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Field season 2021 is already testing our Joshua trees’ limits

A fieldwork update from JTGP collaborator Karolina Heyduk, a plant physiologist and Assistant Professor at the University of Hawai’i Mānoa.

There’s heat, and then there’s Nevada’s 105°F heat that is so dry your skin feels like it should audibly sizzle when you step outside. That’s the kind of heat the Joshua Tree project’s team members found themselves in early June when we all met in the desert to collect some preliminary data for our NSF award. We’re about a year into the project; seeds of Joshua trees were started back in 2020, seedlings were out-planted earlier this spring, and now we’re giving them a year to acclimate to their common gardens before we assess the level to which local adaptation is playing a role in shaping the demography, population structure, and long-term success of this iconic desert species. 

We all met outside of Las Vegas, Nevada, where JTGP scientists Dr. Lesley DeFalco and Dr. Todd Esque, both with the USGS, are based. Todd and Lesley have led the development of our common garden experiments and manage the day-to-day operations of the gardens. Earlier this spring Todd and Lesley led a massive effort, assisted by a small army of USGS researchers, to get the seedlings started and planted in the gardens. The four common gardens are spread across the Mojave and have been planted with Joshua trees (Yucca brevifolia and Yucca jaegeriana) from populations that have different home environments. The seedlings are only a few inches tall, but already we can see differences in “garden effects” – that is, seedlings in the hottest garden are growing much more slowly than those in a wetter, more northerly garden. While we definitely could see garden effects, it’s too early to tell if plants from different populations are faring better in different gardens. That’ll be what we hope to discover with the data we will collect next year in the summer of 2022.

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An uncommon “common garden”

A Joshua tree seedling in one of the gardens.

Spring in the desert means it’s time to plant Joshua trees. Over the past two weeks, Joshua Tree Genome Project collaborators and US Geological Survey staff led by Lesley DeFalco planted thousands of Joshua tree seedlings in gardens spaced across the Mojave Desert. The seedlings were started in greenhouse conditions last year, from seeds collected in Joshua tree populations growing in different climates across the desert. Seedling Joshua trees are delicate, and we start them gently — first the greenhouse, then planting with tilled soil and generous watering, then months of monitoring and more water, to let them put down roots.

But eventually we’ll step back and let the seedlings face the full stress of the different parts of the Mojave where the gardens are planted. It seems mean, but it’s a gold-standard experimental method to understand a key question that will help us protect the species as climate change ratchets up the heat and drought stress across the desert.

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