The following is cross-posted at the Yoder Lab blog.

More than a century ago, in mid-June 1924, the Los Angeles Times devoted a photo spread and several column inches to the second-hand vacation story of some “Cadillac tourists” who saw Joshua trees bearing fruit. It meant, the Times reported, that rain was coming.

M.C. Ellison of Sacramento, who has been touring Southern California in his Cadillac with his family, told the prophecy to J.E. Clark, sales manager for Don Lee. He did not guarantee its accuracy, but admitted that it sounded interesting at least.

“We met an old prospector who was camping among the Joshua-tree forest just beyond the summit of the Cajon [Pass],” Ellison told Clark. “He called our attention to the extremely heavy crop of seeds which the Joshua trees are bearing this year. Almost every tree is loaded with green seed pods.

“‘That means good rains on the desert next season,’ the prospector told us. ‘I have been traveling the Mojave for almost forty-five years now and I have never seen it fail.’”

Joshua trees’ irregular flowering has been a feature of their public image since long before the advent of automobile road-trips. Joshua tree flowers were not observed until some years after the trees were given the formal scientific name Yucca brevifolia in 1871. The same stand of Joshua trees rarely flowers two years in row, and often goes multiple years between big blooms. When the blooms do come, it’s not clear what triggered them. Sometimes Joshua trees flower after a winter of good soaking rains. Sometimes they flower after a run of really dry years. Some people think they need a hard winter frost. Joshua trees will bloom at one site, and a few kilometers away they won’t produce a single bud. Back in 2018, there was a big bloom in Joshua Tree National Park — but only in Joshua Tree National Park — in November, seven or eight months after flowering usually finishes. The LA Times eventually gave up on the idea that the trees forecast the rainfall for the coming year, but newspapers in and around the Mojave Desert continue to report on big bloom years — and years with no flowers to be seen — as a feature of interest for locals and tourists to this day. 

That public interest has given us a way to finally figure out what cues Joshua trees to flower. Making that connection also gives us a new way to study how the trees have experienced climate change trends going back to the early 20th Century, decades before M.C. Ellison and his family left Sacramento to tour the state in their Cadillac. It’s all in a new paper that’s just been published on the website of the journal Ecology Letters, by a team of Joshua Tree Genome Project collaborators.