Individual study: Facilitating plant diversification by sowing the hemiparasite, yellow rattle Rhinanthus minor, in a semi-improved grassland at Little Wittenham Nature Reserve, Oxfordshire, England
Pywell R.F., Bullock J.M., Walker K.J., Coulson S.J., Gregory S.J. & Stevenson M.J. (2004) Facilitating grassland diversification using the hemiparasitic plant Rhinanthus minor. Journal of Applied Ecology, 41, 880-887
On formerly intensively managed farmland, species-rich grassland restoration through the addition of seeds of desirable species is frequently hindered by competition with established competitive species, linked to high soil fertility. Attempts to reduce soil fertility prior to restoration such as continuous cereal cropping, deep cultivation to dilute fertile topsoil and topsoil removal have had limited success. Methods of reducing competition from established species also include grazing and cutting, and gap creation. An alternative may be to increase the density of the hemiparasite yellow rattle Rhinanthus minor, a common component of species rich grassland in the UK, in order to suppress dominant host species.
An experiment was conducted to evaluate the effectiveness of Rhinanthus addition in enhancing the establishment and persistence of introduced forbs, with the aim being to develop a practical management tool for increasing grassland diversity during restoration.
Study site: The study was undertaken at Church Meadow, a semi-improved, species poor grassland of moderate soil fertility, on the Little Wittenham Nature Reserve in Oxfordshire, southern England. Vegetation most closely conformed to UK National Vegetation Classification MG6 Lolium perenne (perennial rye-grass)-Cynosurus cristatus (crested dog's-tail) semi-improved permanent grassland community.
Treatments: Experimental trials of yellow rattle addition began in December 1998. Five treatments, in 10 x 10 m plots, were assigned randomly to five replicate blocks:
i) no treatment (control)
ii) seed mixture sown at 5 kg/ha
iii) R.minor sown at 0.1 kg/ha and seed mixture sown at 5 kg/ha
iv) R.minor sown at 0.5 kg/ha and seed mixture sown at 5 kg/ha
v) R.minor sown at 2.5 kg/ha and seed mixture at 5 kg/ha
The site was managed throughout the study by hay making (late July) and aftermath grazing in autumn by cattle (stocking rate 35 - 50 units/ha).
In December 1998, native seed of R.minor ssp. minor was sown at three rates (treatments 3-5) and pressed into the soil using a flat roller. R.minor populations were allowed to establish over two seasons.
In 2000, all treatments (except the control) were oversown with a commercially purchased seed mix containing: bird's-foot trefoil Lotus corniculatus (14%), ribwort plantain Plantago lanceolata (14%), lesser knapweed Centaurea nigra (14%), rough hawkbit Leontodon hispidus (2.4%), selfheal Prunella vulgaris (14%), cat's ear Hypochoeris radicata (1.6%), autumnal hawkbit Leontodon autumnalis (4%), lesser trefoil Trifolium dubium (8%) and ox-eye daisy Leucanthemum vulgare (14%) at 5 kg/ha.
Vegetation sampling: In June 1999 and 2000, the rooted frequency of R.minor was recorded in each plot in 10 randomly located 1 x 1 m quadrats. In June 2001 and 2002, after the introduction of the other plants, the rooted frequency of all sown species was recorded in the same way, along with vegetation height using 10 randomly located drop disk measurements.
Yellow rattle establishment and colonisation: After the first year there were significant differences in rooted frequencies between all treatments and differences remained between some treatments for the first three years. In year two, R.minor was recorded in plots where it had not been sown (treatments 1 and 2) with a mean frequency of 0.2% (± 0.1). At the higher sowing rates (treatments 4 and 5) the increase in rooted frequency slowed over the three year period. Conversely, frequencies grew increasingly in the unsown and low-sowing rate treatments so that by 2002 there was no significant difference between treatments, all of which had average rooted frequencies of at least 65%.
Effect on sward structure: In both 2001 and 2002 there was a significant inverse relationship between R.minor frequency and sward height. A lag time in the results suggests there is a threshold of R.minor frequency above which there is strong impact on sward height.
Effects on richness and cumulative frequency of sown species: In both 2001 and 2002 the average species richness of sown forbs was significantly lower where no R.minor was sown than where sown. In the treatments unsown with R.minor the dominant species of sown forb was the annual, Trifolium dubium.
The frequency of R.minor had no significant effect on forb richness in the same year for both 2001 and 2002. However, in 2002 R.minor frequency in the previous year had an effect on the species richness, cumulative richness and total frequency of sown forbs.
Effects on frequency of individual forb species: Correlation between the frequencies of individual species and the frequency of R.minor, again in the previous year, were only apparent in 2002. Seven of the 10 species showed significant correlation, all positive, with Plantago lanceolata, yarrow Achillea millefolium and Centaurea nigra showing the strongest correlation. In 2001 only two species of forbs were recorded in the unsown treatments, Trifolium dubium (22.8 ± 6.3%) and Lotus corniculatus (0.2 ± 0.1%). By June 2002 Plantago lanceolata and Prunella vulgaris had also appeared, but at very low frequencies (< 1%).
Conclusions: This study suggests that the establishment and persistence of introduced forbs is facilitated by the presence of R.minor. There are two likely mechanisms through which this occurs:
i) R.minor parasitises and supresses dominant plants decreasing their biomass, thus introduced forbs are better able to compete;
ii) as R.minor is annual, when it dies gaps are left favouring germination and establishment of non-host plants.
Thus, in suitable habiat, R.minor could be used to increase plant diversity. This study indicates that seed should be applied at a high sowing rate (at least 0.5 kg/ha), and allowed to establish for three years before being oversown with other species. Sowing of Rhianthus should take place in autumn so the winter chill can break seed dormancy. Late summer cutting and aftermath grazing (after Rhianthus had set seed) did not have a detrimental effect on establishment; other studies indicate that mechanical cutting and hay removal have a beneficial effect on dispersal and establishment.
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