RESTORATION EVIDENCE

Action: Rewet peatland (raise water table) Peatland Conservation

Key messages

  • Thirty-six studies examined the effect of rewetting (without planting) on peatland vegetation. Fifteen studies were in bogs (two being restored as fens). Fifteen studies were in fens or fen meadows (two were naturally forested). Six studies were in general or unspecified peatlands. Some studies were based on the same experimental set-up or sites as each other: two studies in Germany, three studies in Sweden, two studies in west Finland and two studies in south Finland.
  • Plant community composition (13 studies): Six before-and-after studies (four also replicated) in peatlands in Finland, Hungary, Sweden, Poland and Germany reported changes in the overall plant community composition following rewetting. Typically, drier grassland communities were replaced by more wetland- or peatland-characteristic communities. One replicated, paired, controlled study in a bog in the Czech Republic found that rewetted plots developed a different plant community to drained plots. Three site comparison studies in Finland and Canada reported that rewetted peatlands contained a different plant community to natural peatlands. Three replicated studies in peatlands in the UK and fens in Germany reported that rewetting typically had no effect, or insignificant effects, on the plant community.
  • Characteristic plants (11 studies): Five studies (including one replicated site comparison) in peatlands in Canada, the UK, China and Poland reported that rewetting (sometimes along with other interventions) increased the abundance of wetland- or peatland-characteristic plants. Two replicated site comparison studies in fens or fen meadows in central Europe found that rewetting reduced the number of fen-characteristic plant species. Two studies (one replicated, paired, controlled, before-and-after) in fens in Sweden reported that rewetting had no effect on cover of fen-characteristic plants. Two before-and-after studies in fens in the USA and New Zealand reported that upland plant cover decreased following rewetting. 
  • Moss cover (19 studies): Twelve studies (five replicated, two also paired and controlled) in the UK, Ireland, GermanySweden, Latvia, Canada and Spain reported that rewetting bogs, fens or other peatlands (sometimes along with other interventions) increased Sphagnum moss cover or abundance. Three of these studies reported mixed responses by species. Two additional replicated studies, in bogs in Latvia and forested fens in Finland, reported that rewetting had no effect on Sphagnum cover. Five studies (one paired, controlled, before-and-after) in Finland, Sweden and Canada reported that rewetting bogs or fens had no effect on cover of non-Sphagnum mosses (or mosses/lichens). However, two controlled studies in bogs in Ireland and the UK reported that rewetting reduced cover of non-Sphagnum mosses or bryophytes. One site comparison study in Finland reported that a rewetted peatland had similar moss cover (Sphagnum and total) to a natural peatland, but another site comparison study in Canada reported that a rewetted bog had lower moss cover (Sphagnum and other) than nearby target peatlands.
  • Herb cover (25 studies): Twenty-one studies (including four replicated, paired, controlled) reported that rewetting (sometimes along with other interventions) increased cover of at least one group of herbs. These studies were in bogs, fens or other peatlands in the UK, Finland, Ireland, the Czech Republic, the USA, the Netherlands, Sweden, Germany, China, Latvia, Poland, Canada and Spain. Specifically, rewetting increased other/total sedge cover in 13 of 15 studies, increased cottongrass cover in eight of nine studies, and increased reed/rush cover in five of seven studies. Three of four before-and-after studies in peatlands in the UK and Sweden reported that rewetting reduced purple moor grass cover; the other study reported no effect. One replicated site comparison study in forested fens in Finland reported that rewetting had no effect on total herb cover. Two site comparison studies in Europe reported greater herb cover in rewetted than natural peatlands (overall and sedges/rushes, but not forbs).
  • Tree/shrub cover (13 studies): Ten studies (including two paired and controlled) in peatlands in Finland, the UKGermany, Latvia and Canada reported that rewetting typically reduced (seven studies) or had no effect (six studies) on tree and/or shrub cover. Two before-and-after studies in fens in Sweden and Germany reported that rewetting increased tree/shrub cover. One before-and-after study in a bog in the UK reported mixed effects of rewetting on different tree/shrub species.
  • Overall vegetation cover (4 studies): Of four before-and-after studies (three also controlled) that examined the effect of rewetting on overall vegetation cover, two in bogs in Ireland and Sweden reported that rewetting increased it. One study in a fen in New Zealand reported that rewetting reduced vegetation cover. One study in a peatland in Finland reported no effect.
  • Overall plant richness/diversity (14 studies): Six studies (including one replicated, paired, controlled, before-and-after) in Sweden, Germany and the UK reported that rewetting increased total plant species richness or diversity in bogs, fens or other peatlands. However, five studies found no effect: in bogs in the Czech Republic and Latvia, fens in Sweden and Germany, and forested fens in Finland. One study in fen meadows in the Netherlands found scale-dependent effects. One paired, controlled, before-and-after study in a peatland in Finland reported that rewetting reduced plant diversity. Of four studies that compared rewetted and natural peatlands, two in Finland and Germany reported lower species richness in rewetted peatlands, one in Sweden found higher species richness in rewetted fens, and one in Europe found similar richness in rewetted and natural fens.
  • Growth (1 study): One replicated site comparison study in forested fens in Finland found that rewetting increased Sphagnum moss growth to natural levels.

Supporting evidence from individual studies

1 

A before-and-after study in 1972–1987 in a historically mined raised bog in England, UK (Meade 1992) reported that after rewetting (and diversion of polluted inflow), cover of Sphagnum moss, white sedge Carex curta and cottongrasses Eriophorum spp. increased, but cover of purple moor grass Molinia caerulea decreased. No statistical tests were carried out. Sphagnum was found in 7% of quadrats before intervention but 27% after, white sedge in 0.0% before but 0.8% after, and cottongrasses in 1.1% before but 1.5–1.7% after. In contrast, purple moor grass occurred in 100% of quadrats before intervention but only 74% after. Eighteen other herb, shrub and tree species showed variable responses (see original paper). In 1974, a drained bog was rewetted (surface partially waterlogged) by blocking its water outflow. At the same time, polluted inflow from adjacent farms was diverted off site. The study does not distinguish between the effects of these interventions. Vegetation cover was recorded before (1972–1973) and after (1987) intervention, as presence/absence of species in 8,945 contiguous 4 m2 quadrats covering the whole site.

2 

A paired, controlled, before-and-after study in 1994–1998 in a historically mined peatland in Finland (Tuittila et al. 2000) reported that rewetted plots developed a different plant community to drained plots with lower plant diversity and shrub cover, but similar moss/lichen and total vegetation cover. These results were not tested for statistical significance. Over four years, rewetted plots developed more peatland-characteristic plant communities than drained plots (data reported as a graphical analysis). After four years, plant diversity was lower in rewetted plots than drained plots (data reported as a diversity index). In rewetted plots, shrub cover was 0% (vs 6% in drained plots), moss/lichen cover 13–20% (drained: 19–20%), and total vegetation cover 40–60% (drained: 40–45%). Cottongrass Eriophorum vaginatum cover was 20–34% (vs 11–18% before rewetting) and sedge Carex spp. cover was 12% (vs 4% before rewetting). Before intervention, plots to be rewetted or drained had similar plant diversity, shrub cover (0–11%), moss/lichen cover (2–24%) and total vegetation cover (15–50%). In autumn 1994, four plots were established within one drained peat field. Two plots were rewetted by blocking drainage ditches with peat dams and digging a new input ditch. Two plots remained drained. Every summer between 1994 and 1998, cover of every plant species was estimated in twelve 2 m2 quadrats/plot. The water table was within 18 cm of the peat surface in rewetted plots, compared to 4–31 cm below the surface in drained plots. This study used the same rewetted peatland as (14).

3 

A controlled before-and-after study in 1997–1999 in a historically mined blanket bog in Ireland (Farrell & Doyle 2003) reported that after rewetting, cover of total vegetation, algae, Sphagnum moss and common cottongrass Eriophorum angustifolium all increased, but cover of other bryophytes decreased. No statistical tests were carried out. Total vegetation cover was 40% before rewetting and 93% two years after. Cover of algae was 10% before and 92% after, Sphagnum <1% before and 31% after, other bryophytes 12% before and 1% after, and cottongrass 18% before and 24% after. In control plots that remained drained, cover values remained stable over time (total: 54–56%; algae: 21%; Sphagnum: 0%; other bryophytes: 26%; cottongrass: 31–32%). In August 1997, a 1 m high peat ridge was built around a drained bog to retain water (water table raised to 10–42 cm above peat surface). An adjacent bog remained drained for comparison (water table 0–12 cm below surface). In August 1997 (before intervention) and 1999, vegetation cover was estimated in nine equally sized quadrats/bog.

4 

A before-and-after study in 2001–2002 in a historically mined bog in Quebec, Canada (Cobbaert et al. 2004) reported that fen-characteristic plant species appeared following rewetting. These results are not based on tests of statistical significance. Before rewetting, no vegetation was present. Six months after rewetting, six local fen-characteristic plant species were present. Sixteen months after rewetting, five fen-characteristic species were present. Cover of fen-characteristic plants was 2% six months after rewetting and 10% sixteen months after. Note that the aim of this study was to create a fen, as the post-mining peat chemistry was more like a fen than a bog. In April 2001, a bog was rewetted by blocking the main drainage ditch, unblocking a water supply ditch and building embankments downslope of the bog to retain water. Afterwards, the water table was 1–65 cm below the peat surface during the growing season. In October 2001 and August 2002, cover of every plant species was estimated in ninety 30 x 30 cm quadrats (ten in each of nine 5 x 5 m plots). None of these plots were sown.

5 

A replicated, paired, controlled study in 1997–2004 in a degraded raised bog in the Czech Republic (Lanta et al. 2006) found that rewetted plots developed different plant communities, but with similar species richness, to plots that remained drained. Over four years after rewetting, the overall plant community composition significantly differed between rewetted and drained plots (data reported as graphical analyses). Plant species with greater cover in rewetted than drained plots included Sphagnum mosses, white sedge Carex canescens, sheathed cottongrass Eriophorum vaginatum and marsh thistle Cirsium palustre. Forest-characteristic species had lower cover in rewetted than drained plots. Rewetting had no effect on plant species richness, which fluctuated similarly over time in rewetted plots (5–8 species/m2) and drained plots (4–6 species/m2). In 1997 and 2000, three drainage ditches were blocked with a total of 17 dams, rewetting the peat above. In rewetted plots, the water table was 7 cm below the peat surface on average (vs drained plots: 15 cm below). Over the following 1–4 years, cover of every plant species was visually estimated in 17 pairs of 1 m2 quadrats (one quadrat above and below each dam).

6 

A before-and-after study in 2001–2004 in a degraded fen in Hungary (Timmermann et al. 2006) reported changes in cover of plant community types following rewetting. For example, in the driest areas (winter water level <30 cm) Carex sedge communities dominated 2% of quadrats before rewetting but 31% three years after. Phalaris grass communities dominated 60% of quadrats before rewetting but 2% three years after. In wetter areas (winter water level >30 cm), cover of aquatic vegetation was 0–8% before rewetting and 23–52% after, but for sedge communities was 31–50% before rewetting and 0–9% after. These results were not tested for statistical significance. In 2001, a drained fen was rewetted (water table raised to 0–116 cm above the peat surface) by building dykes to divert river water into it. Before rewetting in August 2001 and annually until 2004, the dominant plant community type was recorded in fourteen 25 m2 quadrats along each of fourteen 100 m transects.

7 

A before-and-after study in 2002–2004 in a degraded fen in California, USA (Patterson & Cooper 2007) found that following rewetting, cover of peatland-characteristic sedges increased whilst cover of grass species preferring drier conditions decreased. Cover of three sedge species characteristic of wet peatlands increased (two significantly or marginally so), from 12–15% before rewetting to 13–20% one year after. Cover of three grass species that prefer drier conditions decreased (two significantly), from 2–6% before rewetting to 1–5% one year after. In July 2003, a fen was rewetted by blocking the main drainage ditch with metal dams. At the same time, channels were dug across a road that previously blocked surface water inflow. The water table was raised, ranging from 55 cm below the peat surface to 15 cm above during the summer. Vegetation cover was estimated in July before (2002) and after (2004) rewetting, in fifty-nine 10 m2 plots.

8 

A controlled, before-and-after study in 2000–2002 in a degraded fen in New Zealand (Sorrell et al. 2007) reported that following rewetting, total vegetation and upland plant cover decreased whilst wetland plant cover was stable. These results were not tested for statistical significance. Total vegetation cover declined in all four rewetted plots (before rewetting: 95–100%; one year after: 45–95%) but was stable in drained control plots (before: 92–100%; after: 90–100%). For two abundant, non-native, upland species, cover declined in all four rewetted plots (before: 5–40%; after: 0–5%) but was relatively stable in drained plots (before: 5–43%; after: 5–40%). For two abundant native species that only grow in wetlands, cover was stable in all rewetted plots (before: 22–90%; after: 24–90%) and all but one drained plot (where cover dropped from 13 to 0%). In March 2001, four plots within a fen were rewetted by blocking the main fen drain with soil dams. Nine plots remained drained. In 2000 and 2002, vegetation cover was estimated in four 4 m2 quadrats/plot.

9 

A replicated site comparison study in 2002 across five fen meadows in the Netherlands (van Dijk et al. 2007) found that rewetting had scale-dependent effects on plant species richness and diversity, and mixed effects on fen-characteristic species. At the site scale, rewetted meadows contained fewer plant species after four years than meadows that remained drained (25 vs 30 species/meadow). However, at the quadrat scale, rewetted meadows had significantly higher species richness than drained meadows (9 vs 7 species/m2) and significantly higher diversity (data reported as a diversity index). Other reported data (not statistically tested) included abundance of sedges Carex spp. (rewetted: in 3–18% of quadrats; drained: in 10% of quadrats), abundance of common reed Phragmites australis (rewetted: 3%; drained: 0%) and number of fen-characteristic species (rewetted: 11/meadow; drained: 15/meadow). In 1998, four fen meadows were rewetted by isolating them from their drainage systems. A reference meadow remained drained. In spring 2002, vegetation cover was visually estimated in ten 1 m2 quadrats/meadow. The water table was 20–30 cm below the peat surface in the rewetted meadows and 45 cm below the surface in the drained meadow.

10 

A before-and-after study in 1995–2006 in a degraded rich fen in Sweden (Mälson et al. 2008) reported that following rewetting, plant species richness, Sphagnum moss cover and tree cover increased, but cover of shrubs and purple moor grass Molinia caerulea decreased. These results were not tested for statistical significance. Plots contained 13–15 plant species before rewetting but 18–27 species four years after. In the plot where it occurred, cover of spiky bog moss Sphagnum squarrosum was 1% before rewetting but 13% after. Tree cover was 9–18% before rewetting but 13–20% after. In contrast, cover of purple moor grass was 58–74% before rewetting but only 32–62% after, and cover of shrubs 11–26% before rewetting but only 1–5% after. There was no clear change in cover of sedges Carex spp. (before: 0–1%; after: 0–2%) or two fen-characteristic moss species (0% before and after). In December 2002, the water table of a drained fen was raised approximately 17 cm by blocking a drainage ditch. Cover of every plant species was estimated in summer before (1995, 1997 or 2002) and after (2006) rewetting, in nine 1 m2 quadrats in each of two plots.

11 

A replicated, paired, controlled study in two degraded blanket bogs in Scotland, UK (Anderson 2010) reported that rewetted plots developed greater cover of sheathed cottongrass Eriophorum vaginatum, and typically less cover of forest mosses, than drained plots. These results were not tested for statistical significance. In three of three comparisons, rewetted plots had greater cottongrass cover than drained plots after five years (rewetted: 19–45%; drained: 11–34%), but less plait moss Hypnum cupressiforme cover (rewetted: 18–44%; drained: 35–57%). Rewetting reduced cover of silk moss Plagiothecum undulatum in one of three comparisons, when plots remained forested (rewetted: 3%; drained: 6%) but had no additional effect in plots where trees were felled or removed (rewetted: 0.6–0.8%; drained: 0.5–0.6%). Six blocks of six 40 x 100 m plots were established in drained bogs forested with spruce and pine. Between 1996 and 1998, six treatments were replicated once/block: rewetting, rewetting with tree felling, rewetting with tree removal, tree felling only, tree removal only, no intervention. Rewetting was achieved by damming plough furrows every 20 m. In rewetted plots, the water table was 8–32 cm below the peat surface during the growing season (vs drained plots: 11–38 cm below). Vegetation cover was recorded five years after intervention (details not reported).

12 

A replicated, paired, controlled study in a degraded raised bog in Scotland, UK (Anderson 2010) reported that blocking plough furrows to help rewet the bog had no (or no consistent) effect on vegetation cover in plots where trees had been felled. These results were not tested for statistical significance. After five years, plots with blocked and open furrows had similar cover of heather Calluna vulgaris (3–15% vs 3–14%) and sheathed cottongrass Eriophorum angustifolium (20–48% vs 15–45%). Grass cover was similar in blocked and open plots when trees had been removed (3 vs 2%), but higher in blocked plots when all tree debris was left in place (6 vs 1%) and lower in blocked plots when tree tops were left in place (7 vs 10%). Grass was mostly wavy hair grass Deschampsia flexuosa. Twelve pairs of 18 x 20 m plots were established in a drained, pine-forested bog. Between 1996 and 1998, plough furrows were blocked in one plot/pair but left open in the other. The water table depth was similar under both treatments (0–22 cm below the peat surface). Trees were felled in all plots, with debris left in place (four pairs), tree tops left in place (four pairs) or all debris removed (four pairs). Vegetation cover was recorded five years after intervention (details not reported).

13 

A replicated, paired, controlled, before-and-after study in 2002–2005 in two degraded rich fens in Sweden (Mälson et al. 2010) reported that rewetting alone led to small changes in plant community composition and cover. These results are not based on tests of statistical significance. The overall composition of the plant community changed over three years following rewetting (data reported as a graphical analysis). Cover of sedges Carex spp. increased in rewetted plots (from 0–2% before rewetting to 1–8% three years after) but was stable in drained plots (0–1%). Purple moor grass Molinia caerulea was common in one fen, where cover decreased in rewetted plots (from 50 to 30%) but was stable in drained plots (50–55%). Sphagnum mosses were common in the other fen, where cover increased in rewetted plots (from 14 to 25%) but decreased in drained plots (from 43 to 28%) – although responses differed between species. In spring 2003, one 50 x 150 m plot in each fen was rewetted by blocking a drainage ditch (water table raised approximately 10 cm). An adjacent plot in each fen remained drained. Both plots were also cleared of trees. Vegetation cover was estimated, in 0.25 m2 quadrats, in the central 100 m2 of each plot: sixteen quadrats in 2002 across the whole 100 m2, and four quadrats in 2005 within subplots that received no additional treatment. This study was based on the same experimental set-up as (18) and (23).

14 

A site comparison study in 2004 in two peatlands in Finland (Soini et al. 2010) reported that a rewetted peatland developed a different plant community to a pristine peatland, with lower plant species richness, lower shrub cover and greater sedge/cottongrass cover (but similar forb and moss cover). Most of these results were not tested for statistical significance. After 10 years, the overall plant community composition differed between the rewetted and pristine peatland (data reported as a graphical analysis and similarity index). The rewetted peatland contained only 15 plant species (vs 18 in the pristine peatland) and 5 species/60 x 60 cm quadrat (vs 9 species). In the rewetted peatland there were no dwarf shrubs (vs 3% cover in the pristine peatland) but sedge/cottongrass cover was 20% (vs 4%). Both peatlands had similar forb cover (5 vs 5%) and Sphagnum moss cover (84 vs 90%), and there was no significant difference in total moss cover (89 vs 90%). In 2004, cover of every plant species was recorded in 15 quadrats, each approximately 60 x 60 cm. Nine quadrats were in a historically mined peatland, rewetted in 1994 (water table 9 cm above peat surface during summer). Six quadrats were in a nearby pristine peatland with similar physical conditions (but a lower water table: 6 cm below surface). This study used the same rewetted peatland as (2).

15 

A before-and-after study in 1996–2007 in a historically mined raised bog in Germany (Bönsel & Sonneck 2011) found that following rewetting, there were increases in the number of plant species, moss cover and sedge/rush cover, but decreases in shrub and tree cover. The number of plant species on the bog increased from 157 before rewetting to 208 ten years after. Over the same time period, total moss cover increased from 31 to 44% and Sphagnum moss cover increased from 15 to 25%. Total herb cover did not change significantly over time (67% before and after). However, sedge/rush cover increased from 7–10% to 19–27% (not statistically tested). Shrub cover decreased from 41 to 22%. Tree cover decreased from 31 to 8%. In 1997, a bog was rewetted by blocking drainage ditches in and around it (water table raised to 5–55 cm below the peat surface). Plant species were counted and vegetation cover estimated before (1996) and after (2007) ditch blocking, in 35 permanent 25 m2 plots. This study was based on the same experimental set-up as (17).

16 

A replicated site comparison study in 2007 in four blanket bogs in Scotland, UK (Bellamy et al. 2012) reported that two rewetted bogs had, after 3–11 years, similar cover of bog-characteristic vegetation and open water (6–26%) to bogs that remained drained (5–16%). This result is not based on a test of statistical significance. Additionally, in one bog (Cross Lochs), transects rewetted eleven years before measurement had greater cover of bog plants/open water (26%) than transects rewetted only four years before (10%). Between 1996 and 2004, two drained bogs were rewetted by blocking most of their drainage ditches with peat and plastic dams. Two other bogs remained drained (ditches were not blocked). In summer 2007, cover of vegetation and open water were recorded in each bog along 30–60 randomly placed 1 m transects. Cover of bog characteristic Sphagnum moss species, common cottongrass Eriophorum angustifolium, dead heather Calluna vulgaris and open water were combined into a ‘bog recovery index’ for analysis.

17 

A site comparison study in 2007 in two bogs in Germany (Bönsel & Sonneck 2012) found that a fully rewetted bog contained more plant species, greater moss cover and greater cover of some herbs than a partially rewetted bog, but less tree and rush cover. After 10 years, there were more plant species on the fully rewetted bog (208) than the partially rewetted bog (68). The fully rewetted bog also had greater cover of mosses overall (44 vs 4%) and Sphagnum mosses (25 vs 14%). Beaked sedge Carex rostrata, purple moor grass Molinia caerulea and one of two Eriophorum cottongrass species were more abundant, relative to other plant species, in the fully rewetted bog than the partially rewetted bog (reported as an abundance index). The fully rewetted bog had less cover of rushes (19 vs 39%) and trees (22 vs 57%). Both bogs had similar cover of herbs overall (67 vs 65%) and shrubs (8 vs 6%). In 1997, drainage ditches in and around both bogs were blocked. In one bog all blockages were successful (water table 17–25 cm below peat surface) but in the other bog only some blockages were successful, so the water table was lower (40–50 cm below surface). In 2007, plant species were counted and vegetation cover estimated in 25 m2 plots: 35 on the fully rewetted and 21 on the partially rewetted bog.

18 

A replicated, paired, controlled, before-and-after study in 2002–2010 in three degraded rich fens in Sweden (Hedberg et al. 2012) found that following rewetting bryophyte and sedge cover increased, but there was no change in species richness, fen-characteristic plant cover or grass cover. Cover of wetland-characteristic bryophytes increased from 33% before rewetting to 46% eight years after. Sphagnum moss cover increased from 10 to 18%. Sedge cover increased from 1 to 3%. There was no significant change in cover of fen-characteristic plants or grasses (data not reported) or plant species richness (from 8 to 10 species/0.25 m2). In plots that remained drained, none of the metrics changed significantly over the eight years. In winter 2002/2003, one 100 x 150 m plot in each drained fen was rewetted by blocking a drainage ditch (water table raised by 12–25 cm). An adjacent plot in each fen remained drained. Trees were also removed from half of each plot. Between 2002 (before intervention) and 2010, cover of every plant species was estimated at 40 points/plot, in 0.25 m2 quadrats. This study was based on the same experimental set-up as (13) and (23).

19 

A before-and-after study in degraded peatland in China (Zhang et al. 2012) reported that following rewetting, cover of wetland-characteristic plants developed. No statistical tests were carried out. Before rewetting, wetland-characteristic plants were confined to drainage ditches (precise cover not reported). After rewetting, wetland-characteristic plants were observed across the peatland. Dominant plants in each part of the peatland were spikesedge Heleocharis valleculosa (80% cover), sedge Carex muliensis (60–70% cover) and Kneiff’s feather moss Leptodictyum riparium (15–80% cover). The blocked drainage ditches were dominated by floating bur-reed Sparganium angustifolium (30% cover). Within the drained and grazed Riganqiao peatland, two drainage ditches were blocked with 12 wooden dams. The water table rose above the peat surface in most areas. After rewetting, vegetation cover was visually estimated in five areas of the peatland (precise methods and dates not reported).

20 

A study in a degraded peatland in China (Zhang et al. 2012) reported that following rewetting, the peatland was colonized by wetland-characteristic plants. No statistical tests were carried out. Wetland-characteristic plants were observed across the rewetted peatland. Dominant plants in different parts of the peatland included sedges Kobresia capillifolia (10–60% cover) and Carex pamirensis (50% cover), rush Blysmus sinocompressus (20% cover) and marsh arrowgrass Triglochin palustre (15% cover). In 2004, the main drainage ditch in Dazhasi peatland was blocked with 19 sandbag dams, raising the water table. The dams failed over winter but were rebuilt each spring. After rewetting (year not reported), vegetation cover was visually estimated in five 4–100 m2 areas of the peatland.

21 

A replicated site comparison study in 2009 in 11 rich fens in Belgium, Poland and the Netherlands (Aggenbach et al. 2013) found that rewetted fens had similar total plant species richness to fens that had never been drained, but lower fen-characteristic species richness, greater herb cover and lower moss cover. Rewetted fens contained a similar total number of plant species to never-drained fens (27 vs 30 species/25 m2) but fewer fen-characteristic species (7 vs 15 species/25 m2). Rewetted fens had greater overall herb cover (52 vs 28%) and tall sedge/rush cover (41 vs 18%), but less cover of mosses overall (50 vs 90%) and fen-characteristic mosses (4 vs 40%). Cover of fen-characteristic vascular plants was similar in rewetted and never-drained fens (34 vs 33%). In summer 2009, cover of every plant species was estimated in sixteen 25 m2 plots: nine plots across five rewetted fens in Belgium and the Netherlands, and seven plots across six never-drained fens in Poland. Details of rewetting were not reported, but the water table in all fens was <10 cm below the peat surface. Plots experienced a range of mowing regimes. The rewetted fens contained more iron and phosphorous than the never-drained fens, which may have affected the vegetation.

22 

A replicated, before-and-after, site comparison study in 2011–2013 in a degraded raised bog in Latvia (Auniņa 2013) reported that rewetting had no effect on plant species richness, increased cover of sheathed cottongrass Eriophorum vaginatum, white beak sedge Rhynchospora alba and Sphagnum moss, and reduced cover of heather Calluna vulgaris (but not other trees/shrubs). Most of these results were not tested for statistical significance. Over six months, rewetting had no significant effect on plant species richness on three of four transects (before: 13.6–15.2 species/20 m2; after: 13.6–15.8 species/20 m2) but increased cover of sheathed cottongrass in 19 of 21 quadrats (by 1–5%). Over 18 months, rewetting increased cover of white beak sedge (before: 1%; after: 4%) and Sphagnum moss (before: 53%; after: 72%), but reduced heather cover (before: 84%; after: 68%) and had no effect on other tree/shrub cover (before: 20%; after: 22%). When vegetation cover changed, it became more like a pristine bog (with 80% Sphagnum, 18% sedge and 9% heather cover). In early 2012, drainage ditches were blocked in a bog remnant. In summer 2012 and 2013, cover of every plant species was estimated in the rewetted bog (in twenty-one 4 m2 quadrats) and in a nearby undisturbed bog (details not reported).

23 

A replicated, paired, controlled, before-and-after, site comparison study in 2002–2010 involving three degraded rich fens in Sweden (Hedberg et al. 2013) reported that rewetting increased plant species richness but found that it had no effect on vegetation height. After eight years, rewetted plots had higher plant species richness than drained plots (not tested for statistical significance). This effect was larger in plots that remained forested (rewetted: 13; drained: 9 species/0.25 m2) than in plots previously cleared of trees (rewetted: 14; drained: 13 species/0.25 m2). Rewetting had no effect on vegetation height: it was similar in both treatments after eight years (rewetted: 5–6 m; drained: 5–6 m) and did not change significantly over time (data not reported). For comparison, a nearby natural (undrained and unforested) fen contained 9 plant species/0.25 m2 and had a canopy height of 1 m. These were significantly greater in the rewetted plots. In winter 2002/2003 at each of three sites, two adjacent 100 x 150 m plots were established: one rewetted above a ditch blockage, and one drained below. Trees were also removed from half of each plot. Before intervention in 2002, then until 2010, plant species and canopy height (ignoring trees present before intervention) were recorded at 40 points/plot, in 0.25 m2 quadrats. The natural fen was sampled in 1978. This study was based on the same experimental set-up as (13) and (18).

24 

A before-and-after, site comparison study in 2004–2009 in a degraded fen in Poland (Kotowski et al. 2013) found that in a rewetted area (also cleared of shrubs and mown), the plant community composition changed in favour of fen and wet meadow species. Over five years, the overall plant community composition in a managed area became more like target fen meadow vegetation (data reported as a graphical analysis; change not tested for statistical significance). The abundance of fen and wet meadow plant species, including sedges Carex spp., increased in the managed area but did not change in the target area (data reported as abundance indices). In 2004, 0.7 ha of drained, overgrown fen was rewetted by blocking its connection to a drainage ditch. After rewetting, the water table was 0–16 cm below the peat surface (during summer). The fen was also cleared of willow Salix cinerea shrubs, then mown annually. The study does not distinguish between the effects of these interventions. The managed area was compared to 0.9 ha of target, shrub-free, fen meadow vegetation (retained in depressions during the drained period, but also affected by the rewetting and mown every other year). Each year between 2004 (before intervention) and 2009, vegetation cover was estimated in 18–22 plots/area. Plots were 20 x 20 m.

25 

A replicated before-and-after study in 2010–2013 in three degraded raised bogs in Latvia (Priede 2013) reported that rewetting had no effect on vegetation cover after one year. These results were not tested for statistical significance. In all three monitored sites, vegetation cover was similar in the three years before rewetting and the year after rewetting. This was true for heather Calluna vulgaris (before: 51–61%; after: 48–60%), Sphagnum mosses (before: 18–30%; after: 19–28%) and three other moss species (3–16% before and after). In 2012, drainage ditches were blocked in three degraded bogs. Each year between 2010 and 2013, vegetation cover was visually estimated in 25–30 permanent quadrats/bog. Quadrats were circular (4 m diameter) and arranged along transects perpendicular to the blocked ditches.

26 

A controlled study in 2006–2012 in a historically mined raised bog in Latvia (Priede 2013) reported that following rewetting, cover of feathery bog moss Sphagnum cuspidatum and white beak sedge Rhynchospora alba increased whilst cover of heather Calluna vulgaris decreased. Most of these results were not tested for statistical significance. Cover of feathery bog moss was 2% one year after rewetting but 28% six years after. No other Sphagnum species colonized. Cover of white beak sedge was 2% one year after rewetting but 39% six years after. Over the same period, heather cover decreased significantly from 35 to 19%. However, heather cover also decreased significantly in plots that were not rewetted (data not reported). In 2006, drainage ditches on part of a historically milled bog were blocked, raising the water table by 60 cm. In the rest of the bog, drainage ditches were left unblocked. Each year between 2007 and 2012, vegetation cover was estimated in permanent quadrats (4 m diameter circles): twenty-one in the rewetted area and seven in the drained area.

27 

A replicated before-and-after study in 2007–2008 in 23 degraded fens in Germany (Zerbe et al. 2013) reported changes in the cover of plant community types following rewetting. These results were not tested for statistical significance. Before rewetting, all sites were dry grassland (precise cover not reported). After rewetting, the fens remained dominated by grasses (most common vegetation type in all 23 fens; overall cover 48%) but cover of other plant groups had increased, including trees/shrubs (in 23 fens; overall cover 13%), common reed Phragmites australis (in 22 fens; overall cover 5%), cattail Typha latifolia (in 23 fens; overall cover 4%) and sedges Carex spp. (in 23 fens; overall cover 2%). Between 1995 and 2008, the 23 drained fens were rewetted by stopping their pump drainage systems (water table raised to 20–50 cm above peat surface). In 2007 and 2008, cover of vegetation groups was recorded in field surveys and/or from satellite images.

28 

A replicated before-and-after study in 2006–2013 seven degraded peatlands in England, UK (Glendinning & Hand 2014) reported that following rewetting, plant community types and purple moor grass Molinia caerulea abundance typically did not change, but Sphagnum mosses became more abundant. These results were not tested for statistical significance. Initially, all seven sites contained wet heath plant communities. After 2–6 years, four rewetted sites were still wet heaths but three had developed peatland plant communities. Purple moor grass abundance was similar (present in 95–100% of quadrats) before and after rewetting in all sites. Sphagnum moss species became more abundant after rewetting in most (21 of 34) comparisons. Abundance of blunt-leaved bog moss Sphagnum palustre consistently increased (six of six comparisons). Sedge Carex spp. and common cottongrass Eriophorum angustifolium abundance showed mixed responses depending on species and site. Between 2008 and 2013, one drainage ditch was blocked in each of seven peatlands. Vegetation (species presence/absence) was recorded before ditch blocking and 3–7 years after. In each site 120–160 quadrats (0.25 m2), arranged along a 30–40 m transect perpendicular to the blocked ditch, were surveyed in summer or autumn.

29 

A replicated, paired, site comparison study in 1993–2010 in three historically mined bogs in Quebec, Canada (González et al. 2014) found that rewetted areas typically had similar moss, herb and tree cover to areas that remained drained, but less shrub cover. In most cases, there was no difference between areas rewetted for 4–17 years and areas that remained drained, for Sphagnum moss cover (17 of 24 comparisons; rewetted: 0–21%; drained: 0–26%), other moss cover (8 of 15 comparisons; rewetted: 0–19%; drained: 0–31%), herb cover (8 of 12 comparisons; rewetted: 0–42%; drained: 0–24%) and tree cover (11 of 12 comparisons; rewetted: 0–8%; drained: 1–17%). However, in most cases shrub cover was lower in rewetted areas (11 of 21 comparisons; rewetted: 0–49%; drained: 1–71%). In the remaining comparisons, rewetted plots had greater Sphagnum and herb cover, lower tree cover, greater or lower moss cover, and similar shrub cover. Between 1993 and 2006, parts of three bogs were rewetted by blocking drainage ditches (water table 7–44 cm above peat surface during summer). Each bog also contained areas that remained drained (ditches not blocked; water table 2–8 cm above surface). In 2010, in each rewetted and drained area, all plant species touching 400–1,800 evenly spaced points were recorded.

30 

A replicated site comparison study in 2009 in 36 forested fens in Finland (Maanavilja et al. 2014) found that rewetting changed the plant community composition towards a more natural state, but had no effect on plant richness or diversity, tree volume or vegetation cover. After 1–14 years, the overall plant community composition in rewetted sites was intermediate between, but significantly different from, both drained and natural sites (data reported as a graphical analysis). In contrast, rewetting had no significant effect on plant species richness (rewetted: 31; drained: 30 species/site), plant diversity (reported as a diversity index), tree volume (rewetted: 235; drained: 335 m3/ha) and Sphagnum moss cover (rewetted: 25%: drained: 9%). Also similar between sites, but not statistically tested, were other moss cover (rewetted: 22%: drained: 25%), shrub cover (rewetted: 9%: drained: 8%) and herb cover (rewetted: 5%: drained: 2%). Compared to natural sites, rewetted sites had lower Sphagnum moss cover (natural: 46%) but greater cover of other mosses (natural: 3%) and greater plant diversity. Of the 36 forested fens studied, 18 had been rewetted in 1995–2008 by filling or blocking drainage ditches (water table raised to 15 cm below the peat surface). Nine fens remained drained (ditches open; water table 40 cm below surface). Nine fens had never been drained (water table 17 cm below surface). In 2009, vegetation cover and species were recorded in 72 circular (30 cm diameter) quadrats/site. Tree volume was measured in one 30 x 30 m plot/site. This study used the same sites as (32).

31 

A replicated site comparison study in 1998–2012 in eight fens in Germany (Görn & Fischer 2015) found that rewetted fens contained a similar plant community of similar height to both drained and natural fens, and had similar plant species richness to drained (but not natural) fens. After 14 years, the overall plant community composition in rewetted fens was not significantly different from drained or near-natural fens (but intermediate between the two; data reported as similarity indices). Vegetation height did not differ significantly between fen types (rewetted: 118 cm; drained: 56 cm; near-natural: 128 cm). Plant species richness was similar in rewetted and drained fens (both 18 species/4 m2), but lower than in near-natural fens (32 species/4 m2). Eight neighbouring fens were compared: two rewetted in 1998 (water table 38 cm below peat surface during summer), three that remained drained (water table 78 cm below surface) and three near-natural (never substantially drained; water table 13 cm below surface). In August and September 2012, cover of every plant species was estimated in thirty 4 m2 quadrats/fen type. Overall vegetation height was measured at 150 points/fen type.

32 

A replicated site comparison study in 2011–2012 in 36 forested fens in Finland (Maanavilja et al. 2015) found that in rewetted sites, Sphagnum moss growth was greater than in drained sites, and similar to undrained sites. After 3–16 years, Sphagnum biomass growth in rewetted sites (147 g/m2/year) was significantly greater than in sites that remained drained (76 g/m2/year) and not significantly different to never-drained sites (128 g/m2/year). The same was true for length growth (rewetted: 6; drained: 3; never-drained: 5 g/m2/year). Of the 36 forested fens studied, 18 had been rewetted in 1995–2008 by filling or blocking drainage ditches, nine remained drained (ditches open) and nine had never been drained. In May 2011, nine 13 x 13 cm plastic nets were installed on the rewetted peat in each site. In May 2012, all Sphagnum growing above each net was harvested, then dried and weighed. Stem length was measured for 20 shoots/net. This study used the same sites as (30).

33 

A before-and-after study in 2008–2013 in a degraded poor fen in Spain (Peralta de Andres et al. 2015) reported that following rewetting (along with cattle exclusion), cover of rushes Juncus spp. increased and new populations of Sphagnum moss appeared. No statistical tests were carried out. Before intervention, the fen was covered by dryland grasses and forbs, with no Sphagnum. Four years after intervention, 81% of the fen area was dominated by rushes: common rush Juncus effusus with some sharp-flowered rush Juncus acutiflorus. Sphagnum mosses also appeared in 3 of 10 monitored quadrats. In 2009, a drained fen was rewetted by blocking drainage ditches, removing a drainage pipe and building a new inflow ditch. The fen was also fenced to exclude cattle. The study does not distinguish between the effects of rewetting and cattle exclusion. Vegetation cover was estimated in 2008 (before restoration) and 2013, in ten permanent quadrats (size not reported) and from aerial photographs.

34 

A replicated before-and-after study in 2007–2015 seven degraded peatlands in England, UK (Glendinning & Hand 2016) reported that following rewetting, plant species richness consistently increased but the plant community type did not change. These results were not tested for statistical significance. Plant species richness increased in all seven sites, from 11–39 species/site before rewetting to 17–49 species/site after 2–7 years. In contrast, the plant community type did not change in any site. Both before and after rewetting, four sites contained wet heath communities, two sites contained dry heath communities and one site contained a dry grassland community. Between 2008 and 2013, one drainage ditch was blocked in each site. Vegetation was recorded before ditch blocking (as presence/absence of species) and 2–7 years after (as cover of every species). In each site 120–160 quadrats (0.25 m2), arranged along a 30–40 m transect perpendicular to the blocked ditch, were surveyed in summer or autumn.

35 

A replicated before-and-after study in 1999–2014 in two historically mined bogs in Sweden (Kozlov et al. 2016) reported that rewetted bogs developed plant communities that included some key bog species. Before rewetting, both bogs were bare peat. In Västkärr bog, vegetation developed within one year after rewetting. The overall plant community composition changed significantly over the 14 measured years (data reported as a graphical analysis). During this period, there were 2–6 plant species/m2 and vegetation cover was 30–112%. After 14 years, vegetation cover included sedges Carex spp. (23%), duckweed Lemna minor (15%), cattail Typha latifolia (10%) and common reed Phragmites australis (1%). In Porla bog, vegetation cover developed 7–14 years after rewetting. During this period, the overall plant community composition did not change significantly (data reported as a graphical analysis). There were 2–4 plant species/m2 and vegetation cover was 40–77%. After 14 years, vegetation cover included Eriophorum spp. (13–32%), Sphagnum mosses (20%), other mosses (<1%), sedges (2%) and common reed (2%). In 1999, both drained bogs were cleared of existing vegetation then rewetted (Västkärr by filling ditches and ceasing pumping, Porla by restoring inflow). Between 2000 and 2014, cover of every plant species was estimated in 1 m2 quadrats: 1–32 quadrats/year/bog. The water table in the sampled areas was 0–20 cm above the peat surface.

36 

A site comparison study in 2008–2014 in a historically mined bog in Quebec, Canada (Rochefort et al. 2016) reported that a rewetted area developed a different plant community to, with less vegetation cover than, nearby natural fens. These results were not tested for statistical significance. Note that the aim of this study was to create a fen, as the post-mining peat chemistry was more like a fen than a bog. After five years, the rewetted area contained a different overall plant community to three nearby natural fens (data reported as a graphical analysis). In the rewetted area, Sphagnum moss was absent (vs 15–25% in natural fens), other moss cover only 8% (vs 12–55%) and vascular plant cover only 24% (vs 59–86%). The rewetted area was dominated by woolgrass Scirpus cyperinus (19% cover; natural fens: 0%) and bog myrtle Myrica gale (8% cover; natural fens: 4–19%). In winter 2009/2010, part of a historically mined bog (abandoned for nine years) was rewetted by blocking drainage ditches with peat. Vegetation cover was estimated in 2008 (donor fen: in 16 quadrats along a transect) or 2014 (rewetted area: in six 25 m2 plots).

Referenced papers

Please cite as:

Taylor N.G., Grillas P. & Sutherland W.J. (2018) Peatland Conservation. Pages 329-392 in: W.J. Sutherland, L.V. Dicks, N. Ockendon, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2018. Open Book Publishers, Cambridge, UK.