing correspondence between the vitality of epiphytic lichens and their phorophytes indi0367-2530/99/194/02-159

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cates that reasons other than acid deposition should be responsible for forest decline. ELLENBERG (1996) took the occurrence of lichens on damaged trees as an indicator that the vitality of the phorophytes was underestimated. Two possible reasons for the occurrence of well developed lichen vegetation on damaged trees have been discussed. Nearly all authors dealing with this subject regard increasing light influx as an important factor (e.g. JOHN 1986, MACHER & STEUBLING 1986). A second important point is the presence of large amounts of decaying bark and wood in dying forest stands. As the decaying substrate was supposed to have a higher waterholding capacity than intact bark, the lichens should be more often and for longer periods in a hydrated state. With regard to these factors growth conditions for epiphytic undoubtedly become more favourable when threes die. But if pollution is a decisive cause of the general impoverishment of lichen flora in Central Europe, an improvement in light and water supplies does not sufficiently explain the phenomenon that lichen diversity, including species known to be pollution-sensitive, is higher in heavily damaged stands. Since the dependence of lichen diversity on pollutant deposition has been established in numerous investigaFLORA (1999) 194

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tions, there is no reasonable doubt that pollution is significant. Two explanations for higher lichen diversity in damaged as opposed to less damaged stands can be taken into account: either the effect of pollutants on lichens is modified by interactions with light and water relations, or lichens in heavily damaged stands are exposed to lower amounts of phytotoxic substances than they are in less damaged stands. We hypothesize that the latter is indeed of considerable importance. This is because stem flow is one of the main pollutant sources for epiphytic lichens on stems and large branches. And the amounts of pollutants in stem flow depend on the total needle (or leaf) surface that is effective in intercepting pollutants from the atmosphere. As this surface decreases with increasing damage, the amounts of pollutants in stem flow also should decrease. We are carrying out an investigation to test this hypothesis; results are presented below.

2. Study site The investigation area is located in the western Harz Mountains in southern Lower Saxony, Germany. The studies are carried out in stands of Norway spruce (Picea abies) on the ridge of the Acker-Bruchberg at an altitude of 790-820 m. The climate of the Acker-Bruchberg area is characterized by a yearly precipitation of 1400-1500 mm and a yearly mean temperature of 4-5 °C. Fog occurs on 130-200 days per year, a closed snow cover on 110-120 days per year (GLASSER 1994). Two stands were selected for investigating chemical parameters: a 145-year-old (in 1995) spruce stand heavily affected by the forest disease ("stand W") and a 156-year-old healthy or only slightly affected forest stand ("stand F"). The distance between these stands is 1 km. The trees in stand W show heavy needle loss; many trees are already dead. No systematic investigations about the causes for the different vitality of the two spruce stands were carried out. The substrate of the soil formation is quarzite in both cases, but the soil of stand F appears somewhat deaper.

3. Methods 3 .1. Vegetation mapping The distribution of epiphytes in healthy and heavily damaged spruce stands on the ridge of the Acker-Bruchberg was recorded. For this purpose, 16 sample plots, each 50 m long and 10 m wide, were selected. At these plots, the epiphytic vegetation on all spruce trees with a diameter at breast height of at least 15 em and a height of more than 5 m was mapped. Eight releves were made at each stem. The stems were divided into four plots based on the four directions NW, NE, SW, and SE. Furthermore, we differentiated between the base of the trees, a height of 0-100 em, and the middle part of the stem (100-200 em). 160

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In this manner a total of 1456 releves were made: 808 in healthy spruce stands on 101 trees and 648 in a heavily damaged spruce forest on 81 trees. The occurrence of all species of lichens, bryophytes and higher plants was registered as percentage of cover. If necessary (especially in the case of the genus Lepraria), thin-layer chromatography was used for species identification according to CuLBERSON & AMMANN (1979). The nomenclature of lichens refers to WIRTH (1994). Calculation of evenness is based on HAEUPLER (1982: 43).

3 .2. Chemical investigations Stem flow was collected on 10 trees from plot F and on 11 trees from plot W using polyurethane circular gutters painted with inert silicone. The water flowed into 10 1 polythene tanks. Incident rainfall was collected in two clearings near plots F and W. Polythene bottles placed at 1 m above the ground and with a collecting surface of 314 cm2 were used. The construction of all water collectors is based on the recommendations of MEIWES et al. (1984). Rainwater samplers were emptied weekly. Results reported here are from the period from July to November 1995. Stem flow and rain were filtered. Each sample was separately analyzed for pH, conductivity, and forNH/, N0 3-, total N, P, S, K, Na, Ca, Mg, Fe, Mn, Al, and Zn. Measurements of P, S, K, Na, Ca, Mg, Fe, Mn, and Al were taken with an Inductively Coupled Plasma Atomic Emission Spectrophotometer (ICP). Zn was analyzed with an Atomic Absorption Spectrophotometer (AAS). NH/, N0 3- and total N were determined colorimetrically based on BLANCK (1991: 85). Organic N is calculated as total minus inorganic N.

4. Results 4.1. Distribution of epiphytes In the 1456 releves, a total of 30 lichen species, 13 bryophytes, and two vascular plants were found. In the intact forests (F plots) 21 epiphytic lichen species were detected; 26 species were found in the heavily damaged stands (W plots). Six of the 30 lichen species have a significantly higher constancy in the type F releves (Table 1). All these species are widespread on acid bark in Central Europe. They are unthreatened in Germany and known as very toxitolerant (WIRTH 1992, 1995, WIRTH et al. 1996). These taxa either have a crustose thallus, or they belong to the genus Cladonia. 16 lichen species have a significantly higher constancy in the W plots. Among these, a number of foliose and fruticose lichens with only moderate toxitolerance are found. Two of the 16 species are threatened in Germany according to WIRTH et al. (1996) and five are regarded as endangered in Lower Saxony (HAUCK 1992). Three species restricted to the heavily damaged forest stands (i.e.

Table I. Epiphytic lichen vegetation in the heavily affected spruce stand W compared to the healthy (or only slightly affected) spruce stand F. Constancy [%] F

w

Mean cover [%]

x2

w

F

Species with higher constancy in stand F : Chaenotheca ferruginea Cladonia digitata Cladonia polydactyla Hypocenomyce caradocensis Hypocenomyce scalaris Micarea botryoides

2.35 56.06 49.01 45.05 10.02 3.22

0.00 19.75 30.71 0.00 0.31 0.00

15.44 197.72 49.83 389.23 63.15 21.23

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

0.00 8.42 67.57 0.00 0.00 0.00 0.50 33.29 0.00 0.25 6.93 0.00 0.12 0.50 0.12 0.00

0.62 37.04 72.99 0.77 2.93 7.10 18.67 38.27 4.48 21.60 32.41 1.08 3.09 14.51 8.95 2.31

5.00 176.62 5.03 6.26 24.00 59.23 151.41 3.89 36.90 186.36 156.32 8.77 22.20 112.45 77.06 18.90

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

0.12 2.10 0.12 0.00 99.75 1.61 0.37 0.00

0.46 1.85 0.00 0.15 100.00 1.54 0.31 0.15

1.51 0.12 0.81 1.25 1.61

0.6± 0.4 8.4 ± 8.6 4.4± 7.0 5.9± 8.8 2.2 ± 5.4 0.6± 0.3

3.4 ± 6.8 *** 5.0± 7.7 0.8 ± 0.4

Species with higher constancy in stand W: Cetraria chlorophylla Cladonia pyxidata s.l. Hypogymnia physodes Hypogymnia tubulosa Lecanora symmicta Lecidea cf. hypopta Lepraria elobata Lepraria jackii Lepraria rigidula Mycoblastus fucatus Parmeliopsis ambigua Parmeliopsis hyperopta Placynthiella icmalea Platismatia glauca Pseudevernia furfuracea Vulpicida pinastri

1.5 ± 1.9 1.5 ± 3.5

1.0 ± 0.7 1.1 ± 1.7 0.5 ± 0.0 0.5 ± 0.1 2.0 0.6±0.3 0.5

0.5 ± 0.0 5.2± 5.9 *** 5.0± 9.0 *** 3.2± 2.5 0.6 ± 0.4 2.9 ± 6.7 6.9 ± 10.5 7.6 ± 11.8 *** 4.8 ± 7.5 2.1 ± 5.5 2.2± 5.7 *** 1.0 ± 0.7 1.7±2.4 2.5 ± 5.4 2.4 ± 6.7 0.5 ± 0.0

Indifferent species : Bryoria fuscescens Cladonia coniocraea Cladonia macilenta s.l. Hypogymnia farinacea Lecanora conizaeoides Micarea prasina Trapeliopsis flexuosa Trapeliopsis granulosa

O.oi 0.04 1.25

0.5 0.7 ± 0.5 5.0 44.9± 22.1 0.7 ± 0.2 0.5 ± 0.0

0.5 ± 0.0 1.5 ± 2.0 0.5 25.2± 20.2 *** 0.6±0.2 0.5 ± 0.0 0.5

Constancy: Presence in the releves from stand F (n = 808) or stand W (n = 648). Statistics: Chi-square test. Mean cover: Arithmetic mean± standard deviation, calculated from all releves with occurence of respective species. Statistics: U-test. Levels of significance: * p ~ 0.05; ** p ~ 0.01; *** p ~ 0.001.

Hypogymnia farinacea, Parmeliopsis hyperopta and Vulpicida pinastri) show a boreal-montane distribution. With regard to cover, tendencies can be observed that are very similar to the constancy data. Except for the extremely toxitolerant crustose lichen Lecanora conizaeoides, all lichen species showing a significantly higher cover in one of the spruce stands also occur with a significantly higher constancy in the respective stand.

L. conizaeoides, which is predominant in the epiphytic vegetation in nearly all releves, covers 45% on average in the F plots, but only 25% in the W plots. Relating the cover of L. conizaeoides to the area of the bark surface overgrown with epiphytic lichens, the mean percentage of cover is 77% in the F plots and 68% in the W plots (Fig. 1). The moderately toxitolerant Hypogymnia physodes is ranked second after Lecanora conizaeoides in the W plots, covering 10% of the epiFLORA (1999) 194

161

Lecanora conizaeoides 77 %

F

- - - others 2 % ------. Hypogymnia physodes 1 % Cladonia polydactyla 3 % " Y - - - - Hypocenomyce caradocensis 8 % "":7"'-----

w

Cladonia digitata 8 %

Lecanora conizaeoides 68 %

- - - others 2 % Cladonia digitata 1 % Parmeliopsis ambigua 2 % Lepraria elobata 3 % Cladonia polydactyla 3 % '------- Cladonia pyxidata s. I. 5 % ~"'------- Lepraria jackii 5 %

=------------.

Hypogymnia physodes 10 %

phytic lichen vegetation. In the F plots, H. physodes forms only 1% of the epiphytic lichen vegetation. In the W plots eight lichen taxa cover at least 1% of the bark surface inhabited by lichens, whereas in the F plots this criterion is met by only five species. This means that more taxa are predominant in the W plots than in the F plots. This fact is expressed by the evenness as well. The evenness of the epiphytic lichen vegetation is E = 61 at the F plots and E = 81 at the W plots. According to HAEUPLER (1982), the higher evenness in theW plots, together with the higher number of species, can be regarded as a measured of major ecological diversity of the lichen vegetation. Bryophytes play a significantly less important role within the epiphytic vegetation of the spruce forests in the Harz Mountains. In the 1456 releves, only 13 species were observed: six liverworts and seven mosses. None of them belongs to the obligatory epiphytes. Instead, all these taxa usually prefer tree trunks, decaying wood and soil as habitats (KoPERSKI 1993). The most frequent species were Tetraphis pellucida, Lepidozia reptants, Calypogeia muelleriana, Dicranumfuscescens and Plagiothecium curvifolium. Bryophytes were recorded in 31% of the releves in the F plots and in 26% of the releves in theW plots. In only seven cases, bryophytes were noticed in the releves at a height of 100-200 em. 162

FLORA (1999) 194

Fig. 1. Average cover of lichen taxa comparing F and W plots. Cover of single species as percentage of mean total cover of all epiphytic lichens.

4.2. Quantities of precipitation Incident precipitation varied between 0.0 and 7.21 m-2 week- 1 (Fig. 2). No significant differences were observed between the study sites F and W. In contrast to incident rainfall, stem flow differs considerably between both sites (Fig. 3). The arithmetic mean of the stem flow volume is 1364 ± 719 ml per tree- 1 week- 1 in stand F, whereas it is 2383 ± 1618 ml tree- 1 week- 1 in stand W. The maximum values in both stands amount to 101 tree- 1 week- 1•

4.3. Element content Results from analyses of incident precipitation are compiled in Table 2. Nitrogen is the most common element in incident rainwater, followed by sulphur. The contents of the analyzed nitrogen compounds decrease in the order [N03-] > [NH/] >[organic N]. Phosphorus occurs in much lower concentrations than the other nonmetallic macronutrients. Among the alkaline cations, sodium and calcium prevail over magnesium and potassium. Aluminium, zinc, iron and manganese contents are considerably lower. The pH values of the incident rain range from 4.1 to 6.4; the mean value of all samples is pH= 4.71.

8000.-----------------------------------------------, 7000 6000

1:

sooo

+------------------------------------------------------------------------------------------------------------------~

::::

E 4000 +----------·--------------·-------------------·--------------------·-----lliiD••-·----------·-·--3000 +---·------------------------------·--·-·---------------·----•t±12000 -+--------···-·--------------·----·---------------·----1000 0 I'0 I'-

r-.: r-.:

0

C! .q: .,....

N

M

cO cO cO cO a) a) a) a) ci ci ci ci ci .,.... 0 0 C! 0 0.q: .,.... C! 0 0 cO r-.: .q: N.,.... C'\i a) cO C") ci cO .,.... co ..0 0 0 M 0 0 0 N N N

C")

Fig. 2. Incident precipitation during the investigation period.

Date 8000~--------------------------------------.

1: 6000

~

G> ~

4000 ----------

----

--------- --

E ---------------------- r------

I oci cO cO 0 C! 0 oci ..r ..N

N

I

.

Jl

...

.,.... ...ci ci ci ci ci .....C") ..- oci ..0 N a> cO ci c.O C")

a> 0a> a> a> 0 0

0

..r 0

..-

N

0

0

Date

These results accord well with those from other investigations in the Harz Mountains (HAUHS 1989, BOUMAN 1991, ANDREAE 1993). Differences between incident precipitation from site F and site W were statistically insignificant in all cases. Element contents of stem flow are much higher than those of incident precipitation (Table 3). The average conductivity of rainwater is 29.3 ~-tS/cm, whereas it is 204 ~-tS/cm (stand F) and 158 ~-tS/cm (stand W) in stem flow. The proportions of the concentrations of the chemical substances are on the whole similar to those in incident rainwater. Nitrogen is the most important element. In contrast to incident precipitation, nitrate has the smallest share in the nitrogen compounds, whereas ammonium has the highest. The sulphur contents are lower than those of nitrogen; the values for phosphorus are 5-10 times below the concentrations of nitrogen and sulphur. Calcium and sodium predominate over magnesium

N

M

0

Fig. 3. Stem flow during the investigation period.

but, not over potassium, as in incident precipitation. Potassium occurs approximately in the same concentrations as calcium and sodium. As in the incident precipitation, the amounts of aluminium, iron, manganese and zinc are significantly lower than the contents of the other metals. Eight chemical parameters of stem flow, in contrast to those of incident precipitation, exhibit statistically significant differences between stands F and W. The concentrations of sulphur, potassium, calcium, manganese, aluminium, zinc, hydronium ions and conductivity are higher in stand F than in stand W. The element doses in the stemflow, calculated for the investigation period, show similar trends as the concentrations (Table 4). But in contrast to the concentrations, the doses do not differ significantly between the stands in most cases. Therefore, they will not be discussed separately, until data from a longer investigation period are available. FLORA (1999) 194

163

Table 2. Element content of incident precipitation in the heavily affected spruce stand W compared to the healthy (or only slightly affected) spruce stand F. Concentrations [[lmol/l] calculated from weekly measurements over the whole investigation period from July to November.

w

F N (total) [[lmol/l] NH 4+ [[lmol/1] N03- [[lmol/1] N (org.) [[lmol/l] P [[lmol/1] S [[lmolll]

108.5 ± 68.0 36.08 ± 36.98 44.54 ± 33.79 27.90± 17.25 2.79± 2.35 64.33 ± 30.59

92.81/69.25 42.12159.97 44.98/41.41 20.70/22.13 2.12/3.65 55.94/23.57

123.2 ± 61.5 41.41 ± 41.84 50.95 ± 32.60 30.80± 17.10 8.34 ± 12.45 82.22 ± 44.79

115.7149.3 38.91/60.69 43.55133.56 23.56123.56 2.4615.69 59.56/58.22

K [[lmol/l] Na [[lmol/l] Ca [[lmol/1] Mg [[lmol/1]

4.32± 5.72 56.78 ± 33.91 29.86 ± 38.51 6.58 ± 3.20

3.6714.76 52.12!46.03 15.95/10.98 6.25/4.74

6.18 ± 6.92 87.59 ± 85.30 22.79 ± 28.02 6.28 ± 5.02

3.93/4.41 51.55148.50 15.46/20.03 4.7516.01

Fe [[lmol/l] Mn [[lmol/1] AI [[lmol/1] Zn [[lmol/l]

0.373 ± 0.469 0.099 ± 0.269 0.712 ± 1.553 0.384±0.164

0.000/0.590 0.000/0.000 0.000/0.000 0.398/0.245

0.384 ± 0.495 0.102 ± 0.376 0.397 ± 1.033 0.393 ± 0.180

0.000/0.780 0.000/0.000 0.000/0.000 0.413/0.253

pH Conductivity [[lS/cm]

4.63 ± 0.31 28.35 ± 12.90

4.70/1.14 23.40/22.30

4.82± 0.58 30.29 ± 16.30

5.28/1.52 23.70127.30

Normal: Arithmetic mean ± standard deviation. Italics: Median!interquartile range. All differences statistically insignificant (p::;; 0.05; median test; U-test, in cases of normal distribution [N (total), N0 3-, Zn, pH] t-test). Number of samples (varying between parameters): n(F) = 13-15; n(W) = 14-15.

Table 3. Element concentrations in stem flow in the heavily affected spruce stand W compared to the healthy (or only slightly affected) spruce stand F. Mean values are calculated from means of 10 trees in plot F and 11 trees in plot W (investigation period from July to November 1995; weekly measurements).

w

F N (total) [[lmol/1] NH4+ [[lmol/l] N0 3- [[lmol!l] N (org.) [[lmol/1] P [[lmol/1] S [[lmol!l]

623.9 ± 324.3 318.6 ± 282.1 147.2 ± 45.8 156.0 ± 47.1 35.32 ± 20.06 599.4 ± 156.3

526.91378.7 218.1/210.7 137.2145.43 163.6/42.4 28.10/23.43 576.61184.6

906.2 ± 975.6 603.2 ± 873.7 136.5 ± 91.8 199.3 ± 199.1 69.16± 101.41 338.2 ± 106.0

K [[lmol/l] Na [[lmol!l] Ca [[lmol/1] Mg [[lmol/1]

228.4 ± 71.3 226.9 ± 55.8 262.2± 69.2 71.46 ± 25.83

227.21100.5 222.11104.1 252.11128.8 76.36/10.55

130.5 ± 109.6 256.3 ± 151.2 189.3 ± 64.7 70.59 ± 25.96

Fe [[lmol/1] Mn [[lmol/l] AI [[lmol/1] Zn [[lmol/1]

11.52 ± 6.67 10.61 ± 4.15 15.47 ± 3.28 4.05 ± 1.08

9.68/5.62 10.44/3.11 15.56/3.24 3.6611.48

8.38 ± 11.18 3.54 ± 1.76 10.16 ±4.78 2.40± 0.39

pH Conductivity [[lS/cm]

3.76± 0.08 203.6± 65.0

3.7610.11 177.2154.0

4.17 ± 0.08 157.5 ± 85.9

*** (n) * * (n) *** ** (n) *** (n) *** (n) *

Normal: Arithmetic mean ± standard deviation; statistics: U-tests; in cases of normal distribution (n): t-test. Italics: Medianlinterquartile range; statistics: median test. Levels of significance: * p::;; 0.05; ** p::;; 0.01; *** p::;; 0.001. Number of samples (varying between parameters): n(F) = 109-129; n(W) = 123-153. 164

FLORA (1999) 194

554.3/660.1 289.0/392.7 90.041173.33 139.0/154.8 32.67!65.18 356.41194.3 76.231117.46 197.01110.3 171.8170.6 65.53/24.41 6.2315.42 3.2112.23 10.38/6.30 2.46/0.66 4.14/0.31 139.0149.5

*** **

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

Table 4. Element content of stem flow in the heavily affected spruce stand W compared to the healthy (or only slightly affected) spruce stand F: accumulated doses [~mol tree-']. Doses are calculated as mean values of 10 trees in plot F and 11 trees in plot W over the whole investigation period from July to November 1995 (weekly measurements).

H+ [~mol] N (total) [~mol/1] NH/ [~mol/1] Non~mol/1]

N (org.) [~mol/!] P [[!mol/1] S [[!mol/1]

F

w

6071.2 ± 3582.1 7880.9 ± 3960.5 2742.6 ± 1802.6 2289.5 ± 1032.3 2 848.8 ± 1596.6 253.0 ± 137.8 14920.4 ± 9104.6

3248.7 ± 3489.7 13493.9 ± 7707.9 6597.1 ± 5405.9 3644.8±3281.5 3 252.0 ± 1729.6 553.8 ± 454.3 8298.5 ± 5272.3

K [[!mol/1] Na [[!mol/1] Ca [[!mol/1] Mg [[!mol/1]

4377.6 ± 2281.5 3 342.8 ± 1906.7 2397.0 ± 571.5 1679.2 ± 1126.3

Fe [[!mol/1] Mn [[!mol/!] AI [~mol/1] Zn [~mol/!]

284.3 ± 142.2 301.0 ± 254.2 425.3 ± 212.7 65.71 ± 46.83

Arithmetic mean ± standard deviation. Levels of significance: normal distribution (n): t-test).

5. Discussion 5.1. Modification of the chemical properties of precipitation in the canopy An enrichment of substances during the passage of rainwater through the canopy is a well-known phenomenon that has been observed by many authors (e.g. NIHLGARD 1970, ULRICH 1986, MATZNER 1988). Two processes are responsible for higher concentrations in stem flow as opposed to incident precipitation: interception of aerosols and gases, and leaching from needles and bark. In addition, the concentration of intercepted substances in stem flow varies due to evaporation from the tree crowns. Leaching strongly influences the contents of potassium, manganese and zinc (NIHLGARD 1970, FASSBENDER 1977, ScHMIDT 1987). The significance of leaching in modifying the amounts of calcium and magnesium is controversial (FASSBENDER 1977, GoDT 1986, BREDEMEIER 1987, MATZNER 1988). With respect to the remaining elements analyzed all authors agree that leaching is not of considerable importance. Sulphur is the element for which interception deposition most strongly influences stem flow. In a spruce forest in North Rhine-Westphalia, GoDT (1986) noticed an interception deposition of sulphur that was 3-6 times the amount deposited by incident precipitation. Since it occurs above all in very small particles, which stay in the atmosphere longer than those with a higher mass,

(n) (n) (n)

*

(n)

2987.6 ± 2999.7 4855.0± 3950.4 1521.0 ± 1351.3 * 1993.2 ± 1626.4 (n) 444.3 ± 1 050.2 106.3 ± 92.5 229.2 ± 176.7 71.70 ± 38.67

* * * (n)

* p :s; 0.05 (U-test; in cases of sulphur is the most common element in aerosols in Central Europe (HOFKEN 1983, GoDT 1986). Futhermore, it is adsorbed as sulphur dioxide from the gaseous phase. Several processes could be responsible for the observation that the order of concentrations of nitrogen compounds changes from [N03-] > [NH/] > [org. N] in the incident rainwater to [NH/] > [org. N] > [N03-] in the stem flow. Interception deposition of ammonium could be higher than that of nitrate, or nitrate could be assimilated by the needles of Norway spruce or by mircoorganisms. Even a reduction of nitrate by bacteria using nitrate as a terminal inorganic electron acceptor is conceivable, but - as these microbial processes only take place in anoxic habitats - only as an artefact in the water samplers.

5.2. Influence of forest dieback on the element content of stem flow As the element contents of stem flow depend on interception as well as on leaching, and as both processes depend on total needle (or leaf) surface, element concentrations in stem flow should change parallel to the leaf area index of forest stands. The significantly lower concentrations of sulphur, potassium, calcium, manganese, aluminium, zinc, and hydronium ions in the stem flow from stand W confirm these considerations. In this context it is interesting that FLORA (1999) 194

165

110 100 ::::::: (/) 900 0 800 E .= 700 s: 600 0 <+= 500 E 400 Q) 300 ( /) 200 100 0 ~

-

•

w

F

• •

• •• ~ • •• • • • 50

100

150

0

50

100

150

200

Incident precipitation [IJmol S/1]

Fig. 4. Correlations between sulphur content of stem flow and incident precipitation.

sodium shows a (statistically insignificantly) higher total content in the stem flow from stand W than in that from stand F. By diffusion into the needles, sodium is kept in the canopy of Norway spruce (FASSBENDER 1977, GoDT 1986). Correspondingly, the amounts of sodium in stem flow increase with a reduction of leaf area. Moreover, a reduced number of trees per area may be responsible for lower concentrations of many chemical substances in the stem flow of damaged forest stands. Under such conditions, precipitation may directly reach not only the very tops of the trees, but larger parts of the crowns and even the trunks. In this way, element concentrations could be further reduced. Additional support for the above considerations comes from examining of correlations between characteristics of incident precipitation and stem flow. The amounts of sulphur, sodium and aluminium in the stem flow are significantly correlated with those in the incident precipitation in stand W, but not in stand F. The same is true for the pH value and conductivity. The correlation coefficient for the relationship between the volume of stem flow and the quantity of incident precipitation is r = 0,95 (p:::;; 0,001) in stand W and r = 0,74 (p:::;; 0,001) in stand F. Higher correlation coefficients in the damaged spruce forest as compared to the healthy stand indicate that these variables are less intensively modified at site W than at site F during the passage through the canopy. As mentioned above, interception deposition is of special importance in the case of sulphur. Correspondingly, the correlation coefficients are particularly different between the sites F and W (Fig. 4). In stand W the sulphur content in the stem flow is closely linked with that in the incident precipitation (r =0,92/p:::;; 0,001). In stand F sulphur content of the precipitation is influenced to such a high degree by interception that no correlation exists (r = -0,08). 166

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5.3. Possible effects on the lichen vegetation The occurrence of many lichen species is restricted to a certain pH range. Today, the prefered pH ranges of the majority of European lichen taxa are well-known (WIRTH 1992). Numerous field studies have shown that acid pollutants as well as alkaline dusts cause a considerable change in lichen vegetation as well as a pH change of the substrate (NASH & WIRTH 1988, WITTMANN & TURK 1988). TURK & WIRTH (1975) showed experimentally that the damaging effect of sulphur dioxide increases with a diminishing pH of the substrate. The buffer capacity of the bark of coniferous trees is generally low (BARKMAN 1958). Thus, even small differences in the pH of stem flow can considerably affect lichen vegetation. Apart from pH, sulphur and zinc are known to affect lichens. Toxic effects of sulphur compounds belong to the best investigated aspects of lichen biology. so2 is injurious to lichens, as are SO/-, HS0 3-, H 2S0 3 and S2 0 52-. The deposition of sulphur compounds results in an enhanced membrane permeability, the disturbance of the conformation of proteins, and the inhibition of photosynthesis, respiration and nitrogenase activity (KERSHAW 1985). The toxicity of zinc to lichens has been proven by BECKETT & BROWN (1983), BROWN & BECKETT (1983) and MARTI (1985). These authors performed short-time experiments with zinc concentrations higher than those we found in stem flow. MARTI ( 1985) established that, after exposing isolated mycobionts and photobionts to a solution of 4 mM zinc sulphate for 16 hours, only the mycobionts, and not the photobionts, were considerably affected. Exposing the lichen bionts to a lower concentration of zinc for a longer period (250 ftM; 20 days) significantly reduced even carbon dioxide fixation and growth of the photobionts. These results point to the

possibility that the lower zinc concentrations occuring in the stem flow also could damage lichens, as zinc is permanently present. BARKMAN (1958: 99) discussed the possibility that the generally poorer epiphytic vegetation of coniferous trees compared with that of the majority of deciduous trees could be due to a high manganese deposition. But experimental evidence is missing (KERSHAW 1985, NASH 1996). The same is true of aluminium. As this element is toxic to vascular plants, however, a detrimental effect on lichens can at least be supposed. The fact that all elements demonstrably toxic to lichens or for which a toxicity can be supposed (i.e. H+, S, Zn, Al, Mn) are less concentrated in the stem flow of the heavily damaged spruce stand W than in stand F could be a decisive precondition for the higher lichen diversity in the former.

6. Conclusions Our results support the hypothesis that a reduction of the needle area in montane spruce forests caused by acid pollutants leads to lower pollutant concentrations in stem flow. It remains to be investigated whether the reduced concentrations of potentially phytotoxic substances contribute to the occurrence of less toxitolerant lichen species, and, thus, to a higher lichen diversity. In order to test this assumption, selected lichen species will be exposed experimentally at the site to higher concentrations of hydronium ions, sulphur, zinc, aluminium, and manganese. Moreover, investigations of the chemical composition of precipitation and stem flow will be continued for at least two more years. Single precipitation events will be considered for a more detailed picture of temporal concentration variations. The spatial heterogeneity of physical and chemical substrate properties as well as of microclimatic conditions will be studied in order to find out whether - and possibly to what extent - these parameters contribute to lichen diversity.

Acknowledgements We thank the administration of the Harz National Park, especially M. HuLLEN, for permission to carry out our field studies in the National Park. Dr. N. LAMERSDORF of the Institut ftir Bodenkunde und Waldemahrung, Gottingen University, supported us with the chemical analyses. Dr. G. GuNTER (Hamburg) helped with statistics and illustrations.

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