Between invasive species, human interference and limited knowledge, native plants are not doing so well. Fortunately, researchers at the University of Hawai‘i at Mānoa are doing their best to protect these beautiful and important elements of the Hawaiian landscape.
Carefully inspecting a fuzzy brown pod, Emily Grave, a second-year master’s student in botany at UH Mānoa, gently plucks it off the branch and untwines the small purple wire around its twiggy end.
“This was an outcross,” Grave says. “Look at how much bigger the pods are. And three of them.”
The three healthy pods have cracked open, revealing hard, bright red seeds like little beans.
Next to it, a single twisted, galled pod quavers in the wind. Grave picks it, even though the pod is still green, so that she can harvest the seed and assess viability before gall wasp larvae eat into it. This one was self-pollinated, and it is obviously smaller and sicklier.
The pods belong to wiliwili (Erythrina sandwicensis), king of the Hawaiian dry forest trees. Their orange-barked trunks, once famously used for surfboard wood, are gnarled and majestic.
A few bright orange flowers remain from this summer’s flowering season, when the trees drop all of their leaves and bloom. Grave estimates that most of the trees in this grove are over fifty years old.
Unfortunately, wiliwili is threatened by an invasive Erythrina gall wasp from Tanzania, which lays its eggs in the leaves and pods of vulnerable native wiliwili.
The Hawai‘i Department of Agriculture released a biocontrol wasp in 2008, which lays its eggs next to the gall wasp’s and eats them before the harmful parasitoid can cause damage to the native tree.
A more recent invasion of bruchid beetles in 2008, which lay their eggs on ripe seeds and hatch into hole-boring, embryo-munching larvae, may be an even bigger threat to wiliwili reproduction in the long term, based on data from the United States Department of Agriculture.
Without beetle larvae holes, wiliwili seed germination rates are high in the lab, Grave says – over 70 percent. When a larva eats just one hole through the seed coat, potentially allowing infection and rot to set in, germination drops dramatically.
Wiliwili trees flower in clusters of showy orange and pink. This summer, Grave and her partner, Tim Kroessig, also a second-year master’s student in botany, performed a series of experimental pollination treatments: autogamy, or self-pollination; geitonogamy, or pollination from another flower on the same tree; outcrossing or pollination from a tree 40 meters away; and an open control.
So far, outcrossed flowers had produced more pods with more seeds across the board, an indication of how important it would have been for native honeycreepers to cross-pollinate the flowers.
On a revisit to her field site at Koko Crater Botanic Garden, Grave is investigating wiliwili reproduction. Which trees flower when and how many? How many seeds are produced by each tree, and is it enough to carry on the population? What pollinators are visiting the flowers and how effective are they at carrying pollen between trees? How is reproduction being affected by invasive species, like the Erythrina gall wasp and the bruchid beetle?
Quickly, the messy reality of field biology sets in. Wind and rain threaten to blow the mesh bags off and take flowers with them, and one well-meaning visitor had already removed all of their mesh bags and thrown them in a nearby trash can.
In the past, Kroessig has observed several novel interactions: plants and animals that have never before been scientifically noted as visiting wiliwili blossoms.
“It’s an amazing assemblage of floral visitors, and 100 percent of them are nonnative,” he said. “The [curved] flowers are obviously bird syndrome – they produce amazingly copious amounts of sugary nectar.”
Kroessig said they might have been pollinated by native honeycreepers in the past. But the nectar-sipping native birds are now virtually extinct in low-elevation wiliwili habitat because of avian malaria spread by mosquitoes. Instead, Japanese white-eyes, myna birds, and invasive ants and bees visit the flowers, often stealing pollen and nectar for food without transporting the precious pollen.
Kroessig continues to juggle several roles in the world of plant conservation. In addition to working at the Lyon Arboretum seed lab and carrying out his own research on Hawaiian Lysimachia, or primroses, he also volunteers as a field assistant for Emily, his girlfriend.
“It’s been really cool to help [Emily] out and be her boyfriend at the same time,” Kroessig says. “We’ve had lots of awesome camping adventures and had some challenges too. But I guess we’re stronger because of it.”
Together, Kroessig and Grave are unraveling the mysteries of the life cycle of native plants. Native plants are irreplaceable parts of our watershed, Kroessig notes.
“Even if you don’t care about native biodiversity, we still need to drink water to live. And many of these species were here long before humans.”
In a world beset by invasive species and other challenges, the wiliwili seem to be hanging on, so far. But the threats to its survival are real. If bruchid beetles continue to decimate seeds, there will be no young wiliwili trees to replace the ancients when they die.
Also, climate change, according to a recent study that assessed climate change risk for native Hawaiian plants, will affect the wiliwili by making its already dry, harsh environment even drier and harsher, reducing the chances of young trees surviving.
“I can’t imagine a Hawaiian dry forest without wiliwili,” Grave says. “They’re the keystone canopy species – they’re the shade for all kinds of understory species.”
Alahe‘e (prized for its hard wood used for spears), lama (a Hawaiian persimmon) and other dry forest species depend on the shade that wiliwili provides.
Hawaiian dry forests have been reduced to less than 90 percent of their original area since human habitation, according to the World Wildlife Foundation; they have been some of the most heavily impacted in the Hawaiian Islands.
On the way out of Koko Crater, Grave points out several young trees. She croons over the babies, which had come up naturally and survived after botanic garden staff irrigated the young saplings. Several were out-planted by garden staff after being grown in the nursery to supplement the population.
Already, several are being attacked by the gall wasp – their leaves are painfully twisted and covered in lesions. But several are healthy, putting out new green leaves and armored spines, ready to face the hot, dry world ahead.
Together, she and Kroessig are learning about what makes for healthy native plants and how to prepare them for their harsh new world.