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An Epic Joshua Tree Roadtrip & the Reproductive Ecology of an Iconic Southwest Plant

Think of your most amazing four-state roadtrip. How much data did you collect between stops at Disney Land and the hotel pool? Did you stargaze in the Mojave Desert or were you too exhausted after a day of running transects through Joshua Tree National Park? Did you look at the famous Joshua trees with wonder and awe, or did you keep your head down and count individual flowers on these episodic bloomers then hastily move on to the next site to keep tallying reproductive metrics? Did you come home to your computer and upload slideshows of vacation snapshots or did you immediately begin writing up notes like:

Despite its prominence in plant communities of the Mojave Desert, surprisingly little has been published on its reproductive and structural ecology. The majority of research on Joshua tree has focused on its highly coevolved pollination relationship with the Yucca moth. Outside its pollination biology only a few studies have been published on its reproductive ecology.


photo: Stacy Egan, Joshua Tree In Bloom, Joshua Tree National Park, CA

Thanks to one amazing roadtrip — with a little help from Disney World and Denny’s — new research is shedding some light on patterns of flowering, fruit production, and stand structure of Joshua trees across the Mojave Desert. I did not realize how “hashtag blessed” my own phenology research was until I read Samuel St. Clair and Joshua Hoines’ new PLoS ONE paper on the reproductive ecology of Joshua trees. My research is a steady annual routine: I study flowering in plant populations that consistently bloom every spring when I arrive in Maine to record them. St. Clair does not have this luxury with Joshua trees — he writes: “episodic blooms make it hard to anticipate a study of its reproduction.” Early in 2013, St. Clair saw Joshua trees blooming at his field sites and called around — the trees seemed to be blooming across their range, he “even heard reports of blooming in Las Vegas and Phoenix yards.”


As it became clear that 2013 was a rare opportunity to study reproductive ecology for an unpredictable study organism, St. Clair jumped to take advantage. “Obviously there was little time to spare. I mapped out a range wide survey of populations, put a travel map together and booked hotels. Took my two sons out of school (ages 10 and 9) for field help in early May and promised them a stop at the Adventure Dome in Las Vegas and a day at Disneyland. We jumped in our car and were off.” St. Clair, a professor at BYU, and Hoines, at the National Park Service, split the fieldwork and covered ten study sites across four states in May and June 2013. At each site they collected data on the population characteristics (population density, tree height, trunk diameter) and reproduction (number of inflorescences and total fruits, percent of trees in bloom, fruit mass, seed number) of 120 Joshua trees. That’s 1200 trees — from 60 100-meter transects! — in under two months. St. Clair shared some memorable moments, “A grasshopper outbreak at Lytle Rach that had the boys in tears, Kids eat free at Denny’s at least 4 or 5 nights and Disney Land was awesome. The boys still talk about the trip fondly.”


The opportunistic rush for reproductive data revealed interesting patterns across the climate gradient of the Joshua tree’s range. At warmer sites, the Joshua trees produced more flowers and seeds, but stand density was lower, while at cooler sites, there were more Joshua trees but fewer flowers and fruit per tree. So while warming temperatures may be good news for reproductive success, the establishment of new Joshua trees seems constrained by warmer temperatures. I asked St. Clair what these results meant for Joshua trees facing climate change. “I think the bigger limitations moving forward will probably be in the seedling establishment and recruitment phases of development.  The fruiting success suggests that the pollinator populations are intact which is good—we’ve see pollination failure due to a lack of yucca moth in populations of Banana Yucca in a recent paper we published.”


The future of Joshua trees in Joshua Tree National Park is not just a concern for scientists. The official twitter account of the Park (@JoshuaTreeNPS) garned five minutes of fame last November when they began tweeting about the potential effects of climate change on the park’s biodiversity. Secretary of the Interior Zinke apparently reprimanded the Joshua Tree National Park superintendant for these social media science lessons.

The idea that a national park should be dissuaded from sharing research on the natural and cultural resources — including, the namesake of that park — with visitors and general public is truly absurd. I think this means that it is our responsibility to tweet out the results and implications of St Clair and Hoines’ new paper and continue the conversation that @JoshuaTreeNPS started.



St. Clair SB, Hoines J (2018) Reproductive ecology and stand structure of Joshua tree forests across climate gradients of the Mojave Desert. PLoS ONE 13(2): e0193248.

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