Directory UMM :Data Elmu:jurnal:A:Agronomy Journal:Vol93.Issue1.2001:
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AGRONOMY JOURNAL, VOL. 93, JANUARY–FEBRUARY 2001
Richardson, J., and J.P. Hall. 1973b. Natural regeneration after disturbance in the forest of eastern Newfoundland. Inf. Rep. N-X-90.
Environ. Canada, Canadian Forestry Serv., St. John’s, NF.
Thompson, I.D., and A.U. Mallik. 1989. Moose browsing and allelopathic effects of Kalmia angustifolia on balsam fir regeneration in
central Newfoundland. Can. J. For. Res. 19:524–526.
Weetman, G.F., R. Fournier, J. Baker, and E. Schnorbus-Panozzo.
1989a. Foliar analysis and response of fertilized chlorotic Sitka
spruce plantations on salal dominated cedar-hemlock cutovers on
Vancouver Island. Can. J. For. Res. 12:1512–1520.
Weetman, G.F., R. Fournier, J. Baker, E. Schnorbus-Panozzo, and A.
Germain. 1989b. Foliar analysis and response of fertilized chlorotic
Sitka spruce plantations on salal dominated cedar-hemlock cutovers on Vancouver Island. Can. J. For. Res. 12:1501–1511.
Weidenhamer, J.D., D.C. Hartnett, and J.T. Romeo. 1989. Densitydependent phytotoxicity: Distinguishing resource competition and
allelopathic interference in plants. J. Appl. Ecol. 26:613–624.
Zackrisson, O., and M.-C. Nilsson. 1992. Allelopathic effects by Empetrum hermaphroditum on seed germination of two boreal tree species. Can. J. For. Res. 22:1310–1319.
Zar, J.H. 1996. Biostatistical analysis. (3rd ed.) Prentice-Hall, New
Jersey.
Zhu, H., and A.U. Mallik. 1994. Interactions between Kalmia and
black spruce: Isolation and identification of allelopathic compounds. J. Chem. Ecol. 20:407–421.
Black Spruce Growth and Understory Species Diversity
with and without Sheep Laurel
Azim U. Mallik*
ABSTRACT
Growth and understory species diversity of black spruce [Picea
mariana (Miller) B.S.P.] planted in central Newfoundland at contiguous sites with and without dense cover of sheep laurel (Kalmia angustifolia L.) were compared. Black spruce stem density and volume per
hectare were calculated by sampling 10 circular quadrats (50 m2), and
the cover of all plant species was determined by sampling 20 quadrats
(1 m2) in each site. In addition, 10 randomly sampled planted black
spruce samplings from each site were analyzed for stem height, basal
diameter, and foliar chemistry. Results showed a significantly lower
stem height and basal diameter (65 and 51%, respectively) at the site
with dense sheep laurel cover (36%) compared with the site with
sparse sheep laurel cover (,1% sheep laurel cover, and henceforth
referred to as the non-sheep laurel site for simplicity). Black spruce
grown at the sheep laurel dominated site contained significantly higher
quantities of Ca, Al, Fe, and K in the needles than that grown at the
non-sheep laurel site. The sheep laurel dominated site also had a
significantly higher mean organic matter depth of 8.3 cm compared
with 5.6 cm at the non-sheep laurel site. Canonical correspondence
analysis (CCA) of the species cover data clearly separated the sheep
laurel dominated plots from the non-sheep laurel plots. The sheep
laurel dominated site had reduced species richness of vascular plants
but increased species richness for lichens compared with the nonsheep laurel site. Allelopathy associated with phenol-induced soil
nutrient imbalance and nutrient stress is a possible cause for black
spruce growth inhibition at the sheep laurel dominated site.
R
apid growth of sheep laurel after clear cutting
and fire in sheep laurel–black spruce communities
has been widely observed in eastern Canada, particularly at sites with organic and coarse textured mediumquality soil types (Page, 1970, p. 7; van Nostrand, 1971,
p. 68; Damman, 1975). The natural regeneration of black
spruce at these sites is poor, and planted black spruce
seedlings exhibit stunted growth (Candy, 1951, p. 224;
Dep. of Biol., Lakehead Univ., Thunder Bay, ON, Canada P7B 5E1.
Received 25 Jan. 2000. *Corresponding author (azim.mallik@
lakeheadu.ca).
Published in Agron. J. 93:92–98 (2001).
Richardson and Hall, 1973a, p. 63, 1973b, p. 46; Wall,
1977, p. 55). Competition and allelopathic effects of
sheep laurel have been attributed to the regeneration
failure and poor growth of conifers (Mallik, 1987, 1990,
1992, 1996; Mallik and Roberts, 1994). In eastern and
central Newfoundland, large areas of moderately productive black spruce forests with sheep laurel understory have been converted into sheep laurel dominated heath following forest disturbance (Mallik, 1995).
A regeneration survey of 5888 plots in black spruce
plantations found that 55% of them contained sheep
laurel (English and Hackett, 1994, p. 12). Black spruce
in sheep laurel infested sites exhibits typical symptoms:
Poor plant height and diameter growth and short and
chlorotic needles, as observed in other conifers in the
presence of different ericaceous plants (Handley, 1963;
Gimingham, 1972; de Montigny and Weetman, 1990;
Fraser, 1993, p. 166; Inderjit and Mallik, 1996a; Jaderlund et al., 1997). Black spruce forests that are dominated by sheep laurel tend to have a reduced species
richness and deficiency in available nutrients (Damman,
1971). Recently, Yamasaki et al. (1998) reported that
black spruce seedlings in close proximity of sheep laurel
(,1 m) experience lower height, biomass, root/shoot
ratio, foliar N and P, and lower mycorrhizal infection
than those growing farther (.1 m) away from sheep
laurel.
Damman (1971, 1975) suggested that long-term occupancy of a site by sheep laurel causes irreversible soil
degradation, leading to a stable heath formation by precluding forest regeneration. Apparently sheep laurel,
like other ericaceous plants, is able to grow in nutrient
poor conditions where black spruce growth is very much
restricted. There is evidence suggesting that the ericaceous plants are able to access the N that is bound
in the protein–polyphenol complex through the ericoid
mycorrhizae, but this N is not available to the conifers
Abbreviations: CCA, canonical correspondence analysis; IAA, indoleacetic acid.
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MALLIK: BLACK SPRUCE GROWTH AND SPECIES DIVERSITY WITH SHEEP LAUREL
through their ectomycorrhizal association (Bending and
Read, 1996). It is also possible that the nutrient requirements of sheep laurel are lower than those of black
spruce. The rapid proliferation of sheep laurel after
clear cutting and fire and the associated black spruce
growth inhibition is a serious problem for forest management in central Newfoundland. Recognizing this
problem, the provincial government of Newfoundland
and Labrador has implemented new forest management
guidelines that discourage forest harvesting in sites with
dense sheep laurel cover. Although poor black spruce
growth in the presence of dense sheep laurel cover has
been widely observed in Atlantic Canada, to this day
no quantitative evaluation of black spruce growth has
been made in plots with and without sheep laurel. The
objectives of the present study were to compare (i) the
growth and foliar nutrient concentrations of planted
black spruce and (ii) the species composition, richness,
and diversity of understory plants in contiguous plots
with and without sheep laurel.
STUDY AREA AND METHODS
The study area belongs to the north-central subregion of
the central Newfoundland ecoregion that is characterized by
a high maximum summer temperature and a lower rainfall
and higher fire frequency than anywhere on the island
(Meades and Moores, 1994, p. 226). Because of the high fire
frequency, the area is dominated by pure black spruce stands
of seed origin and aspen (Populus tremuloides Michaux)
stands originating from root suckering. The soil is typically a
coarse textured humo-ferric podzol. The area has rolling to
undulated topography that is characterized by shallow, medium-quality till with a soil texture ranging from sandy loam
to loam. Black spruce after disturbance in this relatively low
moisture, coarse-textured soil suffers from regeneration failure, particularly when sheep laurel occurs as a dense understory (Meades and Moores, 1994, p. 226).
This study was conducted in a 15-yr-old black spruce plantation in Sandy Pond, central Newfoundland (488509 N, 558249
W; altitude of 153 m). The area was harvested by clear cutting
in 1979, scarified in 1981, and planted with containerized black
spruce in 1982—15 yr before this study. The planting density
was 2100 seedlings ha21. Approximately half of the 10-ha
plantation contained on an average of 36% sheep laurel cover
that was fairly uniformly distributed while the other half of
the plantation had ,1% sheep laurel cover. The sheep laurel
dominated site had a thicker organic layer than the non-sheep
laurel site. Both sites had coarse-textured freely drained sandy
loam soil with a 0.5 to 2 cm thick Ae horizon.
Species Composition and Richness
The cover of all the understory plants was determined by
sampling 10 randomly placed 1-by-1-m quadrats in each of
the sheep laurel and non-sheep laurel sites. The thickness of
the organic and Ae horizon was determined from 25 soil pits
that were randomly dug in each of the sheep laurel and nonsheep laurel sites.
Statistical Analysis
A paired t-test was used to determine the significant difference of the growth parameter and foliar nutrient means of
black spruce at the sheep laurel and non-sheep laurel sites.
PC-ORD (McCune and Mefford, 1995) was used to ordinate
the 20 sampling plots with and without sheep laurel based on
the species cover data. The species–environment relationships
were analyzed using CCA with Pearson correlation.
RESULTS
Black Spruce Growth Response
Black spruce stem height and basal diameter were
significantly less (65 and 51%, respectively) at the site
with dense cover (36%) of sheep laurel compared with
the non-sheep laurel site (Fig. 1). Consequently, after
15 yr there was 85% less black spruce volume at the
sheep laurel dominated site compared with the nonsheep laurel site (Fig. 1). Black spruce height growth
was consistently less at the sheep laurel dominated site
than at the non-sheep laurel site (Fig. 2). The stem
density of black spruce was 34% less at the sheep laurel
dominated site compared with the non-sheep laurel site
(Fig. 1). The current stem density of the two sites consists of planted seedlings as well as natural regeneration
of black spruce. However, the natural regeneration of
black spruce at the sheep laurel site was about onethird (900 stems ha21) of that of the non-sheep laurel
site (2400 stems ha21).
Black spruce grown at the sheep laurel plots contained significantly higher concentrations of Ca, Al, Fe,
and K in the needles than that in the non-sheep laurel
plots (Table 1). The sheep laurel dominated plots had
a significantly higher organic matter depth (8.3 cm) than
the non-sheep laurel plots (5.6 cm). The organic matter
depth was strongly related to the x-axis (r 5 20.841)
while the Ae horizon depth was strongly related to the
y-axis (r 5 298.2). The y-axis did not separate the sampling plots into levels of sheep laurel condition, sug-
Black Spruce Growth Response
2
Ten 50-m circular quadrats were randomly placed in each
of the sheep laurel and non-sheep laurel areas. The stem
height and basal diameter of all black spruce saplings were
determined in each quadrat. From these data, the stem density
and volume of black spruce were determined. Ten randomly
selected planted black spruce saplings were destructively sampled from each site to determine their age and yearly growth
increment by measuring their annual ring widths in two directions perpendicular to each other. The planted black spruce
seedlings were recognized by their presence in the lines with
regular spacing. Foliar samples were collected at the mid canopy level from 1-yr-old branches of black spruce for chemical analysis.
Table 1. Foliar nutrient concentrations of planted black spruce
in sheep laurel and non-sheep laurel sites. Values are the means
of 10 samples 6 SD.
Nutrient
N, %
P, mg kg21
K, mg kg21
Al, mg kg21
Ca, mg kg21
Cu, mg kg21
Fe, mg kg21
Mg, mg kg21
Mn, mg kg21
Zn, mg kg21
Sheep laurel
1.028
8.7211
3495
0.7525
56.949
0.0218
0.2821
9.8706
19.5179
0.3863
6
6
6
6
6
6
6
6
6
6
0.038
0.347
205.95a
0.084a
8.146
0.004
0.018a
0.52
2.437
0.046
Non-sheep laurel
1.028
8.2359
4403
0.5184
56.1466
0.032
0.2247
9.592
15.2522
0.4611
6
6
6
6
6
6
6
6
6
6
0.033
0.278
217.31b
0.053b
4.717
0.005
0.014b
0.689
3.184
0.046
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AGRONOMY JOURNAL, VOL. 93, JANUARY–FEBRUARY 2001
Fig. 1. (A) Mean stem height, (B) basal diameter, (C) stem density, (D) and volume of black spruce in sheep laurel and non-sheep laurel plots
at Sunday Pond, NF, Canada 15 yr after planting.
gesting that this axis had picked up within-site variability.
Black Spruce Crown Closure and Understory
Species Cover, Richness, and Diversity
With smaller black spruce, the sheep laurel dominated
site had relatively open canopy 15 yr after planting, with
only 8.5% black spruce cover. In contrast, the contiguous non-sheep laurel site was approaching canopy closure, with 56% black spruce cover (Table 2). In the
context of the stand density management diagram for
Newfoundland (Newton and Weetman, 1993) black
spruce crown closure approaching at the age of 16 yr
with a stem density of 4500 stems ha21, the non-sheep
laurel site is comparable with Site Index 10 (Newton,
1992). By contrast, the sheep laurel dominated site is
comparable to Site Index 7 (Newton, 1998).
Sheep laurel cover at the two sites was 48.5 and 1.0%,
respectively. The sheep laurel dominated site was also
associated with dense cover of blueberry (Vaccinium
angustifolium Aiton) and schreberi moss [Pleurozium
schreberi (Brid.) Mitt.]—29.5 and 44.5%, respectively,
in contrast to 18.0 and 20.0% at the non-sheep laurel
site. The cover of bunchberry (Cornus canadensis L.),
however, remained similar (12.3 and 14%) at the two
sites (Table 2).
Fig. 2. Mean cumulative black spruce height in sheep laurel and non-sheep laurel plots.
95
MALLIK: BLACK SPRUCE GROWTH AND SPECIES DIVERSITY WITH SHEEP LAUREL
Table 2. Species cover, richness, and diversity in sheep laurel and non-sheep laurel sites. Values are calculated from 10 quadrats (1 by
1 m) in each site.
% Cover
Species
Sheep laurel
Vascular plants
Picea mariana
Betula papyrifera
Larix laricinia
Populus tremuloides
Prunus pensylvanica
Nemopanthus macronuta
Viburnum cassinoides
Kalmia angustifolia
Rhododendron canadense
Vaccinium spp.
Gaultheria hispidula
Cornus canadensis
Linnaea borealis
Soidago rugosa
Epilobium angustifolium
Potentilla anserina
Cypripedium reginae
Vaccinium vitis-idaea
Mitella nuda
Maianthemum sp.
Bryophytes and Pteridophytes
Pleurozium schreberi
Hylocomium splendens
Ptilium crista-castrensis
Sphagnum spp.
Polytrichum spp.
Dicranum scoparium
Dicranum polyseptum
Lycopodium dendroideum
Lycopodium annotinum
Lichens
Cladina rangiferina
Cladina alpina
Cladina arbuscula
Cladina spp.
Permelia sulcata
Cladonia cenotea
Cladonia cornicraea
Cladonia fimbriata
Peltigera apthosa
8.5 6 9.9
2 6 3.5
0
0
0
0.4 6 1.3
0.4 6 1.3
48.5 6 27.0
3 6 9.5
29.5 6 9.6
3.3 6 6.7
3.5 6 5.8
0
0
0
0
0
5.4 6 9.5
3.8 6 4.6
0.3 6 0.95
44.5
1.5
9
1
4.9
1.8
11.7
6
6
6
6
6
6
6
0
0
16
0
0.3 6
0
4.9 6
1.8 6
6.6 6
0
0.5 6
Richness
Non-sheep laurel
56
0.3
2.1
1.8
1.3
1
18.1
10.8
14
0.5
1
0.5
0.5
0.4
1
2.3
6
6
6
6
6
0
0
6
0
6
6
6
6
6
6
6
6
0
6
6
1.6
12
16
1.24
1.23
7
9
1.01
.95
9
5
.69
.89
3.2
4.6
0
6
6
6
0
0
6
6
0
1.8
1.2
Non-sheep laurel
16.7
11.6
6.6
1.6
2.1
1.1
1.6
0.8
3.2
5.4
2.6
5.7
Sheep laurel
2.1
20.1 6 22.2
0.8 6 1.8
2.8 6 3.5
0
0
2.2 6 2.2
9.8 6 4.4
0.3 6 0.95
1 6 3.2
1
4.4
4.8
Non-sheep laurel
25.6
0.95
4.9
3.8
2.2
28.8
3.4
7.4
3.1
5.6
3.5
3.5
0.95
Diversity
Sheep laurel
1.1
4.7
5.0
3.2
1.5
Although the overall species richness of the sheep
laurel and non-sheep laurel sites was comparable with
only 28 to 30 species, the two sites were markedly different in terms of the species composition, richness, and
diversity of the vascular plants and lichens. The sheep
laurel dominated site contained 12 species of vascular
plants and five species of lichens, whereas the non-sheep
laurel site contained 16 species of vascular plants and
nine species of lichens (Table 2). A CCA of the species
cover data separated the sheep laurel dominated plots
from the non-sheep laurel plots (Fig. 3) along the
x-axis (Eigenvalue 5 0.22), which explained 12.7% of
the variance in the species data.
DISCUSSION
Both the stem density and growth of black spruce
were significantly less at the sheep laurel dominated site
(Fig. 1). Significant natural regeneration has occurred
in both sites because the planting density was 2100 seedlings ha21. However, recruitment of black spruce in
sheep laurel dominated site was about one-third that of
the non-sheep laurel site, indicating that the presence
of sheep laurel interfered with the natural regeneration
of black spruce. Although the stem density of black
spruce at the sheep laurel dominated site (|3000 stems
ha21) was less than that of the non-sheep laurel site
(|4500 stems ha21) (Fig. 1), this difference is not critical
from a resource management perspective because a density of 3000 stems ha21 is considered sufficient for black
spruce regeneration. What is more important, however,
is that the height and volume of black spruce in the
sheep laurel dominated plots is consistently less than
that of the non-sheep laurel plots.
Similar growth inhibition of black spruce has been
found in labrador tea dominated sites (Inderjit and Mallik, 1996a). However, at the labrador tea dominated site,
conifer growth tended to improve 7 yr after planting. A
poor early growth and eventual increased growth of
black spruce associated with a labrador tea dominated
site was also reported by LeBarron (1948, p. 60). In the
present study, the sheep laurel dominated site exhibited
significantly slow growth, and no subsequent growth in
height of black spruce was observed 15 yr after planting.
Thus, the growth inhibitory effect of sheep laurel on
black spruce seems to be more long-term than the effects
of labrador tea. Damman (1971) suggested that longterm occupancy of a site by sheep laurel can cause irre-
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AGRONOMY JOURNAL, VOL. 93, JANUARY–FEBRUARY 2001
Fig. 3. Canonical correspondence analysis (CCA) of sheep laurel and non-sheep laurel plots showing the significance of sheep laurel cover and
organic matter depth in their separation.
versible habitat degradation, converting conifer forests
into ericaceous heath. He attributed this vegetation shift
to the high rate of organic accumulation, soil acidification, and nutrient sequestration in the presence of
sheep laurel.
The height and diameter (at breast height, DBH) of
the destructively sampled planted black spruce of the
sheep laurel and non-sheep laurel sites were compared
with the site index curves of naturally regenerating pure
black spruce in central Newfoundland (Newton, 1992).
It was found that the black spruce at the non-sheep
laurel site fit close to Site Index 12 and that of the sheep
laurel dominated site was comparable to Site Index 10.
Using the site index curves of Newton (1992), the projected height of black spruce at the age of 50 in the nonsheep laurel and sheep laurel dominated sites would be
12.18 and 10.32 m, respectively. However, the values
for the black spruce growing at the sheep laurel dominated site may have been overestimated for at least a
couple of reasons. First, trees at the sheep laurel dominated plots were too small to determine a meaningful
diameter at breast height, and secondly, as Newton
(1992) cautioned, the site index curves of age classes 1
to 20 may not be very accurate for the small sample
size of his model. In a subsequent paper, Newton (1998)
presented a more realistic site index for black spruce in
sheep laurel sites by developing successional vectors
based on the size–density relationship (Newton and
Weetman, 1993). He suggested that delayed crown closure due to poor spruce growth and seedling mortality
in the presence of sheep laurel will lower the site index
to 7 or even 4, depending on the black spruce stem
density and the density and longevity of sheep laurel at
a site. He further suggests that even a productive black
spruce–moss forest type on sandy loam or loamy sands
may be degraded into an unproductive sheep laurel–
black spruce type of significantly lower site index if the
site is occupied by dense sheep laurel after forest harvesting.
The primary objective of the present paper was to
quantify the growth differences of black spruce at contiguous sheep laurel and non-sheep laurel sites. Perhaps
it is safe to assume that the contiguous sheep laurel and
non-sheep laurel sites initially belonged to the same site
type, and the invasion of sheep laurel transformed it
into a lower site index type (Damman, 1964, p. 62, 1971).
What is not known for sure is how long sheep laurel
has been occupying the site. A study of disturbanceinduced sheep laurel proliferation at a chronosequence
and associated black spruce regeneration failure and
habitat degradation will elucidate the role of sheep laurel in this vegetation shift.
Inderjit and Mallik (1996a) compared the growth and
foliar nutrients of planted black spruce in labrador tea
and non-labrador tea sites. They attributed the poor
growth of black spruce in labrador tea dominated sites
to a lower foliar N and to a soil nutrient imbalance that
was due to the high phenolic content of the labrador
tea litter. In this study, the black spruce grown at the
sheep laurel dominated site had smaller needles but did
not have lower concentrations of foliar N compared
with the non-sheep laurel site. These values are very
similar and within the adequate range (0.95–1.10%) for
black spruce according to Lowry and Avard (1968, p.
54). Swan (1970), however, considered 1.20% foliar N
to be low for black spruce growth. There is no evidence
in the present data to suggest that foliar N deficiency
is a cause of the growth limitation of black spruce in
the sheep laurel dominated plots. However, other nutrient and heavy-metal imbalances may be responsible.
Significantly higher concentrations of foliar Al and Fe
were found in the black spruce at the sheep laurel dominated site compared with that of the non-sheep laurel
site. Comerford and Fisher (1984) have shown that normal tree growth may be impaired by nutrient imbalances. The high phenolic content of sheep laurel leaf
and litter has been implicated as a soil depositional
factor that reduces N availability elsewhere (Inderjit
MALLIK: BLACK SPRUCE GROWTH AND SPECIES DIVERSITY WITH SHEEP LAUREL
and Mallik, 1996b; Northup et al., 1999), but the importance of this process at this site may depend on further
litter inputs over time. An invasion by sheep laurel
seems to more quickly bring about a reduction of nutrients other than N, and at present, resource deficiency
by a critical concentration of K (Swan, 1970) or increased Fe, Al, and Mn toxicity may have created soil N
deficiency and nutrient imbalance (Inderjit and Mallik,
1996b). This in turn may have created the growth inhibitory effect on black spruce.
At the sheep laurel dominated sites of central Newfoundland, field trials with spot fertilization of black
spruce with three formulations of Gromax Transplant
Fertilizer (TPFS 4, 5, and Gromax Plus) at the time of
planting produced a significant height increase of black
spruce that lasted only for 2 yr (English, 1997, p. 10).
After that, there was no significant difference in black
spruce height between the fertilized and unfertilized
plants, and the author concluded that the fertilizertreated seedlings were not able to capitalize on the initial
height growth boost to overcome the sheep laurel
growth inhibition. These results seem to suggest that
the black spruce growth inhibition phenomenon in the
presence of sheep laurel is more than just a case of
nutrient deficiency.
Results from the ericaceous litter amending experiments of Inderjit and Mallik (1996a, 1997) showed that
sheep laurel and labrador tea litter can lower pH and
increase the total phenolic content of soil; these changes
can reduce the available N and P and increase Fe, Al,
Ca, Mn, Zn, Cu, and Ba (Brady, 1990, p. 619). The
authors attributed this soil nutrient imbalance to the
high phenolic content of the ericaceous litter because
phenolics are known to influence the availability, accumulation, and uptake of nutrients (Rice, 1984; Appel,
1993). The poor black spruce growth that was observed
in the present study may have resulted from the adverse
effects of sheep laurel litter, causing allelopathy and
nutrient imbalance. But this hypothesis must be tested
by further studies. Recent studies have shown that phenolic compounds in soil bind with organic N by forming
phenol–protein complexes, and thus create soil N deficiencies in presence of ericaceous plants (Leak and
Read, 1989; Bending and Read, 1996).
An alternative explanation of the poor growth of
black spruce at the sheep laurel dominated site may be
due to the phenolic acids of the sheep laurel interfering
with the hormonal balance that is necessary for the
normal growth of spruce (Zenk and Muller, 1963; Tomaszewski and Thimann, 1966). Zenk and Muller (1963)
and Tomaszewski and Thimann (1966) showed that phenolic acids in combination with high concentrations
of metallic ions such as Mn can stimulate the decarboxylation of indoleacetic acid (IAA) and thus inhibit
plant growth. For example, p-hydroxybenzoic, vanillic,
p-coumaric, syringic, and phloretic acid are known to
reduce available IAA by promoting IAA decarboxylation (Einhellig, 1995). Zhu and Mallik (1994) identified
p-hydroxybenzoic, genticic, o-hydroxyphenylacetic, vanillic, p-coumaric, m-coumaric, ferulic, and syringic acid
97
from sheep laurel leaves. These authors have shown
that genticic and o-hydroxyphenylacetic acid at 0.5 to
5 mM concentrations, and the others at 1 to 5 mM
concentrations, can inhibit the primary root and shoot
growth of black spruce (Mallik and Zhu, 1995). However, the involvement of these phenolic acids in the
growth inhibition of larger black spruce seedlings under
field conditions has not yet been studied.
A reduced richness and diversity of vascular plants
was obtained in presence of sheep laurel compared with
the non-sheep laurel site. Habitat stress induced by the
ericaceous plants may be suggested as a filtering mechanism leading to heath formation where the species capable of tolerating nutrient stress persist. The failure of
ground-level vascular species to invade sheep laurel
dominated sites allows cryptogams to occupy the soil
surface. It can be argued that the high diversity of stress
tolerant lichens at the sheep laurel dominated site is a
reflection of the stress condition of the habitat
(Grime, 1977).
ACKNOWLEDGMENTS
The work was supported by a research grant from the Natural Science and Engineering Research Council (NSERC). I
thank Abitibi Consolidated, Grand Falls-Windsor for their
logistical help during the field work and Robin Bloom and
Felix Eigenbrod for their help in data analyses. The comments
of Dr. W.H. Carmean and two anonymous reviewers were
helpful in revising the manuscript.
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Black Spruce Growth and Understory Species Diversity
with and without Sheep Laurel
Azim U. Mallik*
ABSTRACT
Growth and understory species diversity of black spruce [Picea
mariana (Miller) B.S.P.] planted in central Newfoundland at contiguous sites with and without dense cover of sheep laurel (Kalmia angustifolia L.) were compared. Black spruce stem density and volume per
hectare were calculated by sampling 10 circular quadrats (50 m2), and
the cover of all plant species was determined by sampling 20 quadrats
(1 m2) in each site. In addition, 10 randomly sampled planted black
spruce samplings from each site were analyzed for stem height, basal
diameter, and foliar chemistry. Results showed a significantly lower
stem height and basal diameter (65 and 51%, respectively) at the site
with dense sheep laurel cover (36%) compared with the site with
sparse sheep laurel cover (,1% sheep laurel cover, and henceforth
referred to as the non-sheep laurel site for simplicity). Black spruce
grown at the sheep laurel dominated site contained significantly higher
quantities of Ca, Al, Fe, and K in the needles than that grown at the
non-sheep laurel site. The sheep laurel dominated site also had a
significantly higher mean organic matter depth of 8.3 cm compared
with 5.6 cm at the non-sheep laurel site. Canonical correspondence
analysis (CCA) of the species cover data clearly separated the sheep
laurel dominated plots from the non-sheep laurel plots. The sheep
laurel dominated site had reduced species richness of vascular plants
but increased species richness for lichens compared with the nonsheep laurel site. Allelopathy associated with phenol-induced soil
nutrient imbalance and nutrient stress is a possible cause for black
spruce growth inhibition at the sheep laurel dominated site.
R
apid growth of sheep laurel after clear cutting
and fire in sheep laurel–black spruce communities
has been widely observed in eastern Canada, particularly at sites with organic and coarse textured mediumquality soil types (Page, 1970, p. 7; van Nostrand, 1971,
p. 68; Damman, 1975). The natural regeneration of black
spruce at these sites is poor, and planted black spruce
seedlings exhibit stunted growth (Candy, 1951, p. 224;
Dep. of Biol., Lakehead Univ., Thunder Bay, ON, Canada P7B 5E1.
Received 25 Jan. 2000. *Corresponding author (azim.mallik@
lakeheadu.ca).
Published in Agron. J. 93:92–98 (2001).
Richardson and Hall, 1973a, p. 63, 1973b, p. 46; Wall,
1977, p. 55). Competition and allelopathic effects of
sheep laurel have been attributed to the regeneration
failure and poor growth of conifers (Mallik, 1987, 1990,
1992, 1996; Mallik and Roberts, 1994). In eastern and
central Newfoundland, large areas of moderately productive black spruce forests with sheep laurel understory have been converted into sheep laurel dominated heath following forest disturbance (Mallik, 1995).
A regeneration survey of 5888 plots in black spruce
plantations found that 55% of them contained sheep
laurel (English and Hackett, 1994, p. 12). Black spruce
in sheep laurel infested sites exhibits typical symptoms:
Poor plant height and diameter growth and short and
chlorotic needles, as observed in other conifers in the
presence of different ericaceous plants (Handley, 1963;
Gimingham, 1972; de Montigny and Weetman, 1990;
Fraser, 1993, p. 166; Inderjit and Mallik, 1996a; Jaderlund et al., 1997). Black spruce forests that are dominated by sheep laurel tend to have a reduced species
richness and deficiency in available nutrients (Damman,
1971). Recently, Yamasaki et al. (1998) reported that
black spruce seedlings in close proximity of sheep laurel
(,1 m) experience lower height, biomass, root/shoot
ratio, foliar N and P, and lower mycorrhizal infection
than those growing farther (.1 m) away from sheep
laurel.
Damman (1971, 1975) suggested that long-term occupancy of a site by sheep laurel causes irreversible soil
degradation, leading to a stable heath formation by precluding forest regeneration. Apparently sheep laurel,
like other ericaceous plants, is able to grow in nutrient
poor conditions where black spruce growth is very much
restricted. There is evidence suggesting that the ericaceous plants are able to access the N that is bound
in the protein–polyphenol complex through the ericoid
mycorrhizae, but this N is not available to the conifers
Abbreviations: CCA, canonical correspondence analysis; IAA, indoleacetic acid.
93
MALLIK: BLACK SPRUCE GROWTH AND SPECIES DIVERSITY WITH SHEEP LAUREL
through their ectomycorrhizal association (Bending and
Read, 1996). It is also possible that the nutrient requirements of sheep laurel are lower than those of black
spruce. The rapid proliferation of sheep laurel after
clear cutting and fire and the associated black spruce
growth inhibition is a serious problem for forest management in central Newfoundland. Recognizing this
problem, the provincial government of Newfoundland
and Labrador has implemented new forest management
guidelines that discourage forest harvesting in sites with
dense sheep laurel cover. Although poor black spruce
growth in the presence of dense sheep laurel cover has
been widely observed in Atlantic Canada, to this day
no quantitative evaluation of black spruce growth has
been made in plots with and without sheep laurel. The
objectives of the present study were to compare (i) the
growth and foliar nutrient concentrations of planted
black spruce and (ii) the species composition, richness,
and diversity of understory plants in contiguous plots
with and without sheep laurel.
STUDY AREA AND METHODS
The study area belongs to the north-central subregion of
the central Newfoundland ecoregion that is characterized by
a high maximum summer temperature and a lower rainfall
and higher fire frequency than anywhere on the island
(Meades and Moores, 1994, p. 226). Because of the high fire
frequency, the area is dominated by pure black spruce stands
of seed origin and aspen (Populus tremuloides Michaux)
stands originating from root suckering. The soil is typically a
coarse textured humo-ferric podzol. The area has rolling to
undulated topography that is characterized by shallow, medium-quality till with a soil texture ranging from sandy loam
to loam. Black spruce after disturbance in this relatively low
moisture, coarse-textured soil suffers from regeneration failure, particularly when sheep laurel occurs as a dense understory (Meades and Moores, 1994, p. 226).
This study was conducted in a 15-yr-old black spruce plantation in Sandy Pond, central Newfoundland (488509 N, 558249
W; altitude of 153 m). The area was harvested by clear cutting
in 1979, scarified in 1981, and planted with containerized black
spruce in 1982—15 yr before this study. The planting density
was 2100 seedlings ha21. Approximately half of the 10-ha
plantation contained on an average of 36% sheep laurel cover
that was fairly uniformly distributed while the other half of
the plantation had ,1% sheep laurel cover. The sheep laurel
dominated site had a thicker organic layer than the non-sheep
laurel site. Both sites had coarse-textured freely drained sandy
loam soil with a 0.5 to 2 cm thick Ae horizon.
Species Composition and Richness
The cover of all the understory plants was determined by
sampling 10 randomly placed 1-by-1-m quadrats in each of
the sheep laurel and non-sheep laurel sites. The thickness of
the organic and Ae horizon was determined from 25 soil pits
that were randomly dug in each of the sheep laurel and nonsheep laurel sites.
Statistical Analysis
A paired t-test was used to determine the significant difference of the growth parameter and foliar nutrient means of
black spruce at the sheep laurel and non-sheep laurel sites.
PC-ORD (McCune and Mefford, 1995) was used to ordinate
the 20 sampling plots with and without sheep laurel based on
the species cover data. The species–environment relationships
were analyzed using CCA with Pearson correlation.
RESULTS
Black Spruce Growth Response
Black spruce stem height and basal diameter were
significantly less (65 and 51%, respectively) at the site
with dense cover (36%) of sheep laurel compared with
the non-sheep laurel site (Fig. 1). Consequently, after
15 yr there was 85% less black spruce volume at the
sheep laurel dominated site compared with the nonsheep laurel site (Fig. 1). Black spruce height growth
was consistently less at the sheep laurel dominated site
than at the non-sheep laurel site (Fig. 2). The stem
density of black spruce was 34% less at the sheep laurel
dominated site compared with the non-sheep laurel site
(Fig. 1). The current stem density of the two sites consists of planted seedlings as well as natural regeneration
of black spruce. However, the natural regeneration of
black spruce at the sheep laurel site was about onethird (900 stems ha21) of that of the non-sheep laurel
site (2400 stems ha21).
Black spruce grown at the sheep laurel plots contained significantly higher concentrations of Ca, Al, Fe,
and K in the needles than that in the non-sheep laurel
plots (Table 1). The sheep laurel dominated plots had
a significantly higher organic matter depth (8.3 cm) than
the non-sheep laurel plots (5.6 cm). The organic matter
depth was strongly related to the x-axis (r 5 20.841)
while the Ae horizon depth was strongly related to the
y-axis (r 5 298.2). The y-axis did not separate the sampling plots into levels of sheep laurel condition, sug-
Black Spruce Growth Response
2
Ten 50-m circular quadrats were randomly placed in each
of the sheep laurel and non-sheep laurel areas. The stem
height and basal diameter of all black spruce saplings were
determined in each quadrat. From these data, the stem density
and volume of black spruce were determined. Ten randomly
selected planted black spruce saplings were destructively sampled from each site to determine their age and yearly growth
increment by measuring their annual ring widths in two directions perpendicular to each other. The planted black spruce
seedlings were recognized by their presence in the lines with
regular spacing. Foliar samples were collected at the mid canopy level from 1-yr-old branches of black spruce for chemical analysis.
Table 1. Foliar nutrient concentrations of planted black spruce
in sheep laurel and non-sheep laurel sites. Values are the means
of 10 samples 6 SD.
Nutrient
N, %
P, mg kg21
K, mg kg21
Al, mg kg21
Ca, mg kg21
Cu, mg kg21
Fe, mg kg21
Mg, mg kg21
Mn, mg kg21
Zn, mg kg21
Sheep laurel
1.028
8.7211
3495
0.7525
56.949
0.0218
0.2821
9.8706
19.5179
0.3863
6
6
6
6
6
6
6
6
6
6
0.038
0.347
205.95a
0.084a
8.146
0.004
0.018a
0.52
2.437
0.046
Non-sheep laurel
1.028
8.2359
4403
0.5184
56.1466
0.032
0.2247
9.592
15.2522
0.4611
6
6
6
6
6
6
6
6
6
6
0.033
0.278
217.31b
0.053b
4.717
0.005
0.014b
0.689
3.184
0.046
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AGRONOMY JOURNAL, VOL. 93, JANUARY–FEBRUARY 2001
Fig. 1. (A) Mean stem height, (B) basal diameter, (C) stem density, (D) and volume of black spruce in sheep laurel and non-sheep laurel plots
at Sunday Pond, NF, Canada 15 yr after planting.
gesting that this axis had picked up within-site variability.
Black Spruce Crown Closure and Understory
Species Cover, Richness, and Diversity
With smaller black spruce, the sheep laurel dominated
site had relatively open canopy 15 yr after planting, with
only 8.5% black spruce cover. In contrast, the contiguous non-sheep laurel site was approaching canopy closure, with 56% black spruce cover (Table 2). In the
context of the stand density management diagram for
Newfoundland (Newton and Weetman, 1993) black
spruce crown closure approaching at the age of 16 yr
with a stem density of 4500 stems ha21, the non-sheep
laurel site is comparable with Site Index 10 (Newton,
1992). By contrast, the sheep laurel dominated site is
comparable to Site Index 7 (Newton, 1998).
Sheep laurel cover at the two sites was 48.5 and 1.0%,
respectively. The sheep laurel dominated site was also
associated with dense cover of blueberry (Vaccinium
angustifolium Aiton) and schreberi moss [Pleurozium
schreberi (Brid.) Mitt.]—29.5 and 44.5%, respectively,
in contrast to 18.0 and 20.0% at the non-sheep laurel
site. The cover of bunchberry (Cornus canadensis L.),
however, remained similar (12.3 and 14%) at the two
sites (Table 2).
Fig. 2. Mean cumulative black spruce height in sheep laurel and non-sheep laurel plots.
95
MALLIK: BLACK SPRUCE GROWTH AND SPECIES DIVERSITY WITH SHEEP LAUREL
Table 2. Species cover, richness, and diversity in sheep laurel and non-sheep laurel sites. Values are calculated from 10 quadrats (1 by
1 m) in each site.
% Cover
Species
Sheep laurel
Vascular plants
Picea mariana
Betula papyrifera
Larix laricinia
Populus tremuloides
Prunus pensylvanica
Nemopanthus macronuta
Viburnum cassinoides
Kalmia angustifolia
Rhododendron canadense
Vaccinium spp.
Gaultheria hispidula
Cornus canadensis
Linnaea borealis
Soidago rugosa
Epilobium angustifolium
Potentilla anserina
Cypripedium reginae
Vaccinium vitis-idaea
Mitella nuda
Maianthemum sp.
Bryophytes and Pteridophytes
Pleurozium schreberi
Hylocomium splendens
Ptilium crista-castrensis
Sphagnum spp.
Polytrichum spp.
Dicranum scoparium
Dicranum polyseptum
Lycopodium dendroideum
Lycopodium annotinum
Lichens
Cladina rangiferina
Cladina alpina
Cladina arbuscula
Cladina spp.
Permelia sulcata
Cladonia cenotea
Cladonia cornicraea
Cladonia fimbriata
Peltigera apthosa
8.5 6 9.9
2 6 3.5
0
0
0
0.4 6 1.3
0.4 6 1.3
48.5 6 27.0
3 6 9.5
29.5 6 9.6
3.3 6 6.7
3.5 6 5.8
0
0
0
0
0
5.4 6 9.5
3.8 6 4.6
0.3 6 0.95
44.5
1.5
9
1
4.9
1.8
11.7
6
6
6
6
6
6
6
0
0
16
0
0.3 6
0
4.9 6
1.8 6
6.6 6
0
0.5 6
Richness
Non-sheep laurel
56
0.3
2.1
1.8
1.3
1
18.1
10.8
14
0.5
1
0.5
0.5
0.4
1
2.3
6
6
6
6
6
0
0
6
0
6
6
6
6
6
6
6
6
0
6
6
1.6
12
16
1.24
1.23
7
9
1.01
.95
9
5
.69
.89
3.2
4.6
0
6
6
6
0
0
6
6
0
1.8
1.2
Non-sheep laurel
16.7
11.6
6.6
1.6
2.1
1.1
1.6
0.8
3.2
5.4
2.6
5.7
Sheep laurel
2.1
20.1 6 22.2
0.8 6 1.8
2.8 6 3.5
0
0
2.2 6 2.2
9.8 6 4.4
0.3 6 0.95
1 6 3.2
1
4.4
4.8
Non-sheep laurel
25.6
0.95
4.9
3.8
2.2
28.8
3.4
7.4
3.1
5.6
3.5
3.5
0.95
Diversity
Sheep laurel
1.1
4.7
5.0
3.2
1.5
Although the overall species richness of the sheep
laurel and non-sheep laurel sites was comparable with
only 28 to 30 species, the two sites were markedly different in terms of the species composition, richness, and
diversity of the vascular plants and lichens. The sheep
laurel dominated site contained 12 species of vascular
plants and five species of lichens, whereas the non-sheep
laurel site contained 16 species of vascular plants and
nine species of lichens (Table 2). A CCA of the species
cover data separated the sheep laurel dominated plots
from the non-sheep laurel plots (Fig. 3) along the
x-axis (Eigenvalue 5 0.22), which explained 12.7% of
the variance in the species data.
DISCUSSION
Both the stem density and growth of black spruce
were significantly less at the sheep laurel dominated site
(Fig. 1). Significant natural regeneration has occurred
in both sites because the planting density was 2100 seedlings ha21. However, recruitment of black spruce in
sheep laurel dominated site was about one-third that of
the non-sheep laurel site, indicating that the presence
of sheep laurel interfered with the natural regeneration
of black spruce. Although the stem density of black
spruce at the sheep laurel dominated site (|3000 stems
ha21) was less than that of the non-sheep laurel site
(|4500 stems ha21) (Fig. 1), this difference is not critical
from a resource management perspective because a density of 3000 stems ha21 is considered sufficient for black
spruce regeneration. What is more important, however,
is that the height and volume of black spruce in the
sheep laurel dominated plots is consistently less than
that of the non-sheep laurel plots.
Similar growth inhibition of black spruce has been
found in labrador tea dominated sites (Inderjit and Mallik, 1996a). However, at the labrador tea dominated site,
conifer growth tended to improve 7 yr after planting. A
poor early growth and eventual increased growth of
black spruce associated with a labrador tea dominated
site was also reported by LeBarron (1948, p. 60). In the
present study, the sheep laurel dominated site exhibited
significantly slow growth, and no subsequent growth in
height of black spruce was observed 15 yr after planting.
Thus, the growth inhibitory effect of sheep laurel on
black spruce seems to be more long-term than the effects
of labrador tea. Damman (1971) suggested that longterm occupancy of a site by sheep laurel can cause irre-
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AGRONOMY JOURNAL, VOL. 93, JANUARY–FEBRUARY 2001
Fig. 3. Canonical correspondence analysis (CCA) of sheep laurel and non-sheep laurel plots showing the significance of sheep laurel cover and
organic matter depth in their separation.
versible habitat degradation, converting conifer forests
into ericaceous heath. He attributed this vegetation shift
to the high rate of organic accumulation, soil acidification, and nutrient sequestration in the presence of
sheep laurel.
The height and diameter (at breast height, DBH) of
the destructively sampled planted black spruce of the
sheep laurel and non-sheep laurel sites were compared
with the site index curves of naturally regenerating pure
black spruce in central Newfoundland (Newton, 1992).
It was found that the black spruce at the non-sheep
laurel site fit close to Site Index 12 and that of the sheep
laurel dominated site was comparable to Site Index 10.
Using the site index curves of Newton (1992), the projected height of black spruce at the age of 50 in the nonsheep laurel and sheep laurel dominated sites would be
12.18 and 10.32 m, respectively. However, the values
for the black spruce growing at the sheep laurel dominated site may have been overestimated for at least a
couple of reasons. First, trees at the sheep laurel dominated plots were too small to determine a meaningful
diameter at breast height, and secondly, as Newton
(1992) cautioned, the site index curves of age classes 1
to 20 may not be very accurate for the small sample
size of his model. In a subsequent paper, Newton (1998)
presented a more realistic site index for black spruce in
sheep laurel sites by developing successional vectors
based on the size–density relationship (Newton and
Weetman, 1993). He suggested that delayed crown closure due to poor spruce growth and seedling mortality
in the presence of sheep laurel will lower the site index
to 7 or even 4, depending on the black spruce stem
density and the density and longevity of sheep laurel at
a site. He further suggests that even a productive black
spruce–moss forest type on sandy loam or loamy sands
may be degraded into an unproductive sheep laurel–
black spruce type of significantly lower site index if the
site is occupied by dense sheep laurel after forest harvesting.
The primary objective of the present paper was to
quantify the growth differences of black spruce at contiguous sheep laurel and non-sheep laurel sites. Perhaps
it is safe to assume that the contiguous sheep laurel and
non-sheep laurel sites initially belonged to the same site
type, and the invasion of sheep laurel transformed it
into a lower site index type (Damman, 1964, p. 62, 1971).
What is not known for sure is how long sheep laurel
has been occupying the site. A study of disturbanceinduced sheep laurel proliferation at a chronosequence
and associated black spruce regeneration failure and
habitat degradation will elucidate the role of sheep laurel in this vegetation shift.
Inderjit and Mallik (1996a) compared the growth and
foliar nutrients of planted black spruce in labrador tea
and non-labrador tea sites. They attributed the poor
growth of black spruce in labrador tea dominated sites
to a lower foliar N and to a soil nutrient imbalance that
was due to the high phenolic content of the labrador
tea litter. In this study, the black spruce grown at the
sheep laurel dominated site had smaller needles but did
not have lower concentrations of foliar N compared
with the non-sheep laurel site. These values are very
similar and within the adequate range (0.95–1.10%) for
black spruce according to Lowry and Avard (1968, p.
54). Swan (1970), however, considered 1.20% foliar N
to be low for black spruce growth. There is no evidence
in the present data to suggest that foliar N deficiency
is a cause of the growth limitation of black spruce in
the sheep laurel dominated plots. However, other nutrient and heavy-metal imbalances may be responsible.
Significantly higher concentrations of foliar Al and Fe
were found in the black spruce at the sheep laurel dominated site compared with that of the non-sheep laurel
site. Comerford and Fisher (1984) have shown that normal tree growth may be impaired by nutrient imbalances. The high phenolic content of sheep laurel leaf
and litter has been implicated as a soil depositional
factor that reduces N availability elsewhere (Inderjit
MALLIK: BLACK SPRUCE GROWTH AND SPECIES DIVERSITY WITH SHEEP LAUREL
and Mallik, 1996b; Northup et al., 1999), but the importance of this process at this site may depend on further
litter inputs over time. An invasion by sheep laurel
seems to more quickly bring about a reduction of nutrients other than N, and at present, resource deficiency
by a critical concentration of K (Swan, 1970) or increased Fe, Al, and Mn toxicity may have created soil N
deficiency and nutrient imbalance (Inderjit and Mallik,
1996b). This in turn may have created the growth inhibitory effect on black spruce.
At the sheep laurel dominated sites of central Newfoundland, field trials with spot fertilization of black
spruce with three formulations of Gromax Transplant
Fertilizer (TPFS 4, 5, and Gromax Plus) at the time of
planting produced a significant height increase of black
spruce that lasted only for 2 yr (English, 1997, p. 10).
After that, there was no significant difference in black
spruce height between the fertilized and unfertilized
plants, and the author concluded that the fertilizertreated seedlings were not able to capitalize on the initial
height growth boost to overcome the sheep laurel
growth inhibition. These results seem to suggest that
the black spruce growth inhibition phenomenon in the
presence of sheep laurel is more than just a case of
nutrient deficiency.
Results from the ericaceous litter amending experiments of Inderjit and Mallik (1996a, 1997) showed that
sheep laurel and labrador tea litter can lower pH and
increase the total phenolic content of soil; these changes
can reduce the available N and P and increase Fe, Al,
Ca, Mn, Zn, Cu, and Ba (Brady, 1990, p. 619). The
authors attributed this soil nutrient imbalance to the
high phenolic content of the ericaceous litter because
phenolics are known to influence the availability, accumulation, and uptake of nutrients (Rice, 1984; Appel,
1993). The poor black spruce growth that was observed
in the present study may have resulted from the adverse
effects of sheep laurel litter, causing allelopathy and
nutrient imbalance. But this hypothesis must be tested
by further studies. Recent studies have shown that phenolic compounds in soil bind with organic N by forming
phenol–protein complexes, and thus create soil N deficiencies in presence of ericaceous plants (Leak and
Read, 1989; Bending and Read, 1996).
An alternative explanation of the poor growth of
black spruce at the sheep laurel dominated site may be
due to the phenolic acids of the sheep laurel interfering
with the hormonal balance that is necessary for the
normal growth of spruce (Zenk and Muller, 1963; Tomaszewski and Thimann, 1966). Zenk and Muller (1963)
and Tomaszewski and Thimann (1966) showed that phenolic acids in combination with high concentrations
of metallic ions such as Mn can stimulate the decarboxylation of indoleacetic acid (IAA) and thus inhibit
plant growth. For example, p-hydroxybenzoic, vanillic,
p-coumaric, syringic, and phloretic acid are known to
reduce available IAA by promoting IAA decarboxylation (Einhellig, 1995). Zhu and Mallik (1994) identified
p-hydroxybenzoic, genticic, o-hydroxyphenylacetic, vanillic, p-coumaric, m-coumaric, ferulic, and syringic acid
97
from sheep laurel leaves. These authors have shown
that genticic and o-hydroxyphenylacetic acid at 0.5 to
5 mM concentrations, and the others at 1 to 5 mM
concentrations, can inhibit the primary root and shoot
growth of black spruce (Mallik and Zhu, 1995). However, the involvement of these phenolic acids in the
growth inhibition of larger black spruce seedlings under
field conditions has not yet been studied.
A reduced richness and diversity of vascular plants
was obtained in presence of sheep laurel compared with
the non-sheep laurel site. Habitat stress induced by the
ericaceous plants may be suggested as a filtering mechanism leading to heath formation where the species capable of tolerating nutrient stress persist. The failure of
ground-level vascular species to invade sheep laurel
dominated sites allows cryptogams to occupy the soil
surface. It can be argued that the high diversity of stress
tolerant lichens at the sheep laurel dominated site is a
reflection of the stress condition of the habitat
(Grime, 1977).
ACKNOWLEDGMENTS
The work was supported by a research grant from the Natural Science and Engineering Research Council (NSERC). I
thank Abitibi Consolidated, Grand Falls-Windsor for their
logistical help during the field work and Robin Bloom and
Felix Eigenbrod for their help in data analyses. The comments
of Dr. W.H. Carmean and two anonymous reviewers were
helpful in revising the manuscript.
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