For years, Susan Cordell has been studying Hawai`i’s dry forests and looking for ways to break what she calls the “grass-fire cycle.” Broadly speaking, that is when grasses get the upper hand over native dry-forest vegetation through the double-whammy of grazing animals and fires.
But now, she told the 200 or so people attending the sixth annual Nahelehele Dryland Forest Symposium, she and colleagues at the U.S. Forest Service’s Institute of Pacific Islands Forestry (IPIF) and the University of Maryland have come upon “a natural fire cycle.”
“The current dogma,” she said, “is that wildfire was a rare disturbance factor in shaping succession and community structure in dry forests. Fires occurred in forests prior to human arrival, yet little is known about the fire history.”
To gain a better understanding of the role of fire in dry forests before the arrival of humans, Cordell, Amanda Uowolo, of IPIF, and Kealoha Kinney of the University of Maryland painstakingly excavated soil pits in an area of Pohakuloa Training Area, in the saddle between Mauna Kea and Mauna Loa on the Big Island. Their work, which examined charcoal from the pits to determine the composition of plants, took them back in time to the Pleistocene era, when plant communities consisted of either low-stature shrubs and grassland or mamane woodland.
“In the last 200 years,” Cordell said, “there’s been a huge amount of fires that we see in the system – a lot of anthropogenic disturbance, a lot of non-native grasses and the like. But also, over time, there’s been a change back and forth between high and low amounts of charcoal, indicating that fire has played a role over the last 8,000 years. We were pretty surprised by that.”
She and her co-workers speculate the area of native grassland they studied “served as a highway for lava flows,” with the mamane shrubland possibly persisting in areas that were more isolated from the effects of lava flows.
Eight millennia in the past, evidence of the presence of alahe`e was found in the charcoal. The finding, Cordell said, was “very strange. This is very much outside the predicted range of alahe`e.”
Either the finding is in error, or “the climate is very different now from what it was eight thousand years ago,” she said.
The research has led Cordell and colleagues to think that “the role of nutrients is probably more important than we originally thought,” a hypothesis that was supported by research conducted after the devastating 2010 fire that occurred in the mamane woodland area adjoining the Saddle Road.
“After the fire, we thought it was the perfect opportunity to look at post-fire restoration,” she said. “The ground was covered with mamane seeds.” She and her colleagues fenced off several areas and seeded them with native species, both in the burned areas and control areas nearby that were not burned. “We got data on the available phosphorus and nitrogen,” she continued, noting there was “a big difference between the burned and unburned areas,” with nitrogen spiking in the burned areas, but phosphorus levels depressed.
“Phosphorus likely limits woodland growth, especially in a mamane dominated system because this species is a nitrogen fixer and likely requires a substantial amount of phosphorus to persist…. This tells us that burned areas are highly unsuitable for the recovery of mamane systems.”
In areas of dodonea (a`ali`i) shrubland, “there was an order of magnitude even less phosphorus,” so low, in fact, that “they may not support trees.”
“Can we ever go back to mamane tree land after repeated fires?” Cordell asked. “They’re nitrogen-fixing trees that require phosphorus. We may need to think of things like fertilization if we’re trying to promote mamane back into these systems.”
Next steps for Cordell and her colleagues is to experiment with applying fertilizers – nitrogen, nitrogen and phosphorus, and phosphorus alone – to experimental and control plots, seed them with natives, and see what transpires.
* * *
Can Mamane Woodlands
Make a Comeback?
Steve Hess, a research biologist with the U.S. Geological Survey’s Pacific Island Ecosystems Research Center, has been studying the subalpine vegetation on Mauna Kea for years. One of the questions he has been addressing is why the mamane woodland exists at all.
“Why is this woodland instead of grassland, or savanna, or steppe – or even thorn steppe?” he asked the audience at the dry forest symposium. (When looks of puzzlement crossed the faces of many of those in the audience, Hess explained the thorn steppe with one word: “gorse.”)
The mamane-dominated woodland was in a precarious position, he continued. Thanks to centuries of depredation by grazing animals and resulting erosion, the organic soil layer that once covered the slopes has disappeared: “Whatever the old soil used to do isn’t done now.” Precipitation – averaging half a meter a year – quickly penetrates what soil is left, making fog-drip under the tree canopy all the more important. “Fog-drip adds 38 percent more precipitation under the tree canopy,” he noted.
Any minor change in climate, precipitation, or temperature in the mamane woodland could “bump that place into a different type of life zone.” And any number of different factors could become that “tipping point:” exotic grasses (“they are very good at robbing moisture, suppressing tree regeneration, and generating fine fuels”); fires (“resulting in the short-term loss of mature trees”); ungulates (“mammalian herbivory is non-existent in the evolutionary history of the mamane woodland, ungulates also pretty clearly suppress tree regeneration, and they alter nutrient cycling”); and finally climate (“long-term temperature and precipitation trends can change, plus you have local loss of fog-drip interception once you lose the tree canopy cover”).
Possible outcomes could transform Mauna Kea into a Yellowstone or Serengeti of the Pacific. “Both are at similar elevations – one tropical, one temperate, both volcanic,” he said. And both have grazing animals.
“Grazers stimulate grass biomass and result in a positive feedback in nutrient cycling,” Hess said. “When grazers crop grasses, plants allocate their energy into roots and less into leaves. So if we look at mamane regeneration and grass cover, we find that where there was more than 60 percent grass cover, mamane weren’t penetrating.”
What you end up with, said Hess, is what he has dubbed “The Ultimate Grass/Fire/Ungulate/Climate Cycle.”
“Ungulates suppress trees and stimulate grasses. Grasses suppress tree regeneration and increase fuels. Fires then promote the loss of the tree canopy and favor pyrogenic grasses. The climate changes, since you get reduced fog-drip interception. You go from woodland, to savanna, to grassland.”
Hess discussed the state’s efforts to remove feral sheep and mouflon from Mauna Kea, displaying a chart that showed increasing numbers of animals removed each year by state-sponsored aerial hunts and public hunting. Since 2005, he noted, “there’s been a dramatic uptick in numbers, with 260 additional sheep per year.”
“We can’t make inferences about the total population,” he said, “but clearly, it’s sufficient to sustain this level of harvest and suggests the population might even be growing.”
“The whole system is a grazing system now,” Hess concluded. “To restore it, you’re talking about going back to a non-grazing system. Quite a few elements would have to be removed to restore that. It’s a big challenge, certainly. But there might be some ways to do that. We don’t know. It really hasn’t been tried. How do you go from having a non-grazing system, then to a grazing system, and then back again?”
* * *
Warning: Be Careful
What You Wish For
To Donald Drake, a professor of botany at the University of Hawai`i at Manoa, the idea that all alien species are bad is not terribly helpful, at least when it comes to promoting the survival of functioning Hawaiian ecosystems.
Take, for instance, the role of honeybees, not native to these parts. At low elevations, he and his colleagues have found, “honeybees are the most important flower visitors at low- to mid-elevations on Mauna Loa, but beyond 1,500 meters elevation, they drop out and the native Hylaeus bees take over.” For native plants that rely on bees for pollination below 1,500 meters, honeybees are key.
“Even within relatively homogeneous plant communities, as you move from place to place across the landscape, the set of pollinators varies,” he told the crowd at the dry forest symposium.
Drake presented a “pollination web” for Pu`u Wa`awa`a, showing pollinators (native and non-native) on one side, and plants (native and non-native) on the other, with lines showing the connections between them. “Alien animals visit all plants, both native and alien,” he noted, but the pattern was very different for native animals, which “visit almost exclusively native plants. They don’t get any resources from alien plants.”
“Does that mean alien plants are not good for native animals?” he asked rhetorically.
“Not necessarily,” he answered. “If you eliminate all alien plants, there is no way to know whether some of the alien animals might put all their attention on the native plants and become strong competitors with native animals.”
On the other hand, if you eliminated all the alien animals, you would eliminate the pollinators of alien plants – “but the problem is, there’s a whole bunch of native plants currently pollinated only by honeybees” and other native plants that rely on alien animals for pollination. “So,” he continued, “this could have potentially negative impacts.”
Alternatively, it’s possible that “if alien animals were eliminated, native animals would start visiting native plants more.”
It is not enough that birds and bees simply visit plants to pollinate them, he noted. It matters just as much how they take the nectar. “On Kaua`i,” Drake said, “the white-eye is the most important pollinator of Cyanea leptostegia [a lobeliad],” which it approaches from the front. “For another lobeliad, Clermontia fauriei, it approaches from the back, stealing nectar without achieving pollination.”
Even rats, almost universally reviled as seed-destroyers, can promote dispersal of some native species, Drake said, adding, “They’re not just seed-demolishing machines.”
The take-home lesson, he concluded, was that to avoid unwelcome surprises resulting from poorly thought-out management actions, “you need to study the full range of alien interactions” with natives. “If you don’t have a good understanding of interactions before you undertake management, you can end up with some nasty surprises.”
— Patricia Tummons
Volume 22, Number 10 April 2012