We present a write-up of just a few of these that seemed to have special bearing on recent events or which challenged received wisdom.
After Broomsedge Is Gone, Things Can Get Worse
Carla D’Antonio, a professor in the Department of Environmental Studies at the University of California, Santa Barbara, has been studying the invasion of Hawai`i Volcanoes National Park by perennial grasses. Do the invaders “represent an alternative stable state?” Or are they “part of a series of changes over time?” Or, as yet a third option, “do they precipitate a series of changes leading to meltdown” of the entire ecosystem?
In the park, broomsedge grass (Andropogon virginicus) has been found to slow the growth of native trees and shrubs and has been blamed for preventing their regrowth in disturbed areas.
So, after a fire took out the broomsedge, natural resource managers and others were hopeful that the natives would have a better chance of returning.
Instead, D’Antonio reported, although the dominant grass declined from 73 percent cover to 10 percent, two other invasives – possibly even nastier than broomsedge – moved in: molasses grass and faya tree.
Overall, said D’Antonio, there was a 50 percent decline in the `ohi`a canopy and a 25 percent decline in pukiawe. “It would have been even greater,” she said, “except that pukiawe loves being next to faya.”
Driving all this are long-term changes in the climate, she said. Starting in the 1980s, the incidence of wet years has declined. “Break it down by season, and you see the dry season is really getting much drier. Since 1997, we have had a lot of very, very dry summers – we’re hardly seeing any summer precipitation at all. We also are seeing a long-term increase in temperatures, so plants see less moisture as well as higher temperatures in the growing season.”
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A Hailstorm of Caterpillar Poop
In early January, staff from the Hilo office of the Natural Area Reserves System, part of the Department of Land and Natural Resources, noticed that koa trees along the Hamakua coast were losing their leaves (more properly, the phyllodes that replace true leaves as the trees mature).
What they saw, and which was later confirmed by overflights, was “a spectacular outbreak of koa looper moth, larger than ever before reported,” said Robert Peck, an entomologist with the University of Hawai`i-Hilo. In areas where the infestation was high, the outbreak “may have long-term impacts,” he added.
The moth (Scotorythra paludicola) is native to Hawai`i and found on Kaua`i, O`ahu, Maui, and the Big Island. Just 12 outbreaks have been recorded in the last 120 years, with the severity of outbreaks (and tree mortality) seeming to be a function of tree health. Mortality can range from 0 to 35 percent, Peck said – with the higher number being recorded after an outbreak at Makawao, Maui, in 1985.
A century ago, the naturalist R.C.L. Perkins wrote of one such outbreak, “Native birds, attracted in thousands by the abundance of this, one of their favorite foods, were gorged to repletion, and the starving caterpillars formed writhing masses on the ground… [T]he dropping of excrement from the trees on the dead leaves beneath made a rattling noise as of a hailstorm.”
By mid-May, the outbreak had spread – south from Hamakua to Ka`u and the Hawai`i Volcanoes National Park, then up the Kona side of the Big Island: Kona Hema, Kona Mauka, and north as far as Pu`uwa`awa`a.
Whatever prompted the outbreak, the trees seem to bounce back fairly quickly. According to Peck, at Laupahoehoe, on the Hamakua coast, refoliation began about six weeks after defoliation.
Stephanie Yelenik, a researcher with the U.S. Geological Survey’s research station in Volcano, has been looking at the effects of the outbreak on soil nutrients and the forest understory.
“Each phyllode got pumped through a caterpillar gut and came out the other end,” she said. “It was a hailstorm of frass. You could watch it collect on your lunch pail.”
The pulse of nitrogen from the frass (the polite word for caterpillar excrement) could facilitate the decomposition of the thick layers of dead phyllodes on the forest floor, she speculated. To verify this, she and a colleague are continuing to study nitrogen in the soil under the affected trees. Also, they are looking at light levels: “When the phyllodes go away, light inputs increase.”
Overall, while the pulse of nitrogen and added light could help native plants, “it might also foster more grass,” Yelenik said.
According to J.B. Friday, the extension forester with the University of Hawai`i, almost every koa tree in the Hamakua area was defoliated by the koa looper. The outbreak is the largest ever recorded, exceeding 30,000 acres, he said.
Rethinking Prickles on Native Poppy
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Kasey Barton, a botany professor at the University of Hawai`i at Manoa, is challenging the notion that a benign, predator-free ecosystem resulted in native Hawaiian plants letting down their defenses.
“We expect island plants to have lower levels of defenses than their continental relatives,” she said. Because Hawai`i has no “herbivorous mammals, mollusks, or reptiles, it leads people to believe defense traits are not needed.”
But this is based on the assumption that the pressure from herbivores is the only reason for the traits, “and generally,” she said, “this is too simple.”
For one thing, she noted, “we do have some native herbivores on the islands,” including gall-forming psyllids and moths, among other arthropods, and also now-extinct flightless birds.
Barton compared the native Hawaiian poppy, pua kala (Argemone glauca), to its “continental sister species,” the Mexican poppy. Both have prickles and latex, “but the island plant is better defended than the continental plant for both prickles and latex – the complete opposite of what you would predict.”
“My two main hypotheses for why pua kala appears to be better defended than the Mexican poppy are that, first, herbivore pressure has been historically strong in Hawai`i on the poppy,” Barton said in a follow-up email to Environment Hawai`i. “It may have had high levels of damage by native insects (especially beetles) and the extinct geese. And two, the prickles may also serve the function of photoprotection and water balance via light reflection and an increase in the boundary layer. I’m researching that aspect this summer to examine the physiological benefits of prickles.” The first hypothesis “is, of course, impossible to test directly.”
Shoots vs. Roots
One under-appreciated defense that Barton is studying is that of tolerance, the ability of plants to survive despite damage. “Seedlings of weedy plants are extremely plastic,” she said. “They can minimize root growth to maximize their above-ground growth following damage, which allows them to recover quickly and have similar growth to undamaged plants.” By “weedy,” Barton later explained, “I do not necessarily mean invasive – just plants with weedy phenotypes (fast growth rates, usually annuals, often found in disturbed habitats).”
Barton’s current research involves studying in her labs the tolerance to herbivory of thousands of seedlings of many native Hawaiian species (including `ohi`a, mamaki, akolea, and haha). She expects their tolerance of herbivore pressure to be low, “since Hawai`i lacks the classic seedling herbivores (mammals, reptiles, herbivorous mollusks).”
In connection with the herbivore pressure, Barton is also studying the effects of drought. “Seedling tolerance to herbivory involves a reduction in root growth in order to maximize shoot growth,” she said, “while drought tolerance involves the opposite pattern – an increase in root growth. Thus, drought may make Hawaiian plants especially intolerant of seedling herbivory.” In other words, by investing in root growth in response to drought pressure, plants are constrained in their ability to reallocate growth aboveground to replace leaf tissue lost to herbivores.
“But these are just predictions at this point,” Barton said. “I’m just starting trials right now with about 2,000 seedlings in pots and another 1,000 to 3,000 to go over the next couple of months.”
“I do hope that this will inform us about restoration by highlighting the effect of climate change on seedling-herbivore interactions and the need to manage for multiple factors simultaneously because of their interactive effects,” she said.
At the same time that she is studying native seedling tolerance, she will be conducting similar experiments on invasive plants to determine whether they are more tolerant of seedling damage and drought than native Hawaiian plants.