Rhinanthus minor is a hemi-parasitic summer annual. It is found in a wide range of grassland habitats on soils of low to moderate fertility, but is most typical of hay meadows. The species germinates in spring, requiring chilling to break dormancy. Flowering is from May to August (Grime, Hodgson & Hunt 1988). Seeds are moderately large (3 × 4 mm; van Hulst, Shipley & Theriault 1987) winged discs held loosely in inflated calyces and are set from June onwards. Regeneration is from seed but Rhinanthus has no persistent seed bank, therefore population size is heavily dependent on the production of seed each year. Rhinanthus is capable of limited autotrophic growth (Grime, Hodgson & Hunt 1988) but in the field parasitism is virtually obligate (van Hulst, Shipley & Theriault 1987). However, Rhinanthus is non-host specific and may preferentially parasitize dominant species (Gibson & Watkinson 1989; Davies et al. 1997).
Leucanthemum vulgare is a polycarpic perennial. Achenes are usually shed while inflorescences are dying in August and September (Grime, Hodgson & Hunt 1988), but some seeds are shed from the outer rim of inflorescences earlier in the flowering season (S. Coulson, personal observation). Achenes are 2–3 × 0·8–1 mm (Howarth & Williams 1968) obovate, ribbed, unwinged (Oomes & Elberse 1976) and lack specialized morphological features facilitating dispersal. Regenerative strategies include vegetative lateral spread, although patches are usually less than 50 mm in diameter, and seasonal regeneration by seed (Grime, Hodgson & Hunt 1988). Germination is in late summer, autumn and spring.
The study took place within a grassland diversification experiment set up in October 1995 at Hill Farm, Little Wittenham, Oxfordshire, UK (15°37′N, 1°10′W). The experimental site was a permanent pasture field on poorly drained Gault Clays of pH 7·4–7·6. The initial vegetation was derived from a resown rye-grass clover ley typical of improved, damp, permanent pasture, dominated by Lolium perenne L., Dactylis glomerata L., Agrostis stolonifera L. and Poa trivialis L. Four treatments were applied; autumn graze only (treatment 1), cut July (treatment 2), cut July and September (treatment 3), cut July and aftermath graze (treatment 4). These reflected hay meadow management options available to land managers. One cut followed by aftermath grazing (treatment 4) is standard hay meadow management. Treatments 2 and 3 comprise cutting only and may be used on farms that do not carry livestock, such as in the east of England where arable farming is predominant and grazing is not an option. This is also true of restoration of grassland on field margins where grazing is not feasible. The second cut (treatment 3) may substitute for an aftermath graze by opening up the sward late in the season to provide establishment microsites. Treatment 1, with only autumn grazing, was carried out to determine whether hay meadow communities can be restored using only grazing. This is an option where farmers can no longer make hay because they have switched to silage production (i.e. they do not have turning machinery or storage facilities). In addition, late grazing is a conservation/restoration management tool specifically designed to facilitate seed dispersal by livestock and to extend the temporal availability of the nectar resource.
Treatments were assigned randomly to four plots in each of five blocks. Each plot measured 20 × 10 m; plots within a block were separated by a 1-m guard row. Blocks were separated by 10-m guard rows. Twenty-eight herb and grass species not found previously in the grassland, including Rhinanthus and Leucanthemum, were slot-seeded into each plot in October 1995, one species per slot. The species were randomly assigned among the slots. A tractor-mounted slot-seeder was used to spray a band of herbicide (approximately 15 cm wide) to kill the existing sward, and to drill the specified species into a slit cut in the ground within the sprayed band. A single pass of the slot-seeder achieved both operations (Wells, Cox & Frost 1989). The slots ran across the 10-m width of each plot, parallel to the plot edge. This design provided relocatable colonization foci for each of the sown species.
Grazing and hay-cut treatments on the plots commenced in 1996. In 1997, a typical year, grazing (treatments 1 and 4) took place between 6 and 31 October at a density of 38·5 ewes ha−1; equivalent to 90·14 livestock units (LSU). In the same year cut plots were cut for hay with an off-line tractor-mounted mower on 16 July (treatments 2–4), the cuttings turned and removed using rakes, and treatment 3 was topped to a height of 10 cm with the same mower on the 19 September.
Seed dispersal measurement
Seed dispersal was measured for Rhinanthus and Leucanthemum in two treatments – graze only (treatment 1) and cut July and September (treatment 3) – to provide a contrast between grazing and hay-cut treatments. Measurement sites were selected where there was a clump of plants along a slot that would provide a substantial seed source (at least two flower heads for Leucanthemum and four plants for Rhinanthus) and where there had been minimal spread of plants away from the slot. The latter was more difficult for Rhinanthus, which had spread in some plots. However, all bands of plants associated with the slots were less than 1 m wide, except for that in block 4 treatment 3, which had spread to approximately 2·8 m. Four measurement sites per species were selected for each of the two treatments across the five blocks. Rhinanthus measurement sites were located in blocks 1, 2, 3 and 4 for treatment 3 and blocks 2, 3, 4 and 5 for treatment 1. Leucanthemum dispersal was measured in blocks 1, 2, 3 and 4 for treatment 3 and blocks 2, 3 and 4 for treatment 1; with two measurement sites in block 4 for treatment 1.
At each site seed traps were placed along a transect perpendicular to the slot and crossing the slot through the centre of the clump of target plants. Traps were placed in pairs at the edge of the target clump of plants (0 cm) and at distances of 30, 60, 90, 120, 150, 200, 250, 300, 350 and 400 cm from the clump in both directions. The seed traps were sunk flush with the soil surface and covered with 15-mm gauge wire mesh to protect against damage by livestock and machinery and to reduce seed predation. Traps were put in place 24–26 June 1997, before seeds had begun to be released.
In the hay-cut treatments, traps were checked twice and seeds of the target species removed prior to hay-cut. Following the hay-cut on 16 July 1997, traps were removed (no seed heads remained on the plants), the contents sorted and seeds of the target species counted in the laboratory. In grazed treatments, traps were checked and seeds removed and counted periodically prior to grazing during the period July to October 1997, during grazing 6–31 October and once after grazing ceased on 4 November 1997.
Seed trap data for each species in the hay-cut treatment were split into pre-hay-cut (wind dispersed), and post-hay-cut. Because there was little dispersal in the grazed treatment after grazing started, the data from all trapping dates were merged. Each of these three data sets was then further split into data representing the two directions of trapping away from the source – 61°, roughly east north east (ENE), and 241°, roughly west south west (WSW), for grazed plots, and cut direction/opposite direction for hay-cut plots – giving six data sets for each species (with four replicated lines each). Dispersal curves were modelled by fitting an inverse power relationship relating seed number s to distance d to each of the six data sets:
- (eqn 1)
where a indicates the number of seeds falling at the source, and bd indicates the rapidity of the decline in seed numbers with distance from the source. this is the unlogged version of the model used by Willson (1993) and many others to describe a variety of seed shadows. To fit the logged model using linear regression, Willson (1993) had to exclude data points comprising zero values for seed numbers. This is an unnecessary exclusion of valid data values, which gives inaccurate parameter estimates. For this reason the unlogged version was used. However, the variance of the seed number data generally increased with the mean. Therefore the data were square-root transformed and this achieved homogeneity of variances. To simplify interpretation of model parameters the right-hand side of equation 1 was also square-root transformed for model fitting.
Equation 1 was fitted to each data set using PROC NLIN in SAS, which fits models using non-linear least-squares estimation (SAS 1990). The following strategy was used to compare the dispersal curves of two data sets (e.g. cut direction vs. opposite direction), i.e. whether ad1 and ad2 differ, and/or whether bd1 and bd2 differ significantly. Parameters ad1 and bd1 were estimated for the first data set. Then addiff and bddiff were estimated, which represented the difference between ad1 and ad2, bd1 and bd2, respectively. The 95% confidence intervals for, ad1, bd1, addiff and bddiff were then used to determine whether the ad and bd estimates differed significantly between the data sets.
The same procedure was used to compare dispersal curves between the two species. Data sets for the two species from the same treatment and period were compared (e.g. pre-hay-cut ENE). An exception was the post-hay-cut, opposite direction data sets, where there was no model convergence for the Leucanthemum data set, so the Leucanthemum post-hay-cut, cut direction data set was compared with the Rhinanthus post-hay-cut, opposite direction data set.
Thirty samples of sheep dung were collected randomly from the entire grazed area of the experiment. These were washed through a 1-mm gauge sieve to separate out any Rhinanthus or Leucanthemum seeds that had passed through the sheep.
Seedling establishment and survival
Seedling establishment was measured for both species, in each of the four treatments, in blocks 3–5 (three out of five blocks were used due to time constraints). A transect running parallel to the seed slots was placed randomly, within the central 6 m of each plot, but avoiding sown slots and, where possible, spreading Rhinanthus. Along the transect, a permanent 50 × 30-cm quadrat was placed every metre, excluding the top and bottom 2 m of each plot and the guard rows, giving seven quadrats per plot. Of these seven quadrats, three were assigned randomly to each species (Rhinanthus and Leucanthemum) and one to an unsown control. Seeds were sown individually using forceps, at 5-cm intervals, forming grids of 10 × 6 seeds in each permanent quadrat. Sowing took place over 2 days, 23–24 August 1997.
Half of the quadrats were checked for seedlings after approximately 3 weeks, the second half after approximately 4 weeks. Thereafter seedling establishment was recorded for each set of quadrats at approximately 3-week intervals, over a period of 9·5 months. Recording of seedlings was restricted to those occurring within 1 cm of the grid position in the quadrat. In control quadrats, establishment was recorded for a grid corresponding to those used in sown quadrats.
The spread of Leucanthemum and Rhinanthus away from the sown slots was measured on 15 June 1998 by laying three transects at random positions across each plot (4 treatments × 5 blocks) perpendicular to and crossing the sown slots. Transects consisted of lines of 1 × 0·5-m quadrats. Percentage cover was estimated for both species in each quadrat, giving data describing the variation in cover of each species across each plot.
Examination of the spread data indicated a negative relationship with distance. Therefore the parameters describing the rate of spread away from the slot were estimated using the same model as for the seed dispersal curves to relate percentage cover c to distance d.
- (eqn 2)
where as is cover at 0[middot]5[nbsp]m from the slot (see below) and bs is the rapidity of the decline in cover with distance d. percentage cover at each distance was calculated as the mean of values from the three transects within each plot, and was arcsine transformed to achieve normality. Equation 2 was fitted separately for each treatment in each direction, with the blocks as replicates, using the PROC NLIN procedure. Cover values at 0 m, the slot position, were excluded from the analyses, because these did not represent spread away from the slot. The two directions within each treatment were compared using the same methods as described for comparing dispersal curves. Only the Rhinanthus treatment 1 spread data showed significant effects of direction on parameter estimates (see below), so the data for the two directions in each treatment were merged and models fitted to each treatment. The parameter estimates for each of the four treatments were then compared in all pair-wise combinations. These comparisons were carried out only on the Rhinanthus data. Leucanthemum showed very little spread beyond 0·5 m from the slot in any treatment (see below), so comparison of model parameters was irrelevant. The same procedure was used to compare spread of the two species within each treatment.