Contrasting response to mowing in two abandoned rich fen plant communities
Introduction
Globally, it is estimated that fens comprise 26% of all wetlands and 42% of all peatlands, which equates to an area of 1.5 million km2 (Joosten, 2002, Ramsar Convention and Secretariat, 2013). Fens are distributed throughout the northern hemisphere and are well represented in North America, Russia, Scandinavia and Central Europe (Lamers et al., 2014). The term ‘rich’ fen refers to minerotrophic peatlands which are base ‘rich’, due to a high concentration of base cations (calcium and magnesium) and are nutrient poor (oligo-mesotrophic) with pH values between 6.0 and 8.0, which subsequently leads to botanically diverse stress tolerant communities (Sjörs, 1950, Wheeler, 1980a, Wheeler and Proctor, 2000). Furthermore, U.K. fens are quite rare and make up just 10% of all British peatlands (JNCC, 2011b). The fens which are the focus for this study, are situated off the north-west coast of Wales (U.K.) on the island of Anglesey, are internationally significant and are designated as Ramsar sites (Jones et al., 2013). In the case of rich fens, their disjunct distribution supports their rarity, as these ground water dependent (GWD) wetlands have strict hydro-geological requirements (Joosten, 2002, Bedford and Godwin, 2003, JNCC, 2011b). It is these unique requirements which characterise the vegetation (Wheeler and Proctor, 2000) and render rich fens to be amongst the most species rich, low production wetlands in the world (Ilomets et al., 2010). Fens also support essential ecosystem services such as water purification, flood protection and climate regulation (Gorham, 1991, Zedler and Kercher, 2005, Lamers et al., 2014) in which their contribution is disproportionate to their size (Bedford and Godwin, 2003).
Historically, U.K. fens were economically important areas in the landscape, where agriculture, peat harvesting and biomass production for thatching ensured these systems experienced regular disturbance regimes. However, in the absence of management, rich fen plant communities have shifted from low competition/stress tolerant plant communities, with a complex assemblage of herbs, low stature sedges and bryophytes, to degraded plant communities which are dominated by graminoids and shrubs (Rodwell, 1992, Billeter et al., 2007, JNCC, 2011b). This change in trajectory from a low to high production community has also led to biomass accumulation from fast growing species, which is deleterious to maintaining fen species richness (Bergamini et al., 2001, White and Jentsch, 2001, Hajkova, 2003, Middleton et al., 2006b). Abandonment also leads to litter accumulation, which covers the peat surface and impedes seedling establishment (Ruprecht et al., 2010). To reverse succession to an earlier sere and increase biodiversity, partial or total removal of biomass is required, which addresses the physical barrier to restoration (Brewer et al., 1997). Mowing increases the space available for species to establish and increases light penetration to the peat surface (Schaffers, 2002, Billeter et al., 2007). Successful re-colonisation by small stature fen plant species is also dependent on the proximity of the restored plant community to a seed source, seed dispersal capability of favoured species and presence of a viable seed bank (Wheeler and Shaw, 1991, Billeter et al., 2003, Levine and Murrell, 2003, Kolos and Banaszuk, 2013). Therefore, rich fen species can only persist where traditional management has been maintained or is reinstated (Westhoff, 1971, Tilman, 1996, Middleton et al., 2006a, Šefferová et al., 2008).
This research was conducted in collaboration with the Anglesey and Llŷn fens EU LIFE project, whose main aim was to rehabilitate 751 ha of degraded fens, in support of improving habitat quality for plants and animals, as well as enhancing recreation value to the local community. The intention for this research was to examine the botanical response to mowing in these previously abandoned fens. The focus was on two European Annexe 1 plant communities, designated for their ecological importance and rarity (Saltmarsh et al., 2006). The first is Cladio-Molinietum (CM plant community), which is a species poor swamp community, situated within topographic depressions within valley floors and is dominated by tall sedge, Cladium mariscus (CM community). The second plant community is located around the fen margin, dominated by Schoenus nigricans and Juncus subnodulosus (SN plant community). S. nigricans is an ecologically important component as it produces elevated tussocks and consequential runnels (shaded channels around the tussock stools) that support a complex suite of fen species that include sedges, distinctive dicotyledonous herbs and an ecological group of “brown mosses” in the families of Amblystegiaceae and Calliergonaceae (Rodwell, 1992, Bedford and Godwin, 2003, Hedenas, 2003).
However due to abandonment, the CM and SN plant communities have reduced in species richness due to an increase in cover from each of the dominant graminoid species C. mariscus and S. nigricans and so machine mowing and hand cutting was selected for each plant community respectively. These methods of management were chosen due to the sensitive nature of fen habitat (Middleton et al., 2006b, Van Andel and Grootjans, 2006, Šefferová et al., 2008). In contrast to North America, burning is not common practice in U.K. fens. It is only undertaken with caution, where fire management has been used historically. This is probably due to their small areal size, which is disproportionate to high conservation value and as many species are associated with the plant litter which is being burnt (e.g. invertebrates, small mammals and amphibians) the risk to biodiversity is high (Middleton, 2002, SNH, 2011).
The aim of this study was to examine the response to mowing in two derelict rich fen communities employing a single mowing application. It was proposed that mowing would encourage colonisation by calcicole (calcium-loving) rich fen plant species, which are tolerant of a low phosphate concentration, associated with the presence of calcium carbonate found in the mineral rich groundwater (Clymo, 1962, Boyer and Wheeler, 1989, Wassen et al., 2005). It was expected that following mowing, species richness would increase due to a reduction in canopy height, standing biomass and litter cover which consequently will increase exposed peat. It was also proposed that mowing would reduce dominant vegetation components, as a reduction in graminoid and shrub cover is expected to be associated with increased herb and bryophyte cover. Therefore, it is hypothesised that due to the mechanisms described above, treatment effects would produce optimum conditions to meet the conservation aim to reduce robust late succession species and increase overall species richness in each plant community.
Section snippets
Site descriptions
The study was conducted within the Anglesey fens Special Area of Conservation (SAC) which comprises 7 Sites of Special Scientific Interest (SSSI) covering 467 ha and which form part of the Anglesey and Llŷn fens Ramsar site (Jones et al., 2013). All experimental sites were situated on the island of Anglesey, which is located off the coast of mainland north-west Wales, United Kingdom. The three study sites comprise: (i) Cors Erddreiniog (53.3125 N, −4.29670 E), a valley head fen system, comprising
Percentage cover of vegetation components
There were no significant differences observed in the baseline vegetation survey for each plant community for vegetation components and species richness. This confirmed that control and pre-treatment plots were botanically comparable. There were also no significant differences in DGPS elevations between treatments for the CM community (P, 0.14) and SN community (P, 0.67), suggesting topography was also equivalent.
Significant reductions were observed in the percentage cover of vegetation
Discussion
This research demonstrates how a single mowing event can have contrasting effects on two botanically different vegetation communities and that the disparity observed is complex. Variation in response to restoration is complicated by ecological gradients between and within sites, which include historical management and plant community dynamics (Turner et al., 1998, Wheeler and Proctor, 2000, White and Jentsch, 2001). Inherent conditions such as nutrient availability, hydrology, peat depth and
Conclusion
This short term study has demonstrated that both communities have exhibited varying degrees of restoration success. The CM community experienced rapid rehabilitation, which resulted in an increase in species richness, although the same progress was not achieved in the SN community. As Bradshaw (1996) and later others (Pfadenhauer and Grootjans, 1999) have stated; restoration is gradual and projecting target community composition is very difficult, given the contrasting influence of abiotic,
Recommendations
Future restoration intervention should include extensive baseline surveys to record environmental variables such as water table depth, N and P inputs (groundwater, surface water) as well as biological surveys. Pre-determining hydro-chemical and/or biological barriers to restoration will provide an insight in to pre-restoration site conditions (Bakker and Berendse, 1999, Holzel et al., 2012, Lamers et al., 2014). Fen systems which are free from deleterious environmental issues should be targeted
Acknowledgements
This project has been made possible by the Anglesey and Lleyn Fens – Restoring Alkaline and Calcareous Fens within the Corsydd Mon a Llŷn (Anglesey & Llŷn Fens) SACs in Wales LIFE07 NAT/UK/000948 project in association with Natural Resources, Wales (NRW). Also this is a Knowledge Economy Skills Scholarship (KESS) which is a pan-Wales higher level skills initiative led by Bangor University on behalf of the HE sector in Wales. It is part funded by the Welsh Government's European Social Fund (ESF)
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