Floral fizzle?


Climate change is sucking the color from the Sonoran Desert. The winter flowers that generally carpet the ground — white woolly daisies, Mexican golden poppies, purple Arizona lupine — are still in hiding. Their seeds lie dormant in the soil, waiting for the rains that are necessary to spark growth.

It usually takes at least an inch of rain to coax the first sprout out of a seed in the Sonoran Desert. Back in the 1980s these "trigger" rains fell in October. By 2007 they had stalled to December. Now it's January 2010, and the 2009 rains have yet to fall.


The Sonoran Desert before winter rains bring out the flowers. Courtesy Jonathan Horst.

When it comes to climate change in the Southwest, rising temperatures go hand-in-hand with the delay of winter rains. And it's having weird effects on the local flora. Even as the region slowly bakes (average temperatures around Tucson have risen 0.05 C per year over the past 25 years), it's the cold-loving plants that are thriving. 

That's the result of a University of Arizona study, which looked at the population of nine common winter plants. Between 1982 and 2007, seven of them declined, while red filaree and popcorn flower — though better suited to cold than heat — actually increased in numbers. 

In the end it all comes back to rain. The later it rains, the colder it gets. And that's good news for chill-seekers, says postdoc Sarah Kimball, the study's lead author who spent months cataloging thousands of plants at a desert lab near the U of A. So while most climate change studies are asking how plants deal with warmer environments, here's a case where rainfall trumps the importance of temperature.   

The Sonoran Desert in bloom after winter rains in 2008. Credit: Jonathan Horst.