A team of researchers led by California State University, Northridge evolutionary biologist Jeremy Yoder partnered with hundreds of volunteer naturalists to reconstruct how 120 years of climate change has affected Joshua trees.

That data will help scientists, conservationists and policy makers develop plans to protect the twisted, spikey trees that populate the Mojave Desert.

“One of the challenges we have studying Joshua trees, which is true of studying a lot of things, is that we have good reasons to think that climate change is affecting them, but we don’t always have easy ways to understand how that affects them,” said Yoder, who teaches in CSUN’s College of Science and Mathematics. “We usually have to track a population over multiple years and see how the health of that population changes as climate changes. We have that kind of data for some species, but not all of them, and usually with those datasets we are looking at limited timeframe — up to a couple decades — and they are in just one location.

“By tapping into crowdsourcing, we have been able to develop a model that provides new insight into Joshua tree population health — a major uncertainty in recent assessments of the species for endangered species protections — for more than 100 years and across the full range of the species,” Yoder said. “We think it’s a method that can be replicated for other plant species, a model that would not be possible without the help of volunteer naturalists around the world. We truly could not do this work without their help.”

Joining Yoder on the project were CSUN undergraduate biology major Ana Karina Andrade; Lesley A. DeFalco, Todd C. Esque and Daniel F. Shryock of the Western Ecological Research Center of the U.S. Geological Survey; climate change biologist Colin J. Carlson with Yale University; digital artist Ray Yeager of Yucca Valley and biology professor Christopher I. Smith with Willamette University. The results of their work, “Reconstructing 120 years of climate change impacts on Joshua tree flowering,” has just been published in the journal Ecology Letters.

Yoder and his collaborators tapped into data collected by everyday people who shared locations, dates and images of flowering Joshua trees through the iNaturalist smartphone app.

“iNaturalist built an app that people with a smartphone can use to identify plants and animals around them,” Yoder said. “They take a photo, and the app makes an educated guess as to what species is in the photo. That creates a record that is then available for researchers to use. Joshua trees are great for this because they’re so distinctive. They also have a very clearly defined geographic range, so we can eliminate a lot of common errors found in crowdsourcing.”

What Yoder and his colleagues were looking for was evidence of Joshua trees flowering, since the species only flowers when there has been significant rain, an indication of weather patterns in the usually parched Mojave. Flowering is an important indicator of the health of a population, the first step towards production of seeds that can grow into the next generation of Joshua trees.

“The flowers grow right at the end of the branches, so they’re very, very visible,” Yoder said. “Even at a distance, you can tell from a tree’s silhouette against the sky if it’s flowering. So, even with low-quality images we can tell what a Joshua tree is doing. That means iNaturalist records are not just recording where people are finding Joshua trees, they’re also recording whether the Joshua trees were flowering, when and where.”

With records of when Joshua trees flowered in a given location during certain periods in a given year, the researchers were able to link that information to weather records for that same location and build a “machine-learning model” that showed what weather conditions look like in locations where the trees flowered and what those conditions looked like that in places where they didn’t.

“Based on the data we got from iNaturalist, we’ve been able to discern a pattern about conditional relationships between the weather, location and the trees,” Yoder said. “Using that information we built a model for understanding what happened before iNaturalist existed, going back 100 years, as long as we had weather records.

“We know that the Mojave Desert has seen substantial climate change during the course of the 20^th century,” he continued. “What we were looking for were visible trends in the weather data that we were already using to make ‘reverse predictions,’ or hindcasts.”

The researchers examined historical herbarium records and even news stories about Joshua tree flowering events dating back more than 100 years in the Los Angeles Times and Orange County Register to compare with hindcasts made by the computer model to see if the ‘reverse predictions’ were accurate. They were.

For instance, Yoder pointed to a news story from 1924 about “Cadillac tourists,” people who took a road trip around Southern California, taking in the sights.

“They told a reporter from the LA Times about driving through the Cajon Pass and seeing all these Joshua trees with fruit,” Yoder said. “That was a flowering event that our model correctly predicted based on weather records from 1924.

“These limited historic records are really important because they let us confirm that the hindcast was working the way we want,” he said.

With the data Yoder and his colleagues have collected, researchers, conservationists and policy makers can make more informed decisions about preservation efforts, including the identification of climate refugia, sites where Joshua tree populations are healthy and conditions will most likely to remain suitable even as the climate changes.

Yoder said the model he and his colleagues created using the data from iNaturalist could be adapted for other species as well.

“iNaturalist has more than 10,000 records of Joshua trees, as of 2022, but more than 30,000 for Saguaro cactus,” he said. “Imagine the possibilities.”