Last year on the Wailuku River, which runs through the middle of Hilo town, Stacy and her team found an unusual group of ‘ohi‘a. Living among the rocks on steep banks, these trees survive submerged for long periods, unlike any other known ‘ohi‘a. What’s more, it turns out they can’t reproduce with other types of ‘ohi‘a, and they have different DNA—which means they could be an unknown species. Trees are thought to have arrived in Hawai‘i no earlier than one hundred thousand years ago, an evolutionary blink of the eye. So the question is, How could a new species evolve in such a short time? Even more tantalizingly, there could be a number of undocumented varieties, some poised to become new species in the near future.
This partly explains why studying ‘ohi‘a in Hawai‘i is so interesting. Each island has slightly different ‘ohi‘a trees because the islands are different ages. That difference in ages creates more variety in landscapes and environmental conditions, which leads to greater diversity among plants within the same family. For example one ‘ohi‘a species, Metrosideros polymorpha (a.k.a. ‘ohi‘a lehua), can grow as straight, tall trees in dry forests, as small shrubs in cloud forest bogs or as enormous, sprawling “walking lehua” in rainforests. The stubby Metrosideros rugosa, or lehua papa, is endemic to O‘ahu and has round, deeply furrowed leaves. Metrosideros waialeale, which grows only on Kaua‘i, has longer leaves that do not gather into rosettes. Such differences, when considered along with the underlying genetic fingerprints, provide Stacy accurate time stamps for when different species could have evolved. That in turn makes it easier to track complex evolutionary processes.
Stacy, who is in her late 30s, has spent most of her professional career unraveling the secrets of trees. A Bostonian who grew up loving nature, she studied pre-veterinary medicine at Penn State University. After graduating she volunteered with a forest research team headed for the Amazon, where she fell in love with tropical rainforests, trees in particular.
Shortly after arriving in Hilo in 2004, Stacy noticed that ‘ohi‘a were ubiquitous on the Big Island. In fact, Stacy discovered, ‘ohi‘a are often the only native tree in some places. “Rarely do you find such dominance by a single species of trees among native flora,” she says. “It was just amazing. I started asking questions and found that very little research had been done on the genetics and evolution of the Metrosideros. It’s a messy family: There are so many hybrids, so many varieties, and the speciation was really unclear. We had the five known species, but there could have been many others. … We just didn’t know.” When Stacy investigated why there wasn’t more research on ‘ohi‘a, a colleague told her that they were complex and scary, meaning there was no obvious path to an amazing discovery. It was a career risk for a scientist. But not for Stacy: “I thought, ‘Cool! That’s a great opportunity.’ I like messes, you see.”
Slowly, one foot at a time, I cross the ridge and then climb the rope. My hands slip, and I have a moment of terror before gaining a toehold. Just above the top of the rope is a patch of Metrosideros rugosa, the aforementioned stubby species that lives only high in the mountains and prefers wickedly windy conditions. Next to the rugosa sits a different ‘ohi‘a. The leaves are a slightly lighter green and more oblong. Stacy rubs the underside of the leaves to see how easily the soft fuzz comes off. “This one … I have no idea. It’s a wild hybrid of some sort. Probably a loser.” A reproductive loser, that is. Stacy hopes to sort the losers from the winners to determine why some hybrid trees become full-blown species capable of reproducing and others don’t.
Occasionally we pass ‘ohi‘a blossoms wrapped in tiny mesh bags — places where Stacy and her UH Hilo students have hand-pollinated a blossom to ensure that it gets fertilized by a specific male. By mating hybrid and non-hybrid trees in different combinations, Stacy can both identify new species and gain insight into how trees evolve. “Hawai‘i is a blank slate upon which organisms can colonize, differentiate and speciate,” she says. “Hawai‘i Island is the youngest, so we can look at what’s going on there and contrast it with what we see on the older islands of O‘ahu and Kaua‘i to look for patterns that develop over time.”
There’s a lot of effort to this sort of science: Stacy and her assistants keep tabs on ‘ohi‘a populations on O‘ahu, Kaua‘i and the Big Island, which involves painstaking record keeping, computerized bioinformatics and genetic analysis and complex GPS location tracking. But perhaps the hardest part, she says, is hand-pollination, especially in conditions like this. “If it’s windy or it’s wet or anything, getting those tiny little bits of pollen into the blossoms is darn near impossible. One time we were up here and the bees were out,” she recalls. “And we were literally racing the bees, because once they land on a blossom, we can’t use it. They carry too much pollen. So we had to pollinate then bag blossoms as fast as we could. We were exhausted.”