In 1877 a woman who signed herself Mrs. Gill came ashore at Garrison, the small settlement on Ascension Island, with her husband David, an astronomer. They had been told about a place called Green Mountain. Standing among the black lava and the red hills, they searched the view for it and found nothing. They decided it was “a fable” and “a mere myth”, and said to each other: “Nothing green.”
The mountain was there. Green Mountain is a volcano and the highest point on Ascension, and today it is known for something unusual: it may be one of very few large forests on Earth that people planted. The ecosystem on its slopes is almost entirely artificial. It began as a plan to bring water to a rock in the middle of the ocean.
Ascension sits near the point in the Atlantic that lies halfway between Africa and South America, a little below the Equator. Writers who saw it two centuries ago described it with real horror. One called it the most barren land his eyes had ever seen, a place so cursed that he believed nothing else in the world was like it. Another wrote that it produced only turtles, rats and sleepless nights. The island is only about a million years old and lies more than 1,500 kilometres from any large landmass, so very few plants ever reached it alone. Scientists count between 25 and 30 native species of vascular plants, and 10 of them grow nowhere else.
People came for the island’s position, not for its plants. The Portuguese sailor João da Nova found it in 1501. Nobody lived there until Napoleon was sent to Saint Helena in 1815, when the British government placed a small garrison on Ascension. The Admiralty kept control of it from 22 October 1815 until 1922. The soldiers and sailors needed water, and there was very little. When the explorer William Dampier was shipwrecked on the coast in 1701, he found the one spring of fresh water the island had, high on Green Mountain. Even then, the 1911 Encyclopædia Britannica recorded, the only vegetation on the island was on that summit, which stayed green because mist so often covered it.
Charles Darwin arrived on the Beagle on 19 July 1836. He described smooth red cones rising out of a dry volcanic landscape, and wrote that “near this coast nothing grows”. He also repeated a French naturalist’s observation that only the English would have tried to make Ascension a productive place, while any other nation would have treated it as a fortress in the sea. Darwin compared the island, run in naval style, to “a huge ship kept in first-rate order.”
Seven years later another young naturalist came ashore. Joseph Dalton Hooker was the botanist on Sir James Clark Ross’s Antarctic expedition, and in 1843 the ships stopped at Ascension on their way home. Hooker proposed covering the island with plants, hoping to increase the rainfall and to make life easier for the garrison. In his view, success depended above all on one practical step: appointing a superintendent to run a farm. Back in Britain, his biographer wrote, he was even ready to sail out again and report on “the Island of Ascension, a barren rock”, as part of the Admiralty’s plan to improve its vegetation.
Trees and seeds began to arrive, and in time they took hold. An Admiralty report of 1865 said of Green Mountain that the island “now possesses thickets of 40 kinds of trees, besides numerous shrubs; through the spreading of vegetation, the water supply is now excellent”. By the early 1900s bananas and guavas were growing wild there.
Mrs. Gill’s book, published in 1878, shows how fragile that water supply could still be. At night she watched thirsty cattle wandering around the house after a long walk down from the mountain in search of food and water. She also told the story of a spring dug in 1830 in a deep ravine, where clouds and mist were trapped by the high ground. Its builder reported that it gave about five tons of water a day, and wrote that the question of supply was now “set at rest”. It was not. The spring stopped flowing, its wells filled up, and only a memory of it remained in 1877, when a long drought forced the islanders to look again. A Marine officer found it and it ran at three tons a day, but by the time the Gills saw it, the flow had fallen to one ton, and people treated it, in her words, “with doubt and suspicion”.
The Gills had come to measure the sky, and Green Mountain made that difficult. Night after night, heavy clouds formed over the mountain and rolled across Cross Hill above Garrison, “sadly interfering with astronomical work”. On one typical evening, she wrote, the mountain was “busy dispatching a train of cloud” directly in front of Mars. In the end they moved their observatory to Mars Bay, a strip of bare rock on the south coast, with the mountain and its clouds far to the north. When they finally took a holiday on Green Mountain, the change was complete: a cart carried them past water tanks and along an iron pipe that brought water down to Garrison, and up into grass and gardens.
Those clouds may be the real source of the forest’s water. Hooker had hoped that trees would bring rain. That probably never happened. What the extra vegetation probably did was catch more of the mist that passes over the mountain. Scientists call this cloud water interception. Tiny drops of fog or wind-driven rain hit leaves and branches, join together into larger drops, and fall to the ground. It is most common on coasts and on islands that are always exposed to sea breezes. The amounts can be large. On the high plateau of Madeira, researchers measured a tree heath at the edge of a forest from 1997 to 1999 and found that it caught almost twice as much water from cloud as fell on it as rain. In a cloud forest in Hawaiʻi, by contrast, intercepted cloud water was about 11 percent of what reached the forest floor, much less than at a more exposed site at the forest’s edge nearby. Location matters, and so does the shape of the forest.
The opposite process is also visible elsewhere. Along the coast of Peru and northern Chile there are landscapes called lomas that depend on fog. Forests there have been cut down since the 16th century, and the land has turned to desert, because without trees the fog is no longer captured.
The forest on Green Mountain has a cost. It is a mix of species from many different climates, with woodland, grassland and shrubland side by side, and the ecosystem that the British introductions created has put some of the island’s few native plants under threat.
How much water the mist really gives is still being studied. In recent research, a scientist named S. Schmitt worked on San Cristóbal in the Galápagos and on Ascension, using stable isotopes, satellite data and studies of plant physiology to follow fog through dry island ecosystems. The work starts from a plain admission: the patterns of fog arriving in such places are poorly understood, and there are few measurements taken on the ground. Nearly 150 years after Mrs. Gill watched clouds come off the mountain and cover Mars, scientists are still trying to learn exactly how much of that cloud ends up as water.