Starting work in December, a small team have been busy installing floating rafts, planting willow and felling trees around the reservoir in Yarner Wood. The reservoir has been transformed from the straight edged concrete pool that was once uninviting to people and wildlife. This is an overview of the management work that has taken place in this watery corner of the National Nature Reserve.

American signal crayfish surveys – good news!
As mentioned in the previous blog, the focus in autumn involved surveying for the invasive American signal crayfish. Surveys occurred daily, over several months, and involved a team effort from both volunteers and staff who took turns to check the baited live traps. We’re pleased to report ‘an all clear’ verdict! This result supports the previous year’s surveying (which also saw an absence of signal crayfish) and allowed us to continue with the planned management.

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Water quality surveys have been taken
Several water surveys have been taken prior to any reservoir work, packed up with ice packs to keep the samples from decaying, and sent off for analysis – we look forward to the results coming back and will be updating you all with results in the future blogs.

Tree felling – now completed
For anyone who has been visiting the reserve and seen the reservoir, tree felling has visually altered its appearance. Although it might look harsh to see many trees down, the benefits of the work are numerous and vegetation regrowth is sure to happen quickly.

The key principle in felling trees around the reservoir is to provide structural variation. In nature, boundaries of habitats are not uniform. In contrast, the reservoir, as an artificial structure is unvarying. Trees have been felled to soften the linear concrete edge and allow for a smooth transition from terrestrial to aquatic zones. Animals don’t like straight lines!

In the far right corner of the reservoir are willow cuttings, and these have been planted to further provide a different vegetation type into the water, and a new habitat for colonisation. With buds already visible, it’s great to see that these saplings appear to be taking well to their new environment.

A tangle of whole trees occupies another corner, mostly composed of oak and birch (the latter species will rot down faster which will provide a habitat that will change and evolve over time) in addition to providing lots of cover for many types of aquatic species. Other areas hold a mattress of oak (a species that is long lasting in water) which supports a layer of planted coir, providing a substrate to many types of aquatic plants to grow.

Finally, water level in the reservoir, like all bodies of water, fluctuates. Trees have been felled at an angle to account for this to ensure that dead wood is not completely exposed if water levels drop. The diversity of life on these trees is clear to see from lichens on the branch that has been left. It might look like a shame but opening and exposing the remaining upright trees to greater sunlight will increase the abundance of lichens further.

These are just some of the aspects of the work that demonstrate how improvements to the reservoir have not been uniform – along each side of the linear concrete edges are a range of techniques to increase structural diversity including angled logs, brash piles, whole trees into the water and large tree trunks with soil and planted wild flowers behind. All these techniques maximise the amount of variation that the reservoir can eventually provide.

Humans, like many animals, have an instinct to ‘tidy up’. It’s a massive problem facing the natural world because the more pristine a green space is, the fewer potential habitats it has for animals to occupy. Take for example, a weed free and extensively mowed green lawn – it typically has maybe one or two uniform grasses and little diversity in the structure, type and age of plant species. Now compare it to a rich wildflower meadow with many types of flowers and grasses which all have different stages of growth and heights. The wildflower meadow has a much higher biodiversity potential than the lawn as it can provide a space for higher number of complex interactions, be it through more habitat availability or more complex predator prey relationships, amongst a wide variety of species.

As a result of this, trees that have been felled have been left in discrete habitat piles rather than being tidied away and so are visible from the reservoir hide. This provides an invaluable source of dead wood for many species, particularly invertebrates which will use them to take shelter. Dead wood is really valuable for many animals and is a vital component in a healthy ecosystem.

Rafts
Throughout March, floating rafts (already planted with vegetation) were installed. Up to 16 different types of plants have been introduced, including species such as Water mint, Devil’s bit scabious and Marsh forget me–not. This was a key milestone in the reservoir improvements as they will form a vital perch and food source for bird and insect species, in addition to being a substrate on which vegetation can grow and later spread. The growing root mass will also begin to draw excess nutrients out of the water, providing better conditions for aquatic life.

Each raft is unique in structure and size, imitating the ‘randomness’ of natural islands. As with all work in the reservoir, this is to increase the number of unique spaces able to be occupied by differing species.

The future
Now management has been completed, it’s exciting to visualise the future impact of the work – not only directly within the reservoir but also beyond its borders into the wider environment as new species colonise. The bird hide adjacent to the reservoir is an ideal place to view these changes for those that are interested, and has been left deliberately unobscured by trees to provide a clear view of a floating island that has been positioned nearby. The team here at the National Nature Reserve look forward to giving you future updates as changes are noticed and tweaks to management are made!  

Written by Katherine Hewkin, Conservation Assistant
Photographs by Fergus Mitchell and Katherine Hewkin

The management work at Yarner Wood reservoir is part of the Moor than meets the eye scheme, with funding support from the Heritage Lottery Fund.