Satellite images have identified scrub encroachment, which poses a threat to this important site, writes Dick Ahlstrom.
Ice-age glaciers helped shape the Burren in Co Clare but grazing livestock made it what it is today. Yet changed farming practices may cause much of this dramatic landscape to disappear under a canopy of trees.
"The Burren would be woodland if it weren't for the grazing by cattle," says ecologist at Teagasc's Johnstown Castle Research Centre in Co Wexford, Dr John Finn. "Undergrazing is probably the greatest threat to the species-rich grasslands of the Burren."
Dr Finn is a researcher in Johnstown Castle's Agrienvironment Research Department. He joined with Dr Grace O'Donovan and Dr Sharon Parr of University College Dublin to conduct a study detailing the various habitats of the Burren using images taken by a Landsat orbiting satellite.
There have been many field studies of the Burren but satellite data has now provided the first baseline study giving information about the Burren as a whole, he explains. The challenge was to be able to equate what the satellite image shows with the actual habitat on the ground.
"It was inspired by the fact that the Burren is an area of international importance. We are trying to understand habitat change and what drives habitat change in the Burren," says Dr Finn.
The spread of scrub cover, mainly hazel trees, is a source of particular concern, says Dr Finn. Grazing by cattle and sheep over the years helped to keep down the growth of tree seedlings, leaving behind what is described as "species rich" natural grassland.
"In the absence of that grazing pressure you see hazel and other tree species starting to grow there. Without grazing there is a competitive advantage for the tree species. Cattle grazing removes that competitive advantage and allows the grasses to grow," says Dr Finn.
"This incidence of scrub is a threat to the very thing that makes the Burren important."
The three set out to prove that satellite-based mapping could actually provide an accurate picture of the Burren's key habitats. The Landsat provides very good infrared data, particularly useful when attempting to picture vegetation, he explains. The current Landsat 7 orbits at about 700 km and Finn and his collaborators settled on a mapping resolution of about 30sq m.
The data sets were studied and their interpretation refined so that the team could differentiate between the natural grasslands and the "improved" or farmed grasslands altered by farmers, he says. "People thought we wouldn't be able to see the difference between the improved grassland and the species rich grassland."
This information then had to be confirmed on the ground and the team conducted "an intensive amount of field work" to show that they could actually measure eight different Burren habitats.
There are four key habitats. The unimproved, species-rich grassland makes up about 31 per cent of the total Burren landscape, they found. Improved grassland accounts for 28 per cent of the total.
Total limestone pavement or "karst" made up a further 20 per cent, with 10 per cent each for bare and vegetated limestone pavement.
The important scrub element accounted for 14 per cent of the cover. This included mixed scrub with hazel, whitehorn, blackthorn and holly as the most common species and also ash-hazel woodland.
The Burren satellite map shown here was made from two composite Landsat images, one from April 2003 and another from August 2000, says Dr Finn. The team found that the ability of the satellite data to reveal ground cover type changed depending on the growth stage of the plants.
"At certain times of the year the grassland type popped out of the satellite image better but at another time of the year the scrub popped out better."
Dr Finn believes that the techniques developed through their Burren study could be reapplied to other important habitats. "This is one of the next projects we would like to pursue."
The question remains, however, what is to be done if more of the Burren falls to scrub cover or woodland as a result of changed agricultural practices. Large parts of the Burren are protected under the EU's Habitats Directive, but if grazing isn't maintained then natural processes will cause the landscape to revert to woodland.
"We now have a baseline set of data that allows us to monitor change over time," says Finn. It will enable researchers to track the evolution of the Burren.
Funding for the project came from the Research Stimulus Fund via the National Development Plan through the Department of Agriculture and Food.