THE EFFECT OF SIMULATED ACID RAIN ON FEATHER MOSSES AND LICHENS OF THE BOREAL FOREST.

T.C. Hutchinson, M. Dixon and M. Scott

Department of Botany, University of Toronto, Toronto~ Ontario, Canada M5S IAI

ABSTRACT. Permanent field plots containing a dominant ground cover of

feather moss (Pleurozium schreberi) and the forage lichen, (Cladina), were established in mature, boreal forest jack pine stands to monitor the effects of simulated acid precipitation. For a five-year period

commencing in 1981, bimonthly sprays (pH range 2.5 to 5.6) were given throughout the growing season. The feather moss wefts were extremely sensitive to simulated rains of pH 2.5 and 3.0; but loss of cover and frond blackening were also observed at pH 3.5. The pH 2.5 treatment killed almost all of the Pleurozium, while the cover remaining in the pH 3.0 treatment after 5 years was reduced by 44%. In laboratory studies

designed to compare the effects of H2S04, HNO 3 and a 2:1 mixture of both, microcosms sprayed with H2SO 4 alone (pH 3.0) were more signifi- cantly affected than fronds treated with HNO 3 alone or pH 5.6 sprays of any ratio. Although less sensitive than Pleurozium, field-sprayed lichens were also visibly damaged. At pHs less than 3.5, C. stellaris and C. rangiferina had reduced podetial height and dry weight; while C. mitis was affected by a combination of the acid rain treatment and other associated factors. While ambient rains of pH 4.2 may not in themselves be harmful to the boreal ground flora, it is apparent that the feather mosses and lichens, lacking a cuticle and true roots, are very sensitive to occasional, extremely acidic rain events.

i. INTRODUCTION

Large areas of northern Canada, the USA, Soviet Siberia, Scandinavia and upland U.K. are covered with boreal forest. In these forests, coni- ferous trees of the genera Picea, Pinus, Abies and Larix predominate. Much of the ground of these forests is covered by dense growths of feather mosses or, in drier, better-illuminated sites, by caribou-forage lichens (Kershaw 1977). In eastern North America, rains of mean pH 4.0 to 4.5 now fall on these forests and on the potentially acid-rain sensi- tive lichens and mosses. Since these lower plants largely lack a cuticle and have no true roots, their cells and chloroplasts are more exposed to direct effects of acidity and leaching. Therefore, deleterious effects on these plants may well show UP before effects on higher plants. In this study, the effect of simulated acid rains on growth, survival and

Water, Air, and Soil Pollution 31 (1986) 409-416. (~) 1986 by D. Reidel Publishing Company.

410 T.C. HUTCHINSON ET AL.

persistence of the feather moss Pleurozium schreberi and the common boreal lichens Cladina stellaris, C. rangiferina, and C. mitis in

northern jack pine (Pinus banksiana) forests is reported.

2. METHODS

Experimental acid rain sprays were made on 5 m x 2 m plots of jack pine forest dominated in the ground flora by either Pleurozium schreberi or in drier sites by the three Cladina species listed above. Sprays equi- valent to a precipitation depth of 2.5 cm were made bimonthly throughout the growing season from June - October in each of five years, 1981-85. Spray treatments of pH 2.5, 3.0, 3.5, 4.0 and 5.6 were applied to each of three replicate 5 m x 2 m plots at each of two sites. Overall, the summer sprays added approximately 30% to the annual growing season precipitation at the sites. 'Rain' was made up using deionised water,

with a 2:1 molar ratio of H2SO 4 to HNO 3 acid and sprayed on to the plots through a plastic garden ho§e With a p±astic diffusion nozzle. In addition, a set of non-spray plots was set up. Various parameters of soil chemistry, stem-flow, throughfall and species persistence and per- formance have been measured and analysed on a monthly or annual basis. We report here on the data for cover of the moss and bare ground esti- mated in August of each year by T.C. Hutchinson. Estimates were made on the same three sub-plots, each 1 m x 1 m, at each of the three replicate

plots - i.e. each cover value is the mean of nine separate estimates. The ground occupied by a vertical projection down was estimated as a percentage for each species. For the mosses only green or yellow fronds were counted in cover and blackened or dark brown shoots were considered dead. For the lichens, podetial height was measured on 30 random indi- viduals (I0 per plot), for each of the six pH treatments. Both total podetial height and height of the living, photosynthetically active portion of the podetium were measured. Measurements were also made of the length of internodes for 15 individuals, i.e. 3 replicate plots x 5 podetia per plot, selected randomly from the original 30. Podetial tips, mid-portions and bases were considered to be the combined heights of internodes 2 and 3, 4 and 5, and 6 and 7, respectively. Bases were already initiated prior to the spraying and have received the longest exposure to the simulated rain events.

A short-term field experiment was performed which compared the effect on P. schreberi of simulated sprays of pH 3.0 and 3.5 (which represent approximately 1% and 7% of the rain events in eastern Ontario) with sprays of pH 5.6. The "throughfall" solutions, consisting of deionized water plus nutrient salts to simulate throughfall of a

mature jack pine canopy, were acidified with a 2:1 molar ratio of H2SO 4 to HNO 3. Seventeen "microcosms", consisting of 30 cm diameter x 30 cm PVC pipes were inserted into the forest floor giving an intact core of moss and soil horizons H, A and B, were randomly located in an area of uniform feathermoss growth. The microcosms were sprayed twice a month from May to October 1983 and 1984. Each spray was equivalent to 0.7 cm of precipitation which, with the additional 8-10 cm of ambient rain,

THE EFFECT OF SIMULATED ACID RAIN ON FEATHER MOSSES AND LICHENS 411

totalled 9.5-11.5 cm of rain per month. Cotton-polyester threads,

serving as growth markers, were tied 0.5 cm from the tip of 35 fronds

per microcosm at the beginning of the experiment. Chlorophyll and ele- mental composition data are reported elsewhere (Dixon 1986).

In addition, a 6-week laboratory experiment was performed to com- pare the effects of different acids on P. scherberi. Fifteen microcosms were removed from a location adjacent to the field site and taken to the University of Toronto greenhouse in October 1984. Five treatments were sprayed daily to a depth of 4.0 cm. Treatments consisted of simulated

throughfall solutions acidified to pH 3.0 with H2S04, HN03, a 2:1 molar ratio of H2SO 4 to HNO 3 or to pH 5.6 made up with the 2:1 mixture. In addition, the last two mixtures (pH 3.0 and 5.6) were sprayed on alter- nate days.

Chlorophyll contents of three samples of the top one cm sections of all fronds from each microcosm were determined using a DMSO - 80% acetone extraction (Dixon 1986).

3. RESULTS AND DISCUSSION

3.1. Effects on Mosses

The acidic sprays in the 2 m x 5 m plots had rapid and severe effects on the growth and survival of Pleurozium schreberi (Table I). By the end

of 1981 the cover of the moss sprayed with four, twice monthly sprays of pH 2.5 was reduced to 66.7% from approximately 85%. The fronds looked yellow or yellow-brown compared with the healthy green of the pH 3.5 or higher treatments. By August, 1982 the lowest pH treatment had had a devastating effect, with all but 1.6% of the cover destroyed. At this time, both sprays of pH 3.0 and 3.5 also had had significant effects. By 1985, pH 3.0 and pH 3.5 had caused Pleurozium to decline from 71% to 40% and from 84% to 64%, respectively. Moss treated with pH 3.0 actually showed some recovery in the very wet 1983, with survival and regeneration of certain, presumably more tolerant, individuals. Acid effects involved destruction of chlorophyll, a blackening of fronds, and total loss of structure. By 1985, in the pH 2.5 treatment it was no longer possible to recognize even decaying moss litter. Bare ground showed an inverse effect. This loss of the moss layer influenced water relations, nutrient cycling and nutrient status of the higher plants and caused changes in soil solution chemistry (described elsewhere). At pH 4.0 and 5.6 the feather moss showed a positive response, possibly due to the increased water supply.

In the field microcosm experiment, the general pattern of response was similar. As shown in Figure i, pH 3.0 sprays had a significantly deleterious effect on frond growth, relative to pH 3.5 and 5.6, which were not significantly different from each other. The unsprayed control had significantly greater growth than any of the sprayed microcosms. Chlorophyll, Mg, Ca and to a limited extent K were reduced in the green growing tips of pH 3.0-sprayed fronds relative to pH 5.6 or unsprayed fronds.

412 T. C. HUTCHINSON ET AL.

TABLE I~ Vegetative cover of Pleurozlum shreherl and hare ground of Site I - recorded annually as % im plots (n = 3 x 3 per p-H~ent).

Year

Treatment (pH spray)

5.6 4.0 3.5 3.0 2.5 Unsprayed .....................................................................................

% Cover

1981 1982 1983 1984 1985

83.9 + 13.2 51.1 + 20.1 83.9 + 16.7 70.6 + 12.9 66.7 + 28.7 86.7 + 12.7 61.1 + 23.3 68.0 + 23.8 47.2 + 20.0 1.6 + 2.5 84.4 + 11.3 87.2 + 9.7 52.8 + 28.1 66.1 + 29.6 33.1 + 20.7 0.7 + 1.6 83.3 + 5.0 90.0 + 7.3 74.4 + 12,8 66.9 + 24.7 ~.7 + 27.6 0.2 + 0.4 76.1 + 16.4 91.0 + 6.7 75.6 + 13.6 63.6 + 25.5 40.0 + 21.0 0.0 81.1 + 9.6

% Bare Ground

1981 1982 1983 1984 1985

3.9+5.5 (21.7+19.0) 4.1+7.2 9.2+17.3 7.9+9.5 0.9+0.9 5.8+5.8 2.2+3.4 10.3+9.6 46.1+11.4 1.8+1.5 2.9+3.2 9.8+12.1 5.4+4.5 27.9+16.1 64.4+8.5 4.8+2.7 0.8+0.8 6.9+7.6 2.2+1.6 24.2+17.8 54.4+33.8 4.7+4.9 0.9+1.1 4.6+3.4 8.0+10.9 25.1+16.2 88.0+14.4 3.6+3.8

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to simulated acid rain of pH 3.0, 3.5 and 5.6 compared with unsprayed plants. Each histogram represents the mean and S.D. of 150 measured fronds marked at the commencement of the experi-

ment. A one-way ANOVA and Duncan's Multiple Range test comparing mean frond growth yielded

an F ratio of 27.23 (P < 0.0001). Means from the pH 3.5 and 5.6 groups were found to be

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* 2:1 refers to a 2:1 molar ratio of H2SO 4 to HNO 3

In the laboratory microcosm experiment, where sprays at pH 3.0 but of different composition were used, significant differences in chlorophyll content were measured (Figure 2). The effect of acidifying the through- fall solution with I12S04 or the H2SO4:HN03 mixture was significantly more destructive to chlorophyll than that of the HNO 3 acidified spray. Alternating the pH 3.0 spray with pH 5.6 ameliorated the acid influence.

The HNO 3 acidified spray caused an initial greening of the fronds but after six weeks (Figure 2) the fronds had significantly less chlorophyll than the pH 5.6 sprayed fronds, as corroborated by a similar field

experiment (Hutchinson 1986). It seems that Pleurozium schreberi, a feather moss that occupies

THE EFFECI OF SIMULATED ACID RAIN ON FEATHER MOSSES AND LICHENS 413

thousands of square kilometres of boreal forest, is very sensitive to acid sprays and that single rain events of as low as pH 3.0 could cause substantial damage to chlorophyll and to growth. However, at the present ambient rain pH of the sites, i.e. pH 4.2, little or no damage would be expected. A problem may arise from the occasional severely acidic events which may act selectively to change the population genetic structure of the moss, or could begin to kill or weaken the critical moss layer.

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Figure 2. The chlorophyll a content of the feather moss Pleurozium schreberi when sprayed daily in greenhouse experiments with one of five spray pH treatments (see text). Values are given as means with S.D. A one-way ANOVA and Duncan's Multiple Range test comparing mean chlorophyll a content resulted in an F ratio of 35.49 ( P < 0.0001). Two treatment pairs (2:1 and H2SO 4 alone; HNO 3 alone and 3.0/5.6) showed significant differences. * 2:1 refers to a 2:1 molar ratio of H2SO 4 to HNO 3

3.2. Effects on Lichens

The responses of the three lichens to the five years of acid treatments were notably less severe than for Pleurozium. Nevertheless, by autumn 1983 browning and abnormal branching of some lichen podetia were noted. Substantial declines in both podetial height and podetial dry weight were observed at pHs less than 3.5 for all three Cladina species (Figures 3 and 4). C. stellaris showed the greatest inhibition of the three species, at pH 2.5, with a reduction in weight of 50% compared with podetia of plants sprayed with pH 3.5 (Figure 4), and a reduction in height of nearly 33%. Such declines may be the result of loss of net photosynthetic potentials at low pH (e.g., Lechowicz 1982). The vertical stratification in age of Cladina podetia is related to the degree of damage from simulated acid rain. Although an apparent growth stimulation occurs in young podetial tips sprayed with pHs less than 4.0, the cor- responding basal podetial tissue shows a reduction in height. Conclu- sions based solely on short-term examination of podetial tips may there- fore he quite misleading. In these experiments, the stunting of lower internodes (Figure 5) may be due to a) a greater number of years expo- sure to acid rain events and/or b) a greater susceptibility to low pH of older tissues which contain less chlorophyll and are photosynthetically inactive (Nash 1980).

14 T . C . HUTCHINSON ET AL.

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Separate studies at the same sites indicate that vascular peren- nials such as Kalmia and Vaccinium (Hutchinson, unpublished data) are

tolerant of simulated rain as low as pH 2.5. In comparison, this study demonstrates that lichen and feather moss communities, which dominate huge expanses of the Canadian boreal forest, are sensitive to simulated rain events and have the potential to be affected by ambient rain epi- sodes of less than pH 3.5. Since both Cladina and Pleurozium species intercept airborne pollutants (Moser et al. 1983; Brown and Smirnoff 1978), possible synergistic interactions between acid precipitation and gaseous pollutants (S02, 03) may occur and could result ~n widespread devastation of boreal understory vegetation.

Although lichens are less sensitive than the feather moss, Pleurozium, it is clear from these data that rare low pH events falling on the boreal forest floor may result in a decrease in growth potential over wide geographic areas.

416 T .C . HUTCHINSON ET AL.

4. ACKNOWLEDGEMENTS

Financial assistance for the study was provided by a Natural Sciences and Engineering Council of Canada Operating Grant to T.C. Hutchinson and a NSERC Post-Doctoral Fellowship to Martha Scott. Logistic support was provided by the Ontario Ministry of Natural Resources and by funding to TCH from the Canadian National Sportsman's Fund and the National Research Council of Canada. Marilyn Feth provided invaluable field assistance with the lichen podetial measurements.

5. REFERENCES

Brown, D.H. and Smirnoff, N.: 1978, 'Observations on the effects of ozone on Cladonia rangiformis', Lichenologist, iO, 91-94.

Dixon, M.: 1986, M.Sc. thesis, Botany Department, University of Toronto.

Hutchinson, T.C., Scott, M. and Dixon, M.: in press, Effects of experi- mental acidification on boreal forest ground flora lichens and mosses. In Proc. NATO Advanced Workshop on "Effects of Acidic Deposition on Forests, Wetlands and Agricultural Ecosystems". Ed. T.C. Hutchinson and K. Meema, Springer Verlag.

Kershaw, K.A.: 1977, 'Studies on lichen-dominated systems. XX. An exami- nation of some aspects of the northern boreal lichen woodands in Canada', Can. J. Bot., 55, 393-410.

Lechowicz, M.J.: 1982, 'The effects of simulated acid precipitation on photosynthesis in the Caribou lichen Cladina stellaris (Opiz) Brodo', Water, Soil and Air Pollution, 18, 421-430.

Moser, T.J., Nash III, T.H. and Olafsen, A.G.: 1983, 'Photosynthetic recovery in arctic caribou forage lichens following a long-term field sulfur dioxide fumigation', Can J. Bot., 61, 367-370.

Nash III, T.H., Moser, T.J., and Link, S.O.: 1980, 'Nonrandom variation of gas exchange within arctic lichens', Can. J. Bot., 58, 1181-1186.