In 1877 a woman who signed her name as Mrs. Gill landed at Garrison, the small settlement on Ascension Island. She came with her husband, David, who was an astronomer. Before they arrived, someone had told them about a place called Green Mountain. They stood among the black lava and the red hills and searched the landscape for it, but they could see nothing of the kind. They concluded that it was “a fable” and “a mere myth”, and said to each other: “Nothing green.”
In fact the mountain did exist. Green Mountain is a volcano, and it is the highest point on Ascension. Today it is famous for something unusual: it may be one of the very few large forests in the world that were planted by people. Almost all of the ecosystem on its slopes was created by humans. The forest started as a plan to bring water to a rock in the middle of the ocean.
Ascension lies in the Atlantic, a little south of the Equator, close to the point halfway between Africa and South America. Writers who visited it two hundred years ago were genuinely horrified by what they saw. One said it was the most barren land he had ever seen, and that it was so cursed that nothing else in the world could compare with it. Another wrote that the island produced nothing except turtles, rats and nights without sleep. The island is only about a million years old, and it is more than 1,500 kilometres from any large area of land. Because of this, very few plants ever managed to reach it on their own. Scientists count between 25 and 30 native species of vascular plants there, and 10 of these exist nowhere else on Earth.
People were interested in the island because of where it was, not because of its plants. The Portuguese sailor João da Nova discovered it in 1501. Nobody settled there, however, until 1815. In that year Napoleon was sent into exile on Saint Helena, and the British government put a small garrison of soldiers on Ascension. The Admiralty, which ran the Royal Navy, controlled the island from 22 October 1815 until 1922. The soldiers and sailors needed water, and there was very little of it. In 1701 the explorer William Dampier was shipwrecked on the coast, and he found the island’s only spring of fresh water, high up on Green Mountain. According to the 1911 Encyclopædia Britannica, even at that time the summit was the only place on the island with any plants. It stayed green because it was so often covered in mist.
Charles Darwin arrived on the Beagle on 19 July 1836. He described smooth red cones standing up out of a dry volcanic landscape, and he wrote that “near this coast nothing grows”. He also repeated a remark by a French naturalist. In this man’s opinion, only the English would have tried to turn Ascension into a productive place, while any other country would simply have used it as a fortress in the sea. Because the island was run in the strict manner of the navy, Darwin compared it 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 expedition to the Antarctic. In 1843 the expedition’s ships stopped at Ascension on their way home. Hooker suggested covering the island with plants. He hoped this would increase the rainfall and make life easier for the garrison. He believed that the most important practical step was to appoint a superintendent who would run a farm. Later, back in Britain, he was even willing, his biographer wrote, to sail out again. His task would be to report on “the Island of Ascension, a barren rock”, as part of the Admiralty’s plan to improve the plant life there.
Trees and seeds started to arrive, and over time they established themselves. In 1865 an Admiralty report described Green Mountain. It said 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 on the mountain.
Even so, Mrs. Gill’s book, which came out in 1878, shows that the water supply could still fail easily. At night she saw thirsty cattle wandering around the house. They had walked a long way down from the mountain, looking for food and water. She also told the story of a spring that had been dug in 1830 in a deep ravine, where the high ground trapped clouds and mist. The man who built it reported that it produced about five tons of water a day. He wrote that the problem of supply was now “set at rest”. He was wrong. The spring stopped flowing and its wells filled up, until people only remembered that it had once existed. Then, in 1877, a long drought forced the islanders to search for it again. A Marine officer found it, and at first it gave three tons a day. But when the Gills saw it, the flow had dropped to one ton, and people regarded it, as she put it, “with doubt and suspicion”.
The Gills had come to Ascension to make measurements of the sky, and Green Mountain made their work hard. Night after night, thick clouds formed over the mountain and moved across Cross Hill, above Garrison, “sadly interfering with astronomical work”. She described one ordinary evening when the mountain was “busy dispatching a train of cloud” right in front of the planet Mars. Finally they moved their observatory to Mars Bay, a strip of bare rock on the south coast, far from the mountain and its clouds to the north. Later, when they at last went on holiday to Green Mountain, they found a completely different world. A cart took them past water tanks and along an iron pipe that carried water down to Garrison, and then up into grass and gardens.
Those clouds may be where the forest’s water really comes from. Hooker had hoped that trees would bring rain, but this probably never happened. Instead, the additional plants probably caught more of the mist that drifts over the mountain. Scientists call this process cloud water interception. Tiny drops of fog, or of rain blown by the wind, hit leaves and branches. There they join together into bigger drops, which fall to the ground. This happens most often on coasts and on islands where sea breezes blow all the time. The quantities involved can be large. On the high plateau of Madeira, researchers measured a tree heath on the edge of a forest between 1997 and 1999. They found that it collected almost twice as much water from clouds as it received from rain. In a cloud forest in Hawaiʻi, on the other hand, the cloud water that was caught amounted to only about 11 percent of the water reaching the forest floor. This was much less than at a nearby, more exposed site on the edge of the forest. So both the location and the shape of a forest make a difference.
In other places, the reverse process can be seen. On the coast of Peru and northern Chile there are landscapes known as lomas, which rely on fog. People have been cutting down the forests there since the 16th century. As a result the land has become desert, because once the trees are gone, nothing captures the fog any more.
The forest on Green Mountain also comes at a price. It contains a mixture of species from many different climates, and woodland, grassland and shrubland grow next to one another. The ecosystem created by the plants the British brought in has put some of the island’s few native species at risk.
Researchers are still investigating how much water the mist actually provides. In a recent study, a scientist called S. Schmitt worked on San Cristóbal in the Galápagos and on Ascension. Schmitt used stable isotopes, satellite data and research into how plants function in order to trace the movement of fog through dry island ecosystems. The research begins with an honest admission: nobody understands well how fog arrives in places like these, and very few measurements have been made on the ground. Almost 150 years after Mrs. Gill watched clouds roll off the mountain and hide Mars, scientists are still trying to find out exactly how much of that cloud becomes water.