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 heard about a place called Green Mountain, but when they looked around at the black lava and the red hills, they could not see it anywhere. They decided it was “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 may be one of very few large forests on Earth that were planted by people. The ecosystem on its slopes is almost entirely artificial. It all started with a plan to bring water to a rock in the middle of the ocean.
Ascension lies in the Atlantic, near the point halfway between Africa and South America, a little below the Equator. Writers who saw it two centuries ago were horrified by it. One 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 on their own. Scientists count between 25 and 30 native species of vascular plants there, and 10 of them grow nowhere else.
People came because of the island’s position, not its plants. The Portuguese sailor João da Nova found it in 1501, but nobody lived there until 1815, when Napoleon was sent to Saint Helena and the British government placed a small garrison on Ascension. The soldiers and sailors needed water, and there was very little. The only spring of fresh water was high on Green Mountain, and the only vegetation on the island grew on its summit, which stayed green because mist covered it so often.
Charles Darwin arrived on the Beagle in 1836. He described smooth red cones rising out of a dry volcanic landscape and wrote that “near this coast nothing grows”. He compared the island, which was run in naval style, to “a huge ship kept in first-rate order.”
In 1843 a young botanist, Joseph Dalton Hooker, stopped at Ascension with Sir James Clark Ross’s Antarctic expedition on its way home. He proposed covering the island with plants, hoping this would increase the rainfall and make life easier for the garrison. Above all, he believed, someone should be appointed to run a farm.
Trees and seeds began to arrive, and slowly they took hold. In 1865 a report from the Admiralty said 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 on the mountain.
Mrs. Gill’s book, published in 1878, shows that water was still a problem. At night thirsty cattle wandered around her house after a long walk down from the mountain. She also described a spring dug in 1830 in a deep ravine. At first it gave about five tons of water a day, but then it stopped flowing and was almost forgotten. During a long drought in 1877 it was found again, but by the time the Gills saw it, it gave only one ton a day, and people treated it “with doubt and suspicion”.
The mountain caused the Gills another problem. They had come to study the sky, but night after night heavy clouds formed over Green Mountain and rolled towards the settlement, “sadly interfering with astronomical work”. One evening, she wrote, the mountain sent a line 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, far from the mountain and its clouds. When they finally took a holiday on Green Mountain, a cart carried them past water tanks and an iron pipe that took water down to Garrison, and up into grass and gardens.
Those clouds may be the forest’s real source of water. Hooker had hoped that trees would bring rain, but that probably never happened. Instead, the extra plants probably caught more of the mist passing over the mountain. Scientists call this cloud water interception: tiny drops of fog 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, a tree heath at the edge of a forest caught almost twice as much water from cloud as it received from rain.
The opposite can also happen. On the coast of Peru and northern Chile, forests that depended on fog have been cut down since the 16th century, and without trees to catch the fog, the land has turned to desert.
The forest on Green Mountain also has a cost. It mixes species from many different climates, and the ecosystem the British created has put some of the island’s few native plants under threat.
Scientists are still studying how much water the mist really gives. A scientist named S. Schmitt has followed fog through dry island ecosystems on Ascension and on San Cristóbal in the Galápagos. The research begins by admitting that fog patterns in such places are poorly understood and that there are few measurements from 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 becomes water.