Letter From The Desert: Joshua trees

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So let’s talk about Joshua trees. My vegetative sensei has been in the news again, partly due to an unsurprising but bleak April study that predicts hard times ahead for the species, at least in its southern populations, and an equally unsurprising and equally bleak decision by the Department of the Interior not to do anything about it.

First, the study, which you can read here. Led by conservation biologist Lynn Sweet of UC Riverside, with whom I have the privilege of serving on the board of the Mojave Chapter of the California Native Plant Society, researchers did Joshua tree counts on selected plots in and near Joshua Tree National Park, then correlated the percentage of young trees in those plots with the local microclimates.

To continue with my oversimplification, the researchers found that tree populations in areas that had higher annual precipitation and dried out less during the warm season, which two conditions are related but not quite the same, were more likely to have higher populations of baby Joshua trees. Of less importance but still significant, the trees reproduced better in spots with lower maximum temperatures in summer and lower ones in winter.

They then took historic climate data and a handful of climate change scenarios to map which parts of Joshua Tree National Park might still be expected to host populations of the park’s namesake vegetable at the end of this century. The results coincide pretty closely with the results of previous such exercises, except more pessimistic. Moderate climate change such as we might expect if all the nations of all the world stopped dithering and started to take urgent measures to limit greenhouse gas emissions would limit the trees to refugia in the higher, moister northwest end of the park. The “business as usual” scenario leads to maybe three trees on Eureka Peak.

The study doesn’t cover populations outside the Joshua Tree area, which can be expected to persist longer if they’re farther north, as for example in the Wee Thump Wilderness in Clark County, Nevada, shown here, with the atmospheric McCullough Mountains in the background. There be desert bighorn in them hills.

The authors further maintain that focusing management efforts on those pockets of Joshua Tree NP that will stay more hospitable to Joshua trees is a worthy goal, saying:

Rather than an ominous prediction of extinction, climate refugia provide land stewards with targets for focusing protective management, giving desert biodiversity places to weather the future.

That protective management is made easier if the federal government offers formal protection of a species, as for instance through listing under the Endangered Species Act. That’s why the group Wild Earth Guardians petitioned the US Fish and Wildlife Service to list the tree as Threatened in September, 2015. (I wrote about that here and here.) And that’s why, seeing that USFWS was three years behind schedule in making its decision whether to grant the Joshua tree Threatened status, the group helpfully forwarded the UC Riverside study to the agency on August 5 of this year.

USFWS responded with a decision that was disappointing but again, not even a little surprising. They declared that the Joshua tree has enough protection already.

USFWS’ “12-month finding,” which took a mere 48 months to complete (to be fair, that’s better than the agency average for deciding whether to list flowering plants), offers the following rationalization:

Because the two species are long-lived, have such large ranges and distributions, mostly occur on Federal land, and occupy numerous ecological settings, we have determined that future stochastic and catastrophic events would not lead to population- or species-level declines in the foreseeable future. As a result, we have determined that neither Yucca jaegeriana nor Yucca brevifolia are in danger of extinction or likely to become so within the foreseeable future throughout all or a significant portion of their ranges. Therefore, we find that listing the Joshua tree as an endangered or threatened species is not warranted.

More on that “two species” thing in a minute.

That decision was reached despite abundant evidence, including that cited above, that climate change is threatening Joshua trees in the south end of their range. Though the trees are doing quite a bit better farther north, a 5°Celsius increase in average temperature would bring the climate of Goldfield, NV, just north of the trees’ range, to a point about the same as that in Twentynine Palms today. And 5°C is a conservative estimate of warming by 2100 in the “business as usual” scenarios commonly accepted by climate scientists these days.

Parenthetically, it’s no surprise that an agency in the He-Who-Shall-Not-Be-Named Administration would deny the risks involved in climate change, but this sort of denialism is a bipartisan trait. The last guy presided over denials of Endangered Species Act Protection for the American pika and the wolverine, both of which are likely more immediately threatened by climate change than the Joshua tree.

Nonetheless, people are paying a lot more attention to the Joshua tree listing denial than they did to the pika or wolverine, including earnest and mildly factually inaccurate editorials in major regional newspapers. In some ways, the climate issue is very convenient for the LA Times: were it not for climate change, the biggest existential threats to the Joshua tree would include habitat loss due to urbanization and renewable energy, and speaking out against either of those is not something the Times usually likes to do.

Then again, a third major threat to the trees is wildfire, and the Times does take brave stands against wildfires on frequent occasions.

An interesting side note: The USFWS also declared that it’s treating the Joshua tree as two distinct species, Yucca brevifolia and Yucca jaegeriana, and while that’s unlikely to change any botanists’ minds without peer review it certainly has prompted some interesting conversations, such as the comment thread on this tweet.

I’ve written a few things on that split and likely will again, but this is already long enough.


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