Trees in a forest above desert in the Oman Mountains have found a way to get water by extracting moisture from low-lying clouds, Massachusetts Institute of Technology scientists report. "In an area that is characterized mostly by desert, the trees have preserved an ecological niche because they exploit a wispy-thin source of water that only occurs seasonally", said Elfatih A.B. Eltahir, professor of civil and environmental engineering, and former MIT graduate student Anke Hildebrandt.

The forest is unique because it "is a water-limited seasonal cloud forest" that is kept alive by water droplets gathered from seasonal passing clouds (fog). The water flushes into the ground and sustains the trees later during the dry season. Trees actually get more water through contact with clouds than via rainfall. Clouds appear in the forest when moist air moves in from over the Arabian Sea and pushes up against the mountains.

Clouds water condenses on the leaves of the treess and the droplets fall to the ground, where the water can be stored and used by the trees in the dry season. "The process works in the same manner as fog getting your clothes wet," explained researcher Elfatih Eltahir.

Cloud forests are not rare. But they usually occur in moist tropical regions where there is a high amount of precipitations. They sustain themselves without the fog, since they get a lot of rainfall. So it is odd to see a cloud forest in a region known for chronic dryness.

So the Omani forest is dependent on its fog. "If you don't have the additional water from the [low-level] clouds, you won't have the trees," Eltahir said.

The researchers studied the area in Oman to learn how the Dhofar Mountain ecosystem "functions naturally, and how it may respond to human activity" that could lead to desertification. Eltahir and his team measured a larger amount of rainfall above and beneath the trees, showing that the fog is essential to the forest.

Eltahir and Hildebrandt said the unusual forest is an interesting remnant "of a moist vegetation belt that once spread across the Arabian Peninsula" in the distant past, during a wetter regional climate.

The forest area in Oman is now semi-arid, and most of the former forest is gone. This small remnant has managed to survive in the Dhofar Mountains. "Although many Omanis have moved into cities and towns as the country has grown rich on oil," Eltahir explained, "a family's prestige still comes from owning many camels, and people now tend to keep more camels than they need, which is part of the problem facing the forest."

"It is an unusual place," Eltahir said. "It's a very good example of a unique and fragile ecosystem" where constant pressure from overgrazing can have consequences beyond defoliation. In fact, the forest illustrates how small changes can lead to major impact on far bigger systems," Eltahir said.

"The trees in wetter ecosystems would recover from small amounts of overgrazing," Eltahir said, but "in this location, due to the nature of the interaction of the canopy structure with the clouds, the trees may not recover."

The overgrazing from goats and camels could damage the forest by decreasing the amount of water they get from the fog. As the foliage goes the trees may lose the ability to pull water from the mist and recharge underground reservoirs. Without the trees that sweep the extra water from clouds, the forest cannot regrow. Grass, even if abundant, cannot collect enough moisture from fog to let a forest regrow. "If you destroy an ecosystem like that, it's hard for it to regenerate," Eltahir said.

The Omani government's Ministry of the Environment is also worried - according to Eltahir - who has been advising them on ways to replant and irrigate the forest to protect it.

Ecosystems dependent on fog are also found in the Namibian desert (in South West Africa) and Atacama Desert (in Northern Chile), but we can't talk about forest in those regions.

Photo credit: Elfatih Eltahir