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Published November 24, 2012 08:19 AM

Forests worldwide near tipping-point from drought

Forests worldwide are at "equally high risk" to die-off from drought conditions, warns a new study published this week in the journal Nature.

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The study, conducted by an international team of scientists, assessed the specific physiological effects of drought on 226 tree species at 81 sites in different biomes around the world. It found that 70 percent of the species sampled are particularly vulnerable to reduction in water availability. With drought conditions increasing around the globe due to climate change and deforestation, the research suggests large swathes of the world's forests — and the services they afford — may be approaching a tipping point.

Water is critical to trees, transporting nutrients, providing stabilizing, and serving as a medium for the metabolic processes that generate the energy needed for a tree to survive. Mechanically, water moves through plants via their xylem, a tissue that can be compared to a system of tubes. Transpiration or release of water from a plant's leaves keeps the system moving. But when water availability is insufficient, the process begins to break down, having substantial impacts on the health of a tree. While this has long been observed, until recently the exact mechanism that triggers drought stress in forests was poorly understood. The new study argues that "hydraulic failure" may be a key factor. Effectively, insufficient water availably leads a tree to start pulling air bubbles — called gas emboli — into its xylem impeding the flow of water. Hydraulic failure is akin to attempting to drink through a broken straw — air bubbles significantly reduce the amount of liquid that reaches the top of the straw.

Rainforest image via Shutterstock.

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