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Keywords:

  • Critical load;
  • mycorrhizal fungi;
  • vegetational composition

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • 1
     Ecosystem recovery after decreased input of nitrogen was examined in two different fertilization experiments where the fertilization had been terminated for 9 and 47 years, respectively.
  • 2
     The species composition of the understorey vegetation showed no signs of recovery 9 years after the fertilization was terminated. Increased sporocarp production of mycorrhizal fungi was seen on formerly fertilized plots compared with plots still receiving N, but the species composition showed large differences compared to control plots.
  • 3
     In the second experiment, examined 47 years after termination of fertilization, N favoured bryophytes such as Brachythecium reflexum (Starke) Schimp., Plagiothecium denticulatum (Hedw.) Schimp. and the leaf-parasitic fungus Valdensia heterodoxa Peyronel (attacking Vaccinium myrtillus L.) was more abundant in the formerly N-treated plots than in controls. The abundance of Hylocomium splendens (Hedw.) Schimp., the most common bryophyte under normal N conditions, showed a contrasting pattern, with less abundance in the formerly N-treated plots than in controls. Sporocarp production of N-sensitive mycorrhizal fungi was lower in the formerly N-treated plots. No difference in plant species composition was noticed for vascular plants.
  • 4
     These results contrast with other studies that have interpreted reduced N leakage and nutrient levels in trees after decreased N input as a rapid ecosystem recovery. The present study suggests that the time needed for recovery of the ecosystem biota may be substantial in originally N-limited ecosystems.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Human-related activities have affected the global nitrogen cycle, with a subsequent increase in N deposition to many ecosystems (Galloway 1995; Vitousek et al. 1997). This has caused eutrophication of a wide range of ecosystems (Bobbink, Hornung & Roelofs 1998; Lee 1998). Generally, slow-growing N-conservative plant species (such as dwarf shrubs) are replaced by fast-growing species with faster N turnover (such as graminoids) (Aerts & Berendse 1988; Aerts et al. 1990). Such changes have been observed in many originally N-limited vegetation types, both as a result of increased N deposition in temperate regions (cf. Bobbink et al. 1998), and following N deposition (Økland 1995) or fertilization of boreal forests (Kellner & Mårshagen 1991; Kellner & Redbo-Torstensson 1995; Mäkipää 1994).

Other groups of organisms are also affected by increased N input. Mycorrhizal fungi are negatively affected by N fertilization and deposition (Boxman et al. 1998a; Brandrud 1995; Brandrud & Timmermann 1998). Changes in mycorrhizal community composition may have large effects on the ecosystem because mycorrhizal associations are important for the nutrient uptake of most plant species in boreal forest (Read 1991). The species composition of mycorrhizal communities changes following increased input of N, both above-ground (sporocarps; Brandrud 1995; Wiklund, Nilsson & Jacobsson 1995) and below-ground (mycelium; Kårén 1997; Boxman et al. 1998a; Brandrud & Timmermann 1998).

The rate of litter decomposition is important for mineralization rate and thus also for plant N availability. Decomposition of litter from fast-growing species generally exceeds that from slow-growing species, and changed species composition following increased N input may increase the nutrient cycling (Berendse 1990; Hobbie 1992; Chapin et al. 1996; Hobbie 1996; Vitousek 1994; Wedin & Tilman 1990). Because of this feed-forward effect, fast-growing plants may be able to persist in a community even after the input of N has been reduced (Bowman & Steltzer 1998; Chapin et al. 1996; Wedin & Tilman 1990). A central issue concerning the effects of N deposition is the recovery rate of ecosystem processes, and the rate of re-establishment of the original flora and fungal community following decreases in N input.

Studies of ecosystem recovery after a decrease in N input have been done either by building roofs that reduce the wet deposition of N (Boxman, van der Ven & Roelofs 1998b), or by using old fertilization experiments in which the fertilization has been terminated (Bowman & Steltzer 1998; Quist et al. 1999). These studies have reported sharp decreases in exchangeable N in soil (Quist et al. 1999) and NO3 in run-off water after decreased N input (Bredemeier et al. 1998; Wright, Lotse & Semb 1993; Xu et al. 1998), indicating that some ecosystem processes might recover rapidly.

To study responses of boreal forests following decreased N input, we examined two old fertilization experiments in northern Sweden. They are suitable for studies of the long-term effects of N additions and of responses following decreased N input because they include plots that still receive N and plots where the fertilization has been terminated. The questions addressed were: (1) how does the understorey vegetation and sporocarp production by mycorrhizal fungi respond to long-time N additions; and (2) how will the biota respond when N addition is terminated?

Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Study sites

The two experimental sites used in this study, the Norrliden site (64°23′ N 19°45′ E) and Hesselman’s site (64°10′ N 19°35′ E), are old forest fertilization experiments. Both sites are located in the middle boreal zone (Ahti et al. 1968). The understorey vegetation is similar at the two sites and is classified as Vaccinium–Myrtillus type according to Kalliola (1973). However there are some obvious differences between them. The Norrliden site was planted with young Scots pine (Pinus sylvestris L.) in 1953 after clear cutting of a Norway spruce [Picea abies (L.) Karst.] stand, while Hesselman’s site is a self-generated old-growth forest dominated by Norway spruce.

Norrliden site

This experiment was established in 1971. Three N treatments (N1–N3) were applied to plots of 30 × 30 m. The treatments were replicated three times (n = 3) with three unfertilized control plots. Nitrogen was added once every year as granules of NH4NO3. The N1 and N2 treatments are still fertilized, but N addition to the N3 plots was terminated in 1990. The total amount of N that the different treatments received until 1999 is: N1, 990; N2, 1980; N3, 2160 kg N ha−1. The average yearly dose of N the plots received is: N1, 34; N2, 68; N3, 108 kg N ha−1 year−1. In the N2 and N3 plots, nitrification and subsequently leakage of NO3 occurred (Tamm 1991; Quist et al. 1999). This suggests that the N2 and N3 plots had reached N saturation, that is, the N added exceeded plant needs. Nitrogen losses have probably been small from the N1 plots, as these plots show no signs of nitrification (Tamm 1991).

Hesselman’s site

At this site N additions started in 1937. A solution of NH4NO3 was irrigated on one experimental plot once every second week throughout the growing season between 1937 and 1951. The total amount of N added to this plot corresponds to 1447 kg N ha−1. This treatment is referred to as N1. One control plot was also established, hereafter referred to as C0. This plot received water in the same amount as the N-treated plot. These plots were 10 × 16 m. Three new treatments were established in 1944: NH4NO3 (referred to as N2); NH4NO3 plus 57 kg phosphorus ha−1 on one occasion (referred to as N + P); and NH4NO3 plus 1000 kg wood ashes ha−1 on one occasion (referred to as N + a). These plots received N on 32 occasions between 1944 and 1946. The N was added by irrigation with water during the growing season. The plot sizes of these treatments were 26 × 30 m, except for the N + P treatment which was 15 × 30 m. The total amount of N added to these plots was 540 kg N ha−1. In addition to the original control plot, extra controls were established in order to obtain a more precise comparison between the formerly fertilized plots and the control (see Vegetation analysis; Sporocarps of mycorrhizal fungi).

For all N treatments at the site, increased tree growth was recorded; this was one of the first experiments to show that N is the limiting factor for tree growth in boreal forests (Malmström 1949; Tamm 1991). Unfortunately there are no data on the understorey vegetation before or during the fertilization period. Malmström (1949) has given a general description of effects on the understorey vegetation. He stated that the N additions resulted in an increased abundance of Deschampsia flexuosa (L.) Trin. This increase was rapid and could be seen 2 years after the start of the experiment; see Fig. 19 in Malmström (1949) and Fig. 2·5 in Tamm (1991), which show a dense sward of flowering D. flexuosa that is not a characteristic feature of late successional boreal forests. The dominating bryophyte Hylocomium splendens (Hedw.) Schimp. was negatively affected by the N treatment (Malmström 1949).

Vegetation analysis

In 1999, an analysis of the understorey vegetation was made at the two sites using a point-intercept method. The vegetation was analysed at 200 random points along each diagonal of the plots at Norrliden (a total of 400 points per plot). At Hesselman’s site the method was adjusted to fit smaller plot sizes. Each plot was examined with the same number of points per unit area as at the Norrliden site. In addition to the formerly fertilized plots and the C0 control, vegetation data were obtained in one extra control (C1) close to the original experimental area.

Sporocarps of mycorrhizal fungi

Sporocarps of mycorrhizal fungi were scored in all fertilized plots and control plots. All species belonging to genera known as mycorrhizal fungi according to Hansen & Knudsen (1992) were collected. Sampling of sporocarps was carried out on one occasion, at the beginning of September 2000. All sporocarps in each plot were collected and identified to species or genus. At Hesselman’s site three extra controls (C1–C3) with a radius of 5 m were examined in addition to the treatment plots and the original control plot (C0).

Data treatment

Prior to statistical analysis of the vegetation data from the Norrliden site, we performed a principal components analysis (PCA) which gave us information on the response of vegetation composition in each plot. The PCA analysis revealed that 93% of the variation in the data was explained by the first principal component axis. The next three axes together explained less than 7% of the variation. Further analysis was made only on the PCA scores from the first axis. The scores from the PCA first axis were used as the dependent variable, and the different treatments as independent variables in a one-way anova.

For the vegetation at the Norrliden site, Simpson’s diversity indices (D) were calculated for each N treatment and for controls. D was calculated by determining, for each species, the proportion of individuals or biomass that it contributed to the total in the sample, that is, the proportion is Pi for the ith species according to:

D = 1/∑Pi2

Due to the lack of replication at Hesselman’s site, only some of the data from this experiment were analysed statistically in the present study (see Results, Hesselman’s site).

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Norrliden

Nitrogen treatment at the Norrliden site has resulted in major changes in understorey vegetation. The fertilized plots (N1–N3) differed from the control plots in species composition and abundance after 28 years’ N treatment (Fig. 1a). A one-way anova based on scores from the PCA revealed a significant effect of N treatment on vegetation (Table 1). The N3 treatment, which had not received N for the past 9 years, did not differ from the other fertilized plots (Fig. 1a). Plots that had received the highest N dose (N2 and N3) also showed the largest shift in species composition (Fig. 1a). Under these treatments the grass D. flexuosa had increased in abundance, while the formerly dominant species Vaccinium myrtillus had decreased compared to control plots (Table 2). The largest change in species composition was observed for bryophytes, their species assemblage being almost completely changed after 28 years’ N addition (Table 2). The dominant species in control plots, Pleurozium schreberi (Brid.) Mitt, was almost absent from the N2 and N3 treatments. Instead, the most abundant bryophytes were species such as Brachythecium starkei (Brid.) Schimp., Brachythecium reflexum (Starke) Schimp. and Plagiothecium denticulatum (Hedw.) Schimp., which all showed marked increases (Table 2). Further, the two reindeer lichens Cladina rangiferina (L.) Nyl. and Cladina arbuscula (Wallr.) Hale & W. Culb. (treated as Cladina spp.) were negatively affected by the N treatment (Table 2). These lichens were absent from the fertilized plots.

image

Figure 1. Response of the nitrogen treatments on plant community composition (a) and biodiversity expressed as Simpson’s diversity index (b). Treatments: C0 control; and N1, 34; N2, 68; N3, 108 kg N ha−1 year−1 until 1990. Different letters above bars refer to significant (P = 0·05) differences between treatments. Vertical bars denote mean ± SE (n = 3).

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Table 1. anova results on data from the Norrliden site showing the effects of nitrogen treatment
Response parameterdfMSTest valueP value
Total vegetation33·510F = 19·0990·001
Simpson’s diversity index33·826F = 28·6170·000
Number of sporocarps of Cortinarius spp.3 χ2 = 8·1940·042
Number of sporocarps of Lactarius rufus3 χ2 = 9·3120·025
Table 2.  Mean abundance (number of hits) ± SE (n = 3) of major plant species under different treatments at the Norrliden site
SpeciesTreatment
CN1N2N3
  1. C, control plots; N1, low-N treatment; N2, medium-N treatment; N3, high-N treatment that has been terminated for 9 years.

Vaccinium myrtillus   400 ± 26·7      392 ± 83·7     226 ± 10·9      146 ± 33·7
Vaccinium vitis-idaea   236 ± 23·7   30·7 ± 18·4   4·70 ± 1·5   2·30 ± 0·7
Deschampsia flexuosa   323 ± 33·41028 ± 72·31129 ± 156·41431 ± 157·5
Brachythecium starkei1·00 ± 1·0   70·3 ± 26·3    78·3 ± 8·80    78·7 ± 17·4
Brachythecium reflexum0·30 ± 0·30   28·3 ± 3·50    35·7 ± 6·40    48·3 ± 4·30
Plagiothecium denticulatum0·00 ± 0·00   2·70 ± 0·90    11·3 ± 5·00    34·0 ± 13·1
Pleurozium schreberi   287 ± 19·1   79·0 ± 6·00    28·7 ± 4·10    57·7 ± 10·30
Cladina spp.11·0 ± 4·60   0·00 ± 0·00    0·00 ± 0·00   0·00 ± 0·00

A one-way anova on the Simpson’s diversity indices revealed a statistically significant decrease in biodiversity as a response to N treatments (Table 1; Fig. 1b). The diversity indices for the fertilized plots (N1–N3) differed from the controls (Fig. 1b). Further, there was no difference in diversity indices between plots that still received N (N1 and N2) and plots in which the treatment had been terminated (N3) (Fig. 1b).

Sporocarp production of mycorrhizal fungi showed large differences between treatments at the Norrliden site (Fig. 2). The plots that still receive N had a lesser abundance of sporocarps than controls or the N3 treatment (terminated 9 years ago). The N3 treatment also showed a large difference in species composition compared with the control. The effect on sporocarp abundance was tested using the Kruskal–Wallis test because the data did not meet the assumptions for an anova. There were significant effects on sporocarp abundance for Lactarius rufus (Scop.) Fr. and for Cortinarius spp. (Table 1; Fig. 2). Cortinarius spp. were abundant in the control plots, while only a few sporocarps were found in the N1 and N3 plots. In the N2 plots no sporocarps of Cortinarius spp. were found (Fig. 2). On the other hand, sporocarps of L. rufus were present only in the N2 and N3 plots (Fig. 2). For Cantharellus tubaeformis Fr. there was a tendency towards a treatment effect, but this was not significant at P ≤ 0·05 (P = 0·052). This species was more abundant in N1–N3 plots than in controls. For Hebeloma spp., Hygrophorus hypothejus Fr., Laccaria spp., Paxillus involutus (Batch.) Fr., Russula spp. and Suillus spp., there were no significant treatment effects.

image

Figure 2. Sporocarp abundance of the mycorrhizal fungi Lactarius rufus (a) and Cortinarius spp. (b) at the Norrliden site. Different codes on the x axis refer to different nitrogen treatments (Fig. 1). Different letters above bars refer to significant (P = 0·05) differences between N treatments. Vertical bars: mean ± SE (n = 3).

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Hesselman’s site

The results from Hesselman’s site (the old fertilization experiment) were more difficult to evaluate because the experiment lacks replication. However, the bryophytes B. reflexum and P. denticulatum were found only in the formerly fertilized plots, and were absent from the controls (Fig. 3). The abundance of H. splendens showed a contrasting pattern, with lesser abundance in the formerly fertilized plots than in control plots (Fig. 3). Further, the abundance of the parasitic fungus Valdensia heterodoxa Peyronel (attacking leaves of V. myrtillus) was also greater in the formerly fertilized plots (without ashes or P) than in the controls (Fig. 3). Vascular plants such as D. flexuosa, V. myrtillus or other species showed no obvious differences in abundance between the formerly fertilized plots (N1, N2, N + a and N + P) and controls (C0–C3) (Fig. 3).

image

Figure 3. Abundance of (a) B. reflexum; (b) V. myrtillus; (c) P. denticulatum; (d) D. flexuosa; (e) H. splendens; (f) the parasitic fungus V. heterodoxa in controls (C0–C1) and formerly fertilized plots (N1, N2, N + a, N + P) at Hesselman’s site.

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Sporocarps of the mycorrhizal fungi Russula spp. were significantly more abundant in control plots than in formerly fertilized plots (Student’s t-test: t = 3·86, P = 0·008, df = 6) (Fig. 4). Cortinarius spp. also tended to be more abundant in control plots, but this was not statistically significant at P ≤ 0·05 (Student’s t-test: t = 3·41, P = 0·052, df = 6) (Fig. 4). No statistical tests were performed for the other species/groups because sporocarps were absent from one or several of the controls and formerly N-treated plots.

image

Figure 4. Sporocarp abundance of the mycorrhizal fungi Russula spp. (▪) and Cortinarius spp. (□) in controls (C0–C3) and formerly fertilized plots (N1, N2, N + a, N + P) at Hesselman’s site.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The recovery of the understorey vegetation following a nitrogen-induced vegetation shift is a slow process. At the Norrliden site no change in either species composition or abundance of individual species was recorded 9 years after the N addition was terminated. The large effects on the vegetation composition caused by the N treatment were still present. This is in accordance with other studies that have found ecosystem effects of nutrient treatment present some years after the treatments were terminated (Milchunas & Lauenroth 1995; Vinton & Burke 1995). The long-term N additions at Norrliden also resulted in reduced biodiversity, expressed as Simpson’s diversity index. A reduced diversity in plant communities after fertilization is known from several experiments (Dirkse & Martakis 1992; Tilman 1988). In our study the reduced diversity was still present 9 years after the termination of the N treatment. This suggests that no rapid recovery of understorey vegetation can be expected in boreal forests after sharp decreases in N input.

Sporocarp formation by mycorrhizal fungi showed clear differences between control and fertilized plots. More sporocarps were present in the control plots than in the still-fertilized N1 and N2 plots at the Norrliden site. This agrees with a study performed at the same site by Kårén (1997), who found that L. rufus had replaced Cortinarius spp. as the dominant mycorrhizal fungus on root tips of P. sylvestris in the N1 plots. In our study sporocarps of Cortinarius spp. were abundant only in control plots, while sporocarps of L. rufus were most abundant in the formerly fertilized plots. This suggests not only that the N treatments severely affected the species composition of mycorrhizal fungi, but also that the sporocarp production of L. rufus responded positively to the decreased N input. Cortinarius spp. are, with Russula spp., among the most sensitive and the first to disappear when N input increases, while L. rufus is less sensitive (Brandrud 1995). Some sporocarps of Cortinarius spp. occurred in the N3 plots, which indicates that Cortinarius spp. have begun to re-colonize the N3 plots.

It has been proposed that reduced production of sporocarps after increased N availability could result from a decreased C/N ratio for soil organic matter (Wallander 1994). This may result in a relative shortage of carbon because the fungi cannot regulate their uptake of N, and reduced sporocarp production could be a way of reducing this shortage (Wallander 1994). Thus the increased abundance of sporocarps observed in the N3 plots may indicate reduced N availability relative to carbon in these plots. However the species composition was different compared to the controls, which suggests that the mycorrhizal community was still affected 9 years after the N treatment terminated.

At Hesselman’s site there were no differences in the abundance of vascular plants between controls and formerly fertilized plots. However, some results showed that the biota is still altered 47 years after the termination of the treatments.

The parasitic fungus V. heterodoxa, which attacks leaves of V. myrtillus and increases in abundance as a response to increased N availability to the host (Nordin, Näsholm & Ericson 1998; J.S. and co-workers, unpublished results) was more frequent in formerly fertilized plots than in control plots. The species composition of bryophytes also showed differences between control plots and formerly N-fertilized plots: the N-favoured bryophytes B. reflexum and P. denticulatum were found only in the formerly fertilized plots. These two species increased dramatically after the N additions at the Norrliden site, and are known to increase in abundance following increased N availability (Kellner & Mårshagen 1991; Mäkipää 1995). Moreover, Malmström (1949) reported that growth of the dominant bryophyte at the site, H. splendens, was negatively affected by the fertilization. This effect of N fertilization persists. In the present study H. splendens was less abundant in formerly fertilized plots than in control plots. In other studies this bryophyte was sensitive to increases in N input (Dirkse & Martakis 1992). The sporocarp abundance of Russula spp. was also less in formerly fertilized plots than in control plots. Russula spp. are sensitive to increased N availability (Brandrud 1995), and the reduced abundance indicates that the N fertilization terminated 47 years ago still affects the mycorrhizal community.

It could be argued that the long-lasting effects of N fertilization on bryophytes and on mycorrhizal fungi at Hesselman’s site may be a result of slow re-establishment of species. The plot sizes at this site were relatively small and, because the surrounding vegetation remained unaffected by the N treatment, it is unlikely that the slow re-establishment of species was caused by dispersal limitations. The rate at which vegetation may re-establish in these plots is likely to be much faster than where larger areas or a whole landscapes have been affected by increased N input.

Several studies have documented a rapid decrease in chemical parameters such as NO3 concentration in run-off water, exchangeable N in soil, and arginine concentrations in leaf tissue following a decrease in N input (Bredemeier et al. 1998; Quist et al. 1999; Wright et al. 1993; Xu et al. 1998). These results have been interpreted as signs of rapid ecosystem recovery, and N enrichment of ecosystems has been regarded as a fully reversible process (Boxman et al. 1998b). The present study indicates that effects of N enrichment on the biota persist for substantially longer periods than effects on chemical parameters. This should be considered when assessing critical loads of N deposition for ecosystems.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We would like to thank Jens Ingerlund for skilful help in the field and Kjell Olofsson for identification of macrofungi. We also would like to thank Nicholas Kruys for help with the PCA, and an anonymous referee for valuable comments on an earlier version of this paper. This study was supported through grants from the ASTA program financially supported by MISTRA (to T.N. and L.E.) and from SJFR (to T.N.).

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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Received 20 December 2000; accepted 2 April 2001