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Thursday, 3 October 2013

Bringing the Levels to the Garden

If you’ve been to the Botanic Garden recently, you may have noticed an area by the pond that has been sectioned off with some ropes. This is the future home of the Somerset Levels and Moors display at the University of Bristol Botanic Garden.  It is one of the mini-habitats of areas that are found here in the West Country that the Garden is replicating as part of its display of rare and threatened plants.

The Somerset Levels run from the foot of the Mendip Hills through to the first rising of the Quantock Hills and occupy an area of about 150,000 acres. The Levels are an interesting habitat, which has formed as a consequence of some rather unique geological features as well as hundreds of years of human modification; they support a rich diversity of species and as a result are of national and international importance.

I had an opportunity to speak with both Andy Windfield, a botanical horticulturist at the Garden, and Nick Wray, the Curator, about the Levels display that’s being constructed and there is an incredible amount of thought and work that goes into it. It’s not simply a matter of taking some of the local species and plunking them down in the garden, Nick and the Garden staff are taking great care to replicate some of the natural processes, such as the seasonal rise and fall of water levels, that make the Somerset Levels such a unique habitat.  

What makes the Levels unique?

The Somerset Levels are a sedge-peat moor, which is very distinct from a sphagnum moss bog that you normally associate with upland areas in the UK or low-lying areas where there is high rainfall and little drainage. Whilst there are some sphagnum mosses on the Levels, it’s mainly sedges and grasses that grow there and this produces a different structure of peat.

Below the peat is an impermeable layer of clay that used to be the bed of a shallow sea. In fact, until the early part of the Middle Ages, large parts of the Levels were still part of an inland sea.  However, local people began to drain the area, creating dykes and ditches to lower the water level.

The Levels are still a catchment for the surrounding hills, which are comprised of primarily carboniferous limestone. This is very hard limestone and it’s hard to weather, making the groundwater running into the Somerset Levels alkaline. This is a unique situation because peat bogs are normally associated with very acidic water, yet the Levels are a sedge peat where the groundwater can be alkaline.

So, how do you replicate all this in the Botanic Garden?

First, Andy takes me out to the pond and we are standing near the rock garden – an area a child visiting the Garden once called ‘wiggly water’ and the name has stuck with all the staff here.  He explains that in 2005, when the Garden moved to this location, one of the first things they did was mark out this main pool.

“Nick started explaining what this was going to be,” explained Andy. “The main pool was constructed with the Levels display already in mind, with this side of the pond 5cm lower than the other edges. So, when the water overflows in the pond it runs down into this display area.”

The Somerset Levels display, with the large pond in the
background, has to settle before planting can begin.
 At the base of the display, they’ve put in a liner with a depth of about 30cm to simulate the impermeable clay bed. This replicates one of the key features of the Somerset Levels - the seasonal rise and fall of the water level.  In the su
mmer this display area will be allowed to dry out, but the liner below will create a reservoir below.

On top of the liner, they’ve put in truckloads of waste sedge peat (a bi-product from a former industry in Somerset). “It took staff and volunteers lots of time to move it in,” says Andy. “We’re now letting this sink and we’ll likely need to add more peat on top before it’s ready to be planted out.”

The rock garden around the wiggly water is comprised of carboniferous limestone and is planted with rare and threatened native plants of the Mendip Hills. It is not a mere coincidence that the Levels display sits adjacent. Nick explains, “Just as in nature the Mendip Hills are next to the Somerset Levels, here at the Botanic Garden the Mendip Hills are next to the Somerset Levels.”

Planting out the Levels

Many of the plants that the Garden will replicate in this area are grasses and sedges and small herbs. There are some shrubby species also and Nick points out a bog myrtle by the pond that was collected for the Garden in the 1970s from Shapwick. It has an incredible smell and not surprising as it comes from the same family as cloves, allspice and eucalyptus (Myrtaceae). Bog myrtle creeps slowly through the peaty soils and creates dense thickets in these peaty soils, so this will be one of the plants included in the display.

Nick also hopes to have a small area of some carnivorous plants because the round-leaved sundew (Drosera rotundifolia) grows in the Somerset Levels, as does the common bladderwort (Utricularia vulgaris). There will also be some ferns such as the royal fern (Osmunda regalis), which is a fantastic foliage plant that grows at the edges of the ditches in the Somerset Levels,

Some of the plants will be sown directly into the peat area once it has settled and been topped up, while others will be sown in little pots or plugs, grown on and then planted out. All seed and plantlet collection is being done with the permission of Natural England and Andy and Nick will be doing the collections themselves.

“One of the exciting plants we’ll be getting in here is a population of marsh orchids,” says Nick. “We already have those marsh orchids growing in our nursery and they’ve been raised from seed from an original collection back in the 1970s of southern marsh orchids. So we want a large population of deep pink and purple orchids.”

Once mature, the main flowering interest in the display will be late June and early July after which it will go to seed and then be cut in August to tidy it up. Then it will be allowed to be wet and grow slowly through the winter months.

Planted in the ordinary soil around the peat area, there will be species of willows, which are common to the area, including the almond willow (Salix triandra), the goat willow (Salix caprea) and, if there’s room, the common osier (Salix viminalis)> These will be coppiced so they don’t get too big.

The sweet track

The sweet track is a very ancient pathway that runs across the Somerset Levels – built in the early Bronze Age.  It consisted of posts of timber pushed into the soft peaty earth above the water with horizontal boards of split timber fixed to them. It created a causeway about 3-4 ft above the water that enabled people to walk easily regardless of the water level.

The timbers, which were embedded into the peat, which is anaerobic, didn’t rot and so the nearly 4,000 year old relics were discovered in the 1970s and at the time were considered the oldest timber trackway in Northern Europe. Nick has contemplated trying to incorporate the sweet track into the Levels display in the Garden as well.

“I would like to find out more about what the timber species were,” said Nick, “and maybe have those species growing in our display, perhaps with a sculptural representation of the sweet track.”

I look forward to seeing the display progress over the next year as the staff and volunteers bring the Somerset Levels to life here in the Botanic Garden.

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