When it comes to one of the most beloved places in California plantsThe prognosis is grim. Estimates suggest that Most of the Joshua Trees are in habitat will become useless by the end of the century because the planet continues to warm,
Now, a new study has revealed that the symbols of the Mojave Desert are facing very different circumstances from the early 20th century, And these conditions are changing the frequency of their reproductive activity.
“This confirms that the impacts of climate change are already present,” the authors said. Jeremy YoderAssociate Professor of Biology at Cal State Northridge, Dr.
This has come to light in a recent study. Published in Ecology LettersJoshua relies on citizen scientists’ observations to reconstruct 120 years of climate change effects on trees. It’s the first systematic investigation of what prompts the spiky, sessile succulents to mast, or flower, the first step in their reproductive process. It’s also the first study to examine how the plants are faring across their entire range.
The results were so surprising that Yoder spent days scouring the data to understand what was going on. He found that, counterintuitively, climate change, with large fluctuations in rainfall and warmer winters, is causing Joshua trees to flower more frequently. While this sounds like a good thing, those same changes are also potentially reducing the survival of delicate Joshua tree seedlings, which need a constant supply of water to grow into robust adult plants.
A piece of fruit hanging from a Joshua Tree in Joshua Tree National Park.
(Gary Coronado / Los Angeles Times)
“We have all these predictive models of what’s going to happen in the future with Joshua trees, but this is showing us some real changes over the last 100 years,” said Kelly Harbinson, executive director of the conservation nonprofit. Mojave Desert Land Trustwho was not involved in the study. “It’s also helping us look at this broader picture, which is that even though they are flowering more frequently, these changes in rainfall and temperature are not necessarily beneficial to tree survival.”
Joshua trees are important to the entire desert community. Yoder likes to refer to them as umbrella species — two species to be exact — because they provide food and habitat for other wildlife unique to the Mojave. The researchers looked at eastern and western Joshua trees, which have some differences — for example, each yucca is pollinated by a different species of insect — but they also are hybrids and occupy overlapping climates, according to the study.
“If we protect Joshua trees, we’re potentially protecting this whole category of things that live in the desert,” Yoder said.
Do you want to become a citizen scientist?
Assessing the broad population health of Joshua trees has long been a challenge for researchers, a task that requires collecting many observations over a relatively long period of time. But Yoder realized that 15 years of data on the reproductive activity of Joshua trees could be collected through the iNaturalist database, where users can upload photos of plants and animals from their smartphones.
Joshua trees are a popular spectacle to photograph: easily recognizable, they are often the largest plants on the landscapes in which they grow. They are also culturally prominent as a symbol of the Mojave Desert, named after the Mojave Desert. Popular National Parkssimultaneously 1987 U2 albumAnd they form large, showy clusters of milky-white flowers that grow at the end of their branches, making the flowers easy to identify in photographs.
Yoder and his fellow researchers used more than 10,200 images of Joshua trees collected from iNaturalist to train a machine learning model to predict flowering from annual weather records. They then used weather records to “predict” flowering activity back to 1900 to see how it changed over time, resulting in the surprising estimate that its frequency has increased from once every five years to more than once every four years.
The model indicated that Joshua trees were more likely to flower when they got a wet year following a drought year, as well as when winter temperatures were above zero. Plants were also more likely to flower when drought stress was relatively low, and temperatures and drought stress were relatively stable over the years, it found.
The researchers validated the results by checking news reports about Joshua tree blooming events, including a 1924 Los Angeles Times article about road travelers who saw seed-laden Joshua trees around the Cajon Pass. The researchers’ model correctly predicted that and other flowering events, Yoder said.
A Los Angeles Times story published on June 15, 1924, reported that “a very large crop of seeds” could be seen on Joshua trees just behind the summit of Cajon Pass, which was said to foreshadow a rainy winter.
(Los Angeles Times)
Yoder said evaluating the health of Joshua trees is a complex task, and more research needs to be done to understand how climate change is affecting each aspect. For example, he said, in an area north of Death Valley, where conditions are cooler but also drier than the rest of the region, researchers have recorded a slight decline in conditions that support flowering frequency. That area has been identified as a potential climate refuge where Joshua trees could survive in the future, even if other parts of the desert become inhospitable.
Still, if cooler conditions favor better plant survival, Joshua tree populations could be healthier than those in locations farther south, where flowering is increasing, Yoder said.
Joshua trees, on the other hand, depend entirely on yucca moths for pollination. Some limited studies have speculated that these moths are less active in cooler, higher-elevation climates. This could mean that in other climate refuges, such as the San Bernardino Mountains, trees are flowering more frequently due to changing conditions, but they are not producing fruit and seeds because pollinators are not showing up, Yoder said.
The news isn’t all bad. Increased flowering frequency could mean conservationists will have more opportunities to collect seeds for restoration purposes after major disturbances like fire. In 2020 and 2023 alone, two wildfires An estimated 1.8 million Joshua trees were destroyed.
The iNaturalist approach is also promising for future studies. For example, since fruit on Joshua trees can only be seen when their pollinators are active, it is possible to use a subset of the iNaturalist data to model the activity of yucca moths, Yoder said.
He added that it’s possible to adapt this modeling approach to other species as well — a postdoctoral scholar in his lab might try it with saguaro cactus.
“Really low-effort, voluntary contributions like participation in iNaturalist have a real-world impact on how we understand the natural world,” Yoder said. “This study wouldn’t have been possible without people who were in the desert, pulled out their phones and said, ‘Hey, that’s cool. I’m going to take a picture of that Joshua tree.'”
