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Though it may seem counterintuitive, desert-dwelling plants and animals who are well-adapted to hot, dry conditions are still threatened by rising temperatures, drought, and other extremes caused by climate change. For some species, like long-lived Joshua trees, a team of researchers including UConn Department of Ecology and Evolutionary Biology Assistant Professor Karolina Heyduk are working to understand how much these plants can respond to climate change. Their findings are published in .

Joshua trees are native to the Mojave Desert in the southwestern United States, and they are only able to grow within their existing range, which seems to be diminishing, says Heyduk. For example, the researchers report seeing fewer young Joshua trees, which indicates that something is affecting their ability to survive. Heyduk explains they set out to try to understand if certain Joshua tree populations may be better adapted than others to the current environment to help focus conservation efforts.

To search for potential differences between Joshua tree populations, the researchers established several test plot gardens where they grew seedlings from plants of both species, Yucca brevifolia and Yucca jaegeriana, from across their ranges. As the seedlings grew, the researchers measured their progress, including physiological and genetic traits, and Heyduk says they made the exciting discovery that may help Joshua trees adapt to the warming climate.

The team's discovery is about an important aspect of photosynthesis. Most plants photosynthesize during the day, when sunlight is available to enable the plant to take up CO₂ and convert it to sugars. Plants take up the CO₂ through small holes in their leaves, and while these holes are open, they also lose some moisture.

Heyduk says that some plants that grow in dry environments have evolved a process called Crassulacean Acid Metabolism (CAM) photosynthesis to take up CO₂ at night so they can store it until daytime when energy from the sun can once again drive photosynthesis.

"We sort of stumbled upon this idea that Joshua trees might use CAM photosynthesis, and we set out to understand the extent to which they do this and the variability of that phenotype across different populations," says Heyduk.

Finding evidence of CAM photosynthesis in Joshua trees is great news, says Heyduk, as it may help the plants tolerate the warming climate across their range. By growing the plants from across their range under the same environmental conditions, the researchers studied the genetic differences between individual plants. Additionally, the researchers found differences in physiology between the two Joshua tree species in how they process carbon and nitrogen.

"We're bridging a lot of fields in biology. We measured a lot of physiological traits, but we also looked at the gene expression of these plants in the gardens, and the accumulation of metabolites, which are compounds that plants make," says Heyduk.

"The science that we're doing is an awesome example of the collaborative research that's bringing together researchers from universities like UConn, researchers from primarily undergraduate universities who bring their students to the field, and also from federal scientists. We couldn't have done this work without that collaborative team."

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This collaborative and comprehensive approach means the team has more assurance that the differences are due to genetics, or genotypes, in the different contexts and they match with what they would expect to observe, called the phenotypes, in the trees.

"It also helps us confirm the use of CAM photosynthesis. We see, for example, the genes that we know are involved in CAM photosynthesis were expressed at the times we thought they should be, so it's another way to verify our phenotypes and interrogate why those phenotypes are showing up at all," says Heyduk.

The researchers are doing follow up studies to better understand the CAM photosynthesis they observed but also to help answer additional questions about Joshua trees.

"One of the mysteries has always been, why do these two species persist? There are differences in their pollinators and their flower structure in response to the pollinators, but there are also some hybrids. In future work we will explore whether or not the physiological differences are also part of why these species continue to exist as two separate entities," says Heyduk.

As the climate continues to warm, research like this will become increasingly important to understand how these ecosystems will respond. Heyduk says the amazing biodiversity of the desert is often overlooked because people assume nothing grows or lives there, which is not the case at all. That can carry through into how people think about climate change.

"We're not focusing on desert species quite as much as other ecosystems. I think partially that's because we think they're already adapted to the super-hot environment and they'll be fine," says Heyduk. "However, the deserts are getting hotter, and in particular the nighttime temperatures are getting hotter."

Another challenging element to studying Joshua tree adaptation is the fact that they live for so long. Heyduk points out that some trees today may have experienced the entirety of the Industrial Revolution within their lifetime, including the climatic changes that it ushered in.

Heyduk says the team's ongoing studies are looking more closely at the question about nighttime temperatures. A current project involves growing seedlings under controlled conditions so the researchers can measure how the plants respond to see if this new stressor is driving the lack of seedlings noted by the researchers.

"What today's seedlings are experiencing is going to be very different than what they might experience in 100 years when they're mature plants," says Heyduk. "We need to understand how they will respond to that change."