Agronomy Journal 93:92-98 (2001)
© 2001 American Society of Agronomy
ALLELOPATHY SYMPOSIUM
Black Spruce Growth and Understory Species Diversity with and without Sheep Laurel
Azim U. Mallik
Dep. of Biol., Lakehead Univ., Thunder Bay, ON, Canada P7B 5E1
Corresponding author (azim.mallik{at}lakeheadu.ca)
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ABSTRACT
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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 non-sheep 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.
Abbreviations: CCA, canonical correspondence analysis • IAA, indoleacetic acid
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INTRODUCTION
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RAPID 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 medium-quality 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; 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; Jäderlund 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 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.
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Study area and methods
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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 (48°50' N, 55°24' 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 ha-1. 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.
Black Spruce Growth Response
Ten 50-m2 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.
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 non-sheep 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.
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Results
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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 non-sheep 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 one-third (900 stems ha-1) of that of the non-sheep laurel site (2400 stems ha-1).
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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
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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 
while the Ae horizon depth was strongly related to the y-axis 
. The y-axis did not separate the sampling plots into levels of sheep laurel condition, suggesting that this axis had picked up within-site variability.
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Table 1 Foliar nutrient concentrations of planted black spruce in sheep laurel and non-sheep laurel sites. Values are the means of 10 samples ± SD
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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 ha-1, 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).
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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
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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).
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 
, which explained 12.7% of the variance in the species data.
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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
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Discussion
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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 ha-1. 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 ha-1) was less than that of the non-sheep laurel site (4500 stems ha-1) (Fig. 1), this difference is not critical from a resource management perspective because a density of 3000 stems ha-1 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 long-term occupancy of a site by sheep laurel can cause irreversible 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 non-sheep 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 disturbance-induced 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 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 fertilizer-treated 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 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).
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ACKNOWLEDGMENTS
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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.
Received for publication January 25, 2000.
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ABSTRACT
INTRODUCTION
