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Where meets sky: of the Wet Tropics

April 2002 It has long been suspected that high Their research highlights the ability of stems, trunks and epiphytes, water altitude forests which are immersed in cloud forests to contribute much which also becomes throughfall or cloud for a large proportion of the time higher volumes of water to catchments stemflow. may in fact ‘strip’ considerable than rainforest at lower altitudes. This amounts of moisture from passing is because of their comparatively low To estimate the water input to the for- . This has now been confirmed rates of water use combined with their est from rainfall and cloud harvesting, by Rainforest CRC researchers from ability to ‘harvest’ significant volumes collars are fixed onto trees to collect CSIRO Atherton who have measured of water from passing clouds. stemflow and troughs are placed water input in rainforest on mountain beneath the canopy to collect tops in the Wet Tropics. Their findings Measuring the harvest throughfall. The amount of water suggest that, in some months of the Measuring cloud harvesting requires collected is then compared to the , up to forty per cent more water that all sources of water input to the amount of water collected in a stand- could be harvested out of the clouds be measured. In a ard raingauge over the same time than is measured as rainfall in a stand- environment falls on the canopy period. The excess is equivalent to ard rain gauge. and then either falls through to the water collected by cloud harvesting. ground as throughfall or runs down the Cloud forests are often difficult and The team of researchers has been first tree trunks as stemflow. In addition, the costly to access and the establishment to accurately measure both the tran- trees also harvest water as clouds blow of throughfall troughs and stemflow spiration of rainforest at different through the canopy wetting the leaves, collars at a large number of sites is time altitudes, and the amount of water consuming, therefore researchers from collected by leaves and branches as the Rainforest CRC are also refining a clouds pass through the canopy of the simple method for measuring cloud high altitude cloud forests. harvesting.

Above: Typical cloud forest environment Left/right: Equipment for measuring stemflow and throughfall being set up in the forest The ‘cloud stripper’ Conserving as well as capturing These threats include conversion of The instrument used is known as a Although these high altitude Wet cloud forest to and pre- ‘cloud stripper’ and is basically a Tropics forests receive huge volumes dicted rises in temperature and cloud modified raingauge with a cylinder of of water from rainfall and cloud har- base altitude as a result of global wire mesh fixed above it and a roof vesting, Centre researchers are also warming. over the top to keep rainfall out. When finding that these forests use very cloud blows through the mesh it little water. Measurements of tree Although Australian cloud forests are collects on the mesh and runs into the water use are showing that the trees largely protected from clearing and raingauge where it is measured. How- of the cloud forests use much less change of land use, the same is not the ever, the mesh of the cloud stripper is water than forests at lower altitudes. case in many other nations like PNG, not representative of the rainforest This is because the leaves of trees are Indonesia and where upland canopy so readings need to be adjusted nearly always wet and the clouds rainforests are under threat from to make them ‘real’. This is achieved block incoming energy from the development and agriculture. In some by analysing the relationship between that would otherwise drive transpira- areas, the loss of cloud-harvesting abil- cloud stripper measurements and tion (tree water usage). ity has already lead to water shortages throughfall and stemflow for the same in the dry . period. Once suitable calibration of Significance and threats cloud strippers is achieved, these Cloud forests cover only a small area Another threat to cloud forests is instruments can be distributed of the Wet Tropics but contribute a global warming. Predicted increases of through the cloud forest of the Wet disproportionally large amount of 1 to 2 degrees in temperature over 50 Tropics to improve spatial estimates of water. Behaving like giant sponges, would cause the cloud base to the contribution of cloud stripping to they capture large volumes of water, rise in altitude. In the Wet Tropics this the hydrology of the region. Initial but then release this water slowly would result in a predicted loss of measurements in cloud forest show throughout the year. For this reason, about three-quarters of Queensland’s that cloud stripping could add billions cloud forests are believed to be of great cloud forests (70,000 hectares). In the of litres of extra flow to tropical river importance both in feeding stream sys- light of current research findings, such systems, boosting the already consid- tems and to the maintenance of stream losses would clearly have a major erable amount of water these forests flows through out the . impact on stream flows, and as a receive from rainfall. result current water allocations are unlikely to remain sustainable. While this research highlights the impor- tance of cloud forest For more information: areas for water sup- Dr David McJannet or Dr Paul Reddell ply, it also raises CSIRO important issues re- Research Centre garding long term PO Box 780 water security and Atherton, QLD 4883 water resource man- Phone: (07) 4091 8800 agement because of Fax: (07)4091 8888 potential threats to Email: [email protected] cloud forest areas. OR [email protected]