Bomere Pool monitoring 2014

Transcription

1 Bomere Pool monitoring 2014 Alex Lockton Ecological Consultant 34 Gordon Road, Whitstable, Kent, CT5 4NF September 2014 Introduction This report covers the fourth year of monitoring of the ecology of Bomere Pool in the light of a planning application in 2011 on the use of the pool (Lockton & Whild, 2011b). The ecological importance of Bomere Pool is described in our previous reports (Lockton & Whild, 2003 & 2011a). It can be summarised as: Clear, mesotrophic waters with healthy populations of aquatic macrophytes such as Luronium natans, Elatine hexandra and Nitella flexilis. A fringe of floating-leaved plants such as Nymphaea alba, Nuphar lutea and Persicaria amphibia. Marginal vegetation characterised by stands of Typha angustifolia, Carex elata and Salix purpurea. A diverse range of Odonata, most significantly a good population of Cordulia aenea. The monitoring programme consists of annual visits by ecological consultants, by groups from Manchester Metropolitan University, and by naturalists involved in the Shropshire Ecological Data Network. Data is collected on the state of the existing vegetation, on the populations of rare species and speculatively on other species of interest that may turn up. The north shore of Bomere Pool in August Note the vigorous regrowth of scrub along the margin The pool was surveyed this year on 25 th August 2014 by Alex Lockton and five students on a Manchester Metropolitan University site assessment module and in July 2014 by Sarah Whild with an aquatic plants group. 1

2 Rare Species 1. Luronium natans, Floating Water-plantain Luronium natans was abundant this year, having spread to much of the north shore as far west as SJ This is actually a new 10 km square for a nationally scarce species, as it has never been recorded in the western part of the mere before (although dots are sometimes plotted for this square as it is the default grid reference for Bomere Pool, used for otherwise unlocalised records). The Luronium was found in flower and spreading vegetatively. Plants had long, robust stolons which appear to have developed in response to the wash caused by speedboats. This undoubtedly has helped to cause its spread across the mere, and the wash is also beneficial in keeping the shoreline clear of other vegetation, which is advantageous to the Luronium. We noticed that the Luronium appeared to be positively associated with Crassula helmsii (Australian Swamp Stonecrop), often growing up through patches of Crassula on the lake bottom. Rosettes of Luronium natans attached to long, thick stolons A plan showing where Luronium was found this year is given below. It shows a considerable increase over previous years, especially along the north shore, which is now in full sunlight and has a sandy substrate. 2

3 Locations of patches of Luronium natans in 2014 (blue dots). 2. Elatine hexandra, Six-stamened Waterwort Elatine hexandra was more abundant than in 2012 and has maintained its colony on the southern shore, but there have been no major changes. 3. Salix purpurea, Purple Willow There appears to be only one patch of Purple Willow, in the north-east corner under a patch of birch trees. It is being shaded out, and it would be a good idea to cut back the birch here to allow the willow to grow. But if this is done it is important to distinguish between the purple willow and other species the photo below shows a close-up of Purple Willow leaves. Left: the patch of Purple Willow on the shore; right: close-up of purple willow leaves. 3

4 4. Bog plants There are a number of plants indicative of bogs and mires present, which suggests that it may be beneficial to attempt to restore this habitat (which was undoubtedly present in the past, as shown by the historical records). The following axiophytes occur mainly at the western end of the mere and it seems quite likely that a small area of Molinia caerulea (Purple Moor-grass) grassland or mire could be recreated if some more trees were removed and the grassland suitably managed (ideally by cutting and grazing). Axiophytes of mire communities found at Bomere in recent years Agrostis canina, Velvet Bent Carex elata, Tufted Sedge C. nigra, Common Sedge C. pseudocyperus, Cyperus Sedge Comarum palustre, Marsh Cinquefoil Dryopteris carthusiana, Narrow Buckler-fern Hydrocotyle vulgaris, Marsh Pennywort Isolepis setacea, Bristle Club-rush Luzula multiflora, Heath Wood-rush Molinia caerulea, Purple Moor-grass Scutellaria galericulata, Skullcap Invasive non-native species There are very few non-native species at Bomere Pool. Excluding archaeophytes (ancient introductions) and plants trees, only Crassula helmsii (New Zealand Pigmyweed), Epilobium ciliatum (American Willowherb) and Conyza canadensis (Canadian Fleabane) have been recorded in recent years. None of these presents any problem for management they are all scattered throughout, in suitable habitats, but not abundant. During our survey this year we noticed that Luronium and Crassula often occurred together in patches around the margin. Their co-occurrence was so remarkable that it seems reasonable to suggest that there could be some positive benefit to the Luronium from the Crassula. This is quite possible, as patches of Crassula might create some stability to the substrate which is usually just bare sand and it could protect Luronium plants from herbivory, as there is so little vegetation under the water. In addition to this, Crassula would help to oxygenate the water, improving the water quality and helping to maintain a balanced fauna. This could be an important factor, as the mere experiences a significant bloom of blue-green algae in the summer, which must have a profound influence on the ecosystem. As Crassula is amphibious (and Luronium is generally not), it is better able to tolerate algal blooms. It therefore seems reasonable to recommend that there should be no attempt to control non-native species, and that any such action should be considered a Potentially Damaging Operation unless a full scientific assessment is undertaken, with a clearly stated objective and a rigorous monitoring exercise (complete with controls) to see whether the stated objective is actually achieved. 4

5 Tree clearance The tree clearance along the north shore has been highly successful, restoring the habitat and resulting in the growth of rare species and axiophtes. In previous reports we have not recommended much tree clearance along the southern shore because there was little evidence that this would be of much benefit, and there was a more urgent need to keep the north shore open. The latter remains the top priority, but much progress has been achieved in this and, in addition, some rare plants have begun to appear along the southern shore where limited clearance has taken place. It therefore seems sensible to suggest that more tree clearance along the southern shore would almost certainly be beneficial, for several reasons: 1. The existing woodland is of low ecological value; 2. Clearing the trees allows sunlight into the shallow margins, where all the species of interest occur; 3. Removing the trees allow the pool margins to erode, creating shallows for aquatic and marginal plants, and changing the substrate from organic leaf litter to bare sand. Displacement of fishing There was initially concern that fishing would be displaced to the western end of Bomere or to Shomere. There is no sign of this happening yet, with new fishing platforms being erected at Bomere and a number of fishermen present at the time of survey. Displacement of motorized boats The concern that boating would adversely impact on other parts of Bomere Pool has not materialised. On the whole, the water skiing has a positive influence on the mere, as the motorboats create waves which help to break up organic matter on the shore; it erodes the shoreline and thereby helps maintain the shallows; and it breaks up mats of vegetation to help maintain the open conditions that Luronium and Elatine require. 5

6 Summary & recommendations 1. There is a need to develop alternative management of the shoreline of Bomere Pool. It needs to be largely surrounded by open habitat, ideally grassland, to a distance of many (10+) metres from the shoreline. It is not possible to create grassland by occasionally cutting down trees an alternative management process must be implemented. The obvious choices are cutting or grazing. Given that cattle are grazing in a field just a couple of metres from the NW shore, the best option might well be to negotiate with the farmer to have the cattle graze a part of the shoreline. Strimming and mowing of other sections is also perfectly acceptable, as long as the cuttings are removed. 2. It seems clear that there is no need to eradicate Crassula helmsii. There is no evidence that Crassula causes any harm at this site, and it is also clear that the attempts to eradicate will not be successful. More importantly, the eradication efforts would almost certainly cause harm to populations of rare native species such as Luronium natans. Create a meadow at the western end, by tree clearance and (if possible) cutting and grazing. Keep the north shore clear of scrub to maintain a sandy, open shoreline. Extend tree clearance to patches along the southern shore to allow light into the shallow margins. Remove clump of birch trees at SE corner to allow the Purple Willows on the water s edge to thrive. 3. It can reasonably be concluded that the tree felling by Natural England has been a notable success so far, and that no problems have been caused by canoeing activities at Bomere. Boating and waterskiing should be encouraged, and the pool can continue to be fished as at present. 6

7 References Lockton, A.J. & Whild, S.J A Botanical Survey of Bomere & Shomere Pools. Whild Associates report to English Nature, Shrewsbury. Lockton, A.J. & Whild, S.J (a). Management of Bomere Pool for protected and ecologically valuable species. Whild Associates report to Barbers Estate Agents, Shrewsbury. Lockton, A.J. & Whild, S.J (b). Bomere Pool monitoring programme Report to Shropshire Council by Whild Associates, Shrewsbury. Lockton, A.J Bomere Pool monitoring. Report to Shropshire Council by Whild Associates, Shrewsbury. 7