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As jewelflowers spread into California from the desert Southwest over the past couple of million years, they settled in places that felt like home, according to a new study from the University of California, Davis. The work, July 1 in Proceedings of the National Academy of Sciences, shows that the ability of plants and animals to adapt to changing climates might be more limited than it appears.

"I was honestly surprised," said Sharon Strauss, Distinguished Professor emeritus in the Department of Evolution and Ecology and corresponding author on the paper. "They haven't evolved as much as you would think."

The study also shows the important role that herbaria—collections of pressed and dried plants—can play in ecological research.

Jewelflowers (Streptanthus) began to spread into California 2 –4 million years ago as the climate became "Mediterranean" with a rainy season from fall to spring and a hot, dry summer. Jewelflower species are now found across the region, in areas that are much colder and wetter overall than the deserts where they originated.

Strauss, postdoctoral scholar Megan Bontrager and colleagues used about 2,000 specimens of 14 species of jewelflowers from the Consortium of California Herbaria, an online resource that draws on multiple plant collections, including UC Davis' own herbarium.

The collections include records of when and where the specimen was collected and often other data as well.

Most jewelflowers are annual plants that germinate with the first significant rainfall of the season. By reconstructing local climate conditions for each specimen, the researchers could therefore estimate when the plant germinated from seed, and how long it had been growing before being collected. For example, a plant collected in June 1935 might have germinated in October 1934.

"We know how far they progressed on their one shot at life," Strauss said.

Tracking the lived environment

Based on the average climate over a year, some jewelflower species live in areas much colder and wetter than others. But when the team looked at the local climate at the time the plants were growing above ground, a different picture emerged. The environments in which the growing plants spent their time were generally warmer and drier than surroundings.

"If you look at the annual climate, you would think that they have diverged a lot, but actually the species are good at tracking hotter, drier times and areas," Strauss said. For example, the plants might favor sunnier, south-facing slopes. At the northern end of their range, jewelflowers are found in areas with drier soils.

The research shows how the "lived" climate can be different from the annual climate when plants take advantage of microclimates or refuges, or use strategies such as changing their timing of germination or flowering.

It also shows how much herbarium specimens can contribute to ecological research.

"The beauty of herbarium specimens is that we have hundreds of years of collections under a wide range of conditions," Strauss said.