The secret world of plants living in our limestone pavements

Anyone out winter walking in the Yorkshire dales will probably be familiar with the dramatic scenery of limestone pavements. Distinctive and beautiful, they can also be found elsewhere in Britain, as well as in mainland Europe and Canada—not to mention in Harry Potter and the Deathly Hallows, where Harry and Hermione set up camp in a rocky clifftop.

Limestone pavements were formed more than 300 million years ago in the Dinantian period, when the ice sheets scraped away soil, leaving limestone rock exposed. Over time, rainfall wore away the rock into the uneven patterns that we see today.

Limestone pavements are made up of large slabs of rock called clints, interspersed with cracks called grikes that can be a few meters deep. Living in these grikes is a secret world of plant life, including rare species such as the pretty white mountain-avens, the poisonous baneberry and the rigid buckler fern, which only ever grows on limestone.

Why these areas matter

The habitats in these grikes are important for conservation, since limestone pavements only cover an area of around 28km² across Great Britain, equivalent to the size of a small town. In the 1970s, a landmark national survey of British limestone pavements led by two scientists, Stephen Ward and David Evans, led to an increase in their protection.

This was necessary because many pavements had been damaged by people removing the stone for building or for decorative rockeries. Individual sites were designated as reserves or sites of special scientific interest. Then, in 1981, the Wildlife and Countryside Act introduced Limestone Pavement Orders, which made it illegal to remove any of the stones.

To see what has happened to the plants that live in and around the pavement and grikes over the last half a century, I have spent five years undertaking a new national survey. In research published in Functional Ecology, I revisited and surveyed the same sites as the 1970s study, including in northern England, Scotland and Wales. I found a very mixed picture.

In some areas, grazing by sheep and cows has stopped or reduced. Whereas in the 1970s, farmers received payments from the then-European Economic Community (EEC) for each animal they grazed, now they receive money for looking after land for conservation.

This reduction in grazing intensity is mostly positive, but in some areas it has led to scrub and trees encroaching on the pavement. This can reduce the amount of light for other plants, and in many cases, I found large reductions in biodiversity. For example, some areas of pavement where there had been large increases in tree cover had lost more than 20 plant species in the years between the two surveys.

On the plus side, the reductions in grazing intensity have also meant that some pavement species that are particularly sensitive to grazing, such as baneberry, seem to be on the increase. Equally, however, not all the species that have increased are desirable. There has been a large increase in thistles, especially creeping thistles, which are not typical of limestone pavements and have the potential to disrupt their ecosystems.

Just as the area of pavements covered by trees and shrubs has increased, so has the area with no trees at all. In these areas, the pavements are actually over-grazed. Sheep, for instance, will put their heads as far down a grike as they can to get to the plants that they want to eat.

This too can affect the ecosystem: Some species have been over-eaten—though I didn’t collect data on which ones—while high nutrient levels in the pavements from sheep dung have increased the abundance of excessively competitive species such as stinging nettles.

More generally, it’s striking that there has been little research into limestone pavements and the plants that grow in them. This is despite their importance for conservation, and the fact that many areas are now nature reserves.

Having evidence to support decisions about how we best manage these natural habitats is important, so building a solid research base is a priority for the future. My survey will play a part, but there’s still a lot to learn, including how these habitats might respond to climate change, so that we can ensure our spectacular limestone pavements stay special for future generations.

This article is republished from The Conversation under a Creative Commons license. Read the original article.