Heat sends Southwest climate back in time

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Bandelier National Monument, N.M. – For 15 years, Craig Allen, a scientist with the US Geological Survey, has monitored a 2.7-acre plot here in northern New Mexico. During that time, he’s witnessed smaller tree species succeeding larger ones. He’s seen dry years, bark-­beetle infestations, large-scale tree dieback, and finally, a shift toward grassland. To Dr. Allen, these changes tell a tale of combined human impacts – overgrazing, fire suppression, and climate change. And they underscore how human activity can amplify the effects of natural cycles to alter a landscape dramatically.

The American Southwest may be drying, one of the predicted consequences of human-induced global climate change. Less water in an already semiarid region will affect how, and for what, people use water. Allen also suspects that tree dieback here may be part of a worldwide phenomenon. As temperature extremes have inched higher in semiarid regions globally, forests have succumbed to heat stress.

But, at least in the Southwest, the news isn’t all bad. Over the past century, fire suppression and grazing pressure have let trees reach a greater density than in times past. But now drought and higher temperatures have, in a sense, prompted the system to reset itself. Savanna will again dominate the landscape. And, given the likelihood of more intense droughts in the future, this means more resilience. Grassland can recover from disturbances more quickly than comparatively long-lived pinyon-juniper forest.



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