For years, Susan Cordell has been studying Hawai`i\u2019s dry forests and looking for ways to break what she calls the \u201cgrass-fire cycle.\u201d Broadly speaking, that is when grasses get the upper hand over native dry-forest vegetation through the double-whammy of grazing animals and fires.<\/p>\n
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\u2019s Institute of Pacific Islands Forestry (IPIF) and the University of Maryland have come upon \u201ca natural fire cycle.\u201d<\/p>\n
\u201cThe current dogma,\u201d she said, \u201cis 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.\u201d<\/p>\n
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.<\/p>\n
\u201cIn the last 200 years,\u201d Cordell said, \u201cthere\u2019s been a huge amount of fires that we see in the system \u2013 a lot of anthropogenic disturbance, a lot of non-native grasses and the like. But also, over time, there\u2019s 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.\u201d<\/p>\n
She and her co-workers speculate the area of native grassland they studied \u201cserved as a highway for lava flows,\u201d with the mamane shrubland possibly persisting in areas that were more isolated from the effects of lava flows.<\/p>\n
Eight millennia in the past, evidence of the presence of alahe`e was found in the charcoal. The finding, Cordell said, was \u201cvery strange. This is very much outside the predicted range of alahe`e.\u201d<\/p>\n
Either the finding is in error, or \u201cthe climate is very different now from what it was eight thousand years ago,\u201d she said.<\/p>\n
The research has led Cordell and colleagues to think that \u201cthe role of nutrients is probably more important than we originally thought,\u201d a hypothesis that was supported by research conducted after the devastating 2010 fire that occurred in the mamane woodland area adjoining the Saddle Road.<\/p>\n
\u201cAfter the fire, we thought it was the perfect opportunity to look at post-fire restoration,\u201d she said. \u201cThe ground was covered with mamane seeds.\u201d 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. \u201cWe got data on the available phosphorus and nitrogen,\u201d she continued, noting there was \u201ca big difference between the burned and unburned areas,\u201d with nitrogen spiking in the burned areas, but phosphorus levels depressed.<\/p>\n
\u201cPhosphorus 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\u2026. This tells us that burned areas are highly unsuitable for the recovery of mamane systems.\u201d<\/p>\n
In areas of dodonea (a`ali`i) shrubland, \u201cthere was an order of magnitude even less phosphorus,\u201d so low, in fact, that \u201cthey may not support trees.\u201d<\/p>\n
\u201cCan we ever go back to mamane tree land after repeated fires?\u201d Cordell asked. \u201cThey\u2019re nitrogen-fixing trees that require phosphorus. We may need to think of things like fertilization if we\u2019re trying to promote mamane back into these systems.\u201d<\/p>\n
Next steps for Cordell and her colleagues is to experiment with applying fertilizers \u2013 nitrogen, nitrogen and phosphorus, and phosphorus alone \u2013 to experimental and control plots, seed them with natives, and see what transpires.<\/p>\n
* * *<\/b><\/p>\n
Can Mamane Woodlands<\/b><\/p>\n
Make a Comeback?<\/b><\/p>\n
Steve Hess, a research biologist with the U.S. Geological Survey\u2019s 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.<\/p>\n
\u201cWhy is this woodland instead of grassland, or savanna, or steppe \u2013 or even thorn steppe?\u201d 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: \u201cgorse.\u201d)<\/p>\n
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: \u201cWhatever the old soil used to do isn\u2019t done now.\u201d Precipitation \u2013 averaging half a meter a year \u2013 quickly penetrates what soil is left, making fog-drip under the tree canopy all the more important. \u201cFog-drip adds 38 percent more precipitation under the tree canopy,\u201d he noted.<\/p>\n
Any minor change in climate, precipitation, or temperature in the mamane woodland could \u201cbump that place into a different type of life zone.\u201d And any number of different factors could become that \u201ctipping point:\u201d exotic grasses (\u201cthey are very good at robbing moisture, suppressing tree regeneration, and generating fine fuels\u201d); fires (\u201cresulting in the short-term loss of mature trees\u201d); ungulates (\u201cmammalian herbivory is non-existent in the evolutionary history of the mamane woodland, ungulates also pretty clearly suppress tree regeneration, and they alter nutrient cycling\u201d); and finally climate (\u201clong-term temperature and precipitation trends can change, plus you have local loss of fog-drip interception once you lose the tree canopy cover\u201d).<\/p>\n
Possible outcomes could transform Mauna Kea into a Yellowstone or Serengeti of the Pacific. \u201cBoth are at similar elevations \u2013 one tropical, one temperate, both volcanic,\u201d he said. And both have grazing animals.<\/p>\n
\u201cGrazers stimulate grass biomass and result in a positive feedback in nutrient cycling,\u201d Hess said. \u201cWhen 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\u2019t penetrating.\u201d<\/p>\n
What you end up with, said Hess, is what he has dubbed \u201cThe Ultimate Grass\/Fire\/Ungulate\/Climate Cycle.\u201d<\/p>\n
\u201cUngulates 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.\u201d<\/p>\n
Hess discussed the state\u2019s 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, \u201cthere\u2019s been a dramatic uptick in numbers, with 260 additional sheep per year.\u201d<\/p>\n
\u201cWe can\u2019t make inferences about the total population,\u201d he said, \u201cbut clearly, it\u2019s sufficient to sustain this level of harvest and suggests the population might even be growing.\u201d<\/p>\n
\u201cThe whole system is a grazing system now,\u201d Hess concluded. \u201cTo restore it, you\u2019re talking about going back to a non-grazing system. Quite a few elements would have to be removed to restore that. It\u2019s a big challenge, certainly. But there might be some ways to do that. We don\u2019t know. It really hasn\u2019t been tried. How do you go from having a non-grazing system, then to a grazing system, and then back again?\u201d<\/p>\n
* * *<\/b><\/p>\n
Warning: Be Careful<\/b><\/p>\n
What You Wish For<\/b><\/p>\n
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.<\/p>\n
Take, for instance, the role of honeybees, not native to these parts. At low elevations, he and his colleagues have found, \u201choneybees 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.\u201d For native plants that rely on bees for pollination below 1,500 meters, honeybees are key.<\/p>\n
\u201cEven within relatively homogeneous plant communities, as you move from place to place across the landscape, the set of pollinators varies,\u201d he told the crowd at the dry forest symposium.<\/p>\n
Drake presented a \u201cpollination web\u201d 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. \u201cAlien animals visit all plants, both native and alien,\u201d he noted, but the pattern was very different for native animals, which \u201cvisit almost exclusively native plants. They don\u2019t get any resources from alien plants.\u201d<\/p>\n
\u201cDoes that mean alien plants are not good for native animals?\u201d he asked rhetorically.<\/p>\n
\u201cNot necessarily,\u201d he answered. \u201cIf 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.\u201d<\/p>\n
On the other hand, if you eliminated all the alien animals, you would eliminate the pollinators of alien plants \u2013 \u201cbut the problem is, there\u2019s a whole bunch of native plants currently pollinated only by honeybees\u201d and other native plants that rely on alien animals for pollination. \u201cSo,\u201d he continued, \u201cthis could have potentially negative impacts.\u201d<\/p>\n
Alternatively, it\u2019s possible that \u201cif alien animals were eliminated, native animals would start visiting native plants more.\u201d<\/p>\n
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. \u201cOn Kaua`i,\u201d Drake said, \u201cthe white-eye is the most important pollinator of Cyanea leptostegia<\/i> [a lobeliad],\u201d which it approaches from the front. \u201cFor another lobeliad, Clermontia fauriei<\/i>, it approaches from the back, stealing nectar without achieving pollination.\u201d<\/p>\n
Even rats, almost universally reviled as seed-destroyers, can promote dispersal of some native species, Drake said, adding, \u201cThey\u2019re not just seed-demolishing machines.\u201d<\/p>\n
The take-home lesson, he concluded, was that to avoid unwelcome surprises resulting from poorly thought-out management actions, \u201cyou need to study the full range of alien interactions\u201d with natives. \u201cIf you don\u2019t have a good understanding of interactions before you undertake management, you can end up with some nasty surprises.\u201d<\/p>\n
— Patricia Tummons<\/p>\n
Volume 22, Number 10 April 2012<\/p>\n","protected":false},"excerpt":{"rendered":"
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 … Continued<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[43],"tags":[],"class_list":["post-458","post","type-post","status-publish","format-standard","hentry","category-april-2012"],"_links":{"self":[{"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=\/wp\/v2\/posts\/458","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=458"}],"version-history":[{"count":0,"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=\/wp\/v2\/posts\/458\/revisions"}],"wp:attachment":[{"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=458"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=458"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=458"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}