Response of tree seedlings to acid precipitation— II. Effect of simulated acidified canopy throughfall on sugar maple seedling growth

E*wiTommOMandExpawu~alBoLetny,VoL 22, No. 3, pp. 385 to 392, 1982

0098-8472182/030385-08 $03.00/0 ~ 1982. Pergamon PressLtd.

Printed in Great Britain

RESPONSE OF TREE SEEDLINGS TO ACID P R E C I P I T A T I O N - II. EFFECT OF S I M U L A T E D ACIDIFIED CANOPY T H R O U G H F A L L O N SUGAR MAPLE SEEDLING G R O W T H D. J. RAYNAL, J. R. R O M A N and W. M. EICHENLAUB

State University of New York, College of Environmental Science and Forestry, Syracuse, NY 13210, U.S.A.

(Received 11 ,~une 1981; acceptedin revisedform 8 jTanuary 1982) R.AYNAL D.J., ROMANJ. R. and EICHENLAUBW. M. Response of tree seedlings to acid precipitation-H. Effect of simulated canopy throughfall on sugar maple seedling growth. ENVmONMENTAL AND EXPERIMENTALBOTANY 22, 385--392, 1982. Sugar maple seedling radicle growth in a laboratory growth apparatus was significantly reduced after exposure to simulated acidified canopy throughfall at pH 3.0 and below. Seedlings exposed to low pH were susceptible to bacterial infection; survival of seedlings transplanted to soil declined with increasing acidity of simulated canopy throughfall. Extension growth and leaf weight gain of established potted seedlings subjected to acidic throughfall was dependent on soil nutrient supplying capacity. Under nutrient-limited conditions, throughfall of pH 3.0 promoted seedling growth although causing foliar damage. At higher fertility levels, reduction in growth was found only after the pH 2.0 treatment. Evaluation of the findings in relation to natural conditions is complicated by (1) the capacity of vegetation and the forest floor to buffer the seedling environment from extreme pH changes, (2) the direct effects of acidity on pathogenic micro-organisms and the predisposition of seedlings to infection, (3) the episodic nature and varying acidity of precipitation, and (4) differential sensitivity of laboratory and field grown seedlings to acidic deposition. However, these studies indicate that sugar maple seedlings are potentially susceptible to direct and indirect effects of acid precipitation.

INTRODUCTION EFFECTS of acidic precipitation on vegetation may

be greatest when early life cycle stages of plants are exposed to acidic rain. Seed germination and seedling emergence, establishment, and growth are potentially among the most sensitive processes affected by acid precipitationJ l'6,7'9'x°'lx) In the northeastern United States precipitation is commonly 10-100 times more acidic than pure rain at equilibrium with CO2 in the atmosphere. There is great concern over the possible influences of precipitation acidification on forests of the

northeast. Whether acid precipitation causes an increase or decrease in forest productivity or alteration in tree regeneration patterns remains to be convincingly demonstrated. ~9) In 1980 and 1981 a demographic survey of sugar maple (Acer saccharum Marsh.) in a 70-yearold regenerating hardwood stand located in the central Adirondack Mountains dominated by American beech (Fagus americana Ehrh.) and sugar maple revealed that sugar maple seed germination begins under full snow cover during February. By mid March, as much as 74% of m a x i m u m germination has occurred (RUTMAN

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D . J . RAYNAL, J. R. ROMAN and W. M. EICHENLAUB

and RAYNAL,unpublished). The exposure of the seedling radicle and hypocotyl to early snowmelt which can be highly acidified may cause seedling injury. Further, the emergence of young seedlings from forest litter after snowmelt but before and during canopy leaf development may subject juvenile plants to rather direct acid precipitation. In a previous paper, ~11) we described the influence ofsubstrate acidity on seed germination of several tree species characteristic of mixed hardwood conifer forests of the northeast. Sugar maple seed germination was found to be insensitive to low substrate pH but possible effects of acidity on the emerging seedling require further study. The present study was conducted to test whether sugar maple seedling growth is influenced by acidic precipitation by evaluating the effects of simulated acidified canopy throughfall on seedling radicle elongation and seedling growth. MATERIALS A N D M E T H O D S

Seedling radicle elongation studies To simulate the effects of acid precipitation on sugar maple seedling radice elongation, an ex-

/b

FIG. 1. Conceptual design of the apparatus for treatment of seedlings to determine effects of acidified throughfall on seedling radicle growth. Dimensions of the box are 48 x 25 x 15 cm. The components are as follows: a, line from carboy holding treatment solution; b, distribution tube; c, adjustable screw clamp; d, capillary tube; e, Plexiglas shelf covered with fiberglass cloth; f, seedling; g, Plexiglas box; h, additional treatment shelves.

perimental growth container fitted with capillary tubes to deliver solution droplets to seedlings was constructed (Fig. 1). This apparatus, measuring 48 × 25 x 14 cm, was fabricated from Plexiglas and assembled so that individual seedlings could be subjected to droplets delivered at a rate of 2 ml h r - a per seedling. Sugar maple seeds were allowed to germinate in darkness at 4°C following stratification. °1) Seedlings with radicles protruding from the fruit coat less than 1 m m and which had germinated within a 24-hr period were positioned beneath a row of capillary tubes on a slanted Plexiglas shelf covered with acid washed fiberglass cloth, one seedling beneath each capillary tube. To provide emerging seedlings with nutrients, seedlings were exposed to droplets of 0.1 strength Hoagland's solution acidified to p H 2.4, 3.0, 3.4 and 4.0 with sulfuric acid or to acidified distilled water of p H 5.3. Droplets drained past the seedlings, providing constant p H and nutrient conditions. The apparatus was covered with a black polyethylene sheet to exclude light and placed in a growth chamber set at 20°C day and 15°C night. Four groups of 15 seedlings each (15 seedlings per shelf, n = 60) were treated simultaneously and two experiments were conducted: Experiment 1 in which the treatment solutions were pH 2.4, 3.0 and 5.3 and Experiment 2 in which treatment solutions of p H 2.4, 3.0, 3.4, 4.0 and 5.3 were used. In each experiment seedlings were exposed to a single p H treatment; each treatment was applied successively since a single experimental apparatus was used. Each experiment was performed twice (Groups I and II). Radicle elongation was measured after 4 days of growth. Analyses of variance evaluating differences in radicle extension at each treatment level were calculated followed by Tukey's test for mean separationJ TM Following the 4-day measurement period, seedlings were planted in flats containing potting loam, divided into two groups, and placed in a greenhouse. One group was watered regularly with tap water while the other was watered once with distilled water containing a fungicide/bacteriacide (Truban, Mallinckrodt Chemical Works, at 0.25 ml/l) and thereafter with distilled water. After two weeks of growth, seedling survival was measured.

RESPONSE OF TREE SEEDLINGS TO ACID PRECIPITATION--II. Seedling growth studies Two experiments were designed to determine the effects of simulated acidified forest canopy throughfall on growth of sugar maple seedlings. In both, 120 newly germinated seedlings were placed singly in 15-cm dia. plastic pots containing a mixture of commercial-grade silica sand and sandy outwash soil from Huntington Wildlife Forest, Adirondack Mountains, New York. Seedlings were watered for one week with distilled water before experimental treatment and then treated with simulated hardwood forest throughfall prepared based on the chemical composition reported for a mixed deciduous forest in New Hampshire ~2) (Table 1). Pots containing seedlings were assigned to one of four treatment groups (n = 30) in a completely randomized block design in which the treatment solution was acidified to pH 4.0, 3.0 or 2.0 with a mixture of sulfuric and nitric acids at a rate of9:1 on a microequivalent basis. A control treatment, simulated throughfall without the addition of acids, had a p H of 5.2. Plants were subjected to these solutions using a treatment chamber measuring 1 x 1 × 2.5m constructed of 2.5 × 5.0cm lumber frame supporting polyethylene walls. A nozzle was mounted in each of the corners of the frame at a height of 1.5 m and pointed up and toward the centre of the frame so that distribution of droplets of treatment solution was rather uniform. Pots were placed randomly in the chamber on a Table 1. Chemical composition of simulated forest canopy throughfall used in seedling growth studies Reagent

Concentration (peq/1)

NH4C1 NH4NO3 K2SO 4 CaSO4.2H20 MgSO4.7H20 Na3PO 4.12H20 H2SO 4 HNO 3

40 30 150 80 50 10 900 100

Composition based on that reported for a mixed deciduous forest by EATON et al. ~2)

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hexagonal "lazy susan" which was periodically rotated throughout the rain treatment. Treatment rainfalls were applied weekly. During treatments temperature was maintained at approx. 20°C and light intensity at 50-70 microensteins m -2 sec -x. Solutions were applied over a 20-30 min period (equivalent to 2.5 cm of rain per week, the approximate average weekly rainfall in the central Adirondack mountains). When not being treated, seedlings were kept in a growth chamber at 20°C day and 15°C night with an 18-hr photoperiod at a light intensity of 150 microensteins m - 2 sec- x. In an initial experiment (Experiment 3), no water was applied to the seedlings other than the throughfall simulant. In a second experiment (Experiment 4), nutrient differences caused by elevated nitrate and sulfate concentrations at the adjusted low pH levels were nullified by supplementing simulated rain with nutrient solution. One hundred millilitres of one-tenth strength Hoagland's solution was applied to the soil of potted seedlings between treatment rains. Consequently, any possible fertilization effect of the acidified throughfall would be diminished with all treatment groups receiving substantial nutrient input. Seeds used in the first experiment were obtained from Schumacher Seed Co., Sandwich, MA 02563, while in the second experiment, seeds from the U.S. Forest Service Laboratory, Burlington, V T 05401 were used. In Experiments 3 and 4, the height of each seedling and the length and width of one of the two primary leaves was measured weekly. A visual estimation of degree of injury, including occurrence of necrotic areas or missing leaf segments, was also made. Seedlings were grown, treated, and measured for a 9-week period. At the end of the experiments, plants were harvested and separated into leaves, shoots and roots. Oven-dry weights of each portion of each seedling were then determined. Results were analysed by analysis of variance followed by Tukey's test for mean separation.(~2) The potting soils were analysed before and after the experiments for cation exchange capacity (CEC), p H and calcium concentration. Reaction (pH) was measured electrometrically using a 1 : 1 soil to distilled water mixture. Total

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D . J . RAYNAL, J. R. ROMAN and W. M. EICHENLAUB

cation exchange capacity was determined by the a m m o n i u m replacement method (5) and exchangeable calcium was extracted with neutral normal a m m o n i u m acetate, and analysed using atomic absorption spectrophotometryJ s)

RESULTS

Effects on seedling radicle elongation Sugar maple seedling radicle growth was adversely affected by simulated acidified canopy throughfall. Growth was significantly reduced at exposure to treatments of p H 3.0 and 2.4 compared with p H 5.3, 4.0 and 3.4 (Table 2). While no significant growth reduction was observed at the p H 3.4 treatment level, extension growth was reduced between 29.1 and 37.9% at p H 3.0 and 47.2 and 67.2% at p H 2.4 compared to seedlings grown at p H 5.3 (Experiment 1). Elongation was reduced between 10.4 and 37.5% at pH 3.0 and between 41.7 and 60.0% at p H 2.4 compared to seedlings grown at p H 4.0 (Experiment 2) (Table 2). Seedlings exposed to p H 3.0 and 2.4 solutions developed a black soft ring near the radicle tip. Microscopic examination of tissue from this blackened region revealed the presence of bacteria. These were neither isolated nor identified. When transferred to a greenhouse potting medium and watered with tap water, 7% of the pH 2.4 treatment seedlings survived while 53, 93 and 100% of the seedlings treated at p H 3.0, 3.4 and 4.0 respectively survived. When seedlings were grown in soil treated with fungicide/ bacteriacide, all seedlings survived. Fungicide/ bacteriacide treated-seedlings with radicles

having black, necrotic rings developed callus tissue and healthy secondary roots developed at this callus.

Effects on seedling growth The growth response of sugar maple seedlings to treatment with acidified canopy throughfall was dependent on the nutrient supplying capacity of the soil. Where no supplemental nutrient solution was provided (Experiment 3), seedlings exposed to p H 3.0 throughfall grew better than those at p H 5.2 as indicated by greater leaf weight and height ater 9 weeks (Table 3). Seedlings exposed to p H 2 . 0 treatments were greatly reduced in size compared with all other treatments. Significant differences between treatments were first noted after 5 weeks of weekly exposure (Table 3). In addition to differences in growth, degree of foliar damage differed among the treatments. Although all seedlings exposed to pH 2.0 throughfall suffered necrotic lesions and missing leaf portions, only 35% of the seedlings treated at p H 3 . 0 were damaged. About 4% of p H 4 . 0 treatment seedlings showed similar visible damage. When nutrient solution was added to pots between experimental treatments (Experiment 4), differences in seedling growth were less pronounced compared with that of the more nutrient-limited conditions in Experiment 3 (Table 4). No effect of exposure of seedlings to throughfall o f p H 4.0 and 3.0 was observed. Under p H 2.0 treatment, total seedling weight was reduced compared with that at p H 4.0 and 3.0 but not compared with control (pH 5.2). This reduction primarily resulted from a decrease in root

Table 2. Mean radiclegrowth (mm) of sugar mapleseedlingsmeasuredafterfour days of exposure to dropletsof simulated canopy throughfall of differing pH Treatment pH Experiment 1 2

Group (n = 60)

2.4

3.0

3.4

4.0

5.3

F

I II I II

6.7a 3.8a 4.8a 2.8a

9.0a 7.2a 7.5a 4.3a

--13.3b 6.8b

--12.0b 4.8b

12.7b 11.6b ---

12.2 14.5 22.6 12.1

Significance P P P P

~< 0.001 ~< 0.001 ~< 0.001 ~< 0.001

Experiments 1 and 2 differed only in treatment levels. Dashes indicate no treatment. Values not followed by the same letter are significantly different (P ~< 0.05) by Tukey's test.

RESPONSE OF TREE SEEDLINGS T O ACID P R E C I P I T A T I O N - - I I .

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Table 3. Mean valuesfor several variables describing sugar maple seedlinggrowth and injuryfoUowing exposure to simulated acid throughfall in Experiment 3 Treatment pH Variable Leafwt 9 wk, mg Height 4 wk, cm Height 5 wk, cm Height 9 wk, cm % with visible damage at 9 wk n

2.0

3.0

4.0

5.2

F

Significance

41.0a 10.0a 16.7a 19.2a

139.0b 13.3a 29.6b 37.2b

93.0c 11.8a 24.2bc 31.2bc

91.0c 12.8a 22.9c 29.0c

15.3 1.6 10.5 18.5

P ~< 0.001 NS P ~< 0.001 P ~< 0.001

100 30

35 30

4 30

0 30

Values not followed by the same letter are significantly different (P ~< 0.05) by Tukey's test.

weight ( T a b l e 4). Seedling height a n d leaf weight a n d w i d t h d i d not differ between treatments. However, average leaf length was r e d u c e d in the p H 2.0 t r e a t m e n t due to leaf tip d a m a g e . As in E x p e r i m e n t 3, foliar injury was p r o n o u n c e d in seedlings exposed to p H 2.0 throughfall b u t u n d e r higher n u t r i e n t levels d a m a g e was m u c h less severe t h a n when nutrients were not available. M a n y seedlings subjected to the p H 2.0 t h r o u g h fall p r o d u c e d s u p p l e m e n t a l leaf g r o w t h due to activation of the t e r m i n a l b u d following foliar d a m a g e ( T a b l e 4).

E v a l u a t i o n of the chemical attributes of the soils from E x p e r i m e n t s 3 a n d 4 both prior to a n d following simulated throughfall treatments revealed little c h a n g e ( T a b l e 5). T h e only c h a r a c teristic that differed significantly after t r e a t m e n t was the p H of soil exposed to the p H 2.0 solution. C a t i o n exchange increased slightly b u t not significantly when subjected to throughfall of p H 4.0 or less c o m p a r e d with p r e - t r e a t m e n t soil. C a l c i u m c o n c e n t r a t i o n declined very slightly u n d e r acidity levels o f p H 4.0 or less. A similar p a t t e r n o f c h a n g e in soil p H was observed when nutrients were

Table 4. Mean valuesfor several variables describing sugar maple seedling growth and injuryfoUowing exposure of seven weeks to simulated acid throughfall in Experiment 4 Treatment pH Variable Leaf wt, mg Twig wt, mg Root wt, mg Total wt, mg Height, mm Leaf length, mm Leaf width, mm % with visible damage ~o with supplemental leaf growth n

2.0

3.0

4.0

5.2

F

Significance

214a 54a 96a 363a 41.7a 73.6a 49.2a

244a 58a 148b 450b 45.2a 87.0b 51.8a

242a 58a 160b 460b 46.6a 87.8b 51.4a

236a 61a 132b 429ab 44.6a 84.9b 51.5a

1.23 0.85 9.85 3.75 1.83 7.95 0.72

NS NS P ~< 0.001 P ~< 0.01 NS P ~< 0.001 NS

87.0

3.8

0

4.0

34.8 30

0 30

0 30

0 30

Values not followed by the same letter are significantly different (P ~< 0.05) by Tukey's test.

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D . J . RAYNAL, J. R. ROMAN and W. M. EICHENLAUB Table 5. Mean valuesfor hydrogen ion concentration (pH), cation exchange capacity ( CEC), and calcium concentrationfor soils prior to and following treatment with simulated canopy throughfall Treatment pH Before treatment

2.0

Experiment 3: without added nutrients pH 4.9a 4.2b CEC 3.9a 4.6a (meq/100 g) Ca 0.12a 0.08a (meq/100 g) Experiment 4: with added nutrients pH 5.2a 4.5b CEC 2.3a 4.4a (meq/100 g) Ca 0.07a 0.14a (meq/100 g)

3.0

4.0

5.2

4.9a 4.8a

5.1a 4.6a

5.1a 3.8a

0.08a

0.09a

0.12a

5.2a 3.9a

5.3a 3.2a

5.4a 3.5a

0.12a

0.12a

0.13a

Levels before and after treatment were tested by Tukey's test. Values not followed by the same letter are significantly different

(P ~< 0.05).

added (Experiment 4). Again only the soil pH after exposure to the pH 2.0 solution differed significantly following treatment. The slight increase in cation exchange capacity was more pronounced with addition of nutrient solution but final cation exchange capacity was somewhat less than in the previous experiment because initial soil cation exchange capacity was low. The slight increase in calcium concentration differed from the change in Experiment 3 where some decrease was observed. DISCUSSION

Because effects of acid precipitation on plants may be life cycle stage dependent, determination of possible acute effects of acid precipitation on vegetation requires evaluation of influence of acidity on the processes of seed germination, seedling emergence, and early seedling growth. Our studies of sugar maple presented here and in a previous paper (11) revealed differential sensitivity of juvenile plant stages to acid conditions. Whereas sugar maple seed germination is unaffected by substrate acidity as low as pH 3.0,

seedling radicle elongation was reduced when exposed to simulated throughfall at pH 3.0 or less in a laboratory treatment apparatus. Susceptibility of emerging radicles to bacterial infection increased with exposure to increasing acidity of throughfall. Thus seedlings were adversely affected by simulated acid precipitation, and both direct and indirect effects were observed. Growth of seedlings established in soil was not affected by acidic throughfall until after four weeks of weekly exposure to pH 3.0 treatment after which time simultaneous promotive and inhibitory effects were observed. Seedling response differed depending on the nutrient supplying capacity of the soil. At a low fertility level, acidic throughfall of pH 3.0 was found to stimulate seedling growth. This response could result from nutrient input effects primarily caused by nitrate in the simulated throughfall. Sulfate is not a limiting nutrient in most Adirondack soils although it is possible that seedling growth in the amended soils used in this experimentation was affected by sulfate fertilization. When nutrient solution was provided to seedlings so that nutrient

RESPONSE OF TREE SEEDLINGS TO ACID P R E C I P I T A T I O N - - I I . input differences caused by treatments were nullified, no promotive effect of simulated acid throughfall was found. Seedling injury was observed only at the p H 2.4 treatment. Even where growth was increased by simulated acidified throughfall, foliar lesions and leaf tip or margin distortions similar to that reported by Evans and colleagues ~3,4) were observed. O t h e r workers ~6'x3, a4) have reported similar competitive promotive and inhibitory effects of acid precipitation on tree seedling development. Extrapolation of our experimental findings to natural forest conditions m a y be complicated by several factors. First, acidity levels to which plants were subjected in the laboratory m a y not occur similarly in the forest. T h a t is, the buffering capacity of organic matter, mineral soil and vegetation m a y largely prevent exposure of emerging seedlings to throughfall or substrate acidity levels as low as p H 3.0. However, because they typically emerge near the leaf litter surface of the forest floor beneath snow cover, sugar maple seedlings m a y be exposed to highly acidic snowpack meltwater. Careful field measurements of the p H and ionic character ofsnowmelt and of leaf litter solution during the period of seed germination and seedling emergence are needed to further evaluate the seedling chemical microenvironment and actual conditions of exposure of seedlings to hydrogen ion concentration. T h e direct effects of hydrogen ions on seedling emergence m a y be of limited occurrence and promotive effects of acidic throughfall caused by a seedling fertilization response m a y normally occur where nutrients are limited. Second, the direct effects of acidity on pathogenic bacteria and other micro-organisms and the predisposition of seedlings to infection m a y intensify d a m a g e to emerging seedlings. Third, due to the episodic nature and varying acidity of natural precipitation events application of findings based on single dosage levels and constant treatment frequency m a y be difficult. Finally, possible differences in sensitivity of laboratory-grown seedlings and those growing naturally should be recognized. T h e character of precipitation chemistry and timing of occurrence of precipitation in relation to forest phenology and plant life cycle stage sensitivity m a y influence the effects of acid precipi-

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tation on sugar maple seedlings. O u r studies indicate that developing sugar maple seedlings are potentially susceptible to direct and indirect effects by acid precipitation. Further investigations coupling studies of effects of acid precipitation on seedling growth and on soil nutrient and toxicity status (s) are required to more fully evaluate effects of acid precipitation on tree regeneration. Acknowledgements--This research was supported by New York State Energy Research and Development Authority, Agreement No. ER-400-78/79 EHS. We are grateful to Drs. L. S. EVANS and J. j. LEE and two anonymous reviewers for valuable comments on an earlier manuscript and to F. S. RALEIOHfor technical assistance.

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of acid precipitation on an Adirondack forest ecosystem. New York State Energy Research and Development Authority Publication No. 80-28. 11. RAYNALD. J., ROMANJ. R. and EICHENLAUBW. (1982) Response of tree seedlings to acid precipitation--I. Effect ofsubstrate acidity on seed germination. Environ. Exp. Bot. 22, 377-383. 12. STEELER. G. D. and TORRIEJ.H. (1960) Principles and Procedures of Statistics. McGraw-Hill, New York. 13. WOODT. and BORMANNF. H. I1974) The effects of

an artificial mist upon the growth of Betula alleghaniensis Britt. Environ. Pollut. 7, 259-268. 14. Wood T. and BORMANNF. H. (1976) Short-term effects of simulated acid rain upon the growth and nutrient relations ofPinus strobus L. Pages 815-825 in L. S. DOCHINGERand T. A. SELIGA (eds) Proc. First Int. Symp. on Acid Precipitation and the Forest Ecosystem. Department of Agriculture, Washington D.C.