in Eurasian boreal forest biomes (Walter and Breckle, 1989; Havas and Kubin,. 1983). When found in dense clonal monospecific stands, bilberry can interfere.
Journal q["Chemical Ecoh~gy, Vol. 22, No. 5, 1996
EFFECTS OF BILBERRY (Vaccinium myrtillus L.) LITTER ON SEED GERMINATION A N D EARLY SEEDLING GROWTH OF FOUR BOREAL TREE SPECIES
ANDERS
J,~DERLUND,*
OLLE
MARIE-CHARLOTTE
ZACKRISSON,
and
NILSSON
Suvctish Uni~vrsity zlf Agricultural &'iences 1;?wulty of Forestry, Department of Forest Vegelaliot~ Evology S-901 83 Umefi. Sweden (Received July 14, 1995; accepted January 3, 1996)
Abstract--Laboratory and greenhouse bioassays were used to test tbr inhibitory effects of senescent and decomposed leaves and aqueous extract from bilberry" (t"accinium ,~rrtittus L. ) against seed germination and seedling growth of aspen (Popu/us tremula L.), birch tBetula pendttla Roth.), Scots pine (Piuus syh,estris L.), and Norway spruce I Picea abies (L.) Karst.J. Aqueous extracts from bilberry leaves were inhibitory to aspen seed germination and seedling growth and also induced root damage and growth abnormalities. Addition of activated carbon removed the inhibitory effects of exlructs. Senescent leaves reduced pine and spruce seed gemaination, but rinsing of seeds reversed this inhibition. Senescent leaves were more inhibitory than decornposed leaf litter, suggesting that the inhibitory compounds in bilberry leaves are relatively soluble and released at early stages during decomposition. Spruce was generally less negatively affected by litter and aqueous extracts than the other tested species. This study indicates that chemical effects of bilberry litter have the potential to inhibit tree seedling recruitment, but these effects were not consistently strong. Phytotoxicity is unlikely to be of critical importance in determining success lot spruce seedling establishment. Key Words--Vacvinium myrtillus, regeneration failure, seed germination, seedling establishment, activated carbon, germination inhibitors, phenolic compounds, Populus tremuh~, Pitltts syh,estris, Picea abies, Bettda penduht.
*To whom correspondence should be addressed.
973 {I(19841331/96/{15{xl-(g)735ng.50/o
~c. 1996 Ptcnuln Publishing Ct~c'pt~rall~ltl
974
JA.DERLUND, ZACKRISSON, AND NILSSON INTRODUCTION
Bilberry (Vaccinium myrtitlus L.) is a dominant component of ground vegetation in Eurasian boreal forest biomes (Walter and Breckle, 1989; Havas and Kubin, 1983). When found in dense clonal monospecific stands, bilberry can interfere negatively with tree recruitment, and these effects can be severe (Sarvas, 1950; Sir6n, 1955; Pellissier, 1994). Several possible explanations for failure in seed germination and seedling establishment have been proposed, including resource competition, chemical interference, and humus accumulation (Tir6n, 1949; Lehto, 1969; Andr6 et al., 1987; Pellisier, 1993, 1994; Kuuluvainen, 1994). Seedlings of deciduous and coniferous trees are seldom observed in monospecific bilberry sites (Amborg, 1943; Sir6n, 1955), and it is therefore reasonable to expect that litter interferes with tree establishment. The large amounts of senescent and decomposing leaves of bilberry found in dense stands could potentially interfere with seed germination and growth of tree seedlings through both physical and chemical interactions. The aim of our study was to test this hypothesis by using senescent and decomposed bilberry leaf-litter in a series of germination and root growth bioassays with pioneer and late successional boreal trees as target species.
METHODS AND MATERIALS
Collection of Senescent Bilbert 3, Leaves. Bilberry leaves were sampled in October 1990 during senescence while they were still attached to shoots. The site used for litter collection was a late successional mixed pine-spruce tbrest of Vaccinium mvrtillus type (sensu Amborg, 1990), near Ume~_ in northern Sweden (63°50'N, 20°22'E). The leaves were air-dried on plastic trays at 20°C until no further weight loss was recorded and then stored in a deep freezer ( - 1 8 ° C ) until the experiment started. Seed Source. Seeds of the following four tree species were used in bioassays: aspen (Populus tremula L.) seeds from a natural stand (.Alidhem 63°50'N) with a viability of 96 %; Scots pine (Pinus sylvestris L.) half-sib plantation seeds (Skaholma 66°90'N), with a viability of 96%; Norway spruce [Picea abies (L.) Karst.] half-sib plantation seeds (Lillpite 65°00'N), with a viability of 98%; and birch (Betula pendula Roth.) seeds from a natural stand (Mattismyran 66°03'N) with a viability of 42%. Bioassays with Aqueous Extracts. This experiment was performed to test the effect of aqueous extracts of senescent bilberry leaves on seed germination and radicle growth. Extracts were prepared by soaking 12.5 g of dried leaves in 100 ml distilled water with slow continual stirring for 48 hr at 20°C. The solution was then passed through a filter paper (Munktell No. 3) and the extract
EFFECTS OF BILBERRY LITTER
975
was frozen and stored at - 18°C until used. This represents the 12.5% extract, and from this, extracts of 10%, 7.5%, 5.0% and 2.5% were prepared. Two milliliters of each water extract were added to 50-mm-diameter Petri dishes containing one sheet of filter paper (Munktell No. 3) and 50 seeds of either aspen, Scots pine, or Norway spruce. An additional treatment was set up in which 0.5 g of finely powdered activated carbon (Labasco) was added to Petri dishes with 2 ml of 10% extract (and containing 50 seeds of aspen). The carbon was expected to detoxify the extract. Distilled water was used as a control. All treatments were replicated five times. The total number of germinated seeds was counted daily for all species until there was no further germination. Aspen root necroses and abnormal seedlings (i.e., those growing in a prostrate manner) were recorded seven days after sowing. Root length was measured for 10 randomly chosen seedlings per Petri dish at the end of the experiment. Measurements of pH and osmotic effects were made to eliminate the possibility that these might influence observed trends. The osmotic pressure of the 10% bilberry water extract, measured using a vapor pressure osmometer (Wescot 5500), was 84 mmot/kg, while the pH of the extract was 4.3. Germination tests with polyethylene glycol showed that the lowest molality required to cause negative effects on germination of aspen, Scots pine, and Norway spruce seeds was 154 mmol/kg, 146 mmol/kg, and 130 mmol/kg, respectively, which means that the osmotic potential of our solutions was too low to cause inhibition of our test species. Bioassavs with Senescent Bilberry Leaves. An experiment was set up to test for possible inhibitory effects of bilberry leaves against germination of aspen, Norway spruce, Scots pine, and birch. To each of twenty-seven 50-ram-diameter Petri dishes, 0.5 g of air-dried leaves and 8 ml of distilled water were added, and this was left to incubate for 48 hr at 20°C. To five of the Petri dishes, later to be sown with aspen seeds, 0.5 g of finely powdered activated carbon (Labasco) were added after 44 hr. The Petri dishes were sealed with Parafilm during the incubation, Petri dishes with 10 filter-papers and 8 ml distilled water were used as controls. After incubation one sheet of filter paper (Munktell No. 3) was placed on top of the material used in the different treatments and 50 seeds of aspen, Norway spruce, Scots pine, or birch were sown. Five replicates were used for aspen and birch, and six replicates for Scots pine and Norway spruce. The test was run in a greenhouse with a temperature of 20°C and a day-night light ratio of 17:7. Seed germination was measured daily, 7 days for aspen and 21 days for Norway spruce, Scots pine, and birch. To investigate whether Scots pine, Norway spruce, and birch seeds that had not germinated at the end of the test were nonviable or only quiescent, a water cleaning treatment was performed. The seeds were moved to new Petri dishes containing 20 ml distilled water, and after 3 hr they were rinsed in running tap water for 15 sec. The seeds were then placed in a clean Petri dishes, each
970
JAt)FRI ~,~Nt). ZA('txRISS()N. AND NII.SS(~N
containing one filter paper and 2 ml distilled water. The Petri dishes were placed in the greenhouse, and total seed germination was measured alter 17 days. Bioassavs with Senescent and Decomposed Bilberry Leaves. An experiment was set up to compare rooting ability and root growth of pregerminated Norway spruce and Scots pine seeds in senescent and decomposed bilberry leaves. Decomposed bilberry leaves were prepared by placing 21 g air-dried leaves in nylon-net litterbags on the ground under snow in bilberTy-dominated vegetation in January 1991. In May the same year, the litterbags were collected and the leaves dried at room temperature (20°C) for 60 hr. The loss of weight after decomposition was 15% compared to senescent leaves. A corresponding treatment was set up in which senescent leaves were repeatedly soaked in distilled water. It was assumed that soaking removes possible toxicity of the leaves but that the leaves have the same physical structure as unsoaked leaves. The soaked leaves were prepared in the following way. One hundred grams of dried bilberry senescent leaves were added to 1000 ml of distilled water and stirred for 8 hr. The extracts were decanted and another 1000 ml of distilled water was added, stirred at 8 hours, with the soaking process repeated 21 times in seven days. The leaves were then dried on plastic trays at 20°C. Subsamples of 1.5 g dried leaf material and unlimed, unfertilized commercial peat, which served as control, were put in 50-ram Petri dishes, each filled with 10 ml distilled water. All Petri dishes were sealed with Parafilm and incubated in 20°C for48 hr, so that all samples had the same moisture conditions at the start of the experiment. Pots (100 ml) were then half filled with sand (Silversand 90), and distilled water was added to saturation. The incubated leaves and peat were then placed on the sand surface and 10 pregerminated Scots pine and Norway spruce seeds, each with a radicle length of I ram, were placed on lop of peat or leaves in each of the pots. All pots were covered with Paratitnl and placed in a greenhouse at 20°C and with a day-night light ratio of 17: 7. Pots were sprayed with distilled water daily. There were six replicates of each treatment for each species. The total numbers of rooted seedlings per treatment were counted daily for six days. A seedling was considered to be rooted when the radicle had penetrated the organic substrate and the cotyledons grew so that there was no contact with the organic substrate. The length (root plus hypocotyte) of each seedling was measured six days after the test had started.
RESULTS
Effects of Bilber O" Aqueous Extract. Aspen seed gennination was significantly reduced by all extracts compared to the distilled water control and the activated carbon treatment (Figure 1). The germination rates in the distilled
EFFECTS OF BILBERRY LITTER
~
977
50 40 ao
"~ 2 0
0
50
100
150
200
250
Hours after sowing
(Populus tremula) e x p o s e d to a q u e o u s extract o f myrtillus) s e n e s c e n t l e a v e s . T r e a t m e n t s : i-~, 10% e x t r a c t treated with 0, distilled w a t e r c o n t r o l ; A , 2 . 5 % e x t r a c t , ,, 5 . 0 % extract: L
FI~;. I. Seed g e r m i n a t i o n o f a s p e n bilberry ( V a c c i n i u m activated carbon; 7 . 5 % extract;
0,
1 0 . 0 % extract; and e ,
1 2 . 5 % extract. See T a b l e
1 for statistical
analysis. N = 5.
water control and the activated carbon treatments were not significantly different (Table 1). Initiation of germination was delayed in all extract treatments when compared with germination in the distilled water control. In all extract concentrations, the root length of aspen was significantly reduced compared to the activated carbon treatment (Figure 2). Root length was reduced by 71% in the most diluted extract and by 97% in the 10% extract compared to the activated carbon treatment. TABLE | .
myrtillus)
ANALYSIS OF VARIANCE TABLE FOR EFFECTS OF BILBERRY
(Vaccinium
AQUEOUS EXTRACTS AND EXTRACTS TREATED WITH ACTIVATED CARBON ON SEED GERMINATION OF ASPEN
(Populus tremula)" Hours
Extract concentration
24
36
48
56
72
120
144
168
192
216
240
10% + act.carb, Control 2.5% 5~0% 7.5% 10,0%
a a a a a a
a a b b b b
a b c d d d
a b c d e e
a a b c d e
a a b c d e
a a b b c d
a a b b c d
a a b b c d
a a b b c d
a a b b c d
12.5%
a
b
d
e
f
f
e
e
e
e
e
"Control = distilled water. Treatments with the same letter within each column do not differ significantly in Tukey's multiple range test at P -< 0.05. See Figure t for germination data.
978
J,~DERLUND, ZACKRISSON, AND NILSSON a
lO E
I
E
6~
I
O~ t-
o
i
I
8 ¸
II
2
10+
!
b
I 2.5
'
cI
cd i
5
d J
7.5
I
10
carbon Extract
concentration
(%)
FIG. 2. Root length of aspen ( P o p u l u s tremula) exposcd to aqueous cxtracts of bilberry senescent leaves and extract treated with activated carbon. Vertical line = standard error, Treatments with the same letter do not differ significantly in T u k e y ' s multiple range test at P < 0.05.
(Vaccinium myrtillus)
Aspen roots in all extract treatments were necrotic, while those in the distilled water control and activated carbon treatments were fresh and normal in color (Table 2). All aspen seedlings in the 10% extract had abnormal prostrate growth forms (Table 2), while 49% of the seedlings had abnormal prostrate growth form in the most diluted solution. Seed germination of Scots pine and Norway spruce were also affected by the extracts, but less so than aspen. Scots pine germination was slightly (but not significantly) reduced in the 10% extract compared to the control (Table 3). Norway spruce germination was inhibited only by the 12.5% extract compared to the control (Table 3). TABLE 2. PROPORTION OF ASPEN (Populus tremula) SEEDLINGS SHOWING ROOT NECROSIS AND PROSTRATE GROWTH IN AQUEOUS EXTRACTS OF SENESCENT BILBERRY (Vaccinium myrtiltus) LEAVES"
Extract concentration
Root necrosis ( %)
Prostrate grown ( %)
10% + carbon Control 2.5% 5.0% 7.5% 10.0% 12.5%
0 0 100 100 100 100 NS
0.4 3.7 48.7 77.5 78.9 100 NS
"Control = distilled water. NS = no seedlings available.
EFFECTS OF BILBERRYLrrTER
979
TABLE 3, SCOTS PINE (Pinus sylvestris) AND NORWAY SPRUCE (Picea abies) SEED GERMINATION IN AQUEOUS EXTRACTOF SENESCENT BILBERRY (Vaccinium myrtillus) LEAVES AFTER |4 DAYSa Germination (%) Extract concentration Control 2.5% 5.0% 7.5% 10.0% 12.5%
Pine 100 a 99.6 91.5 92.4 93.2 NS
Spruce a a a a
100 a 99.6 98.8 I00 a 98.8 83.0
a ab ab b
"Numbers within columns followed by different letters are significantly different at P -< 0.05 (Tukey's multiple range tcst). NS = no seedlings available. Control = distilled water.
Effects of Bilberry Senescent Leaves. Bilberry senescent leaves had significantly inhibitory effects on seed germination o f Scots pine, Norway spruce, and birch when compared to the distilled water control (Table 4). Bilberry leaves delayed the start o f germination by seven days for Scots pine and 10 days for Norway spruce and birch. After the seeds exposed to the leaves had been rinsed in water, germination increased from 24% to 93% for Scots pine and from 22% to 79% for Norway spruce (Table 4), Rinsing caused Scots pine germination to increase to the level o f the control. Birch germination did not increase significantly after rinsing. Seven days after sowing, the total germination o f aspen was 80% in treatments with senescent bilberry leaves and 97 % in the activated carbon treatment TABLE 4. SCOTS PINE (Pinus sylvestris), NORWAY SPRUCE (Picea abies), AND BIRCH (Betula pendula) SEED GERMINATION IN SENESCENT BILBERRY (Vaccinium myrtillus) LEAVES AND DISTILLED WATER CONTROL AFTER 21 DAYS" Germination (%) Treatment
Scots pine
Norway spruce
Birch
Control Senescentleaves Rinsed seeds
100a 24.0 b 92.6 a
100a 21.9 c 79. l b
100a 58.1 b 66.6 b
"Rinsed seeds = germination 17 days after water cleaning treatment. Treatments with the same letter within each column do not differ significantly according to Tukey's multiple range test at P _< 0.05.
980
J,~DERLUND, ZACKRISSON, AND NILSSON
50
a
m
g 2o
o 0
1
2
3
4
5
6
7
Days after sowing
FIG. 3. Seed germination of aspen (Popu/us tremula) in senescent bilberry (Vaccinium myrtil/us) leaves and leaves treated with activated carbon. I I, Senescent leaves; z~, senescent leaves treated with activated carbon: and, ,, distilled water control. Trcatments with the same letter do not differ significantly in Tukey's multiple range test at P _< 0.05, N = 5.
when compared to the distilled water control (Figure 3). None of the treatments differed significantly from the distilled water control. Effects of Senescent and Decomposed Leaves on Root Development. The proportion of rooted Scots pine and Norway spruce seedlings was significantly reduced by both senescent and decomposed bilberry leaves compared to controls with peat and soaked leaves (Figure 4a and b). Senescent leaves were more inhibitory than decomposed leaves. Root growth was also significantly inhibited by senescent and decomposed bilberry leaves when compared to soaked bilberry leaves and peat (Figure 5a and b). The effects of the senescent leaves were the strongest, and these reduced root growth by 79 % for pine and by 67 % for spruce when compared to peat. Treatments with senescent bilberry leaves resulted in 17% mortality of Scots pine and 13% mortality of Norway spruce seedlings. No mortality occurred in treatments consisting of decomposed leaves, soaked leaves, or peat. DISCUSSION Bilberry litter has the potential to inhibit tree seedling establishment of the tested species, although these effects were weak for Norway spruce. Seed germination, growth, and root condition of aspen were negatively affected by even the most diluted extracts. Neither the pH nor the osmotic pressure of the bilberry
981
EFFECTS OF BILBERRY LITTER
a)
a
o~ 1 0 0
g
~_
80
13
~
60
~
40
E
2o
cL
o 1
2
3
4
5
6
Days after start
A
I O0
b)
a
t~
c
80
~
60
~0
40
E
2o
if)
~"
o 1
v
!
!
!
!
2
3
4
5
6
Days after start
FIG. 4. Proportion of upright rooted (a) Scots pine (Pinus syh,estris) and (b) Norway spruce (Picea abies) seedlings in senescent and decomposed bilberry (Vaccinium mvrtillus) leaves. Control = peat and soaked bilberry leaves. III, senescent leaves; ~,, decomposed leaves, J I, soaked leaves; , ,, peat. Treatments with the same letter do not differ significantly in Tukey's multiple range test at P _< 0.05.
water extract are likely to be responsible for the observed negative effects on seed germination we observed (Nilsson, 1992; Rikala and Jozetek, 1990). The production and mode of action of phytotoxins present in senescent litter remains unknown. Our bioassays suggest that the inhibitory compounds are water soluble and released from senescent leaves during early decomposition. It is also possible that microorganisms could mediate transformation of compounds, causing inhibition both during the extraction process and later when the
982
JADERLUND, ZACKRISSON, AND NILSSON
E
== (D 03
0 O_
=>
a) 50 ............ I
40 30
t
20 d
10
+
0 0 ¢r
E E
Peat
Soaked leaves
Decomp. leaves
Senescent leaves
b) 5O
r-
c m 0J
o o Q.
=>
40
a
a i
30 20
C
10
+
0 0 er
Peat
Soaked leaves
Decomp. leaves
Senescent leaves
FlG. 5. Root and hypocotyle length of (a) Scots pine (Pinus syh'estris) and (b) Norway spruce (Picea abies) seedlings in senescent and decomposed bilberry (Vaccinium myrtillus) leaves. Control = peat and soaked bilberry leaves. Vertical line = standard error. Treatments with the same letter do not differ significantly in Tukey's multiple range test at P _< 0.05. water extracts were used in different bioassays. However the sterilization of extracts prepared from senescent leaves by 0.45-#tm and 0.22-p.m sterile filtration did not alter the degree of inhibition observed (data not shown). Rinsing of exposed pine and spruce seeds removed the negative effects of water-soluble compounds extracted from bilberry litter. The results of the litterbag study with decomposing senescent leaves also indicates that when the litter produced in the autumn is allowed to decompose under subnival conditions during winter, between 34 and 74% of the inhibitory effect is lost. We interpret this to mean that natural rinsing of coniferous seeds and water-soluble compounds from bilberry litter by snowmelt and rain during normal field conditions
EFFECTS OF BILBERRY LITTER
983
may reduce most of the inhibitory effects found in senescent litter. In experiments with Empetrurn hermaphroditum, another dominant boreal species that forms clonal mats, Zackrisson and Nilsson (1992) found that the toxic compounds leached from leaf glands and absorbed by seeds could not be removed by washing exposed seeds. They also concluded that toxins were quickly absorbed by seeds and caused seed mortality before germination. The low seed mortality and radicle growth inhibition found in Scots pine and Norway spruce seedlings in our study indicates that the phytotoxic effects by bilberry litter are relatively limited, especially in comparison with results obtained in similar experiments with Empetrum hermaphroditum (Nilsson and Zackrisson, 1992). The slower germination rate and radicle growth for seeds present in senescent leaves compared to that of the control treatments can be important for successful natural establishment of all species, but especially those with microbiotic seeds such as aspen. During the dry conditions prevailing in spring, the effect can be severe as radicle and whole seedlings are susceptible to drought and the litter may be more difficult to penetrate. Quick radicle penetration of litter and humus, which accelerates the opportunity of the seedlings with low nutrient resources to reach moist soil, can strongly reduce the susceptibility to water deficit under dry conditions (Grime and Curtis, 1976). In all of our experiments, Norway spruce seeds and seedlings were less affected by bilberry litter and water extracts than were the other tree species. Norway spruce is a late successional climax species (Sirrn, 1955; Havas and Kubin, 1983) and can be expected to be more tolerant of inhibitory effects from phytotoxic compounds than early successional species. However, it is also tempting to speculate that the lower level of inhibition found in spruce is an effect of coevotution between spruce and bilberry, since these two widespread boreal species may have coexisted in distinctive plant communities at least since the end of the Tertiary period (Walter and Breckle, 1989; Tallis, 1991). The deciduous broad leafed tree, Populus tremula, was more strongly inhibited by water extracts of senescent bilberry leaves than were the two coniferous species. Betuta pendula seed germination in litter was relatively high, but rinsing had a much lesser revitalizing effect on these seeds than on the coniferous seeds following exposure to inhibitory compounds. Our interpretation is that the two boreal deciduous species we tested are more sensitive to litter of bilberry at the seedling stage than are coniferous species. Both aspen and birch are pioneer species that naturally colonize by seed in early successions after fire or other disturbances (Zackrisson, 1985; Grime et al., 1988). Fire has played an evolutionary important role in the maintenance of most boreal forest ecosystems dominated by bilberry (Sirrn, 1955; Kuusela, 1990). Fire return intervals of less than 100 years have been determined for Norway spruce-bilberry forest types (Zackrisson, 1977). Nutritional changes (increased cation concentration and higher pH) caused by combustion of bilberry litter and organic soil following
984
JADERLUND, ZACKRISSON, AND NILSSON
fire (Vim, 1974; Pietik/iinen and Fritze, I995) can facilitate the establishment of pioneer deciduous trees such as aspen and birch. Species diversity also increases substantially in early postfire succession on previous bilberry sites (Schimmel, 1993). We postulate that this is not only a question of nutritional release, but also an effect of reduction of bilberry litter and humus that cause inhibitory effects on seedling establishment of early succesional species. In any ecosystem in which fire is a common element, one of its functions could be the destruction or deactivation of phenolics and other phytotoxic compounds found in litter and soil. Addition of pure activated carbon in our bioassays resulted in significantly higher seed germination and seedling growth of aspen. These results support previous studies (e.g, Zackrisson and Nilsson, 1992; Mahall and Callaway, 1992; Yambe et al., 1992; Nilsson, 1994) and suggest that the activated carbon adsorbed inhibitory substances and deactivated them. The technique of using activated carbon to detoxify inhibitors therefore enables us to separate phytotoxic and nonphytotoxic effects. There is no reason to suppose that the carbon powder is itself a stimulatory factor in our experiments since distilled water amended with activated carbon did not cause stimulation of seed germination (Eliasson, 1959; Zackrisson and Nilsson, 1992). It has previously been shown by Gallet and Lebreton (1995) that bilberry contains a range of phenolics including tannins, caffeic acid, p-coumaric acid, and flavonoids. Many of these also have been detected in high amounts in litter leachates from bilberry. These are all compounds with documented inhibitory effects on seed germination and seedling growth (Rice, 1984; Blum et al,, 1984, 1985: Einhellig, 1987). Most of the phenolic compounds identified in bilberry have also been detected in soil (Blum and Rice, 1969; Whitehead et al,, 1983; Kuiters and Danneman, 1987). Even if many of the phenolic compounds present are found only in low amounts in litter and soil, they can have an additive or synergistic effects on seed germination and growth (Rasmussen and Einhellig, 1977: Williams and Hoagland, 1982; Blum et al., 1985, 1989; Einhellig, 1987; Lyu et al., 1990; Lehman et al., 1994). Our results provide evidence that phytotoxic effect of bilberry can be important in inhibiting seedling recruitment. However, these effects are not necessarily strong, and decomposing leaves do not appear to have a significant role in suppressing conifer tree seedling establishment. Our results also showed that the tree species we considered did not respond equally, and in particular spruce was less sensitive than the others. Acknowledgments--We thanks Gisela Norberg for technical assistance, Christiane Gallet fi~r valuable comments on the manuscript',and David Wardle lot constructivecomments on the manuscript and for correctingthe English. Research was supportedby the Swedish Council of Forestry and AgriculturalResearch.
EFFECTS OF BILBERRY LITIER
985 REFERENCES
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