Corky-fruited Water-dropwort Oenanthe pimpinelloides in Suffolk: an update

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CORKY-FRUITED WATER-DROPWORT OENANTHE PIMPINELLOIDES IN SUFFOLK: AN UPDATE FROM THE MAIN POPULATION AT BOURNE BRIDGE, IPSWICH TIM GARDINER & CHRIS STRACHAN Introduction The main location from which corky-fruited water-dropwort Oenanthe pimpinelloides is currently known in Suffolk is in the immediate surroundings of Bourne Bridge (grid ref: TM160419) where Belstead Brook flows into the Orwell Estuary. Despite being classified as of Least Concern in England’s Red Data List (Stroh et al. 2014), this tall growing perennial plant is listed as ‘rare (but common elsewhere)’ in the Suffolk Rare Plant Register (Sanford 2005). At Bourne Bridge, the plant is found on and around raised clay embankments which act as tidal flood defences next to a set of sluice gates. A track which runs from the road to the sluice has a large concentration of plants which benefit from annual mowing to allow access by vehicles and machinery.

The grassy flood defence embankments are largely covered in scrub and trees.

The plant is noted as occurring on neutral soils in dry pastures, hay meadows, roadside verges, and banks (Leach & Southam 1994). Despite its tall growing form, it can persist under quite intensive management on village playgrounds and football pitches. Its springy flower stems tend to escape mowing which may explain its abundance on the mown track at Bourne Bridge. Research suggests that gaps in the turf (bare earth) may also be important for seedling establishment; therefore, the track used by vehicles provides these opportunities in the sward.

It is the aim of this short paper to provide an update on recent flood defence management of the site by the Environment Agency (EA) and efforts to conserve O. pimpinelloides at its most important Suffolk site.

Effects of scrub clearance Management During February 2012, scrub and trees were cleared from the embankment using chainsaws and brushcutters to allow inspection of the structural integrity of the flood defence by EA engineers. Woody species such as ash Fraxinus excelsior, dog rose Rosa canina agg., hawthorn Crataegus monogyna and pedunculate oak Quercus robur were removed from the embankments.

Survey method Annual surveys of O. pimpinelloides at the site were undertaken before and after the clearance works (once annually in September 2011-14 and once in early October 2015). Counts of the number of O. pimpinelloides seed heads were made in several distinct areas of the site including the boat store, grassy track verges and two scrub cleared embankments. The counts involved a general sweep of the area. These counts serve as an estimate of the total number of plants and are likely to be a significant underestimate due to the particularly high concentration of plants in some areas such as the boat store. Results are presented as the total number of seed heads counted in each area and as numbers per m2 (number of seed heads divided by size of each area in m2).

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Results Approximately 65 flowering seed heads were recorded (a density of approximately 0.4 seed heads per m2) in 2011 from the two embankments cleared of scrub, compared to an astonishing 705 (c. 4.8 seed heads per m2) in 2012, an increase of more than 10-fold (Table 1). An area from which no significant disturbance occurred (boat store = control, no scrub clearance) during a recent sluice gate replacement was also monitored and the number of plants ranged from 891 (c. 1.3 per m 2) in 2012 to 166 (c. 0.2 per m2) in 2015. The extremely wet 2012 growing season seems to have prompted growth in the population of the plant on the flood embankments.

Clearance of woody vegetation also appears to have increased the number of plants at least initially. Particularly notable at Bourne Sluice was an area of embankment from which no O. pimpinelloides plants were observed pre-clearance in 2011, but 336 seed heads were recorded in 2012 where woody species such as C. monogyna, F. excelsior, Q. robur and R. canina agg. had been removed.

The number of seed heads decreased in 2013 and 2014 on both of the scrub cleared embankments (Table 1), but the number in 2015 still represented a substantially higher level than in 2011 before scrub clearance had commenced. This indicates that the increase in the number of seed heads on the scrub cleared embankments represents a generally positive response to the increased availability of grassland habitat on the slopes and crests of the earthen flood defences. However, since the 2012 clearance, scrub has begun to encroach back onto the embankments and in the unmanaged grassland of the boat store. Scrub clearance will be necessary in future years to restrict encroachment and conserve the grassland habitat of O. pimpinelloides.

Table 1: Estimated number of seed heads of O. pimpinelloides from different areas of the Bourne Bridge site from 2011-2015 where scrub was cleared in February 2012 and a control area (density per m2 in brackets) Effects of machinery and vehicular access during sluice gate replacement Site area Scrub cleared embankments Boat store (control area) Overall number

2011 65 (0.4) 238 (0.3) 303 (0.4)

2012 705 (4.8) 891 (1.3) 1596 (1.9)

Year 2013 212 (1.4) 203 (0.3) 415 (0.5)

2014 1 (0.0) 200 (0.3) 201 (0.2)

2015 144 (1.0) 166 (0.2) 310 (0.4)

Management A major sluice gate replacement was undertaken during the summer of 2012 (JuneAugust). To replace the gate, a crane was required to lift it from the sluice. The crane and associated machinery gained access along the grassy track with a large population of O. pimpinelloides. To facilitate access and provide a safe platform for the crane and other vehicles, crushed concrete material and clay was placed over parts of the track near the access gate, and a crane pad was constructed adjacent to the sluice. All materials laid down were removed at the end of the works and the track was allowed to naturally regenerate its grassland sward cover from the seed bank.

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Survey method To determine the impact of the access improvements and crane pad on the underlying grassland habitat containing a large population of O. pimpinelloides an annual quadrat survey was undertaken in September 2011-14 and once in early October 2015. The quadrat used was a standard 0.5 x 0.5 m frame with 100 divisions to allow estimates of the percentage ground cover of O. pimpinelloides to be obtained. A frame quadrat was used to provide standardised estimates of the plant’s abundance as seed head counts were not possible due to the low growing form of O. pimpinelloides on the track. The seed head counts were also not sufficiently scientific enough to allow a determination of the impact of machinery access on the plant.

Quadrats were placed at 1 m intervals along a central belt transect on the track starting from the entrance gate to the site (north edge) and moving southwards towards the sluice. A total of 60 quadrats were recorded along the belt transect on each survey. In each quadrat the number of squares (total = 100) in which O. pimpinelloides plants (parts of leaves and stems) were present in was counted. This roughly equated to the percentage cover of O. pimpinelloides in each quadrat. Mean percentage cover was then estimated for each survey of the track. It was possible to divide the belt transect into three sections with slightly differing management treatments regarding access during the 2012 sluice gate works: 1) from gate to start of ramp (1-32 m along transect), a section of track covered in topsoil; 2) (33-45 m along transect), a section of track not covered in topsoil or other material acting as a ramp leading to the crane pad; 3) (46-60 m), the crane pad area covered by matting to allow its safe operation.

Results In 2011, the season before works commenced, the overall mean percentage cover of O. pimpinelloides on the track was 81.4% ± s.e. 3.8% (Table 2; Fig. 1). After works on the sluice had finished in September 2012, the overall percentage cover ‘crashed’ to 3.4% (s.e ± 1%). Worse was to follow in 2013 when no plants at all were recorded in any of the quadrats. It seemed that a year after sluice gate works had finished, O. pimpinelloides had been all but eliminated from the access track. However, in 2014 the cover of the plant began to recover, leading to an overall percentage cover of 27.4% (s.e ± 4.1%) in 2015.

Table 2: Mean percentage cover of O. pimpinelloides on three sections of the belt transect (mean % cover +/- standard error) from the pre-work’s survey (2011) and post-works (2012-15) Transect section (no. quadrats) 1: topsoil covered track (32) 2: ramp (13) 3: crane pad (15) Overall (60)

2011 79.7 ± 5.5 88.6 ± 5.0 78.8 ± 8.8 81.4 ± 3.8

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2012 0.1 ± 0.1 8.9 ± 3.7 5.5 ± 1.9 3.4 ± 1.0

Year 2013 0 0 0 0

2014 0.4 ± 0.2 9.0 ± 3.1 24.9 ± 4.2 8.4 ± 1.8

2015 9.0 ± 2.6 67.2 ± 6.8 32.2 ± 8.1 27.4 ± 4.1

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Figure. 1: Percentage cover of Oenanthe pimpinelloides along the belt transect preworks (2011) and four seasons after works had been completed (2015).

A breakdown of the quadrat data along the belt transect into the three distinct sections revealed that the highest percentage cover of O. pimpinelloides pre-works in 2011 was in section 2 (Table 2). It appeared that immediately post-works in 2012, O. pimpinelloides was almost absent from the topsoil covered track (section 1), compared to the ramp (2) and crane pad (3) areas where the cover was higher, although dramatically reduced from the pre-work’s level. O. pimpinelloides was absent from the quadrats in 2013 as previously stated. However, by 2015, the plant had begun to reappear on all three transect sections, with the recovery well advanced on sections 2 and 3 (Table 2). A visual inspection of the belt transect data shows the significant recovery of O. pimpinelloides in sections 2 and 3 (from 33-60 m; Fig. 1). In two quadrats in 2015, 100% cover was achieved, a level not recorded since the 2011 survey.

Conclusions and the future It appears that the population of O. pimpinelloides at Bourne Bridge had been affected in two ways by recent EA management of the site: 1) scrub clearance on the earthen embankments in 2012 led to an increase in abundance due to the creation of a grassy sward suitable for O. pimpinelloides; 2) vehicular and machinery access along the grassy track during sluice gate replacement in summer 2012 appeared to have eradicated the plant one year after works had finished. Fortunately, the plant has recovered since, particularly at the southern end of the track near the sluice. It seems that scrub clearance has had the desired effect of increasing the abundance of

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O. pimpinelloides on the embankments which may go some way to offset its cover reduction on the track. The boat store and flood defence embankments have important populations of O. pimpinelloides where the plant should be safe from future sluice works. The regrowth of scrub after cutting needs to be addressed to maintain the plant on the embankments, while encroachment of woody vegetation in the boat store needs to be controlled.

The surveys also illustrate the difficulty in managing the sluice gates and surrounding embankments to maintain the flood defence structures, while reducing impacts on the O. pimpinelloides population. Mitigation measures during future works will need to be carefully planned to avoid further detrimental impacts on O. pimpinelloides. It is proposed to restrict the working corridor for machinery and perhaps investigate the reseeding of the track to allow O. pimpinelloides to recolonise after works have finished.

In the spring of 2014, half of the access track was utilised by excavators and vehicles to allow maintenance works to the nearby railway line to be undertaken. The additional disturbance to the track (section 1; Table 2) has probably not had a significant impact on the recovery of the plant as the cover of the species was higher in this section in 2015 than in 2014.

Acknowledgements The authors would like to thank members of the EA’s Fisheries, Biodiversity & Geomorphology (FBG) Team who have contributed to the site surveys, along with colleagues from other functions (Amy Butcher and Anna Lewis).

References Leach, S. J. & Southam, M. J. (1994). Oenanthe pimpinelloides L. In Stewart, A., Pearman, D. A. & Preston, C. D. Scarce Plants in Britain, JNCC, Peterborough p.

283.

Sanford, M. (2005) Suffolk Rare Plant Register. Suffolk Natural History 41: 122–150.

Stroh, P. A., Leach, S. J., August, T. A., Walker, K. J., Pearman, D. A., Rumsey, F. J., Harrower, C. A., Fay, M. F., Martin, J. P., Pankhurst, T., Preston, C. D. & Taylor, I. (2014) A Vascular Plant Red List for England. Botanical Society of Britain and Ireland, Bristol.

Tim Gardiner & Chris Strachan Environment Agency, Fisheries, Biodiversity & Geomorphology (FBG), Iceni House, Cobham Road, Ipswich, Suffolk IP3 9JD. tim.gardiner@environment-agency.gov.uk

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