Rainforests Harvest The Skies

It is giving the upland rainforests a whole new environmental significance in the landscape, as “cloud harvesters” which add billions of litres of extra flow into tropical river systems, says CSIRO’s Dr Paul Reddell.

Dr Reddell and Dr David McJannet lead a team in the Rainforest Co-operative Research Centre, involving scientists from CSIRO and James Cook University.

“From our early work, it looks as though rainforests may pull up to 40 per cent more water out of the clouds than is measured as rainfall in a standard rain gauge,” they say.

In high, wet tropical regions above 900 metres, low cloud, mist and fog blow constantly through the forest, condensing on trees and running or dripping down them to the ground.

This condensation is additional to normal precipitation and does not occur where the forest has been cleared.

“The role of rainforests in the water cycle is poorly understood. We know forests are important in regulating the flow of water through the landscape – and we know that, by holding back water, they contribute to the dry-season flow of coastal rivers” said team member Dr David McJannet.

“Now we have clear evidence of another role, cloud-stripping, in which the forest actually harvests large volumes of moisture additional to the rainfall.”

A serious issue is that, when rainforests are cleared, the amount of moisture reaching the soil decreases significantly due to the removal of the cloud-stripping effect.

Also, because these upland rainforests transpire very little, they contribute a disproportionately large volume of water to their catchment. This contribution is greatly reduced when the forest is cleared and the water can evaporate.

Dr Reddell also warns of the consequences for water security in the event of global warming: “if the cloud banks which currently contribute water to the forest via cloud stripping rise in altitude - as they are forecast to do - there could be a major loss in water gathering by the forest and its catchment, with consequences for communities downstream that rely on these resources.

To work out how much moisture falls as rain and how much is harvested by the forest, the team set up rain gauges in open areas at Longlands Gap and Mt Lewis in North Queensland, and “throughfall” troughs and collar gauges round trees in the forest. The throughfall troughs measure water which directly reaches the forest floor, dripping through the canopy. The collars measure “stemflow”, or water running down the tree trunks.

The difference between what’s in the rain gauge and throughfall plus stemflow is the amount of extra water stripped by the forest.

The team are now looking at developing gauges that can be used to estimate cloud stripping at a much wider range of sites – and so build a fuller picture of the wet North Queensland rainforest’s contribution to the wet tropic’s hydrological cycle.

Dr Reddell says that the information will be valuable in several ways:

· it will contribute to more accurate water balances, and so to the development of sustainable water allocation policy

· it will make the public more keenly aware of the value of rainforests and the services they provide to the environment and people

· it will alert the world to a new aspect of the losses sustained when upland rainforests are cleared, and contribute to better management of tropical catchments and rivers

· it may encourage people to replant trees in cleared areas of the wet tropical uplands

· it could even generate income from sale of the extra water harvested from a replanted forest.