- California’s native jewelflowers, a group of plants that belong to the mustard family, grow in widely diverse landscapes and microclimates across the state. But until now, scientists didn’t understand what allowed their wide distribution.
- To understand this, researchers analyzed information from nearly 2,000 specimens; dug into climate and geological databases; and amassed field observations to understand the climatic conditions that 14 species of jewelflowers need to grow and reproduce.
- Their study found that, despite living in different landscapes, from desert to valleys and mountains, jewelflowers prefer hotter and drier climates, timing their sprouting and flowering accordingly. Even those species growing in colder regions adjust their life cycle to flower later in the summer and seek drought-prone soils.
- The research shows how plants distributed across vast geographies may require specific microclimates and habitats to survive, which are potentially at risk in a warming world.
The small, bell-shaped flowers that dangle like gems in dazzling hues of yellow, maroon, purple, brown and white give jewelflowers their name. Yet these plants, from the genera Streptanthus and Caulanthus, belong to the humble mustard family. Native to California, more than 60 species dot the state’s myriad landscapes, splashing the hillsides of the Sierras, prettifying arid deserts, splattering the grasslands with colors, and bedazzling coastlines.
Most species are highly endemic. For instance, Farnworth’s jewelflower (Streptanthus farnsworthianus) lives in the foothills of the Sierra Mountains, and the Mount Tamalpais jewelflower (Streptanthus batrachopus) grows around the peak just north of San Francisco — and nowhere else.
These plants originated in arid southwestern deserts some 2 million to 5 million years ago. Over time, they spread northward, thriving in various climates and at different altitudes, from Baja California to Oregon. Much of California has a Mediterranean climate, where summers are hot and dry; it rains during the cooler months, from fall to spring — quite different from the dry deserts the plants come from.
Until now, scientists thought jewelflowers moved into new territories by evolving and adapting to a range of temperatures and rainfall patterns. But they wondered if there was another reason for their successful move to new landscapes.
A recent study found an unexpected answer: Yes, jewelflowers evolved to survive in different habitats, but not nearly as much as scientists had thought. Instead, they changed their habits. These plants adjusted their schedules, altering the timing of when they sprouted and flowered to coincide with the drier, hotter part of the year — irrespective of where they grew.
Megan Bontrager, study co-author and botanist at the University of Toronto, said she was “really surprised” by the results.
Every one of the species the researchers studied showed the same climate preferences. Even those growing in colder regions tuned their life cycle to sprout and flower later in the summer. In regions such as northern California, where it’s wet and temperatures are mild, these plants seek drought-prone soils.
“Jewelflowers are really amazing plants,” said Bontrager, who conducted the study as part of her postdoctoral research at the University of California, Davis. “It’s one of the groups of flowers that have figured out how to complete their life cycle under the constraints of that super dry spring and summer.”
The phenomenon of syncing life stages with climate isn’t unique. Bears hibernate during the winter when food is scarce and temperatures plummet. Many birds migrate to mate and breed in warmer climes, and some plants, like grasses, attract seed dispersers that sow them in particular microhabitats where they can thrive.
But this is the first time scientists have observed this phenomenon in jewelflowers.
Deciphering jewelflowers’ preferred climate
The 14 jewelflower species the researchers studied are annuals, whose seeds sprout after the season’s first rainfall. Researchers pulled information from nearly 2,000 herbarium specimens, available online at the Consortium of California Herbaria. Each specimen included details of when and where it was collected. When paired with historical data on rainfall, the researchers were able to estimate when that jewelflower sprouted and how old it was when it was collected. They also gathered average climate data from areas where the specimens came from.
The aha moment came when they compared the lived experience of the plants, pairing their life events with temperature and precipitation averages where they grew. Regardless of their location, all 14 species sprouted and flowered when the climate turned hot and dry, however long or short that window was.
In geographies where that option wasn’t possible, these plants sought arid microclimates where the seeds can establish and grow. For example, jewelflowers in northern California thrive in pockets with coarser soil that doesn’t hold much water. Overall, the researchers found that the jewelflowers live in areas that are much like the deserts they came from.
The study highlights the importance of peering into the past, as the researchers did with herbarium collections, to understand the different ways plants evolve or adapt to their surroundings.
“This study is an example of creatively using natural history collections to better understand ecology and evolution,” said Isabela Lima Borges, an expert in rare plants at the Santa Barbara Botanic Garden, who wasn’t involved in the study. She noted that seemingly widespread plants, such as the jewelflowers, “can still have very specific habitat needs.”
A precarious future in the face of climate change
Jewelflowers are rare gems among California’s plants, but their very particular habitat needs also makes them vulnerable to climatic changes, placing them at risk in a warming world where weather patterns are increasingly erratic. The California jewelflower (Caulanthus californicus) and the Tiburon jewelflower (Streptanthus glandulosus niger) are already endangered due to habitat loss, overgrazing, urban development and loss of pollinators. Climate change poses an additional threat.
With climate change, scientists expect California to have its first rains of the season — a vital cue for jewelflowers to sprout — much later in the fall, shortening the window these plants have to grow and flower. That means these plants will have to grow faster to catch up with summer, Bontrager said. If they can’t flower in time and produce seeds, fewer and fewer plants will appear in subsequent years.
Human-caused climate change is hitting California hard, with intense heat waves and long droughts punctuated by heavy rainfall — driven by atmospheric rivers — which then cause floods and landslides. State-wide, wildfires are larger and more frequent. These trends are expected to worsen in the future.
This doesn’t bode well for these iconic flowers. “As climate change alters temperature and precipitation patterns across California, this study suggests that jewelflowers are unlikely to adapt,” Borges said. If they can further change their body clocks, she added, it could put them out of sync with their pollinators and seed dispersers. Insects that pollinate jewelflowers might arrive earlier than the flowers bloom, and there may be fewer seeds to sow for the next year.
As climate change impacts grow, Borges said, “It’ll be important to preserve the locations where these plants can take refuge.”
Banner image:The California jewelflower (Caulanthus californicus) is an endangered species in the state and protected federally because of habitat loss, overgrazing, urban development, and loss of pollinators. Research shows that changing climate poses an additional threat to their survival. Image by emmaranthaceae via iNaturalist (CC BY-NC 4.0).
Spoorthy Raman is a staff writer at Mongabay, covering all things wild with a special focus on lesser-known wildlife, the wildlife trade, and environmental crime.
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Citation:
Bontrager, M., Worthy, S. J., Ivalú Cacho, N., Leventhal, L., Maloof, J. N., Gremer, J. R., . . . Strauss, S. Y. (2025). Herbarium specimens reveal a constrained seasonal climate niche despite diverged annual climates across a wildflower clade. Proceedings of the National Academy of Sciences, 122(28). doi:10.1073/pnas.2503670122
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