Restoration of magnesian limestone grassland: optimizing the time for seed collection by vacuum harvesting

Summary

Magnesian limestone grassland is a rare plant community in the UK, with many sites lost to quarrying. Its re-establishment, for example at former quarry sites, may be enhanced by sowing with seed collected from established communities. This study evaluated the effectiveness of methods of seed harvesting for restoration purposes and assessed optimum time for seed collection based on phenology of plants at the donor sites.

Seed collection sites: In summer 1999, seed was collected from two sites:

1) Thrislington National Nature Reserve was the main seed donor site. It was dominated by blue moor grass Sesleria caerulea and sheep’s fescue Festuca ovina (with quaking grass Briza media, kidney vetch Anthyllis vulneraria, bird's-foot trefoil Lotus corniculatus, small scabious Scabiosa columbaria, burnet saxifrage Pimpinella saxifraga, glaucous sedge Carex flacca and harebell Campanula rotundifolia). Species composition and percentage cover was recorded on 20 July within five randomly distributed 4 m² quadrats.

2) Wingate Quarry , abandoned in the 1930s and with many characteristic plants including: F.ovina, B.media, C.flacca, S.columbaria, Yorkshire fog Holcus lanatus, meadow oat grass Helictotrichon pratensis, downy oat grass Helictotrichon pubescens, cock's-foot Dactylis glomerata, L.corniculatus and wild thyme Thymus polytrichus.

Seed collection and cleaning: A hand-held vacuum harvester was adapted to collect seed. It consisted of a 90 cm long, 15 cm diameter plastic collection tube with motor-driven blades at one end to generate suction and a seed collection bag (emptied every 20-min to maintain suction). The donor areas were divided into strips, each 3 × 24 m. At each, three strips were randlomly chosen on each collection date and harvested for 2 h. A strip was only harvested once. At Thrislington, five collections were made at 2-week intervals from mid-July to early September, and at Wingate Quarry on 27 July and 7 September.

Harvested seed was spread on aluminum trays, dried for two days at room temperature and sieved to remove detritus. The seed was stored in paper bags at room temperature until September. In total, 5.043 kg of seed were collected during the 42 h of harvesting (120 g/h).

Germination: Three 1g seed samples from each of the 21 collections were sown into separate seed trays on soil from a magnesian limestone field. Three trays had no added seed (to assess any emergence from seed already present in the soil). Trays, watered as necessary, were kept in a greenhouse from September to early January and then moved to cold frames. Seedling emergence was recorded.

Peak seed production: To ascertain peak seed production, phenological data for 14 species were collected at Thrislington from mid-July to early September. 

Differences between sites in seed harvest: Differences between the Thrislington and Wingate sites were based on the average number of seedlings that germinated from seed collected on 27 July and 7 September. Few differences were found in seed composition. In the 27 July sample, significantly more ribwort plantain Plantago lanceolata germinated from Wingate seed (average 23.8) than Thrislington seed (average 2.0); while significantly more yellow rattle Rhinanthus minor germinated from Thrislington seed (average 20.0) than from Wingate seed (average 10.7). In the 7 September sample, more rough-stalked meadow grass Poa trivialis germinated from Wingate seed (average 2.4 ) than from Thrislington seed (average 0.3). For many species, statistical comparison was not possible as seeds germinated in insufficient quantities.

Species composition: Two species, Briza media and Festuca sp. accounted for 74% of all seedlings. Twelve species accounted for around 96% of the harvested seed, with a further 24 making up the remaining 4%. The commonest forbs were mouse ear hawkweed Pilosella officinarum, P.lanceolata and Hieracium sp.  (8% of seedlings). At Thrislington, B.media, L.corniculatus, S.caerulea and red clover Trifolium pratense each had a cover of more than 10%, and Festuca sp. almost 50%. Most species however covered 5% or less of the donor area. Half of the species (27 of 54) at Thrislington were present in the harvested seed. Festuca sp. and B.media were over-represented in the collected seed, compared to the donor site, whilst all other species were under-represented. Seed of some common species (e.g. S.caerulea, C.rotundifolia and P.saxifrage) were either absent or only present in very small quantities. Vegetation at both site was significantly more diverse than the species germinating from the harvested seed.

Peak seed production: Peak seed production for most species occurred between early August and early September. Earlier peaks were found for R.minor (late July), B.media (mid-July) and S.caerulea (mid-June or earlier), and later peaks for S.columbaria and greater knapweed Centaurea scabiosa (early September).

Conclusions: Collection of seed by the vacuuming method trialled is not sufficient by itself for seeding large areas of magnesian limestone grassland because of its low collection efficiency (only 120 g seed/h). However, it is superior to hand collection. A vacuum harvester could be used to target particular species that cannot be purchased and guarantees the provenance of such species. It has the advantage that it is easily transported and can be used at sites (e.g. quarries) that are inaccessible to a tractor-towed harvester.


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