Succession and Nickey Line Management
July is a month when growth is rapid. We can easily forget its consequences:
Open Views
When the Nickey line was first established, the view from the train between Harpenden and Hemel would have been over grassy banks to open fields. The fields are still there of course, but if a train were able to pass down the route now the view would be obscured by high hedges and tall trees.
It is only when you look at old photographs that you can see the difference - such as this shot from 1905 of the (underpowered so short-lived) combined loco / carriage Railmotor viewed over Keens Field just 'round the corner' from Hemel Station. Contrast the vegetation ahead (to the right) of the Railmotor to the 'treed' view below from the same section of the Line just north of Queensway Bridge. (The photos are from Winter and Autumn respectively rather than July of course but hopefully the point remains!)
The Effect on Wildlife
The change of grassy bank to tall trees affects the whole of the wildlife in the area.
As the bank gets overgrown, grassland feeders disappear, along with their predators, and a whole food chain of herbs, insects, small mammals and even birds disappears.
Not all is lost, of course, because new conditions provide new opportunities, and a replacement group of organisms establishes. In fact the replacement continues from grasses through scrub vegetation and on until the trees are established. Once they are though, the habitat remains much the same and the process (known as succession) reaches a dynamic equilibrium in which there is replacement but only with much the same, and species composition changes little.
To increase the biodiversity along the line there must be a re-establishment of the more open habitats and that means managing the line to provide those earlier conditions. Establishment of the banks largely of grass and scrub, provide the opportunity for the animals and plants that inhabit them to recolonize.
Cutting down trees along the line is not just about creating views, it is about providing the biological diversity that can help maintain our environment. Keeping grass short allows associated plants and animals to inhabit that area. Not all techniques are about pruning, lopping and mowing. One more subtle technique is to encourage the grass to stay short by the introduction of yellow rattle. The yellow rattle is a semi-parasitic plant on grass, weakening its growth, and keeping it short. In areas of ‘maintained’ grassland as by the interpretation board by Knott Wood. you may spot the flowers of yellow rattle.
In late winter of 2006, EDF, who owned the power cables running alongside about 1km of pathway near Knott wood had their contractors drastically remove the vegetation - such that all the trees on the west side were either coppiced or cut to a height no higher than the fence which runs along the pathway. This opened up fabulous views across the Ver Valley.
Nickey line volunteers have since been managing parts of this unique habitat to the Nickey Line, including areas (scallops) where the grass is cut and raked annually to remove nutrients and encourage the appearance of grasses and wild flowers. Cowslips have since appeared in some of these areas, and continue to increase annually.
The Fight for the Top - It's a Jungle Out There!
Now that all plants are in leaf, the great scramble for light is on. Every plant needs to gain as much food from photosynthesis as it can so that it will be able to spend more energy on reproduction (both flowering and fruiting). One way up to light without using precious food for support, is to use other plants as scaffolding. Climbing plants are therefore common, and the Nickey Line shows a great range of them. Here are some of the means by which they climb;
- using the stalks of the leaves to wrap round supports like the old man’s beard (wild clematis),
- using stems to coil round, like black bryony and the bindweeds; these plants are called twiners, but some twine to the right and some to the left,
- using the terminal leaflets of a compound leaf; these are modified into tendrils, like the vetch,
- using tendrils to pull the plant closer to its support, like white bryony (see below),
- using adventitious roots to burrow into cracks and crevices in the bark of trees, like ivy,
- using slightly backward projecting spines along the main stem, which snag on other plants helping to hold the climbing plant up, like bramble or wild rose,
- using minute hooks covering almost the entire surface of the plant which make it feel sticky, like goose grass (sweethearts).
White Bryony is one of the most sophisticated climbers. Once the tendril has established connection, the middle section of the tendril begins to rotate, coiling up the rest and so shortening the distance between the ends, pulling the plant closer to its contact. It is an easy plant to find, because of the characteristic shape of the leaves and its white flowers.
