Pressed plants from long ago yield data on climate change

Scope Correspondent

The primary act of social media—whether Twitter, tumblr, or Instagram—is virtual curation. Around the turn of the 20th century, though, the curation fad was literal: people roamed fields and forests to collect plant specimens and preserve them in plant libraries called herbaria. Now those old specimens are helping scientists reconstruct how trees have responded to shifts in the climate.

Scientists have recently gleaned data from New England herbarium specimens on historical timing of leaf-out—the time in spring when leaves unfurl, an important biological indicator of climate change. A team from Boston University used 1,599 plant specimens from 27 different tree species, dating from 1875 to 2008, to determine past leaf-out dates in New England. By combining herbarium specimen data with weather station data from the same time period, they found that trees leafed out 2.7 days earlier for each degree Celsius increase in April temperature.

Leaf-out is one of the “most easily identifiable indicators of climate change,” says Eli Malaas, a postdoctoral researcher at Boston University and an author of the study. Leaf-out can be measured from satellite data, but that only goes back to the 1970s. Before that, there are really no resources other than herbarium specimens.

Claude Lavoie, an authority on the uses of herbarium specimens who was uninvolved in the study, says there have been “a lot of papers” using herbarium specimens to determine flowering times, but for leaf-out times, “this is certainly the first one.”

Herbaria are similar to libraries, filled rows and rows of metal cabinets. But inside the these cabinets are leaves, fruit, and flowers neatly pressed, labeled, and stuck to sheets of paper. Professionals and amateurs alike collected them in a fad that peaked between the 1890s and the 1920s. And just like people on social media today, they did it to create a certain impression. “Imagine people having their own home herbaria with their collections,” says Elizabeth Ellwood, an author of the study and a postdoctoral researcher with IDigBio at Florida University. “You have your friends over and you check them out and, ‘Hey, I got this species this year, how about you?’”

Herbarium specimens are useful to scientists as “a permanent record of the way the natural world was in the past and is today,” says Richard Primack, professor at Boston University and an author of the study, adding that, in particular, they are “very useful for climate change research.” Herbarium specimens are usually dated and labeled with their collection location (though the level of detail varies), and because they are physical pieces of plants or even whole plants, they can be sources of information from that their collectors may not have imagined, including DNA.

Before this study, Lavoie was skeptical that herbarium specimens would include enough young leaves to actually determine leaf-out times. Plant collectors did not consider young leaves very interesting, so they rarely sought them out to collect.

On the other hand, collectors were keenly interested in flowers. And when they broke off a branch of flowers, that branch often included some leaves. So the researchers ingeniously found their young leaf specimens by looking for flower specimens of tree species—including birches, oaks, elms, and maples—that leaf out at the same time as they flower.

Thanks to these dried bits of plants that were once a sort of social capital, we now know more about how trees could respond to future climate changes. “To look to the future and to understand that, we need to have an understanding of the past,” says Ellwood. Maybe in the future, whatever we curate and collect—Twitter feeds, Pokémon—will also turn out to have an unexpected use for scientists trying to understand the world.