Wessex Group: Wimbleball Lake, Brendon Hills, Somerset

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13 November 2022

Recent heavy rain meant that there was some debate as to whether the extensive drawdown zone at Wimbleball Lake reservoir on the south-western edge of Exmoor would still be there. The view from the road as you approach the lake immediately confirmed that large expanses of shoreline were still accessible. The reservoir had been so low in recent months, following the exceptionally dry year, that it had made the local press, with water at less than 20% capacity.

A small but enthusiastic group of five, Marion, Alan, Pete, Paul and myself, gathered at the car park near Bessom Bridge along Wimbleball’s northern shore. Before heading down to the lake edge, we paid our respects to a large patch of Neckera pumila on the trunk of a beech by the cars. The weather was unseasonably mild and soon we were on our hands and knees in the sunshine peering at the mosaic of tiny mosses on the higher parts of the drawdown zone. We were soon picking up species, such as Pseudephemerum nitidum, Bryum dichotomum, Trichodon cylindricus and, amongst these, the mats of a tiny but mature Ephemerum. It was assumed this was E. stoloniferum (E. serratum), which was confirmed that evening under a microscope. Ephemerum crassinervium subsp. sessile (E. sessile) was found under Bessom Bridge during a recce in October, and appears to be the rarer of the two species at this location.

Slightly farther down the shore we started to find large patches of a Physcomitrium. The expected P. patens was colouring the lower muddy areas a bright green, but this was a smaller species, in dense mats with the shiny protruding capsules, ranging in colour from green, through yellow and orange, to a mature black. Fortunately, whilst checking the site earlier in the week I had come across this moss, and after some checking had come to the conclusion that it was a candidate for P. readeri (later confirmed as a new VCR by Sharon). This is an enigmatic species that this autumn has been discovered at a number of reservoirs in southern England. Has it always been here or has its population recently exploded? It is found in Asia, North America and Australasia (it has the common name of Billabong Earthmoss!), but was confirmed at Lindley Wood Reservoir, West Yorkshire, new to Europe, in 2006. Looking at the large areas of Wimbleball Lake where this was the dominant species and the abundant ripe capsules, it was easy to image how quickly it could colonise the drawdown zone given sufficient time above water. Perhaps the recent dry summers had helped it to spread.

Higher up the shore line towards Bessom Bridge the group explored some of the eroded bank edges and upper shore, finding Pogonatum aloides, Weissia controversa, Bryum rubens, Fissidens taxifolius and Philonotis fontana. Alan found a large moss that was forming creeping tufts amongst the shale. After some puzzling, it was realised that this was Climacium dendroides without its ‘palm tree habit’. Moving on to a patch of willows on what should have been the water’s edge, but was currently high and dry, we searched the branches and trunks for epiphytes. There was a good coating of species, mostly various species of Metzgeria, Radula complanata, Hypnum andoi, Orthotrichum pulchellum, Plenogemma phyllantha and Ulota bruchii. Marion found a Schistidium on some boulders which on closer inspection was identified as S. rivulare, nearby there was a nice patch of Cinclidotus fontinaloides.

After lunch we headed round to the north-east shore, first stopping to inspect some willows. These were again coated in epiphytes, but no additional species, other than Microlejeunea ulicina and Zygodon conoideus, were noted. A nearby open patch of the upper shore was covered in an intricate carpet of tiny species. Here were bright green patches of Weissia rostellata. with its egg-shaped dark capsules hidden amongst the leaves. There was also more Climacium. Further down the shoreline was a tideline of Physcomitrium, mostly P. readeri, with clusters of Pseudephemerum and Ephemerum. A steeper higher section with some flushed areas turned up much Fossombronia wondraczekii (confirmed later from the spore ornamentation), Alan spotted Bryum pseudotriquetrum, and a puzzling pleurocarp was considered to be a form of Drepanocladus aduncus. A tiny Pohlia was later confirmed as P. camptotrachela. Also, a rather untidy pale green acrocarp turned out to be Pleuridium subulatum, with its tiny male branches just developing.

The mild weather continued throughout the day and a total of around sixty species were recorded, but the vast carpets of tiny mosses on the shore of the reservoir will be the enduring memory of the day.

Andrew Branson

Meeting details

Leader: Andrew Branson

With all the recent dry weather this autumn may be a good time to explore the draw-down zone of this reservoir. We will have a look at the north-east shore where there are also some nice areas of sallow carr and stands of poplars (always good for epiphytes). Recent records from the reservoir have included Phaeoceros laevis, Ephemerum stoloniferum, Physcomitrium patens, Climacium dendroides and Weissia rostellata, as well as species of Fossombronia and Pohlia. Wellies essential!

Meet at the car park at the north end of Wimbleball Lake at SS974318 by Bessom Bridge, accessed from the minor road off the B3190 west of Ralegh’s Cross.

Please remember to book your place with Andrew Branson andrew(at)3alpacas.co.uk


Wimbleball Lake (SS974318)