Installation of surface grips for the restoration and management of wet grasslands on mineral soils

A key tool for restoration and management of lowland wet grasslands is the control of ditch water levels. Past research has shown that ditch water level control alone may have limit effect on in-field water table levels in soils of low hydraulic conductivity that restrict lateral movement of water between the ditches to the intervening land units. The installation of grips or foot-drains (shallow channels that mimic natural surface topography) has been proposed and implemented as a means of linking ditch water levels and in-field wetness. This project set out to test the effectiveness of grips of different spacing on lowland grazing marshes underlain by mineral soils at three sites across southern England (Pawlett, Somerset; Otmoor, Oxfordshire; Berney, Norfolk), using water table levels and soil moisture measurements to index hydrological response and invertebrates as a measure of ecological response. The three sites had different soil properties (Otmoor heavy clays, Berney and Pawlett had well-structure soils). Volumetric soil moisture content showed a strong relationship with water-table depth in the well structured soils of Pawlett and Berney, but this was a weaker relationship at Otmoor with its low permeability soil. Overall, the results suggest that installation of grips can increase invertebrate numbers and biomass in wet grasslands on mineral soils, particularly where the soils are not well structured. Optimum grip spacing varies with soil type; in poorly structured soils (such as Otmoor) grips need to be closely spaced (5m) to drain surface water, but will have little impact on the soil water table. In well-structure soils (such at Pawlett) grips can be about 20 m apart and still strongly influence the water table. Where soils have high permeability (such as Berney) grips or ditches can even be 60-70 m apart. An adequate source of water, infrastructure to maintain specific seasonal water levels and a means of evacuating excess water are essential requirements for a controlled water-regime. The case studies illustrated the need for a site-specific approach because the combination of these factors pertaining at any particular site will vary. The bird behaviour studies at Otmoor showed that pools of shallow water with slight slopes and a patchy vegetation structure provide valuable feeding habitats for both redshank and lapwing. The cross-sectional profile of the grips should therefore be shallow to maximise margins. This was support by the land managers at each site who indicated that shallow grips made it easier to use machinery on the site, such as mowers. By the end of the project, the grips at Otmoor were already showing infestation by emergent vegetation such as Typha latifolia that would reduce conveyance. Grips would need regular management to maintain their hydrological properties, such as clearance or possibly re-cutting every few year