What is scrub, and why is it there?
Scrub is a word we use for when trees start to invade a grassy area, known as a grassland. This usually happens when livestock haven't grazed the land down too much.
In most farmer’s fields, grass is cut or grazed very short, to maximise milk or meat production. This means that scrub can’t establish and grow.
On our reserves, we choose to have fewer animals grazing for a shorter time. This is because longer, more tussocky grass means more places for invertebrates or voles to hide away in, and therefore more food for things higher up the food chain. Also, cutting and grazing too much usually means that some species of flowers, pollinating insects and birds are lost. This in turn results in a poorer grassland with less biodiversity.
Having low numbers of livestock grazing or allowing the grass to grow all year before cutting it means that trees can start to get a toehold. Jays bury oak trees, or other birds poo out hawthorn or other seeds. Often the first trees to get established are thorny ones that will then be avoided by sheep and cattle. The trees then grow up and produce more seed so the scrub spreads.
Why do we remove scrub?
Grasslands can be very diverse for flowers and invertebrates, especially in areas which are managed for conservation, like hay meadows.
Some ground nesting birds like to have open grasslands without scrub, so places with lots of nesting lapwing or curlew (birds that have declined in the UK) would probably be kept free of scrub.
We also remove scrub to provide a good habitat for wildflowers and other wildlife. This is in short supply in the UK - 97% of flower-rich grassland has been lost since WW2. This means that removing scrub from the remaining good wildflower sites is very important.
Why do we want scrub in some places?
Scrub can be a valuable habitat in it’s own right. Although it is generally less diverse than open grassland, it can be home to birds like willow warbler. Some hoverflies and other invertebrates like to hang around in the sunny but less windy areas on the edge of scrub.
The problem with scrub is that it doesn’t hang around for long. The trees keep growing and in a few years all the open spaces can close up, and 10-20 years later it is young woodland.
Woodland is also an important habitat, but new woodland is generally less valuable than ancient woodland, as ancient woodland usually has a greater diversity of flowers and insects.
How do we decide what is more important, scrub, grassland or woodland?
For some areas that we manage, we have an aim for them to be grassland, transitional scrub or woodland. This helps us to make decisions about what we do to manage it.
Deciding on what an area should be managed as is very individual to a site. A good guide is what wildlife is or has been there in the past, but also envisioning what could be attracted there in the future by allowing a different habitat to develop.
The size of a site also matters - a small hay meadow, like Fen Carr near Doncaster (4 hectares in size), needs the scrub along the hedgerow removing frequently. This is because the flowers are the main interest and allowing a 10m scrub margin along the hedgerow would be a large proportion of the meadow. However, on a larger site like Carr Lodge (31 hectares), there's space for an area to develop scrub alongside other habitats.
The ultimate aim is trying to protect the important species of a nature reserve, or allow encourage new species to breed and thrive there.
Understanding what species’ needs are and the way the land changes over time are important considerations. There isn’t always a single ‘right answer’ to what a site should be, but as managers of land Yorkshire Wildlife Trust have to make what they think is the best decision for the land and the wildlife.