Joshua Trees at the Climate Edge

The tree the park is named for is one of the most specific living things in the American West. Joshua trees, Yucca brevifolia, can only germinate in the narrow elevation band where the Mojave and Sonoran deserts meet: cold enough for a winter frost, hot enough for a Mojave summer, at the precise altitude where those two regimes overlap. Move too far upslope, and the conditions are not right. Move too far down, and they are not right either. The tree occupies a zone measured in hundreds of vertical feet, not thousands.

They grow slowly. At lower elevations, a Joshua tree may add as little as an inch of height per year. The large specimens you see in the park, the ones with multiple branches and visible age, have been growing for a long time.

The problem is that the zone they require is moving. Climate change is shifting temperature gradients upslope across the Mojave and Sonoran desert regions. The lower edge of viable Joshua tree habitat is retreating upward. Studies conducted through the twenty-tens projected that the park could lose most of its Joshua tree cover at lower elevations by the end of this century under high-emissions climate scenarios.

What that means, visually, is that the park might eventually contain its namesake tree primarily at higher elevations and in smaller numbers, while the lower desert areas you are driving through now return to something the Joshua tree cannot live in.

The park is named for the most climate-vulnerable thing within it. That was not known when Mormon settlers crossing the desert sometime in the mid eighteen hundreds are said to have decided the strange branched yuccas reminded them of Joshua raising his arms in prayer, though no contemporary record confirms exactly when or how the name began. The name came first. The reckoning came later.