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ALLELOPATHY SYMPOSIUM

Growth Inhibitory Effects of Salal on Western Hemlock and Western Red Cedar

^a Dep. of Biol., Lakehead Univ., Thunder Bay, ON, Canada P7B 5E1
^b Dep. of Forest Sci., Univ. of Br. Columbia, Vancouver, BC, Canada V6T 1Z4

Corresponding author (azim.mallik{at}lakeheadu.ca)

	ABSTRACT

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Salal (Gaultheria shallon Pursh) regenerates vigorously after harvesting in the cedar–hemlock (CH) forests of coastal British Columbia, Canada. Eight years after harvesting, regenerating conifers on salal-dominated sites exhibited stunted growth and chlorotic foliage. The potential inhibitory effect of salal on hemlock was tested by germinating and measuring the primary growth of seedlings in the presence of salal litter and different concentrations of the litter leachate. Effects of aboveground and belowground competition of salal and its presumed growth inhibitory effects on western hemlock [Tsuga heterophylla (Raf.) Sarg.] and western red cedar (Thuja plicata Donn ex D. Don) were tested with pot experiments under greenhouse conditions. Seed germination and primary growth of hemlock seedlings were not significantly affected by water leachates of salal leaf or litter, nor were they affected when grown directly on partially decomposed salal litter. The primary root growth of hemlock seedlings was significantly increased in CH humus. Salal litter had no significant effect on hemlock and cedar seedlings. Combined aboveground and belowground competition from salal had the strongest negative effects on both hemlock and cedar, causing significant reductions in stem height, basal diameter, and biomass. Belowground competition from salal affected the conifers more than aboveground competition. The results suggest that the growth inhibitory effect of salal on conifers is primarily due to competition for nutrients rather than allelopathic effects of litter.

Abbreviations: ANOVA, analysis of variance • CH, cedar–hemlock • HA, hemlock–amabilis fir

	INTRODUCTION

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SALAL IS AN EVERGREEN ERICACEOUS SHRUB that forms a dense understory in the CH forests of coastal British Columbia (Fraser et al., 1993). After forest harvesting on these sites, salal regenerates rapidly by vegetative means, and 6 to 8 yr later, both naturally regenerated and planted conifers such as western hemlock, Sitka spruce [Picea sitchensis (Bong.) Carr.], and to a lesser degree, western red cedar experience severe growth inhibition (Fraser, 1993, p. 166; Weetman et al., 1989a,b). Foliar analysis showed that the affected conifers suffer from low levels of N and P (Prescott et al., 1993). Trees that are established on adjacent hemlock–amabilis fir [Abies amabilis (Dougl.) Forbes] (HA) sites that are free of salal do not exhibit these symptoms (Weetman et al., 1989a,b). Significant improvement in conifer growth has been achieved through the N fertilization of sites that exhibited conifer growth inhibition (Prescott and Weetman, 1996, p. 85).

Inadequate natural regeneration and poor growth of planted black spruce [Picea mariana (Miller) B.S.P.] in the presence of another ericaceous plant, sheep laurel (Kalmia angustifolia L. var. angustifolia), has been reported from nutrient poor sites of eastern Canada (Page, 1970, p. 7; Richardson, 1975, p. 34; Richardson and Hall, 1973a, p. 63; Richardson and Hall, 1973b, p. 46; English and Hackett, 1994, p. 12). The regeneration failure of black spruce in the presence of sheep laurel was attributed to the combined effects of allelopathy, competition, and soil nutrient imbalance (Mallik, 1995, 1998). It was found that the humus and leaf leachate of sheep laurel inhibits the primary root elongation of black spruce, balsam fir [Abies balsania (L.) P. Mill], and red pine (Pinus resinosa Ait.) (Mallik, 1987; Thompson and Mallik, 1989; Mallik and Roberts, 1994). Inderjit and Mallik (1996a)( b, 1997) reported that water soluble phenolic compounds released from the leaves of sheep laurel and labrador tea (Ledum groenlandicum Oeder) create nutrient imbalances in soil by reducing the amount of available N and increasing the amount of Fe, K, Ca, Mg, and Mn.

Poor natural regeneration of Norway spruce [Picea abies (L.) Karst.] in the presence of bilberry (Vaccinium myrtillus L.) has been reported by several authors from the subalpine spruce forests of France (Pellissier, 1993). Pellissier (1993) suggested that the potential causes of the regeneration failure of Norway spruce in these habitats are the seed predation and reduced seed germination and seedling growth due to allelopathy and competition from bilberry. A low-growing member of the Empetraceae, crowberry (Empetrum hermaphorditum Hagerup), was found to be responsible for the inhibition of the germination and seedling growth of Scots pine (Pinus sylvestris L.) in northern Sweden (Nilsson and Zackrisson, 1992; Zackrisson and Nilsson, 1992). Using these plants, Nilsson (1994) was able to separate the allelopathic effects of E. hermaphorditum from the effects of competition.

The autecology of salal is similar to that of the other ericaceous shrubs described above, and all of these shrubs have growth inhibitory effects on conifers. The water soluble organic substances from the fresh leaves and partially decomposed litter and humus of salal might interfere with the seed germination and seedling growth of hemlock, as occurs with the germination of Norway spruce in the presence of bilberry leaf leachate and humus (Jäderlund et al., 1996; Pellissier, 1993). Therefore, the objectives of the present study were to determine (i) if salal leaf leachate and CH humus affect the seed germination and seedling growth of hemlock, (ii) whether conifer growth inhibition in the presence of salal is due to competition or allelopathy from its litter, and (iii) whether aboveground or belowground competition is most important in the conifer growth inhibition process.

	Materials and methods

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Two sets of experiments were conducted to test (i) the seed germination and seedling growth response of western hemlock in the presence of salal leaf leachate and CH humus and (ii) the aboveground and belowground competition and allelopathic effects of salal on western hemlock and western red cedar in pot cultures.

Seed Germination and Seedling Growth
Leachates were collected after soaking 10, 20, 40, 60, and 100 g of fresh salal leaves or an equivalent amount of partially decomposed salal litter in large glass beakers with 400 mL of distilled water for 72 h. Hemlock seeds were stratified by soaking in running water over night followed by surface drying and storing the moist seeds at 4°C for 6 wk. Eight stratified hemlock seeds were planted in a petri dish that was lined with Whatman No. 1 filter paper and received 5 mL of one of the leachates. Seeds that were sown in petri dishes on filter paper soaked with distilled water were used as the control. In addition to the leachate treatments, hemlock seeds were planted directly on salal litter and CH humus. For these treatments, the petri dishes were half filled with fresh litter and CH humus before sowing. Each treatment had five replicates. After the initial treatment, the seeds were kept moist by adding distilled water as required. The number of seeds that germinated was recorded every day for 21 d, at which time the root and shoot lengths were measured. The dry weights of the roots and shoots were determined following oven drying of the samples at 72°C for 24 h. No red cedar seeds were used in this germination experiment.

Competition and Allelopathy
This experiment was conducted with transplanted hemlock and cedar seedlings that were grown with salal in HA humus in pot cultures under greenhouse conditions. Approximately 2 m³ of HA forest-floor humus was collected from three mature forests that are approximately 25 km west of Port McNeill, Vancouver Island (50°30' N, 127°22' W; 150 m above sea level). The forests were dominated by 80- to 100-yr-old western hemlock and amabilis fir, with very few understory species other than pleurocarpous mosses such as Brachythecium frigidum, Kindbergia oregana, Rhytidiadelphus spp., and Hylocomium splendens. At each of the forest sites, humus was collected from 10 random locations. After removing the living moss layer, a 15 to 20 cm thick humus sample was cut out in blocks with a knife and transported to a truck. The collections were done in small forest openings that were free from salal understory. The composite samples were transported to the University of British Columbia campus and left to air-dry for 3 wk in a greenhouse with occasional mixing. When the humus lost approximately 80% of its moisture, it was passed through a garden mulcher and thoroughly mixed to get uniform texture.

Fifteen-liter round plastic pots (29.5 cm i.d. and 23 cm deep; 5T-15, Listro Products, Vancouver, BC) were used for this experiment. First, a thin layer (2 cm) of crushed rock was put in each pot for drainage of excess water, and then the pots were filled with the thoroughly mixed HA humus. A 6-cm space was left on top for watering and adding treatment substances. A total of 104 pots were prepared, and they were divided into two sets. One set of 52 pots was used to test the effect of salal competition and allelopathy on western hemlock, and the other 52 pots were used to determine the salal effects on western red cedar. Each set of 52 pots was divided into four groups, and the following four treatments were applied: (i) no salal (Control 1), (ii) root and shoot competition from salal, (iii) shoot competition, and (iv) root competition (Fig. 1) .

View larger version (22K): [in this window] [in a new window]   Fig. 1 Treatment structure differentiating salal root and shoot competition and allelopathic effects

The same experimental design was followed with red cedar and salal, except in the allelopathy test for root and shoot competition from salal, shoot competition, and root competition. For these treatments, the activated-charcoal treatment was replaced with fireweed (Epilobium angustifolium L.) litter (70 g pot^-1). We presumed that the easily decomposable fireweed litter would have little or no allelopathic effects; thus, this treatment would test the potentially positive effect of salal litter amendment on conifer growth.

One-year-old salal grown in plastic pots that were 9 cm square and 8 cm deep were purchased from Langley tree nursery in British Columbia. Each nursery container had an average of six salal seedlings with an average height of 21 cm.

At the start of the experiment, the average stem height of the hemlock seedlings was 24.2 (4.2 SD) cm, and the average basal diameter was 3.0 (0.2 SD) mm. The average dry weights of the root, needle, and stem were 0.92 (0.3 SD), 1.23 (0.3 SD), and 0.55 (0.1 SD) g, respectively. For the 3-mo-old red cedar, the mean stem height and basal diameter were 8.65 (0.6 SD) cm and 0.88 (0.1 SD) mm, and the dry weights of the root and the needle plus shoot were 0.02 (0.0 SD) and 0.01 (0.2 SD) mg, respectively.

The control hemlock and red cedar seedlings were grown as a single plant in the middle of a 15-L pot. For the treatment receiving both aboveground and belowground competition from salal, two sets of container-grown salal seedlings were planted opposite each other near the margin of the pots while the hemlock or red cedar seedling was planted in the middle of each pot. At the start of the experiment, the following start values for salal in each pot were estimated: Mean height, 16.5 cm; number of stems, 36; and dry weights of the roots, leaves, and stems, 9.3, 14.3, and 4.8 g, respectively. These values were determined by destructively sampling 10 sets of salal that were contained in the nursery pots. The root competition of salal was separated from that of cedar and hemlock by growing the conifers within thin-walled (2 mm) round plastic pots with a diameter of 13.5 cm, a height of 18 cm, and 8 holes (3 cm diam.) at the bottom (Nilsson, 1994). These root exclusion containers were placed on the crushed rock in the middle of the larger pots. Root competition from salal was achieved by growing the conifer with salal in the same pot and placing a conical wire fence (mesh size 2 cm) in each pot. The fences were 29 cm high with an upper and lower diameter of 25 and 18 cm, respectively. Each fence was attached with three metal stakes that were pushed into the humus. All of the aboveground parts of salal were kept outside the fence. The experiment was conducted at the University of British Columbia Horticultural Greenhouse from April to December 1998, with a mean temperature and relative humidity of 20.5°C and 71%, respectively.

The stem height and basal diameter of hemlock and red cedar as well as the salal stem height and total number of stems per pot were determined at planting in April 1998 and at the end of the experiment in December 1998. The dry weight of the aboveground and belowground components of salal and the conifers were determined at the end of the experiment by oven-drying the samples at 72°C until they were a constant weight.

Humus samples were collected at the end of the experiment from pots containing seedlings of western hemlock and red cedar to examine the effect of belowground competition from salal on N availability. The samples were taken (i) without salal, (ii) from inside the root exclosure, and (iii) from outside the root exclosure (i.e., in the presence of salal roots). Four samples of each humus type per species were extracted with 1 M of KCl, and the concentrations of NH₄–N and NO₃–N in each extract were measured with an Alpken autoanalyzer (Karla and Maynard, 1991).

Data Analysis
Data on the seed germination and seedling root and shoot growth of hemlock at the end of the germination trial were analyzed with a one-way analysis of variance (ANOVA), and significant differences among the means were determined using Tukey's HDS test. Arcsine transformation was done to normalize the percent seed germination data before the ANOVA (Zar, 1996, p. 662). Data for the allelopathy and competition experiments were also analyzed with a one-way ANOVA. All statistical analyses were done using the SPSS 7.0 software.

	Results

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Seed Germination and Seedling Growth
Hemlock seed germination ranged from 85 to 92.5%, and there was no significant difference among the treatments (data not shown). The root length of germinated hemlock did not differ significantly among the treatments, with the exception of the CH humus treatment, where an increase in root length occurred compared with all other treatments (Table 1). The shoot length of the hemlock seedlings grown in CH humus was also significantly greater than that of the control and the 20, 30, and 50% leachate treatments (Table 1). There was no significant difference in the dry weight of shoots, but the dry weight of hemlock roots that received the 20% leachate treatment was significantly greater than that receiving 5 and 50% leachate and salal litter and CH humus treatments.

View larger version (33K): [in this window] [in a new window]   Fig. 2 Mean ± SE of height, root length, basal diameter, and stem, needle and root dry weights of western hemlock seedlings after 9 mo of receiving (C) no competition (control), (R + S) root-plus-shoot competition, (S) shoot competition, and (R) root competition from salal. Histograms in each experiment topped by different letters are significantly different at (Tukey's multiple-range test)

In the absence of salal root competition, salal litter and salal litter plus charcoal caused a significant reduction in the basal diameter of hemlock compared with the no litter and charcoal-added treatments (Table 2). In the presence of root competition, hemlock had a significantly smaller root length and total biomass compared with no litter, salal litter, and charcoal-added treatments. No significant difference was found in the other parameters of hemlock following the litter and charcoal amending treatments (data not shown).

View larger version (29K): [in this window] [in a new window]   Fig. 3 Mean ± SE of height, root length, basal diameter, root, stem plus needle, and total dry weights of cedar seedlings after 9 mo of receiving (C) no competition (control), (R + S) root-plus-shoot competition, (S) shoot competition, and (R) root competition from salal. Histograms in each experiment topped by different letters are significantly different at (Tukey's multiple-range test)

Response of Salal to Allelopathy and Competition Treatments
There were no significant differences in the salal height, aboveground biomass, or number of stems per pot among the different litter treatments. The stem height of salal was significantly greater in the pots that received fireweed litter compared with other treatments (in the absence of salal shoot competition) (data not shown).

When the salal response was compared among the competition treatments, a significant increase in the salal stem number per pot was observed in the root competition treatment compared with the other two treatments (data not shown). However, the salal stem height was significantly less in the root competition treatment compared with the shoot competition treatment and the control (no salal). Significantly less aboveground biomass of salal was observed in the shoot competition treatment compared with the root competition treatment and control (Table 4).

	Discussion

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Response of Hemlock to Salal Allelopathy and Competition
In the presence of both root and shoot competition from salal, the addition of salal litter had no significant effect on hemlock growth. Therefore, it is concluded that salal litter had no apparent allelopathic effect on hemlock. However, in the absence of root competition from salal, the addition of salal litter and salal litter plus charcoal caused a significant decrease in the basal diameter of hemlock compared with the no litter treatment (Table 2). But in the absence of salal shoot competition, the addition of salal litter and charcoal caused a significant reduction in the root length and total biomass of hemlock. This result is contrary to the general expectation that the charcoal would absorb the soluble phenolic compounds (Nilsson, 1994) while the litter was expected to provide nutrients, and hence improve the seedling growth. It is possible that microbial decay of the litter immobilized the nutrients during the course of the experiment. Nutrient immobilization is common during the early stages of litter decomposition (Aber and Melillo, 1991). Coupled with competition for N with the extensive fine root mass of salal in these pots, this treatment may have increased the severity of N deficiency for the seedlings.

The hemlock seedling growth was most greatly affected by the combined effects of salal whole plant competition, followed by root competition and shoot competition (Fig. 2). The affected seedlings were chlorotic with very poor root development. Increased shoot growth is a common response of seedlings to N fertilization (Cormer and Jarvis, 1990; Hinckley et al., 1992). Thus, the greater seedling shoot growth in the absence of salal roots may be indicative of a greater availability of N in the humus from which salal roots were excluded. The measures of KCl-extractable N in humus at the end of the experiment corroborated this suggestion, as N levels were lower in the humus where salal roots were present. Water was maintained at nonlimiting levels throughout the experiment. Thus, the increased seedling shoot growth in the absence of salal roots suggests that the negative influence of salal on hemlock seedling growth is primarily an effect of competition for N.

Response of Western Red Cedar to Allelopathy and Competition
There was no evidence of an allelopathic effect of salal litter on the growth of cedar seedlings. The small increase in the stem height and basal diameter of cedar amended with salal litter, fireweed litter, and salal litter plus activated charcoal is likely due to the release of nutrients from the litters. The growth of cedar seedlings was, however, even more affected by salal competition than hemlock. This may be because the cedar seedlings were smaller at the start of the experiment (only 3-mo-old compared with the 1-yr-old hemlock seedlings), and hence more sensitive to competition.

Response of Salal to Allelopathy and Competition
Generally, all allelopathy experiments test the effects of donor plants on the target plants (Mallik, 1987; Nilsson and Zackrisson, 1992; Zackrisson and Nilsson, 1992; Weidenhamer et al., 1989; Nilsson, 1994; Jäderlund et al., 1996). In this experiment, we studied the effects of allelopathy and competition treatments reciprocally on both the target plants (conifers) and the donor plant (salal). Litter-amending allelopathic treatments had no significant effect on the salal stem number or biomass, indicating no autotoxic effects of salal litter. There was some reduction in salal growth in the shoot competition treatments, likely due to the limited space for root growth in these pots.

Forest Management Implications
We found that the negative influence of salal on conifer growth was through competition—primarily root competition for nutrients rather than allelopathic effects of litter leachates. This has implications for the management of cedar–hemlock sites to improve the productivity of conifers. If competition for nutrients is the main effect of salal, fertilization to alleviate the nutrient supply problem should be more effective than vegetation control to eliminate salal. A combination of fertilization and vegetation control would be even more effective at alleviating competition from salal.

	ACKNOWLEDGMENTS

 
The research was made possible by a sabbatical leave of the first author from Lakehead University and a partial financial support from a Forest Renewal Br. Columbia (FRBC) grant awarded to the Salal Cedar Hemlock Integrated Research Program (SCHIRP). We appreciate the help of L. Blevins, K. Bothwell, J. Lavery, and L. Zabek for field assistance and L. Blevins, K. Cockle, S. Harper, A. Plautz, C. Staley, and S. Vetterlein for the greenhouse studies. We also thank J. Bennett and L. Lavkulich for their valuable discussions and Western Forest Products for field logistics and accommodation.

Received for publication January 25, 2000.

	REFERENCES

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (6) Citing Articles via Google Scholar Google Scholar Articles by Mallik, A. U. Articles by Prescott, C. E. Search for Related Content PubMed Articles by Mallik, A. U. Articles by Prescott, C. E. Agricola Articles by Mallik, A. U. Articles by Prescott, C. E. Related Collections Crop Growth and Development Allelopathy