United Kingdom: England: Restoration of Lowland Grassland on Ex-arable Land

Project Stage:
Completed

Start Date:
1994-01-01

End Date:
1998-12-31

Primary Causes of Degradation

Degradation Description

Post-war intensification of agricultural management together with land-use change have resulted in the large-scale degradation and fragmentation of this habitat. It has been estimated that unimproved grassland declined in area by 97% in lowland England and Wales between 1930 and 1984, so that no more than 865 sq km of this habitat now remain.

Reference Ecosystem Description

The project was conducted on five Environmentally Sensitive Areas in lowland England. The plant communities that were sought as targets, as defined under the NVC are: 1) Bromus erectus grassland, with a Centaurea nigra subcommunity; 2) Cynosurus cristatus-Centaurea nigra grassland, Lathyrus pratensis subcommunity; 3) Cynosurus cristatus-Centaurea nigra grassland, Danthonia decumbens subcommunity.

Project Goals

The project tried to examine the effectiveness of practical treatments to overcome possible limitations on diverse grassland creation. These treatments were designed specifically to investigate the relative importance of the following hypotheses: (i) diverse plant communities described by the NVC are useful ecological targets for restoration on ex-arable land; (ii) the assembly of these plant communities is seed limited; (iii) reduction of soil fertility can promote assembly of diverse communities; and (iv) establishment of desirable species can be enhanced by the amelioration of abiotic and biotic conditions through sowing a nurse crop. In order to test the generality of the treatment effects, they were applied at five sites distributed across different regions of southern England, with differing climates, management histories, soil types and nutrient status. Therefore a further hypothesis was tested by this meta-analysis: (v) these treatment effects are reproducible over a range of sites.

Monitoring

The project does not have a monitoring plan.

Stakeholders

The UK Biodiversity Action Plan (BAP) requires the sympathetic management of all remaining grassland fragments together with the recreation of 2000 ha of various types of species-rich grassland over the next decade. Much of this will be achieved within the agri-environment schemes such as the Environmentally Sensitive Areas (ESAs) designation. Carefully targeted habitat creation can be a useful tool for the conservation of such biodiverse habitats. However, the results of previous restoration under the agri-environment schemes have been inconsistent. The project used the NVC to derive target diverse grassland communities that were appropriate to the location, soil, hydrology and proposed management of each site. The National Vegetation Classification (NVC) is a systematic phytosociological description of British plant communities. The NVC describes 860 communities and subcommunities that have been derived from the analysis of 35,000 quadrats interpreted using information on management and environmental variables. Such classification systems provide a useful template for selecting appropriate ecological targets for habitat restoration and for monitoring progress towards their attainment.

Description of Project Activities:
The project consisted of a randomized experiment with four replicate blocks, each with seven restoration treatments at each site. The treatments were: 1. natural regeneration from cereal stubble; 2. shallow cultivation + species-poor ESA seed mixture; 3. shallow cultivation + species-rich NVC seed mixture; 4. shallow cultivation + species-rich NVC seed mixture + nurse crop; 5. deep cultivation + species-poor ESA seed mixture; 6. deep cultivation + species-rich NVC seed mixture; 7. deep cultivation + species-rich NVC seed mixture + nurse crop. The core of each plot measured 6x4 m with a discard area at one end. The plots were separated by 1-m guard rows. The cereal stubble was left intact for the natural regeneration treatment (1). Shallow cultivation was achieved with harrows or discs depending on the soil type. This was contrasted with deep cultivation to a depth of 30 - 40 cm using a plough, a method that was intended to bury fertile topsoil and the seed bank of undesirable species. A seed mixture designed to create the composition and structure of each NVC target community was made up from commercially available seed of native species. All forb species and the majority of the grasses were of lowland British provenance. These seed mixtures typically comprised between 25 and 41 species, of which 80% by weight were grasses and 20% forbs. The seed rate was between 24 and 28 kg ha. The performance of this species-rich NVC seed mixture was contrasted with that of the more species-poor seed mixture, which was recommended in the Department of Environment, Food and Rural Affairs (DEFRA) guidelines for establishing moderately diverse grassland on arable land. This prescription varied slightly between each ESA. Typically it recommended sowing six to eight common grasses at between 20 and 31 kg ha. However, the seed mixture recommended for the South Wessex Downs ESA required the inclusion of nine forbs with the basic mix of eight grass species. Finally, the effect of an annual nurse crop on forb seedling recruitment in the NVC seed mixtures was investigated by sowing the rye-grass Lolium multiflorum at a rate of 20 kg ha. In the first year after sowing the vegetation was cut and removed in early June to prevent the nurse crop from persisting. In subsequent years cutting was carried out in July and the herbage was left to dry for several days before removal to allow some seed return. In all years the aftermath growth was grazed by sheep between October and December at an annual stocking rate of 0·4 - 0·7 livestock units ha - 1 year - 1 depending on the grass growth. This typically equated to between 25 and 40 sheep ha - 1 grazing for 6 - 8 weeks. Soil samples were collected from each plot following cultivation in September 1994. Ten samples were taken at random from each plot using a 6-cm diameter and 20-cm deep auger. The samples were split into 0 - 5 cm and 6 - 20-cm depth fractions and bulked. A subsample of around 500 cubic cm was analysed for soil nutrients according to the methodology described by Allen et al. (1974). Soil sampling and analysis were repeated in September 1998. In early June of each year three quadrats were placed at random within each plot, avoiding a 1-m strip around the edge. Each quadrat measured 40x40 cm and was subdivided into 16 10x10-cm cells. The presence of all vascular plants was recorded in each cell. Abundance of each species was expressed as the percentage occupancy of the 48 cells per treatment. Species were classified as sown and unsown grasses and forbs. These were summed to provide a measure of plant species richness. both gave identical qualitative results.

Ecological Outcomes Achieved

Eliminate existing threats to the ecosystem:
The soil of the five arable reversion sites contained moderate to large residual amounts of P and K, and in most cases low %N. This is typical of soils with a history of intensive arable agriculture supported by inorganic fertilizers. There is good evidence to suggest that high residual soil fertility places severe constraints on the restoration and long-term maintenance of plant species diversity. Elevated nutrient levels are often associated with high productivity, which enables a small number of more competitive species to dominate the vegetation and exclude other species. However, this project suggests that the establishment of diverse grassland communities may not be constrained by high residual soil fertility on ex-arable land where N is initially in short supply. Monitoring is continuing in order to investigate the effects of elevated nutrient concentrations on the rate and direction of succession in the longer term. Nevertheless, this project has shown that deep cultivation of ex-arable soil can cause a rapid and significant reduction in available P and K concentrations in the 0 - 5-cm depth fraction across a range of soil types. It is likely that this reflects the burial and dilution of fertile upper layers of the soil with less fertile soil from below. Analysis of individual sites showed that deep cultivation was most effective on shallow chalk soils, where up to 50% reductions in both nutrients were achieved. However, it is important to note that the resultant nutrient concentrations achieved by this treatment were still considerably higher than those typically found in the unimproved grassland target communities. Furthermore, after 4 years P concentrations fell in the shallow cultivation treatments to those achieved by deep cultivation in the first year. Also, K concentrations recovered in the deep ploughed treatments so that after 4 years there were no significant differences between cultivation treatments. This might be explained by weathering and release of nutrients from the inorganic pool, as well as the mixing of the soil by macrofauna and the redistribution of nutrients by plant roots. It is perhaps surprising that %N declined by a small amount at both depths over the period of the experiment. Several studies have shown N and associated mineralization rates increase during old-field succession. However, little is known about soil processes during the early stages of old-field succession. It is possible that the rate of assimilation of mineralized N into the rapidly growing perennial vegetation and multiplying soil microbial populations initially exceeds the amount returned by processes such as leaf fall and root excretion. Deep cultivation caused a significant reduction in the number and abundance of unsown grass species. Many grasses have a light requirement for germination and therefore need to remain at or near the soil surface to establish. The beneficial effect of deep cultivation on sown forbs may therefore relate to decreased competition from unsown grasses, the short-term reduction in soil fertility, and the improved physical structure of the seed bed. Monitoring is continuing to see whether species richness will be maintained despite these comparatively high nutrient levels. The vegetation communities resulting from natural regeneration and those sown with typical ESA seed mixtures had significantly fewer desirable species than the treatments sown with the NVC seed mixture throughout the experiment. This confirms that the creation of diverse vegetation on ex-arable land is highly seed limited and there is little potential for the colonization of later successional grassland species. Sowing a seed mixture (ESA or NVC) led to a decrease in the number and individual abundance of unsown weedy species, but there was no evidence to suggest that sowing a species-rich NVC seed mixture was any more effective in suppressing weedy species than the more species-poor ESA seed mixture. This conflicts with the findings of another multi-site study where high-and low-diversity seed mixtures were sown on five ex-arable sites across north-west Europe (van der Putten et al. 2000). Plots sown with 15 species contained fewer "˜natural colonizer' species than those sown with four species. In our experiment the species-poor and species-rich seed treatments were sown with an average of eight and 37 species, of which an average of 8·6 and 17·9 species established, respectively. The relative similarity in the number of sown species establishing between our species-poor and species-rich treatments may explain the lack of support for the results of the latter experiment. However, these differences were sufficient to cause large differences in productivity between species-rich and species-poor treatments after the first year. The plots sown with the NVC seed mixtures provided a source of desirable species to colonize the other plots. This allowed us to determine whether sowing a grass-rich, but forb-poor, mixture, as the ESA mix was, makes the vegetation less susceptible to colonization by desirable forb species compared with the vegetation produced by natural regeneration. This may be a function of species richness but also the fact that the ESA mix contains later successional species than the ruderal-dominated natural colonizers. Thus the former may be less susceptible to colonization by other late-successional species. However, there was no evidence that vegetation produced by sowing the low-diversity ESA seed mixture was any less susceptible to colonization by desirable later successional grassland forb species than the natural regenerating vegetation. A simple measure of restoration success was applied to this experiment, namely a comparison of the composition of the restored vegetation to pre-defined desirable target communities described by the NVC. However, these communities were originally described from sample stands of high nature conservation value, so they represent an ambitious target for restoration on ex-arable land after 4 years.

Factors limiting recovery of the ecosystem:
The natural regeneration and ESA treatments corresponded poorly to the target vegetation types. This reflects the importance of the constraint of seed limitation on community development. The treatments sown with the NVC seed mixtures were reasonably similar in composition to the diverse target communities on neutral soils. Correspondence with the defined target subcommunities on the acidic and calcareous soils was much lower, reflecting the poor performance of the indicator species for these target vegetation types. In order to resemble truly the target ecosystem, a restored system must be similar in terms of the following attributes, which can be considered as measures of restoration success: (i) community structure and composition of all other biota, including non-vascular plants, invertebrates, soil microbiota; (ii) ecosystem function, including measures of nutrient utilization efficiency, retention and cycling, productivity of different trophic levels, community respiration, and water use; and (iii) the maintenance of communities and functions over time (stability, resilience and resistance).

Socio-Economic & Community Outcomes Achieved

Economic vitality and local livelihoods:
This project has demonstrated that the most effective means of restoring diverse plant communities to ex-arable land was deep cultivation followed by the application of a diverse seed mixture comprising ecologically appropriate species. The most important constraint on achieving the assembly of these plant communities was seed limitation: high soil fertility appeared to be a less important constraint, the amelioration of which can have small beneficial effects on the assembly of diverse communities. The establishment of desirable species was not significantly enhanced by sowing a nurse crop for amelioration of abiotic and biotic conditions. Overall these treatment effects were found to be reproducible over a range of sites. The NVC proved to be a useful means of tracking the course of vegetation succession towards a pre-defined target. The success of restoration, as measured by these means, was greatest on neutral soils.

Long-Term Management

Further research is required to enhance the establishment of species with a preference for calcareous and acidic soils, for this project, monitoring is continuing to ensure that these conclusion are upheld in the longer term.

Sources and Amounts of Funding

The project was funded by a commission (BD1404) from the Department of Environment, Food and Rural Affairs.

Other Resources

Pywell, Richard F. et al. 2002. Restoration of species-rich grassland on arable land: assessing the limiting processes using a multi-site experiment. Journal of Applied Ecology 39: 294-309.

Primary Contact

Organizational Contact