Individual study: The effects of mowing and grazing on a cowslip Primula veris population growing in calcareous grassland near Voeland, Limburg, Belgium
Brys R., Jacquemyn H., Endels G. De B. & Hermy M. (2004) The effects of grassland management on plant performance and demography in the perennial herb Primula veris. Journal of Applied Ecology, 41, 1080-1091
European calcareous grasslands are under threat due mainly to changing land-use practices including grazing abandonment and conversion to arable. These grasslands have a high diversity of rare plants and a rich invertebrate fauna, and as such many are considered of high conservation value. At a site near Voeland (eastern Belgium) an experiment was undertaken to evaluate the effectiveness of different management strategies for population enhancement of cowslip Primula veris (a rosette-leaved plant typical of lowland calcareous grassland) and to design an effective long term grassland management strategy.
Traditional grazing management previously undertaken was intended to suppress grass growth and to encourage rosette-leaved plants and other herbs. This consisted of periodic cattle grazing restricted to short periods (< 2 weeks) during the early summer. With an estimated 10,000 adult plants, cowslips are one of the commonest rosette-leaved plants at the site, and the species is considered a good indicator of the health of the ecosystem. However, there has been a great decrease during the last decade, hence the intiation of the experiment and the search for a more effective management regime.
Mangement regimes: Four regimes were applied to five replicate plots measuring 3 x 3 m, located in the central area of the site. Other than those which were grazed, all were fenced. To avoid edge effects, only the central 1 x 1 m area of each plot was sampled. The management regimes were:
i) Cattle grazing - restricted to short periods (less than two weeks) during the summer growing season. Part way through the experiment managers cautioned that grazing too early in the growing season was causing a decline in cowslips. Consequently, from 2001 onwards grazing was restricted to later in the growing season i.e. early July onwards;
ii) Summer mowing in mid-July after flowering, using shears, cut material removed;
iii) Autumn mowing in mid-October, using shears, cut material removed;
iv) No management.
This programme was continued for a five year period (1999 to 2003).
Monitoring: Plots were evaluated annually at the end of May, when adult cowslips had grown to their full size and seedlings had emerged. A total of 1,083 plants were monitored over the study period. All adults and seedlings were mapped. Each year 30 individuals per treatment were randomly selected to study the effect of management on the performance of reproductive individuals: plant size was calculated using a combination of leaf size, stem and flower dimension measurements; the effect of management on fecundity was quantified using 15 individuals randomly selected from each plot annually, and the number of flower heads and seeds produced by each, counted.
Cowslips showed considerable variation in performance and population growth under different management regimes. Each regime tested is discussed below in order of their effectiveness, and results are summarised in Table 1 (attached).
Autumn mowing: Autumn mowing was the most favorable management scenario. Over five years the density of flowering cowslips increased (average annual population growth rate 1.21). Additionally, plants produced more flowers and seeds than any other of the regimes. Autumn mowing created optimal conditions for seedling recruitment, the removal of established vegetation effectively opening up the sward and reducing competion.
Summer mowing: The effect of summer mowing on flowering and seed set was similar to autumn mowing. Adult populations and seed output were increased. However, mowing early allowed vegetation to recover during the growing season, this resulted in a taller and denser vegetation the following spring. Under these conditions seedling recruitment was reduced and population growth rates consequently lower, though still positive (average annual population growth rate 1.05).
Grazing: Under a regime of early summer grazing (stopped early in the experiment), cowslip numbers were reduced (average annual population growth rate 0.86). Flower stalks were destroyed by cattle, reducing the number of flowering individuals. The removal of leaves early in the growth cycle also resulted in reduced seed production per plant. Additionally there was no seedling recruitment. There was an annual population decline of 11%. In contrast, with later grazing (early July onwards), flowering probability and seed set increased and populations grew (average annual population growth rate 1.08).
No management: Lack of management was by far the worst management strategy. With an annual population decline of 35%, this would have rapidly led to cowslip extirpation. Adult plants had little chance of flowering due to competition from surrounding vegetation and reduced light penetration. Seed germination and seedling establishment were also considerably reduced.
Annual survival: Annual survival rates varied considerably with management regime, especially when plants were in the seedling and juvenile stages. Grazing and autumn mowing resulted in the highest annual survival rates (70-90%). Survival was very low in the unmanged control plots, being less than 20% for seedlings and less than 50% for juveniles. For adult plants no significant differences in annual survival between management regime were found; annual survival was consistently high (mostly above 80%).
Conclusions: Autumn (mid-October) mowing was the most effective management method trialed. It increased the number of flowering plants, provided suitable conditions for seed germination and seedling establishment, and led to the greatest increase in cowslip numbers over the 5-year study period.
Note: If using or referring to this study, please read and quote the original paper, this can be viewed at: http://onlinelibrary.wiley.com/doi/10.1111/j.0021-8901.2004.00981.x/full