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FOREST, RANGE & WILDLAND SOILS

Nitrogen and Water Availabilities and Competitiveness of Bluejoint: Spruce Growth and Foliar Carbon-13 and Nitrogen-15 Abundance

^a Dep. of Renewable Resources, 442 Earth Science Bldg., Univ. of Alberta, Edmonton, AB T6G 2E3, Canada

^* Corresponding author (scott.chang{at}ualberta.ca).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
How resource availabilities affect the competitiveness of Canada bluejoint grass [Calamagrostis canadensis (Michx.) P. Beauv., hereafter referred to as bluejoint] is poorly understood. Bluejoint is a widespread grass species in boreal forests and competes with tree species such as white spruce [Picea glauca (Moench) Voss] for belowground resources (e.g., soil N and water) when their supply is limited. In this greenhouse-based study, we tested the following hypotheses: (i) bluejoint competition reduces white spruce growth when belowground resource availabilities are limited; (ii) greater N and water availabilities may increase bluejoint competition and its adverse effects on white spruce growth; and (iii) white spruce foliar ¹³C and ¹⁵N are affected by soil N and water availabilities and bluejoint competition. A 2 x 2 x 2 (competition x N availability x water availability) factorial experiment was conducted using pots of planted white spruce seedlings with or without bluejoint. Bluejoint competition reduced the volume index (diameter² x height) of white spruce by 50%. The competitiveness of bluejoint appeared to be independent of resource availabilities, but bluejoint had greater growth response to increased N availability than white spruce. Bluejoint competition depleted white spruce foliar ¹³C and ¹⁵N by 1.2 and 1.2, respectively, even under adequate water supply, indicating that N deficiency caused by bluejoint competition had a dominant effect (increasing ¹³C discrimination during photosynthesis) compared with the potential effect of drought stress on foliar ¹³C, and that strong NH₄ uptake by bluejoint may have prevented significant soil N losses and ¹⁵N enrichment through nitrification and subsequent denitrification.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Canada bluejoint grass is a widespread understory grass species that grows vigorously on mesic to hygric and nutrient-rich sites in boreal forests (Eis, 1981; Beckingham et al., 1996). Since this grass species can aggressively compete for resources including nutrients, water, and light, bluejoint competition may reduce the growth of white spruce, one of the most important commercial tree species in Canada (Lieffers et al., 1993; Ländhausser and Lieffers, 1998; Bell et al., 2000). Our previous studies conducted in the field showed that: (i) bluejoint infestation significantly reduced the diameter growth of white spruce even without competition for light, i.e., where white spruce was taller than the competing vegetation and was thus free to grow; (ii) bluejoint affected soil N cycling processes such as N mineralization and nitrification by decreasing soil temperature; (iii) bluejoint decreased soil N availability as a result of its strong competition for available N; (iv) bluejoint reduced soil gravimetric water content in a dry year; and (v) the reduction of white spruce growth may be related to soil N availability that was greatly depleted by bluejoint (Matsushima and Chang, 2006, 2007). These results are consistent with the fact that bluejoint could immobilize significant amounts of soil available N (Staples et al., 1999; Robinson et al., 2001), as its root system could rapidly take up NH₄⁺ and NO₃^– ions in the soil solution (Hangs et al., 2003).

Understory competition has been reported to be intensified by N fertilization in young regenerated western red-cedar (Thuja plicata Donn ex D. Don) stands on the West Coast in British Columbia (Chang et al., 1996) and in other systems (Staples et al., 1999; Ostertag and Verville, 2002; Goodman and Hungate, 2006). Nitrogen fertilization has also been found to increase bluejoint biomass growth in boreal forests (Robinson et al., 2002; Matsushima and Chang, 2006).

Bluejoint prefers a moisture-rich environment, but its competition for soil water or the interaction between bluejoint competition for N and water has been less studied (Lieffers et al., 1993). Hogenbirk and Wein (1991) reported unchanged growth of bluejoint under drought conditions, while Ländhausser et al. (2003) found the greatest bluejoint leaf area under water-saturated conditions. Interactions between soil N and water availabilities may influence plant competition since soil water content directly affects N movement in the soil and consequently N supply to plants (Casper and Jackson, 1997; Blank et al., 2002), yet we do not understand how soil N and water availabilities interact to affect the competitiveness of bluejoint.

The predicted global climate change may result in changed plant water availability and therefore a changed competitive relationship between different plant species in future environments (Casper and Jackson, 1997; White et al., 2001). Fay et al. (2003) reported that grass species have different sensitivities to changes in precipitation amount and pattern. In Canada, the mean annual precipitation during the last 40 yr has increased, and the patterns of precipitation events (e.g., light vs. heavy rainfalls) have also been changing (Akinremi et al., 1999). When we consider future ecosystem changes and the consequent adaptation of forest management practices under global climate change, the aggressive competition from a bluejoint-dominated understory should be taken into account for boreal forest ecosystems. Furthermore, we need to understand how bluejoint responds to altered resource availabilities (such as N and water) and how that changes the effects of bluejoint on white spruce growth. Precise control of water availability concomitant with the control of N availability are difficult to achieve in open field conditions and, therefore, experiments using carefully controlled environmental conditions are essential to simulate these changes.

The abundance of ¹³C (expressed as ¹³C) in plant tissues may reflect the effects of environmental conditions on plant gas exchange, as it is affected by stomatal conductance and CO₂ fixation activities (Livingston et al., 1999). In C₃ plant species, heavier isotope (¹³C) discrimination occurs during photosynthesis due to its slower physical and chemical reactions during CO₂ diffusion and carboxylation by RuBisCO (Farquhar et al., 1989). Drought stress can lead to stomatal closure and increase ¹³C fixation, resulting in less discrimination against ¹³C. Therefore, less negative ¹³C may indicate drought stress, and plant tissue ¹³C is known to be correlated with plant water use efficiency (Sun et al., 1996). Nitrogen availabilities can also affect plant tissue ¹³C, and N deficiency has been reported to result in greater discrimination against ¹³C (resulting in more negative ¹³C; Livingston et al., 1999).

In addition to ¹³C, ¹⁵N abundance (expressed as ¹⁵N) in plant tissues has been used to examine environmental effects on soil N cycling (Choi et al., 2005). The discrimination of ¹⁵N against ¹⁴N occurs in the soil mainly through nitrification, denitrification, NO₃ leaching, and other N transformation processes because the lighter isotope, ¹⁴N, has a faster rate of reaction than the heavier ¹⁵N (Högberg, 1997), and these N transformation processes would make the substrate more enriched with ¹⁵N. Specific responses of ¹⁵N to environmental changes are less clearly understood than that of ¹³C. Understory competition can affect tree ¹³C through changing soil N, water, and other resource availabilities, and therefore foliar ¹³C and ¹⁵N values may reflect physiological responses of trees to understory competition (Fotelli et al., 2003; Kume et al., 2003). The foliar ¹³C and ¹⁵N values of trees under bluejoint competition and changing N and water availabilities may clarify the competitiveness of bluejoint under changing environmental conditions.

The objectives of this greenhouse-based study were: (i) to study the effects of bluejoint competition and soil N and water availabilities on white spruce growth; (ii) to examine the interactions between the availabilities of soil N and water and how that affects interspecific competition; and (iii) to determine the effects of competition and N and water availabilities on plant foliar ¹³C and ¹⁵N. We tested the following three hypotheses: (i) bluejoint competition reduces white spruce growth when soil N or water availabilities are limited; (iii) greater N and water availabilities reduce white spruce growth when planted with bluejoint as the growth and competitiveness of the latter would be disproportionally increased; and (iii) white spruce foliar ¹³C decreases under N stress and increases under drought stress, depending on N and water availabilities and bluejoint competition, while foliar ¹⁵N increases under high soil N and water availabilities due to greater potentials of N loss.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Experimental Design
This study was conducted in growth chambers with controlled environmental conditions. The experiment used three treatments (bluejoint competition, N fertilization, and watering), each with two levels in a 2 x 2 x 2 factorial design with five replications (five blocks), with a total of 40 pots. The first treatment included two levels of bluejoint competition, with (C1) or without (C0) competition, assigned to pots planted with white spruce seedlings. The second treatment included two levels of N concentrations in the watering solution: 25 and 100 mg N L^–1 (N25 and N100, respectively), where conifers would be provided with sufficient N at 100 mg N L^–1 (Utriainen and Holopainen, 2001). Nutrient solutions containing these levels of N and optimal levels of other essential nutrients were applied to the pots (Table 1; after Utriainen and Holopainen, 2001). The third treatment included two levels of watering regime to maintain 20 and 40% of volumetric soil water content (W20 and W40, respectively) when pots were watered, where the volumetric water content of the growth media at field capacity is around 50%. The concentration of nutrients in solution applied in the W20 treatment was twice that in the W40 treatment for the respective N treatment so that each pot received the same amount of nutrients for the same fertility level at each watering time. Nutrient solutions were applied every 2 or 3 d to maintain soil volumetric water content at the desired levels. Soil water content at each watering time was measured using a Field Scout TDR 100 Soil Moisture Meter (Spectrum Technologies, Plainfield, IL) or occasionally by weighing each pot to double-check the watering treatment. In between waterings, soil water draw-down was small, consistent with the short interval between waterings. The extent of water content depletion depended on the growth of tree and grass in each pot; soil water content was drawn down more quickly between waterings in the latter stage of plant growth than in the beginning and more quickly in C1 than in C0. The N treatment did not affect water consumption in the pots.

The white spruce seedlings used in this experiment were containerized stock type PSB 412 1+0. Thirty-two (making up four of the five blocks) of the 40 seedlings were provided by the PRT Nursery in Beaverlodge, AB, and eight (making up one block) by Coast to Coast Reforestation Nursery in Edmonton, AB. The seedlings were stored in a refrigerator (4°C) until they were transplanted to pots on 4 Mar. 2004. Two different white spruce stocks were used, as shortly following the initial planting some pots had to be replanted. This should cause no effects on the data interpretation as the differences between the two stock types were blocked out in the experimental design.

Rhizomes of bluejoint were collected from the field experiment site (Matsushima and Chang, 2006) on 28 Sept. 2003 and stored in a refrigerator (4°C) until planting. After being washed with water to remove soil, rhizomes were cut into pieces that were about 5 cm long and planted in peat moss to germinate. About 30 d after germination, four to five shoots of bluejoint were transplanted into each pot of the C1 treatment on 24 Apr. 2004.

The experiment was conducted in a controlled environmental chamber in the Department of Biological Sciences, University of Alberta, where the conditions were maintained with an 18/6 h light/dark period, a day/night temperature regime of 24/15°C, and a light level of 300 µmol m^–2 s^–1. The 40 pots planted with white spruce seedlings were watered to 45% of volumetric soil water content without any nutrients for 50 d, and then competition, N, and watering treatments were applied for 114 d. When the experiment was halfway through (i.e., 54 d after the treatments were applied), bluejoint was found to require more light for its optimum growth, and at that point the pots were moved to another growth chamber (16/8 h, 23/14°C) that had a higher light level (350 µmol m^–2 s^–1) for 60 d. The chambers had semicontrolled humidity, which was maintained at about 60% relative humidity, but humidity was not recorded.

Plant Measurements
The diameter and height of the seedlings were measured using a caliper and a tape measure, respectively, on the first day and 114 d after treatments were applied. The increments of height and diameter were determined as the difference between the initial and final measurements. Volume index (diameter² x height) was calculated using the diameter and height measurements at the end of the experiment.

Aboveground bluejoint and white spruce current-year foliar biomass (needles flushed after transplanting, hereafter referred to as current-year growth) were collected, dried in an oven at 70°C for 24 h, and weighed. The determination of root biomass for both species was not feasible, because dense bluejoint roots fully occupied the growth medium in each pot and the root systems of the two species were intermingled together. As a result, complete separation between white spruce and bluejoint roots was not possible. Therefore, only roots of white spruce were carefully collected for determination of root N concentrations. Needles of white spruce were separated from branches, and then 100 needles from each seedling were randomly selected and weighed. The shoots of bluejoint and needles and roots of white spruce were ground by a ball mill (Retsch GmbH, Haan, Germany) and analyzed for C and N concentrations and C and N isotope compositions using a continuous-flow stable isotope ratio mass spectrometer (IsoPrime-EA, Micromass, Manchester, UK) linked to a CN analyzer (NA Series 2, CE Instruments, Milan, Italy). Plant foliar ¹³C and ¹⁵N were calculated as

where the standards for C and N are the Pee Dee Belemnite standard and atmospheric N₂, respectively. The accuracy and reproducibility of the measurements of ¹³C and ¹⁵N, checked using an internal reference material and Chinese cabbage (Brassica campestris L.) sample, were better than 0.2 and 0.1 for ¹³C, respectively, and 0.3 and 0.2 for ¹⁵N, respectively. The internal reference material (¹³C = –28.3 ± 0.1; ¹⁵N = 3.4 ± 0.1) was calibrated against NIST SRM 8542 (sucrose, –10.5) for ¹³C and against IAEA-N2 for ¹⁵N.

Statistical Analyses
Normality of the data set was tested by the univariate procedure, and the Shapiro–Wilks test using the SAS software (Version 8.2, SAS Institute, Cary, NC) showed that every data set was normally distributed. The Bartlett test conducted using the GLM procedure showed that the data had heterogeneous variances. The general linear mixed model in SPSS (Version 13.0, SPSS, Chicago) was used to analyze the volume index data and the MIXED procedure in SAS was used to analyze the other measurements to overcome some heterogeneous variances found for some of the variables measured. For statistical significance, = 0.05 was chosen.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Response of Bluejoint to Nitrogen and Watering Treatments
Bluejoint aboveground biomass, N concentration, and N uptake were consistently greater in the N100 than in the N25 treatment (Table 2; P 0.001, Table 3). The interaction between N and watering (W) treatments affected (Table 2; P = 0.007, Table 3) bluejoint aboveground biomass: increased water availability decreased bluejoint aboveground biomass under low N supply while it had no effect under high N supply. The increased bluejoint aboveground biomass and N concentration in the N100 treatment resulted in greater aboveground N uptake than in the N25 treatment.

Bluejoint and White Spruce Foliar Carbon-13 and Nitrogen-15 Abundance as Affected by Competition, Nitrogen, Watering Treatments
Bluejoint shoots generally had greater (less negative) ¹³C than white spruce foliage; the overall average ¹³C in bluejoint shoots was –28.5, compared with –32.1 in white spruce needles under competition with bluejoint (Fig. 1a and 1b ). The N25 treatment resulted in lower ¹³C ( –28.7) in bluejoint shoots compared with the N100 treatment (–28.2, Fig. 1a) (P = 0.002, Table 3); and the W20 treatment resulted in higher ¹³C (–27.8) in bluejoint shoots than the W40 treatment (–29.1, Fig. 1a) (P < 0.001, Table 3).

View larger version (33K): [in this window] [in a new window]   Fig. 1. Foliar 13C of (a) bluejoint and (b) white spruce as affected by competition (C), N application level (N, mg N L–1), and watering level (W, % of volumetric soil water content). Error bars are SEs.

Neither N nor W levels significantly affected foliar ¹⁵N for bluejoint or white spruce (Fig. 2a and 2b). Only bluejoint competition significantly decreased white spruce foliar ¹⁵N (P = 0.001, Table 3, Fig. 2b). The average white spruce foliar ¹⁵N values were 2.0 in C0 and 0.8 in C1. There were consistent differences in foliar ¹⁵N between bluejoint and white spruce, and bluejoint generally had lower ¹⁵N (0.5) than white spruce grown under the same conditions (C1, 0.8) (Fig. 2a and 2b).

View larger version (31K): [in this window] [in a new window]   Fig. 2. Foliar 15N of (a) bluejoint and (b) white spruce as affected by competition (C), N application level (N, mg N L–1), and watering level (W, % of volumetric soil water content). Error bars are SEs.

View larger version (22K): [in this window] [in a new window]   Fig. 3. Relationships between (a) white spruce foliar N concentration and 13C and (b) white spruce foliar N concentration and 15N as affected by competition (C1, competition; C0, no competition), N application level (N, mg N L–1), and watering level (W, % of volumetric soil water content).

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

The ¹³C data clearly showed the ecophysiological response of bluejoint to the N and watering treatments (Fig. 1a). The increased (less negative) ¹³C value in the N100 compared with the N25 treatment is in agreement with many previous reports, which showed that increased N availability and N uptake resulted in increased carboxylation capacity and decreased discrimination against ¹³C (Livingston et al., 1999; Choi et al., 2005). On the other hand, increased soil water availability (the W40 treatment) in this study decreased bluejoint ¹³C compared with the W20 treatment (Fig. 1a), reflecting the increased stomatal conductance that increased intercellular CO₂ concentration and subsequently increased ¹²CO₂ fixation (Farquhar et al., 1989). This indicates that photosynthetic activities and stomatal conductance of bluejoint responded to changes in resource (i.e., N and water) availabilities, even though the treatment effects did not show up in the aboveground biomass data, again reflecting that N limitation probably controlled bluejoint biomass production. Because this experiment was of a short-term nature, we suspect that the water level treatment effect might show up if the experiment was continued for a longer period, such as for another growing season. We had expected that N and water availabilities would interact to affect bluejoint growth, since water is essential to mineral N diffusion in soils and its uptake by bluejoint; however, the effects of N and water availabilities on bluejoint growth and C and N isotope ratios appeared to be independent of each other, even though the interaction between N and water availabilities significantly affected the growth and N concentration in aboveground biomass (Table 2). Whether a lower (than that studied in this experiment) soil water availability (e.g., at 20% of field capacity) will lead to a different result still needs to be tested.

Response of White Spruce to Nitrogen and Watering Treatments
The foliar N concentration for optimal white spruce growth was reported to be between 15 and 25 g N kg^–1 (Nienstaedt and Zasada, 1990), and white spruce foliar N concentrations in the no-competition treatment in this study fall within this range (Table 4), with those in the C0-N100 treatment combination close to the upper range of the suggested optimum foliar N level. Therefore, the N100 treatment should not have caused toxicity and the N25 treatment provided adequate N for white spruce when there was no bluejoint competition. The expected positive effects of increased N and watering levels were not found for height or diameter increment, but were found for current-year foliar biomass and foliar N concentration (Table 4). The lack of response in height or diameter increments to the treatments could be due to the short-term nature of this experiment and the fact that white spruce is a determinate species. Determinate species have buds containing all growth units preformed, as opposed to indeterminate species that are capable of adding further leaf primordia during the growing season. The positive response to the N treatment in current-year foliar biomass production and foliar N concentration would indicate that a positive height or diameter growth response could be expected in the longer term. Also, white spruce may have high plasticity in response to drought (Silim et al., 2001). It is possible that both the low N supply and low watering levels were not low enough to cause an immediate reduction in white spruce growth.

White spruce foliar C isotope composition showed, however, that the W40 treatment had a significantly more negative ¹³C than the W20 treatment, indicating increased stomatal conductance and decreased water use efficiency under the W40 treatment (Farquhar et al., 1989), a response similar to what was displayed by bluejoint to the same treatment. The increased N level (N100) showed an opposite nonsignificant trend (decreased ¹³C discrimination in response to increased N availability) to that of the increased watering level (Fig. 1b), again indicating potentially increased carboxylation capacity in response to increased N availability (Livingston et al., 1999).

Bluejoint Competition Effects on White Spruce Seedlings
Bluejoint competition significantly reduced every growth and nutrition parameter of white spruce (Table 4), consistent with previously reported adverse effects of bluejoint competition on white spruce growth (Lieffers et al., 1993; Ländhausser and Lieffers, 1998; Bell et al., 2000). This result therefore supports our first hypothesis about bluejoint competition effects on white spruce. Reductions in tree growth or N uptake due to bluejoint competition were also found in previous greenhouse studies (Robinson et al., 2002; Hangs et al., 2002).

Robinson et al. (2002) observed that the negative influence of bluejoint on jack pine (Pinus banksiana Lamb.) photosynthesis and N uptake increased as N application rate increased, consistent with the fact that bluejoint is frequently found in nutrient-rich sites (Lieffers et al., 1993) and that it is more competitive where N availability is high. In this study, bluejoint exhibited vigorous growth in the treatment with greater N availability, with its aboveground biomass about 90% greater in the N100 than in the N25 treatment (Table 2). Since the interactions (C x N, C x W, and C x N x W) were not significant, however, there was little evidence to suggest that the increased N or water availabilities facilitated bluejoint competitiveness and its adverse effects on white spruce growth, rejecting the second hypothesis. In addition, the N and W treatments had very similar effects on white spruce foliar ¹³C, whether bluejoint competition was present or absent (Fig. 1b). It is clear, however, that bluejoint growth had a greater response to increased soil N availability than white spruce, suggesting that higher N availability may have increased bluejoint's competitiveness (Tables 2 and 4), which did not yet show up in effects on white spruce growth.

Several studies have investigated plant ¹³C response to plant competition with somewhat conflicting results (Lucero et al., 2000; Staples et al., 2001; Kume et al., 2003). For example, Kume et al. (2003) reported that understory competition increased foliar ¹³C of Japanese red pine (Pinus densiflora Sieb. et Zucc.), indicating that competition caused a water-limited condition for the red pine. Together with the N use efficiency data, they showed that understory competition caused both drought and N limitation to trees, as competition also reduced tree foliar N content. In contrast, Staples et al. (2001) found that understory competition decreased white spruce foliar ¹³C (more negative), and this result did not reflect soil moisture conditions that were affected by the vegetation treatments. They suggested that increased photosynthesis and ¹³C fixation during photosynthesis in the understory-removed treatment resulted in increased white spruce foliar ¹³C. This result of Staples et al. (2001) is consistent with Sun et al. (1996), who reported that white spruce foliar ¹³C reflects changed carboxylation rate and photosynthetic capacity. Since increased N availability has been reported to increase ¹³C (Livingston et al., 1999; Choi et al., 2005), the Staples et al. (2001) data probably indicated that understory removal improved soil N availability for white spruce. In our study, white spruce foliar ¹³C was depleted by 1.2 due to bluejoint competition, a result consistent with that of Staples et al. (2001), suggesting that bluejoint competition caused both N and water stress to white spruce seedlings, probably more so with N deficiency than with water stress, as was reflected in the significant reduction in foliar N concentrations for white spruce under competition. White spruce foliar N concentration explained 45% of the variation in foliar ¹³C (Fig. 3a). The other 55% of the variation in ¹³C may be attributed to changes in soil moisture and other nutrient availabilities and white spruce photosynthesis capacity as affected by the treatments (Fig. 3a).

Effects of Nitrogen Cycling on Foliar Nitrogen-15 Abundance of Bluejoint and White Spruce
Generally speaking, the ¹⁵N of plant tissues reflects source N (e.g., soil available N) and soil N transformation processes (Högberg, 1997), and as such can be affected by N fertilization and irrigation regimes (Choi et al., 2005). There have been few plant competition studies that involved foliar ¹⁵N analysis. White spruce foliar ¹⁵N was reduced by 1.2 by bluejoint competition, but was not affected by N and watering levels (Table 3, Fig. 2a and 2b). What factors of the competitive environment might have affected white spruce foliar ¹⁵N? We know that N losses, such as NH₃ volatilization, nitrification, denitrification, and NO₃ leaching cause ¹⁵N enrichment in soils due to substantial N isotope fractionation and, as a result, plant tissues become enriched in ¹⁵N (Högberg, 1997). Our previous field experiment showed that soils in plots with bluejoint had a low net nitrification rate compared with bluejoint-removed plots, most likely because bluejoint was an effective competitor with nitrifiers for the NH₄⁺–N in the soil (Matsushima and Chang, 2007). Direct comparisons between the field and this pot experiment should be made cautiously, but we believe that the effects of bluejoint proliferation on soil biological processes such as nitrification may be similar between field and pot experiments. We suggest that the growth medium in the C1 treatment might have had less nitirification and subsequently less denitrification loss of N, resulting in less ¹⁵N enrichment of mineral N in the growth medium and subsequently lower white spruce foliar ¹⁵N (Fig. 2b). This is consistent with the observation in Hangs et al. (2003) that bluejoint and white spruce prefer NH₄ over NO₃. Moreover, N-starved plants, such as white spruce planted with bluejoint under low N supply, minimize N losses and maximize N retention and consequently result in lower ¹⁵N values in soil mineral N and foliar samples (Robinson et al., 2000; Chang and Handley, 2000). Such reasoning is supported by the significant relationship between white spruce foliar N concentration and ¹⁵N; white spruce foliar N concentration explained 22% of the variation in foliar ¹³C (Fig. 3a).

We do not have a good explanation for why N fertilization did not reverse the competition effects on ¹⁵N discussed above. In addition to the differences in source ¹⁵N abundances as affected by soil N transformation, factors that might have influenced the variations in white spruce foliar ¹⁵N include physiological conditions as affected by the tree's nutritional status. For example, Pritchard and Guy (2005) proposed that discrimination against ¹⁵N in white spruce depends on demands for N on assimilation under NH₄ nutrition when NH₄ supply is sufficient and on N uptake capacity under NO₃ nutrition or at low NH₄ availability. Further investigations (especially under field conditions) are needed to study natural ¹⁵N abundance of trees competing with bluejoint to elucidate how bluejoint proliferation affects soil N cycling, tree nutrition, and consequently, forest ecosystem dynamics.

The lower ¹⁵N for bluejoint than for white spruce (Fig. 2) might have been caused by different mycorrhizal fungi associations in roots, as a few studies have reported mycorrhizal associations in bluejoint (Visser et al., 1998; Thormann et al., 1999). Mycorrhizae can concentrate ¹⁵N in their fungal body and provide relatively ¹⁴N-enriched N to the host plant in a N-limited environment (Hobbie and Colpaert, 2003). It is possible that mycorrhizae-infected bluejoint roots play an important role in N cycling in ecosystems with low N availability. Further research is needed to elucidate the mechanisms involved in affecting bluejoint ¹⁵N.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
We can conclude that the first hypothesis is accepted and that bluejoint is a very competitive understory species and its competition dramatically reduces white spruce growth and N uptake. Compared with the low N and watering level (without bluejoint competition) treatments, bluejoint competition had a determinant role in affecting the growth of white spruce seedlings in this greenhouse experiment. In the second hypothesis, white spruce growth was expected to be decreased to a greater degree by competition under resource-abundant than under resource-limited conditions. Those expected interactions among competition, N, and watering treatments were not significant, however, and therefore the second hypothesis is rejected based on this short-term greenhouse experiment. Even though the high N application rate increased bluejoint growth, that did not translate into greater competitive effects of bluejoint on white spruce growth, which may change if the treatments are applied to seedlings for a longer period. Therefore, the second hypothesis is rejected. The third hypothesis is accepted, adding to the growing literature that ¹³C is a sensitive parameter for evaluating plant ecophysiological changes in response to nutrient and water limitations or additions. It is commonly agreed that the current state of knowledge of ¹⁵N in plant and soil samples is at a stage of pattern finding and hypothesis testing about N cycling. This study again illustrated that plant ¹⁵N data reflects soil N cycling processes.

	ACKNOWLEDGMENTS

 
We thank Drs. P.G. Comeau, S.M. Landhäusser, V.J. Lieffers (University of Alberta, Canada), W.-J. Choi (Chonnam National University, Korea), and K. Inubushi (Chiba University, Japan) for consultation on the study. Funding for this study was provided by the University of Alberta (Faculty of Graduate Studies and Research and the Department of Renewable Resources), Natural Science and Engineering Research Council (NSERC) of Canada, the Canadian Foundation for Innovation, the Weyerhaeuser Company Ltd., and Weldwood of Canada Ltd.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Current Address: Graduate School of Science and Technology, Chiba Univ., 648 Matsudo, Matsudo-City, Chiba-Ken 271-8510, Japan

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Received for publication November 9, 2006.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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