Ecologists have discovered that timber plantations in Hawaii use more than twice the amount of water to grow as native forests use. Especially for island ecosystems, these findings suggest that land management decisions can place ecosystems and the people who depend on them at high risk for water shortages.
"Scientists used to think that forests in same environments use water in the same way," says Lawren Sack of The University of California at Los Angeles, who coauthored the study with graduate student Aurora Kagawa in the September issue of the ESA journal Ecological Applications. "Our work shows that this is not the case. We need to know the water budget of our landscape, from gardens to forests to parks, because water is expensive."
Although forests like these Hawaiian timber plantations can be valuable for their contributions to human society, such as fiber, fuel and carbon sequestration, they are dominated by non-native vegetation.
Kagawa, Sack and their colleagues compared the water use of trees in native forests, composed mostly of native ohia trees, with water use in timber plantations containing exotic eucalyptus and tropical ash. The team inserted heated and unheated probes into the trees' trunks and monitored the temperature differences between the two as sap flowed past them. This technique allowed them to determine the rate of sap flow through the tree. A faster flow rate means that the tree is using more water.
"The way plants grow determines how fast they can take up water," says Sack. "Plants open their leaf pores, called stomata, to take in carbon dioxide. But when these pores are open, the plants also lose water. Like a wet towel on a clothesline, the insides of the leaf can dry pretty quickly." Since fast-growing exotic plants typically have more open leaf pores than native, slow-growing trees, Sack says, they lose more water in less time.
The researchers found that individual
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Ecological Society of America