In 1877 a woman who signed her name as Mrs. Gill arrived at Garrison, the small town on Ascension Island. She came with her husband David, an astronomer, a scientist who studies the stars and planets. People had told them about a place called Green Mountain. They looked across the black lava and the red hills, but they could not find it. They decided it was “a mere myth”, and they said to each other: “Nothing green.”
But the mountain was there. Green Mountain is a volcano and the highest point on the island. Today it is famous because it may be one of very few large forests on Earth that people planted. The natural world on its slopes was almost completely made by humans.
Ascension lies in the Atlantic Ocean, about halfway between Africa and South America, a little south of the Equator. Two hundred years ago, visitors hated it. One writer said it was the emptiest land he had ever seen. Another wrote that it gave people only 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 reached it. Scientists count between 25 and 30 native plant species, and 10 of them grow nowhere else in the world.
A Portuguese sailor, João da Nova, found the island in 1501. Nobody lived there until 1815, when Napoleon was sent as a prisoner to Saint Helena and the British put a small group of soldiers on Ascension. The soldiers needed water, but there was very little. When the explorer William Dampier lost his ship on the coast in 1701, he found the island’s only spring of fresh water, high on Green Mountain. The only plants on the island grew on that summit, which stayed green because mist often covered it. When Charles Darwin visited in 1836, he wrote that “near this coast nothing grows”.
In 1843 a young scientist called Joseph Dalton Hooker stopped at Ascension on his way home from the Antarctic. He was an expert on plants. He suggested covering the island with plants, because he hoped this would bring more rain and make life easier for the soldiers. Trees and seeds began to arrive. A report in 1865 said the island now had 40 kinds of trees and that the water supply was excellent. By the early 1900s, bananas and guavas were growing wild there.
Mrs. Gill’s book, which came out in 1878, shows that water was still a problem. At night she saw thirsty cattle walking around the house.
The clouds from the mountain also made the Gills’ work difficult. Night after night, heavy clouds came off the mountain and covered the sky. One evening a line of cloud moved right in front of the planet Mars. In the end, the Gills moved their observatory to Mars Bay, a place of bare rock on the south coast, far from the mountain. Later they took a holiday on Green Mountain and found grass and gardens there.
Those clouds may be the real source of the forest’s water. The trees probably never brought more rain, as Hooker had hoped. Instead, the extra plants probably caught more of the mist that passes over the mountain. Tiny drops of fog hit leaves and branches, join into bigger drops, and fall to the ground. This happens most on coasts and on islands with constant sea winds.
The forest has a cost. It is a mix of plants from many different climates, and it has put some of the island’s few native plants in danger.
Scientists are still studying how much water the mist really gives. One researcher, S. Schmitt, has studied fog on Ascension and on San Cristóbal in the Galápagos. There are still very few measurements taken on the ground. Nearly 150 years after Mrs. Gill watched a cloud cover Mars, scientists are still trying to learn how much of that cloud becomes water.