By Robert Sanders, Berkeley News

At a time when climate change is making many areas of the planet hotter and drier, it’s sobering to think that deserts are relatively new biomes that have grown considerably over the past 30 million years. Widespread arid regions, like the deserts that today cover much of western North America, began to emerge only within the past 5 to 7 million years.

Understanding how plants that invaded these harsh deserts biomes were able to survive could help predict how ecosystems will fare in a drier future.

An intensive study of a group of plants that first invaded emerging deserts millions of years ago concludes that these pioneers — rock daisies — did not come unequipped to deal with heat, scorching sun and lack of water. They had developed adaptations to such stresses while living on dry, exposed rock outcroppings within older, more moist areas and even tropical forests, all of which made it easier for them to invade expanding arid areas.

The study by University of California, Berkeley, researcher Isaac Lichter-Marck is the first to provide evidence to resolve a long-standing evolutionary debate: Did iconic desert plants, like the stately saguaro cacti, the flaming ocotillos and the Seussian agaves, adapt to arid conditions only after they invaded deserts? Or did they come preadapted to the stresses of desert living?

The question has relevance today, Lichter-Marck said, because accelerating aridity due to climate change is challenging plants to adapt much more quickly than they have in the past. Already, about one-fifth of Earth’s land surface is desert. If adaptation to arid conditions was only possible for plants that had already evolved to deal with such stresses, then many today may not be equipped with an adequate genetic tool kit to survive.

A carpet of ephemeral wildflowers paints the desert yellow in Anza Borrego State Park. These blooms are dominated by a few species of plants in the daisy family (Compositae). Many daisies possess an annual life cycle that allows them to germinate, flower and go to seed within the short window of favorable conditions that follow rain, effectively escaping the harsh, dry conditions that typically characterize desert life. Rio, a blue heeler or Australian cattle dog, was an important part of the field team that helped locate rock daisies across wide swaths of rugged desert terrain. (Photo credit: Isaac Lichter-Marck)

“If you think about aridity only as a stimulus to plant evolution, then in many cases people could say these plants are survivors, they are adaptable, and they will be fine. They will take advantage of these new conditions, and they will thrive,” said Lichter-Marck, who is also a National Science Foundation postdoctoral research fellow at UCLA.

But the history of rock daisies suggests that “when the deserts emerged, those plants that had the necessary preadaptations to take advantage of new conditions were the ones that thrived,” he said. “Adding more aridification to the system doesn’t necessarily mean more rapid adaptive evolution will occur. There’s a limited source of lineages that can take advantage of new levels of aridity, and that is important for understanding the effect of climate change on biodiversity.”

Lichter-Marck and Bruce Baldwin, UC Berkeley professor of integrative biology, curator of the Jepson Herbarium and chief editor of The Jepson Desert Manual: Vascular Plants of Southeastern California (2002), published their study about the evolution of rock daisies in North American deserts this week in the journal Proceedings of the National Academy of Sciences.

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