Study

A comparison of seed origin and sowing treatments in establishment of an MG13 grassland on a grazing marsh with high phosphorous availability, Barn Elms, Greater London, England

  • Published source details Gilbert J.C., Gowing D.J.G. & Bullock R.J. (2003) Influence of seed mixture and hydrological regime on the establishment of a diverse grassland sward at a site with high phosphorus availability. Restoration Ecology, 11, 424-435

Summary

High phosphorus availability in soil is associated with low plant species diversity in grassland swards. This study investigated whether creation of a diverse sward is possible on a soil with very high phosphorous concentrations at a grazing marsh in southeast England. Three experiments (see also Cases 564 and 565) were undertaken to investigate the influence of seed mixture on the establishment of a target grassland community. The experiment summarised here compared autumn- and spring-sown commercial seed mixtures with seed harvested from a nature reserve with respect to their ability to produce an inundation grassland community similar to that described by the British National Vegetation Classification (NVC) as Agrostis stolonifera (creeping bent)–Alopecurus geniculatus (marsh foxtail) grassland (NVC code MG13).

Study site: The comparison of seeding treatments on the establishment of anAgrostis stolonifera (creeping bent)–Alopecurus geniculatus (marsh foxtail) grassland (NVC code MG13, Rodwell 1992) was undertaken on the grazing marsh at the Wildfowl and Wetland Trust's (WWT) Wetland Centre, Barn Elms, London.

Seeding treatments: Three seeding treatments (spring sown commercial, autumn sown commercial and sown with seed of wild origin) and a fourth left unseeded to allow natural regeneration to occur were compared to assess which resulted in development of a sward most similar to the desired MG13 grassland target community. Seed mixes were broadcast onto the bare soil surface by hand in 1997. The seeding treatments were sown over equal areas (20, 1 m² randomly distributed permanent quadrats per treatment). Only one area of 10 permanent quadrats was monitored for the assessment of natural regeneration because the area available for this unseeded treatment was limited.

Wild seed was collected from existing MG13 grassland at WWT Welney on the Ouse Washes, Norfolk (Ordnance Survey Grid Reference TL 546 946), eastern England. The species content was ascertained using a germination test. As seeds only represented approximately half of the bulk of this mix, the rest being grass cuttings, a high sowing rate was used. The commercial seed mixes used were designed to contain seeds of grass species occurring in the MG13 community at proportions related to their frequency and abundance as defines the vegetation community. However, there were some differences between the autumn and spring sown seed composition. Some of the required species were not commercially available, most notably A.geniculatus, which along with with A. stolonifera, are the most frequent MG13 grassland species. The spring-sown mix sowing rate was lower as it was anticipated that A.geniculatus seed would become available for over-sowing later in the year, but by autumn 1997 insufficient quantities had been produced and seed of this species was not therefore sown. The autumn sown commercial seed mix differed slightly from that of the spring sown due to variations in seed availability. Their composition is given in Table 1 (attached). The differences in these mixtures was however only slight, thus it was thought that any effects on results would have been small.

The areas sown with the different seed mixes and the unsown area were adjacent, so over time seed was able to move between the areas, especially during mowing or periods of flood water inundation.

Germination tests: Germination tests were undertaken prior to sowing. Five replicates of 20 seeds of each species from each batch purchased were placed on moist filter paper in a petri dish (giving 100 seeds tested per species). These were kept between 20 and 24ºC under natural light with the filter papers moistened regularly. After 6 weeks a count was made of all the seeds that had germinated.

Plant monitoring: A visual estimate of percentage cover of all species within the 1 m² quadrats were recorded each June over 3 years. The location and surface elevation of each quadrat was also recorded to enable estimation of the water regime.

Calculation of similarity coefficients to vegetation communities: The percentage cover estimates for all the species recorded in the quadrats were entered into a vegetation analysis program which compares the vegetation to that of NVC floristic tables.

Hydrological regime: A network of ditches intersects the grazing marsh and influences the water table. A hydrological model was produced to predict the water table depth. This required weekly records of rainfall, evapotranspiration and ditch water level, plus a range of soil parameters (hydraulic conductivity, depth of soil layers, drainable porosity) and the elevation and distance from the ditch of each location. Ditch water level and rainfall were measured by WWT staff, and 18 dip wells were installed to enable the water table to be measured in representative locations. Evapotranspiration data were provided by the nearby weather station at the Royal Botanic Gardens, Kew. These data were used to validate the water-table predictions produced by the model.

Germination test results: Germination percentages were used to calculate the proportion of the sward each species would be expected to occupy if there were no other factors influencing germination success except for seed viability. The actual and expected percentage of the sward occupied by each species was quite similar for the autumn sown commercial mix but less so for the spring sown commercial and 'wild' Welney mixes. This could be due to the conditions after spring sowing being less amenable to germination than in autumn, as spring 1997 was very dry. Infact, the grazing marsh had to be watered with sprinklers to assist germination but promoted a rapid growth of annual weeds approaching 100% cover, which competed with the grass seedlings. In general, the cover occupied by unsown species decreased each year, although the area sown in autumn with commercial mix always had the lowest unsown species cover and the Welney seed always the highest.

MG13 similarity coefficient: The results show that in 1998 (the year following sowing) the Welney and natural regeneration treatments had a significantly lower similarity to the MG13 community than did the commercial seed mixes. This however, ceased to be significant by year 3. By year 3 a significant relationship had developed between the MG13 similarity coefficient and aeration stress.

Species diversity: Species diversity was heavily influenced by the number of weed species present. To avoid this effect, analysis was undertaken using only the species in the MG13 floristics table. By year 3 the Welney seed had achieved close to its predicted diversity, but the commercial mixes were significantly below predicted diversity. Overall species diversity increased from year 1 to year 2 but decreased in year 3 (Shannon-Weiner Diversity Index: Year 1 – 0.86; Year 2 – 0.96; Year 3 – 0.82). The proportion of the sward covered by MG13 species was stable between years 2 and 3 (spring sown c. 91-93%; autumn sown c. 98-97%; Welney c. 78-85%). In combination with the fall in species diversity, the relative proportions of the MG13 species changed from year 2 to year 3, with the dominance of some species increasing at the expense of others.

Conclusions: The results from this seeding trial show that seed mixture, timing of sowing and seeding rate had an initial effect on the vegetation that established. By the third year of monitoring there were no significant differences between the treatments, and hydrological regime had become the most important factor in determining species distribution. Autumn sowing produced the least weed cover. The vegetation was less diverse than predicted from germination tests and decreased in diversity over the monitoring period. This was perhaps due to the water regime being unsuitable for most species or the extremely high P concentration in the soil allowing the most competitive species to dominate. The Barn Elms studies highlight the need to be aware of soil and hydrological conditions of a site before choosing a target community and designing a seed mixture. It appears that in attempts to achieve the desired MG13/ MG8 grassland community, the proportion composition of the seed mix was less critical to vegetation establishment than the water regime and soil nutrient status.


References
Rodwell J.S.E. (1992) British plant communities. Volume 3. Grasslands and montane communities. Cambridge University Press, Cambridge. UK.


Note: If using or referring to this published study please read and quote the original paper, this can be viewed at: http://www.blackwell-synergy.com/journal.asp?ref=1061-2971


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