January 2, 2010 – Original Source: Arizona Daily Star
When it comes to climate change, you have to expect the unexpected.
That’s what researchers at Tumamoc Hill, near downtown Tucson, learned after studying how nine species of flowering winter annual plants behaved as temperatures rose and rainfall declined over a 25-year period ending in 2007.
The plant varieties included asters, sunflowers, plantains, stork’s bills, popcorn flowers and others.
While most of the species declined during that time, those whose populations rose or remained stable did so for a reason that the researchers found very surprising:
The plants that germinate and grow better in cold winter temperatures did the best — even as the area’s year-round temperatures kept rising.
These findings appear in a study just published in a scientific journal, Global Change Biology. It’s one of the few peer-reviewed studies done so far on the effects of climate change on Sonoran Desert plants.
"The fact that most of them were declining is a big deal, but the big message of paradox is that if the climate changes, we think we know what will happen. Species will move north, or warm-weather adapted species will do better," said Larry Venable, director of research at Tumamoc Hill’s Desert Research Laboratory, which has been monitoring desert plants for more than a century.
"But this study points out that you don’t know what will happen. You don’t know what living organisms will do. Who would have thunk it?" he added.
Specifically, the study looked at the nine most abundant plant species found on 72 plots, each about one-tenth of a square yard, in the desert lab area. The study found that:
- The nine species studied in detail represented 74 percent of all the plants in the area. Of those nine, six declined, two increased and one stayed stable in numbers over the 25 years.
- Overall, there was a decline in numbers when the researchers considered all 40 species living on the research lab test plots that blossom during the winter.
- Temperatures rose and rainfall dropped by small percentages during the winter annual plant growing season, from Sept. 1 through May 5, with the biggest rainfall declines felt during the critical plant germination season from October through December.
- Plants often began germinating as early as October during the study’s early years. The germination shifted to December by the 2000s decade as the onset of the winter rainy season slowly pushed back. The date of the first wave of plant generation was pushed back an average of .267 days per year during the study period.
- Because plants germinate later than they used to, there has been a dramatic decrease in the average temperature at which the first plants are germinated.
- The plants’ response to climate changes is determined less by growing season temperatures than by the timing of storms that trigger plant generations.
The species studied are "the bread and butter flowers that you see everywhere," Venable said. Some are called "belly flowers" because they are best seen close up, unlike less common, showier annuals such as poppies and lupines.
"Even though the overall winter growing season has been getting warmer, the rain events that trigger germination have been getting colder," said Sarah Kimball, a University of Arizona evolutionary ecologist and post-doctoral researcher who collaborated with Venable and two other researchers on this study.
"Maybe we would have expected all species to shrink," Kimball said. "It wasn’t too surprising that so many species decreased in abundance during warmer, drier weather, because these are annual wildflowers that require rain to germinate and grow and reproduce. They are only active for a short time of the year.
"But once we thought about it, it seemed obvious" as to why the winter-adapted plants were doing best, Kimball said — because winter is when they’re being germinated as rainfall patterns change.
This winter, the researchers have been holding their breaths, because so far, no new plants have been germinated on the Tumamoc sites due to the sparse rainfall that has persisted all year, Venable said. About one-third of an inch fell nearly two weeks ago. But as of the middle of this week, there were no new plants to show for it.