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Keywords:

  • habitat restoration;
  • raised bog communities;
  • scale-dependent processes;
  • secondary succession;
  • vegetation classification

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. LITERATURE CITED
  9. Supporting Information

The natural recovery of vegetation on abandoned peat extraction areas lasts for decades and the result of restoration succession can be unpredictable. The aim of the study was to specify environmental factors that affect the formation of the pioneer stages of mire communities and, therefore, be helpful in the prediction of the resulting ecosystem properties. We used the national inventory data from 64 milled peatlands in Estonia, distributed over the region of 300 × 200 km. This is the first national-scale statistical evaluation of abandoned extracted peatlands. During surveys, vascular plants, bryophytes, and residual peat properties were recorded on three microtopographic forms: flats, ditch margins, and ditches. The microtopography was the main factor distinguishing the composition of plant communities on flats and ditches, while ditch margins resembled flats. The extracted indicator species suggested two successional pathways, toward fen or raised bog community. A single indicator trait—the depth of residual peat, which combines the information about peat properties (e.g. pH, ash content, and trophicity status), predicted the plant community succession in microtopographic habitats. We suggest that peatland management plans about the cost-efficient restoration of abandoned peat mining areas should consider properties of residual peat layer as the baseline indicator: milled peatfields with thin (<2.3 m) and well-decomposed residual peat should be restored toward fen vegetation types, whereas sites with thick (>2.3 m) and less decomposed residual peat layer should be restored toward transitional mires or raised bogs. Specific methodological suggestions are provided.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. LITERATURE CITED
  9. Supporting Information

Peatlands have been used for peat extraction for centuries. Ombrotropic mires (raised bogs) are the most efficient areas for industrial peat milling because of a thick (several meters) layer of homogeneous peat. In the European Union, mires with peat thickness of more than 30 cm cover only about 2.8% of the land area; 2.1% of it is in exploitation at present (Joosten 2008). After the peat extraction is completed, the extracted peatlands are sometimes used for agricultural purposes and forestry, but quite often they are left for natural re-vegetation. The spontaneous re-vegetation of abandoned extracted peatlands, however, is a slow process (Lavoie et al. 2003; Triisberg et al. 2011) and, consequently, the area of un-restored ecosystem functions constantly increases. These abandoned and poorly vegetated peatlands generate a threat to the local environment and have also more global effects; for instance, milled peatlands are sources of greenhouse gas emission (Paavilainen & Päivanen 1995; Laine & Minkkinen 1996) and are in a high risk of fires. Therefore, the recovery of fen or bog vegetation on milled peatlands is a vital task for environmental restoration, and particularly, the recovery of peat accumulation processes (Rydin & Jeglum 2006; Clarke & Rieley 2010). Considering that (1) peat extraction areas are largely created in raised bogs and, (2) the raised bogs provide valuable service as fresh water reservoirs and carbon sinks (Keddy 2010), the main target of restoration should be directing the re-vegetation succession toward raised bogs.

In extracted peatlands, the hydrological regime has been changed and the viable seed bank has been destroyed; only immigration from the surrounding areas can support the re-vegetation, but the quick establishment of new arrivals is held back by many critical factors and ecological filters (Salonen 1987; Campbell et al. 2003; Price et al. 2003; Beleya 2004). The recovery of vegetation and ecosystem functioning depends on the seasonal and yearly fluctuation in the water table (Girard 2000; Lavoie et al. 2005), wind erosion (Campbell et al. 2002), frost heaving (Groeneveld & Rochefort 2002), and/or specific residual peat chemistry (Salonen 1994). Landscape scale-limiting factors are the distance to the nearest neighboring seed source habitats, which are usually not natural raised bogs, but various other habitats, such as forests, wetlands, or agricultural areas.

The cost-efficient restoration planning should consider all critical processes and drivers, which might affect the natural re-vegetation; for that, some robust and easily measurable indicators are required. The water table is the most widely used indicator for restoration planning of peatfields (Wheeler & Shaw 1995; Price et al. 2003; Graf et al. 2008). However, the water table has large yearly and seasonal fluctuations and therefore its adequate estimation is complicated and rather costly. Alternatively, characteristics of the residual peat (depth, pH, etc.), local microtopography, and the structure of surrounding landscape offer more stable and robust indicators for restoration planning.

Our aim was to look for drivers, which could predict the secondary succession on peatfields. We expected that the chemical properties and the depth of residual peat, landscape structure, and peatland microtopography can serve as potential indicators for restoration planning. The pinpointing of succession driving indicators will give an ecologically reasoned basis for suggestions to improve the restoration methodology in disturbed mire ecosystems.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. LITERATURE CITED
  9. Supporting Information

Study Sites and Data

In Estonia, located in North-Eastern Europe, mires cover 5.5% (245,000 ha) of territory, peat is actively extracted on more than 20,000 ha (Paal & Leibak 2011), and the total area of extracted abandoned peatlands is 9,371 ha (3.8% of the area of raised bogs). In the coming decades, the area of abandoned peatfields will double because of the depletion of excavation areas in use. The annual precipitation in Estonia varies from 600 to 700 mm/yr−1. Temperature ranges from 17°C in July to −6°C in February.

The study consists of an inventory of almost all abandoned extracted peatlands in Estonia (64 peatlands, Fig. 1) carried out by the Geological Survey of Estonia by the request of Ministry of the Environment. Studied peatlands were abandoned 5–50 years before the survey, mostly 20–30 years ago, that is in the period when there was no statutory obligation to restore the areas after the cessation of peat excavation. Numerous extracted peatlands were sparsely re-vegetated with a mean projective cover of 10–20% (Orru 2010). The small-area peatlands were inventoried as one study unit. In the larger-area peatlands, where different peatfields were abandoned at different time or had a contrasting outlook because of treatments, peatfields were inventoried as separate study areas. We did not include the data of those peatlands, which were without established plant species and/or areas where peat excavation history was unknown. In total, we had 114 peatfields as survey data.

image

Figure 1. Location of studied abandoned extracted peatlands in Estonia (centroid of study areas 58°41′42′′, 25°25′59′′, radius approximately 150 km).

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In all inventory areas the presence or absence list of the plant species was compiled separately in three types of microtopographic habitats: flats (central parts of peatfields, maximum width of the peatfield was 16 m), ditch margins (0–2 m from the ditch), and ditches. In analysis of microhabitats, we used only data, where at least two species were recorded (112 flats and ditch margins, and 49 ditches). Approximately in the center of each inventory area the water table, the depth of slightly (decomposition degree <25%) and well-decomposed peat (decomposition degree >25%; according to the Records of Mineral Resources of the Environmental Register), and the total depth of residual peat layer were measured. The water level of ditches was obtained from neighboring flats. From the uppermost 50 cm residual peat layer the peat sample core was taken for the laboratory analyses. In laboratory the following parameters were estimated: (1) peat botanical composition (described with microscope), (2) ash content (measured with the loss of weight in burning at 450°C), (3) pH (measured in the pHKCl), and (4) degree of decomposition (estimated by centrifugal method). According to the botanical composition, that is by the fragments of mosses and vascular plants and peat decomposition degree, the trophicity level of upper peat layer (0–0.5 m) was ascertained as: (1) oligotrophic, (2) mesotrophic, or (3) eutrophic, which refers to the gradient from the rain-fed type to groundwater-fed type peatland.

Environmental factors such as (1) time since extracted peatland abandonment, (2) the total area of abandoned extracted peatfields in the bog, (3) visual signs of burning, and (4) special management operations were considered in the analysis. Special management operations used to promote the re-establishment of bog vegetation on some peatfields were cutting of young birch trees, sowing of seeds of some species (Oxycoccus palustris, Vaccinium angustifolium, Betula sp., Pinus sylvestris, Picea abies), or fertilization with P2O5 mixed with sawdust. Supplementary characteristics of the landscape around the excavation areas and the distance from the sea were obtained from maps and aerial photos, available from the WMS-service of the Estonian Land Board Web site (www.maaamet.ee). The dominant habitat adjacent to the extracted peatlands was categorized as (1) an active peat mining area, (2) mire in natural state, (3) forest, or (4) area of intermixed habitats.

Occurrence of certain species groups, such as (1) trees and bushes—Pinus sylvestris, Betula spp., Picea abies, Salix spp., (2) dwarf shrubs—Andromeda polifolia, Calluna vulgaris, Empetrum nigrum, Ledum palustre, Vaccinium uliginosum, V. myrtillus, V. vitis-idaea, (3) fen species—Carex spp., Juncus spp., Eriophorum angustifolium, Typha latifolia, Phragmites australis, Potentilla palustris, (4) bog bryophytes—Sphagnum magellanicum, Polytrichum strictum, Aulacomnium palustre, and (5) lichens were considered as binary categorical factors in the evaluation of composition.

The nomenclature of vascular plant species follows Tutin et al. (1964–1980), and that of bryophytes Hill et al. (2006), and that of lichens Randlane & Saag (1999).

Data Processing

The generalized pattern in floristic composition was analyzed by the detrended correspondence analysis (DCA; McCune & Mefford 1999). Cluster analysis was applied to define the species community (resp. assemblage) types for microtopographic forms. The chord distance was used as the dissimilarity measure, and the flexible beta method (β = −0.8) as the grouping algorithm (McCune & Mefford 1999). The distinctness of community types was tested by the multi-response permutation procedure (MRPP; McCune & Mefford 1999), taking into account the Bonferroni correction for multiple comparisons in pair-wise tests. In the multivariate analyses (DCA and clustering), the relevé data were filtered by selecting species occurring in at least three micro-habitats. For each revealed community type, the characteristic species were ascertained according to the indicator species analysis (Dufrêne & Legendre 1997) implemented in PC-Ord ver. 5.2 (McCune & Mefford 1999).

To obtain the normal distribution of residuals, the environmental variables such as the total area of abandoned extracted peatfields in bog, the distance from the sea, the depth of residual peat layer, the depth of slightly decomposed peat, the depth of well-decomposed peat, and ash content in residual peat were log-transformed.

The difference in the mean values of environmental variables in established community types was tested by means of ANOVA and Tukey HSD test (StatSoft Inc. 2004).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. LITERATURE CITED
  9. Supporting Information

In total, 111 vascular plant species and 70 cryptogam (moss and lichen) species were registered. The first ordination analysis identified two distinct subsets: (1) flats and ditch margins and (2) ditches (Fig. 2; MRPP test p < 0.001). Therefore each subset was analyzed separately.

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Figure 2. Scatterplot of abandoned extracted peatland vegetation relevés by the DCA.

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Flats and Ditch Margins

Several species were present rather frequently, e.g. Betula spp., Pinus sylvestris, Eriophorum vaginatum, Calluna vulgaris, Empetrum nigrum, Polytrichum strictum, and Pleurozium schreberi. Cluster analysis of the data of flats and ditch margins established four clusters (Fig. S1). These clusters do not overlap on the DCA-ordination biplot (Fig. 3) and have distinct (p < 0.001) species composition, as confirmed the MRPP test. About 30 species occurred to be indicative for those clusters (Table S1). Therefore, we defined these clusters as community types, and used the most contrasting indicator species to label these types: (1) the Phragmites australis–Calamagrostis canescens type, (2) the Eriophorum vaginatum type, (3) the Calluna vulgaris–Polytrichum strictum type, and (iv) the Ledum palustre–Sphagnum magellanicum–lichens type. DCA biplot (Fig. 3) indicates that the variation in vegetation composition is correlated with an increasing pH, ash content, and the peat trophicity of the upper layer of residual peat (0–0.5 m), but also with a decrease in the depth of the slightly-decomposed peat and the total depth of the residual peat layer (p < 0.05; Table 1). At the same time, the comparison of the average estimates of environmental factors between the four community types revealed only few significant differences in environmental factors. A thinner layer of residual peat and therein of slightly decomposed peat layer is associated with the Phragmites australis–Calamagrostis canescens community type, whereas the communities of Calluna vulgaris–Polytrichum strictum and Ledum palustre–Sphagnum magellanicum–lichens type occur on the deeper layer of slightly-decomposed residual peat (Table 1). Time since abandonment appeared to be a significant factor: on the most recently abandoned peatlands communities of the Eriophorum vaginatum and Calluna vulgaris–Polytrichum strictum types are characteristic, while for the development of the Ledum palustre–Sphagnum magellanicum–lichens type communities, more time is needed. The impact of other environmental factors, including the water table level, was fairly low.

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Figure 3. DCA biplot of the environmental factors and vegetation data of flats and ditch margins. Trophicity, Ash, pH—trophicity level, ash content, and pH of the residual peat upper layer, Depth—depth of the residual peat layer, Sl-dec—depth of the slightly decomposed peat, S—species richness.

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Table 1. Correlation of environmental variables with DCA ordination axes and the value of environmental variables (M ± SD) in habitats of different type communities on flats and ditch margins, significance of their differences by the univariate ANOVA and Fisher LSD test
VariablesDCACommunity TypepANOVA
rI axisrII axisPhr aus–Cal canEri vagCal vul–Pol strLed palSph mag–Lichens
  1. r, correlation coefficient; pANOVA, significance level by univariate ANOVA; S, species richness; Trees and shrubs, Dwarf shrubs, Fen species, Bryophytes, Sphagnum species, Lichens, occurrence frequency respective plant groups; Time, time since peatland abandonment; Area, size of the abandoned mined area; Distance, distance from the sea; Water level, water level in residual peat layer; Slightly decayed, depth of slightly decayed residual peat; Well decayed, depth of well-decayed residual peat; Trophicity level, Decomposition degree, Ash, pH, the respective characteristics of the upper-layer (0–0.5 m) residual peat. Other notations as in Table S1.

  2. *p < 0.05, **p < 0.001, ***p < 0.001, with superscript letters are marked the homogeneous groups by Fisher LSD test.

S−0.24**0.11ns6.8 ± 3.8b4.4 ± 1.5a5.7 ± 2.6ab10.8 ± 3.6c<0.001
Trees and shrubs−0.13ns−0.41***0.7 ± 0.5ab0.8 ± 0.4b0.4 ± 0.5a0.9 ± 0.3b<0.001
Dwarf shrubs−0.58***0.08ns0.3 ± 0.5a0.6 ± 0.5ab0.7 ± 0.5bc1.0 ± 0.2c<0.001
Fen species0.50***0.10ns0.8 ± 0.4b0.3 ± 0.5a0.4 ± 0.5a0.2 ± 0.4a<0.001
Bryophytes−0.45***0.32***0.4 ± 0.5a0.3 ± 0.5a0.9 ± 0.3b0.8 ± 0.4b<0.001
Sphagnum species−0.46***0.44***0.1 ± 0.3<0.001
Lichens−0.46***0.04ns0.2 ± 0.4a0.2 ± 0.4a0.4 ± 0.5ab0.7 ± 0.5b0.011
Time (yr)−0.18ns−0.08ns23 ± 11ab18 ± 8a22 ± 8a28.2 ± 9b0.002
Area (ha)0.22*−0.11ns82 ± 10876 ± 5459 ± 3161 ± 42ns
Distance (km)0.12ns0.05ns33 ± 39556 ± 5550 ± 4741 ± 42ns
Water level (m)−0.12ns0.02ns0.6 ± 0.30.5 ± 0.20.6 ± 0.20.6 ± 0.2ns
Depth of residual peat (m)−0.24**0.22**2.0 ± 0.7a2.3 ± 0.8ab2.6 ± 0.8b2.6 ± 1.0b0.010
Slightly decayed (m)−0.26**−0.03ns0.7 ± 0.40.9 ± 0.61.0 ± 0.61.0 ± 0.7ns
Well decayed (m)−0.12ns0.35***1.3 ± 0.51.4 ± 0.51.6 ± 0.61.7 ± 0.7ns
Trophicity level0.26**0.09ns1.5 ± 0.91.5 ± 0.91.4 ± 0.81.2 ± 0.5ns
Decomposition degree (%)0.09ns0.03ns21.9 ± 7.519.0 ± 7.421.3 ± 7.818.3 ± 6.5ns
Ash (%)0.26**−0.11ns5.5 ± 5.64.4 ± 3.34.5 ± 4.24.1 ± 4.2ns
pH0.34***0.08ns3.4 ± 0.73.3 ± 1.03.4 ± 0.92.9 ± 0.3ns

The species richness is the highest in the Ledum palustre–Sphagnum magellanicum–lichens type and the lowest in the Eriophorum vaginatum type (Table 1). If we grouped relevés by the presence or absence of the generalized ecological groups of plants species on the ordination plot (Fig. 4), the MRPP test results revealed that these functional groups reflected distinct composition patterns (MRPP test p < 0.001). On the ordination plot, relevés with dwarf shrubs, lichens, and bog bryophytes are clustered on the left side and relevés with fen species on the right side (Fig. 4). The occurrence frequency of trees is the lowest in the Calluna vulgaris–Polytrichum strictum type communities, while the frequency of other three community types is twice as large. The occurrence frequency of dwarf shrubs is the lowest in the communities of the Phragmites australis–Calamagrostis canescens type and the highest in the Ledum palustre–Sphagnum magellanicum–lichens type communities. The communities of the Phragmites australis–Calamagrostis canescens type have many fen species, while for the Ledum palustre–Sphagnum magellanicum–lichens type communities are characteristic species of transitional mires and raised bogs.

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Figure 4. Presence of dwarf shrub, fen species, bog bryophytes, and lichens in flats and ditch margins by DCA.

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Ditches

The most common species in ditches were Typha latifolia, Carex pseudocyperus, Eriophorum vaginatum, Sphagnum riparium, and Warnstorfia fluitans. Cluster analysis defined four community types for the vegetation relevés sampled in ditches (Fig. S2). They have distinctive species composition (MRPP test p < 0.001), as the DCA biplot illustrates (Fig. 5). The extracted community types were labeled using statistically significant indicator species (Table S2): (1) the Betula–Salix type, (2) the Phragmites australis type, (3) the Sphagnum cuspidatum type, and (4) the Carex rostrata type. The Carex rostrata type contains also some sphagna, e.g. Sphagnum magellanicum, S. fimbriatum, S. capillifolium, S. squarrosum, or S. balticum. Communities of the Sphagnum cuspidatum type and the Carex rostrata type were recorded in sites with thicker layer of slightly decomposed peat, or where the overall residual peat layer was relatively deep (Table 2). Residual peat of the Sphagnum cuspidatum communities has lower ash content of the uppermost peat layer than other three types (peat pH is also the lowest, but contrast is nonsignificant). The Betula–Salix type ditch communities are characteristic on the most recently abandoned peatlands, being almost half as old as the Sphagnum cuspidatum or Carex rostrata type communities. Ditch vegetation has the largest composition-environmental covariation along the first ordination axis (Fig. 5), which is determined by the overall depth of the residual peat and by the slightly-decomposed peat layer within it in one direction and the pH-level of the residual peat in another direction (Table 2). However, in ditches, time since abandonment appeared to be the strongest factor of vegetation composition, because it is significantly correlated with both axes (Table 2).

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Figure 5. DCA biplot of the environmental factors and vegetation data of ditches. Time—time since peatland abandonment, Depth—depth of the residual peat layer, Sl-dec—depth of the slightly decomposed peat, pH—pH of the residual peat upper layer.

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Table 2. Correlation of environmental variables with DCA ordination axes and the value of environmental variables (M ± SD) in habitats of ditches, significance of their differences by the univariate ANOVA and Fisher LSD test
VariablesDCACommunity TypepANOVA
rI axisrII axisBet-SalPhr ausSph cusCar ros
  1. Notations as in Table 1 and Table S2.

S−0.24ns−0.22ns3.8 ± 0.44.7 ± 0.33.7 ± <0.15.3 ± 0.5ns
Trees and shrubs−0.06ns0.79***1.0 ± <0.1a0.2 ± <0.4b<0.1 ± <0.1b<0.1 ± <0.1b<0.001
Fen species−0.60***−0.32ns0.7 ± <0.50.6 ± 0.50.6 ± <0.51.0 ± <0.1ns
Sphagnum species0.68***0.38*0.1 ± 0.4a0.1 ± 0.3a1.0 ± <0.1b0.7 ± 0.5b<0.001
Time (yr)0.43**−0.61***15 ± 8a20 ± 8ab28 ± 6b29 ± 11b0.007
Area (ha)−0.24ns0.31ns50 ± 5089 ± 7271 ± 3743 ± 29ns
Distance (km)−0.38*0.08ns14 ± 9a24 ± 26a69 ± 43b9 ± 3a<0.001
Water level (m)−0.16ns0.22ns0.5 ± 0.40.5 ± 0.30.4 ± 0.40.3 ± 0.4ns
Depth of residual peat (m)0.40*0.33ns1.6 ± 0.5a2.2 ± 0.7ab2.8 ± 0.5b2.7 ± 1.0b0.003
Slightly decayed (m)0.39*−0.24ns0.5 ± 0.5a0.6 ± 0.4a1.2 ± 0.6b1.1 ± 0.6b0.022
Decomposition degree (%)−0.10ns0.21ns24.4 ± 7.025.1 ± 9.221.4 ± 7.422.8 ± 7.6ns
Ash (%)−0.34ns0.05ns5.7 ± 3.2b7.7 ± 6.5b1.8 ± 0.7a7.5 ± 6.0b0.010
pH−0.42*0.15ns3.6 ± 0.73.8 ± 0.92.9 ± 0.23.5 ± 0.9ns

Species richness in all types is almost equal (Table 2), but the functional groups had varying occurrence—shrubs were most common in the Betula–Salix type, fen species in the Phragmites australis and Carex rostrata types, and Sphagnum species occurred most frequently in the Sphagnum cuspidatum and Carex rostrata types. In the two latter types, trees and shrubs were absent. According to that correlation pattern, fen species occupy the left side (thin residual peat and high pH) and bog bryophytes the right side (thick layer of residual peat and low pH) on the ordination biplot (Fig. 6). Presence of tree and shrub species is restricted to lately abandoned peatlands with thin residual peat layer (Fig. 6). Relevés grouped according to the presence of fen plants, trees, and moss functional groups differ significantly in their composition (MRPP test p < 0.001).

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Figure 6. Presence of tree and shrub species, fen species, and bog bryophytes in ditches by DCA.

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Analysis for Restoration Application

Considering that ecosystem rehabilitation is usually planned at the scale of a complex of peatfields, but the community dynamics are determined at the microtopographic level (as we showed above), we made an attempt to generalize the obtained knowledge and to test some common indicators encompassing the whole peatland. We classified the studied areas into four peatfield groups using two criteria common for both microtopographic subsets (combining results in Tables 1 and 2): (1) the depth of the residual peat layer with the threshold value 2.3 m (shallow and thick layer of residual peat) and (2) the time elapsed since the peatland abandonment with the threshold of 23 years (young and old abandoned peatlands). The critical values were estimated as average values of two extreme community types within both subsets of microtopographic habitats.

The results of the MRPP test for species composition between the four predefined peatfield groups indicated a significant difference between peatfields on shallow and thick residual peat (p = 0.006), as well as between peatfields of young and old abandonment history (p = 0.009). However, within the peat thickness classes, the field abandonment age did not show a differentiation effect in the composition. Nevertheless, each peatfield group had significant indicator species. The number of indicator species (Table 3), and also the species richness (Table 4) are higher for groups with thicker residual peat layer. The significance of residual peat thickness is confirmed by the results of factorial ANOVA (p < 0.001), while the effect of time since abandonment is nonsignificant (Table 4). Phragmites australis was present in all groups, and Carex flava, Vaccinium vitis-ideae, and Pleurozium schreberi were found in young and old areas with peat depth >2.3 m. A transitional mire species Molinea caerulea was common in three groups except for old areas with thick peat layer (Table 3).

Table 3. Indicator species of different groups of abandoned peatfields and species frequency in each group
SpeciespPeat < 2.3 mPeat > 2.3 m
<23 yr>23 yr<23 yr>23 yr
1234
  1. p, Significance level.

Calamagrostis epigeios0.04912300
Juncus bufonius0.03001100
Carex flava0.01100194
Melampyrum sylvaticum0.01800130
Molinea caerulea0.00135310
Phragmites australis0.049328388
Pyrola rotundifolia0.00230318
Vaccinium vitis-idaea0.047602516
Calliergonella cuspidata0.00300190
Carex acuta0.01100016
Populus tremula0.018001320
Aulacomnium palustre0.005631332
Pleurozium schreberi0.0479113840
Sphagnum coll.0.00318304472
Table 4. Average species richness and value of environmental variables (M ± SD) in site groups
VariablesPeat < 2.3 mPeat > 2.3 mpANOVA
<23 yr>23 yr<23 yr>23 yr
  1. Notations as in Tables 2 and 3.

S8.1 ± 4.2a9.2 ± 5.3a13.0 ± 4.5b12.2 ± 5.8b<0.001
Trees and shrubs0.8 ± 0.40.8 ± 0.40.9 ± 0.30.9 ± 0.3ns
Dwarf shrubs0.5 ± 0.50.7 ± 0.50.8 ± 0.40.7 ± 0.5ns
Fen species0.6 ± 0.50.6 ± 0.50.6 ± 0.50.6 ± 0.5ns
Transitional mire species0.6 ± 0.50.7 ± 0.50.9 ± 0.30.6 ± 0.5ns
Sphagnum species0.2 ± 0.4a0.3 ± 0.5ab0.4 ± 0.5bc0.7 ± 0.5c<0.001
Time (yr)15 ± 4a17 ± 5a33 ± 8b32 ± 5b<0.001
Area (ha)69 ± 4672 ± 52103 ± 14141 ± 18ns
Distance (km)43 ± 5459 ± 4736 ± 3534 ± 38ns
Depth of residual peat (m)1.5 ± 0.4a3.0 ± 0.6b1.8 ± 0.3a2.9 ± 0.8b<0.001
Slightly decayed (m)0.5 ± 0.3a1.3 ± 0.6b0.5 ± 0.3a1.3 ± 0.5b<0.001
Well decayed (m)1.1 ± 0.3a1.7 ± 0.6b1.4 ± 0.4a1.7 ± 0.7b<0.001
Decomposition degree (%)22.3 ± 8.319.3 ± 7.919.3 ± 8.020.2 ± 5.2ns
Ash content (%)5.2 ± 3.54.6 ± 5.14.4 ± 3.14.4 ± 5.1ns
pH3.5 ± 0.83.2 ± 0.83.2 ± 0.83.1 ± 0.6ns

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. LITERATURE CITED
  9. Supporting Information

The first species established on extracted peatlands of the northern hemisphere are common pioneer species, such as Betula spp., Pinus sylvestris, Calluna vulgaris, and Eriophorum vaginatum (Groeneveld & Rochefort 2002; Campbell et al. 2003; Triisberg et al. 2011). Birches, particularly, are the best adapted to the changing water table and can persist in these areas (Heathwaite 1995; Lavoie & Saint-Louis 1999). Once established, these species create suitable conditions and facilitate the establishment of more demanding species (Boudreau & Rochefort 1998; Soro et al. 1999; Tuittila et al. 2000). Our results on flats and ditch margins are in good concordance with these observations, because we detected these species in all community types, but not in ditches, where they were found only in one type of ditch community (the Betula–Salix type) on thin residual peat. Probably ditch depth limits the establishment of these species.

The extracted abandoned peatlands are habitats for several pioneer species characteristic of mineral soils. In our analysis, for example, those species were Calamagrostis epigeios, C. neglecta, Juncus bufonius, Pyrola rotundifolia, Populus tremula, and Carex flava. This indicates that extracted peatlands are prone to biological invasions from the large species pool from the neighborhood (Lavoie & Saint-Louis 1999; Bérubé & Lavoie 2000). These non-peatland species, however, cannot be considered as a threat for bog restoration, as usually they do not form dense populations in abandoned peatlands and they will disappear from the community when natural bog vegetation forms (Salonen 1990; Poulin et al. 2005).

Results of our study indicate that milled peatlands have several successional pathways of spontaneous re-vegetation, predicted mostly by the thickness and properties of the residual peat. In ditches, particularly, the community types can be ordered along the gradient from eutrophic to oligotrophic, indicating that residual peat parameters have strong influence on the vegetation succession there. The effect of time in formation of ditch vegetation must be considered, as in course of the decades of nonmanagement they will be filled by sediments or collapsed. A thin layer of slightly-decomposed or thick well-decomposed peat in the upper layer appears to support the vegetation to develop in the direction of fens, while a thick layer of slightly-decomposed peat promotes the development of vegetation typical to raised bogs. In that way the communities of the Phragmites australis and Carex rostrata community type in filled ditches, and also the Phragmites australis–Calamagrostis canescens communities on flats and ditch margins represent the succession toward meso-eutrophic communities, called Magnocarici–Phragmetetalia (Succow & Joosten 2001) or Phragmites–Carex fen (Ilomets 1997). Something similar suggests the occurrence of the Betula–Salix type in ditches in peatlands with thin residual peat—they will probably develop into a strip of meso-oligotrophic woodland, where the pioneering birch will later be at least partly replaced by pine and spruce (Ilomets 1997). The Eriophorum vaginatum communities on flats resemble the respective typical communities in natural transitional mires. Communities of Calluna vulgaris–Polytrichum strictum type and Ledum palustre–Sphagnum magellanicum–lichens type correspond to the path resulting in communities similar to oligotrophic bog vegetation (Paal & Leibak 2011). Most likely these successional pathways are not specific for Estonian conditions only, but also for other regions (Kovalinkova & Prach 2010).

Present day restoration practices of abandoned excavated areas concentrate mostly on restoring the water table by closing ditches, because expectedly the raised water table will ensure a permanently high moisture level and direct the secondary succession toward oligotrophic or ombrotrophic bogs (Wheeler et al. 1995; Price et al. 2003). We found that the water table is inefficient in predicting the vegetation recovery, because the closure of ditches is a uniform practice everywhere, but results vary largely (Bretschneider 2012). Instead, plans to re-vegetate extracted abandoned peatlands should consider the depth and decomposition degree of residual peat. It is reasonable that the parts of abandoned peatlands with thin and well-decomposed residual peat be restored in a way that enhances their development toward fens, and in sites with thick and less decomposed residual peat layer, restoration of bogs should be set as a target.

In the case of thick residual peat and restoration planning for transitional mires or bogs, in addition to embanking of ditches, we suggest that ditches should be filled with peat from the flat parts of the fields around ditches to avoid massive invasion of wetland plants (such as Typha latifolia or Phragmites australis) into the ditches, which will cause the encroachment and inhibit the establishment of light-requiring bog species. Filling ditches and avoiding the formation of wetland patches would reduce the seed dispersal pressure of invasive species into flat areas later. In some observed areas the invasion has already happened and in such areas bog formation is inhibited. Ploughing the peatfield to fill neighboring ditches also increases the microtopographic variability within flats and that will support the establishment of bog species in microhabitat shelters.

Additionally, the recovery process can be accelerated by introducing plant and moss propagules (rhizomes, fragments, or seeds and spores) or transplanting undisturbed vegetation blocks from the areas where mining is prepared (Quinty & Rochefort 2003). We suggest that the selection of source habitat for plant material should be made adaptive, according to the depth of the residual peat and the planned choice of the successional direction (i.e. target community). We suggest sowing into partly (max 2/3 of depth) filled ditches, because microhabitat conditions in these hollows will favor species establishment.

We conclude that the easily measurable and seasonally robust indicators, such as the depth of the residual peat layer, its decomposition rate, and some simple peat chemical properties (e.g. pH), will provide cost-efficient information for restoration planning and the choice of methods. The general rule in predicting the speed and directing the succession of spontaneous re-vegetation in abandoned milled peatlands in the boreo-nemoral region is the depth of the residual peat layer (<2.3 or >2.3 m).

Implications for Practice

  • Milling companies should have a restoration target added to their management plan, including the criteria for stopping peat extraction from peatlands. The stopping criteria should include the depth and/or properties of residual peat with critical value related to the restoration target community: fen (<2.3 m) or raised bog (>2.3 m).
  • After peatfield abandonment, ditches should be partially filled (ca 2/3 of depth) with peat from the neighboring flats to avoid a massive invasion of large-sized wetland species (mainly reed).
  • Ploughing or any other mechanical disturbance of peatfield flats could be applied to increase microtopographic and -climatic heterogeneity, which will enhance plant establishment.
  • The choice of plant propagules (plant rhizomes, moss fragments, seeds or spores) for introduction should be based on the properties of residual peat and restoration target community type.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. LITERATURE CITED
  9. Supporting Information

The inventory of abandoned peat production fields was financed by the Estonian Environmental Foundation: Grant nr K-13-4-2005/443. 01.006.05. This study was financed by target-financed projects SF0180012s09; SF0180025s12 and by the Estonian Science Foundation grants 7878, 8060 and by the EU Regional Development Fund (Centre of Excellence FIBIR). We thank Kersti Unt for the English revision.

LITERATURE CITED

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  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. LITERATURE CITED
  9. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. LITERATURE CITED
  9. Supporting Information
FilenameFormatSizeDescription
rec12030-sup-0001-FigureS1.pdfPDF document17KFigure S1. Dendrogram of flat and ditch margin sites.
rec12030-sup-0002-FigureS2.pdfPDF document12KFigure S2. Dendrogram of ditch sites.
rec12030-sup-0003-TableS1.pdfPDF document13KTable S1. Indicator species of community types estimated on flats and ditch margins.
rec12030-sup-0004-TableS2.pdfPDF document11KTable S2. Indicator species of community types estimated in ditches.

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