|
The other possibility is that more species went extinct in the recent past than scientists think. At first glance, the pattern from the Pleistocene seems to hold into the deeper past: Plants do seem to be more resistant to extinction than animals. Roots make plants individually tough, and seeds make plant species collectively tough. Luke Mander, a paleobotanist at the Open University in the United Kingdom, describes this idea by comparing the hardiness of plants in a botanical garden to animals in a zoo. If you hit all the animals on the head with a hammer, you’ll probably be left without a zoo. But if you do the same to the plants, he says, you’ll still have a botanical garden the next day.
On the other hand, Mander says, the story of Critchfield’s spruce suggests scientists may simply have missed many floral extinctions. Due to the difficulty of identifying the small, scattered fragments of the fossil record to the level of species, scientists who study extinction in the past often work in higher taxonomic rankings — at the level of “spruce,” for example, rather than of “white spruce” or “red spruce.” This means that, if a plant goes extinct but has many surviving relatives, as Critchfield’s spruce does, it could be easy for scientists to miss its disappearance from the fossil record.
For now, it remains unclear whether the spruce is a hint that more extinctions remain undiscovered — that the dire predictions of the models are right — or whether it is the exception that proves the rule of floral resilience. “Did I happen to stumble on the one major tree species that bought it at the end of the Pleistocene?” Jackson says. “Or is it the tip of an iceberg we can’t access because we haven’t applied the tools, or we lack the tools? I think that’s still an open question.”
Two decades after Jackson and Weng announced their discovery of Critchfield’s spruce, the tree remains an enigma. It often gets mentioned at scientific conferences, Gill says, held up as a curiosity. “I think it could be a bit of a Rorschach test, where we can project on this tree sort of what we want to see,” she says. One could say that extinction is natural, and that we’re fortunate that only this species, and not other species of spruce, went extinct, she says. “Or you could look and say, ‘It’s a warning, a canary in the coal mine. As the climate changes, we need to make sure we’re helping species not end up like this example,’” she says.
But scientists may still arrive at a better understanding of the tree itself. In the early 2010s, a group of researchers at the University of Illinois Urbana-Champaign and the Illinois State Museum developed a method that could potentially be used to more easily track the tree’s whereabouts with pollen. Pollen can last for thousands of years in low-oxygen environments like peat bogs and lake beds, and is far more widespread than larger fossils like the needles and cones that Jackson used to identify Critchfield’s spruce. The problem is that the pollen of different species of spruce, shaped like a half-deflated Mickey Mouse head, is hard to tell apart by eye. The researchers took a statistical approach, using the minute differences in average dimensions of the pollen grains of the various spruce species to train a computer to sort between white and black spruce.
In a 2014 paper, the research group showed that pollen of Critchfield’s spruce has a distinct morphology from other spruces’ pollen, meaning a computer may be able to learn to pick it out as well. The main purpose of the work was to be able to investigate the dynamics of the population over time, according to Mander, the study’s lead author. Being able to reliably identify Critchfield’s spruce pollen could show the tree’s whereabouts and abundance through time, and help pinpoint when it went extinct.
|
