The pine-juniper woodlands of the southwestern U.S. could be wiped out by the end of this century due to climate change, while conifers throughout much of the Northern Hemisphere may be on a similar trajectory, according to a study by Los Alamos (N.M.) National Laboratory.
Ecologist and first author Nate McDowell and his team found that dominant evergreens in the Southwest died when tree predawn water potential fell to levels that impaired the transport and stores of water and carbon. This predawn water potential, a measure of water stress, is the water status of the tree that results in part from soil water availability and atmospheric water demands on plant water use.
Trees, a precious carbon sink, become a carbon source when they die, so knowing how they interact with the climate and the carbon cycle is imperative to the climate's delicate balance.
The very mechanism that a tree uses to preserve its water stores during prolonged drought can be its undoing: the tree closes the stomata on its needles to prevent water loss, but this prevents the tree's food source, C[O.sub.2], from entering, halting photosynthesis.
As the air becomes hotter and drier, subsequent pressure...