Early seral communities in a limestone quarry: an experimental study of treatment effects on cover and richness of vegetation

Summary

Abandoned limestone quarries in Britain can develop vegetation communities of high conservation interest as they may support many local and rare species. However, low seed inputs, low nutrient levels and grazing by rabbits Oryctolagus cuniculus have all been cited as factors constraining plant colonization. This study describes the effects of seeding with two native plant species, fertilizer application and rabbit grazing treatments that were investigated to assess their effects on the colonization of a limestone quarry floor from an initial average vegetation cover of less than 10%.

Study site: The study was carried out in Clipsham Old Quarry (National Grid ref: SK 980152) in Leicestershire, eastern England. Quarrying ended in 1941 producing a pit about 160 m long by 40-80 m wide. A sparse vegetation had since developed, with surrounding banks of clay overburden well vegetated with tor grass Brachypodium pinnatum and yellow oat-grass Trisetum flavescens. The quarry was surrounded by a mix of arable land, ley grassland, active quarry workings and woodland. There was a well established rabbit population.

Experimental design and treatments: In 1980 a sparsely (<10%) vegetated area was selected and divided into four blocks, each with 8 x 1 m² plots separated by a 0.5 m wide strip. Eight treatments were allocated at random within each block to study the separate and combined effects of:

i) covering the plots with cages (1.25 x 1.08 x 0.22 m with 5 cm galvanized wire mesh over a steel rod frame) (C) to exclude rabbit grazing (from 20 January 1981);

ii) sowing (S) with false brome Brachypodium sylvaticum and bird's-foot trefoil Lotus corniculatus (26 March 1981);

iii) adding fertilizers (F) (26 March and 28 May 1981 and 2 April 1982).


Sowing: B.sylvaticum and L.corniculatus (a legume), were chosen as they are two widespread, early and persistent grass and herb colonists of chalk and limestone quarries in England, and because of the long term beneficial effects of the legumes on soil nitrogen. Both were present in the quarry but no B.sylvaticum and only a few small L.corniculatus plants were present in the experimental area in 1980. Sowing rates were 500 Brachypodium/m² and 250 Lotus/m², equivalent to 22.5 and 3.4 kg/ha respectively. Laboratory germination tests were done prior to sowing to test seed viabilty.

Fertilizer application: Fertilizer was applied in 1981 as mixed powders of ammonium sulphate, triple superphosphate, potassium chloride and kieserite (2.1 g/m² of actual N:P:K:Mg at 2 :2 :1:1) in two half doses. In 1982 a single application of two slow-release, granular fertilizers was made (8.8/gm² of N:P:K:Mg at 4.4:1.8:1.6:1).

Vegetation recording: Vascular plant recording was undertaken three times during 1980 in late May pre-treatment, the beginning of July and the beginning of September. In order to make the plots as similar as possible without disturbing the soil itself, all established perennial plants and most annuals were gradually eliminated by spraying with 'Roundup' herbicide (glyphosate) after each recording. From 1981 to 1983, after the treatments had been applied, the plots were recorded three times a year using a quadrat frame subdivided into twenty-five units (20 x 20 cm). Within each unit, the percentage vegetation cover was estimated visually and the presence of all rooted species was recorded. The records from these 25 units were pooled to estimate the following:

(i) % cover;

(ii) total number of species/m² (species-richness);

(iii) local rooted frequency for each species (the number of units in which a species occurred);

(iv) combined rooted frequency (the sum of all the values of variable (iii)).

During 1981 and 1982, flowering species were noted within each m² unit, but from July
1983 onwards flowering was recorded within each 20 x 20 cm unit and summed to give a
'combined flowering frequency' comparable with (iv) above.

Counts of B.sylvaticum and L.corniculatus plants in were made in July 1981 and May 1982 to estimate establishment and survival. B.sylvaticum height was recorded in July 1983. Mosses were recorded in April 1982.

Soil characteristics: Four random samples of surface material were taken separately for mechanical analyses, studies on soil moisture retention, and bulk density measurements. Chemical analyses were carried out on 16 samples (four per block, each consisting of 20 soil cores 2-4 cm deep). Extractable K, Ca, Mg and Mn were determined; Fe, P and total N, were measured after extraction.

Plant cover: Compared to controls (<10% cover) when one or two factors were applied plant cover increased by up to seven-fold, and by 15 times when all three (i.e. seeding, fertilizer and rabbit exclosures) were applied simultaneously, reaching 78% cover in 1984. Mosses contributed little or nothing to cover during the summer months and were not included in cover estimates.

Plant species and rooted frequency: The number of species recorded in the experimental area rose from 46 in 1980 to 65 in 1984. The average number per plot rose from 22 to 32/m² but there was no significant difference between treatments in 1983 or 1984. Flowering was enhanced by several of the treatments, especially by cages, in 1984.

Rooted frequency (all species combined) was at first increased significantly by the sowing treatments but by 1984 was greatest in the cage + fertilizer treatments.

Conclusions and discussion: This experiment showed that nutrients, rabbit grazing and seed input were all limiting vegetation development on the quarry floor. The alleviation of either or both of the first two constraints had only marginal benefits for increased plant cover as most of the species present in the plots had very limited potential for response. Sowing with B.sylvaticum and L.corniculatus produced only moderate growth in the absence of rabbit exclosures or fertilizers. Only when all three were applied simultaneously did cover reach high values (70-80%). Such high cover occurred elsewhere on the quarry floor but tall grass communities where absent outside the cages. Depending on the situation, the use of cages/exclosures could be applied seasonally and fertilizer levels could be adjusted to suit different conditions or objectives.


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