EMERALD GREENS

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Europe's plant refugees have found an Irish Ellis Island.

A standard stop for many an Irish-American tracing his ancestral roots is the
Burren-"the world's largest rock garden," say the brochures-a bleak,
wind-blasted steppe of limestone jutting out of the west coast of Ireland into
the Atlantic. Visitors slide up in their commodious tour buses, scamper out
among the lunar limestone dotted with blue harebell and bloody cranesbill,
mountain avens and maidenhair, bee orchids, fly orchids, frog orchids, returning
to their custom coaches eager to quit this eerie land for the nearest pint o'
Guinness.

Botanists are likely to have a very different reaction: The list of the Burren's
inhabitants makes them reel in disbelief. Alpine species grow side by side with
flowers normally found in Spain. Arctic flowers share the same scant soil with
Mediterranean ferns; limeloving plants cohabitate with those that thrive in
acidic soils. In fact, more than 75 percent of all plant species found in
Ireland are native to the 100 stark, ostensibly uninviting square miles of the
Burren.

Why do such seemingly disparate species coexist in such close quarters? Theories
are nearly as plentiful as the specimens themselves.

Some botanists say seeds were blown here westward across the continent, or
southward from the Arctic; other scientists assert that they were carried here
in the fur of animals when Ireland was connected by land bridge to Europe, or
fell here in the droppings of migratory birds; or perhaps the seeds were
gathered in the south by the great glacier and deposited here as the ice sheets
receded northward.

David A. T Harper, senior lecturer in geology at University College, Galway, has
made it his business to discover why the area is hospitable to so many species.
He believes the 1,000-meter-deep limestone is the key to the Burren's diversity.
Composed of calcium carbonate derived from the heavily compressed shells of
marine organisms deposited during the Carboniferous Period some 350 million
years ago, limestone accounts for many of the Burren's unique geological
features. Because rain easily erodes calcium carbonate, erosion produced the
Burren's system of clints (upstanding elements) and grykes (depressions).
Alternating bands of thick volcanic ash layered upon even thicker bands of
limestone weathered in a stepped pattern because limestone and volcanic clays
erode at different rates.

On top of all this, Harper says, the receding glacier at the close of the last
ice age "scoured the Burren, like rubbing a Brillo pad across its surface." The
limestone face exposed by this geomorphic sculpting makes it attractive to
opportunistic species: It retains heat, keeping its inhabitants warm through the
chilly Irish winters; the lime encourages bacteria, which in turn provide
nutrients; and the calcium carbonate allows for only a certain amount of soil to
form, ensuring that the environment does not become too attractive to weeds and
grasses, which would otherwise overrun the more delicate ferns and orchids.

Harper's own theory regarding the presence of the Alpine spring gentian and the
Mediterranean maidenhair fern owes as much to geography as it does to geology.
"Look at a globe," he says, "and you see that Ireland really sticks out into the
Atlantic; the island is perched at the extreme edge of Europe."

Once the seeds were transported here, the plateaus of limestone became the final
destination for a polyglot colony of European botanical emigrants. Unlike their
human fellow travelers, the plants could not make the leap across the Atlantic
to North America, so the ferns and flowers of Europe gathered and made their
melting pot in a barren place called the Burren.