Hawai`i\u2019s forests are under siege. The dry forests have long been held up as the poster child for endangered native landscapes, but a recent study suggests the islands\u2019 wetter rain forests face imminent peril as well.<\/p>\n
\tThe study\u2019s principal author, Gregory Asner, said in an interview that the aerial research he and his team have done carries a clear and urgent message for anyone involved in managing Hawai`i\u2019s mauka lands: \u201cWe should do a full-court press now, or get out of the business.\u201d<\/p>\n
\tWhat Asner and his team have done is describe in heart-breaking detail the way in which invasive plants are restructuring native rain forests. As Julie Denslow, a botanist who has studied invasive plants for decades, says, the study doesn\u2019t really surprise anyone who has conducted research on the ground, but it \u201cdramatically confirms impressions from the ground that invasive exotic species are changing the structure of Hawaiian forests in a major way.\u201d<\/p>\n
\tAuthors of the study included Asner and Robin Martin, both of the Carnegie Institution of Washington, based at Stanford University, Stanford\u2019s Peter Vitousek, and Flint Hughes of the U.S. Forest Service\u2019s Institute of Pacific Islands Forestry, based in Hilo. Using an array of sophisticated instruments, the team conducted aerial surveys of over a million acres of forested lands at elevations ranging from the coast to the tree line. All the areas surveyed were in one or another category calling for protective management, such as state-owned Natural Area Reserves or federal National Park Service lands.<\/p>\n
High-Flying High Tech<\/i><\/b><\/p>\n
One of the most remarkable things about the research is the technology it uses. Over the last decade, Asner has worked to develop a means of studying forests using remote sensing technology, including satellites and planes. Satellites provide too little detail, however, and even high-resolution digital photographs from overflights don\u2019t provide enough information to be able to identify vegetation at the level of its species composition.<\/p>\n
\tBut by combining a high-powered laser system (Light Detection and Ranging, or LiDAR) with souped-up spectrometers, Asner has overcome both those drawbacks. A small plane outfitted with the instruments as well as a precise Global Positioning System is used as the research platform. From an altitude of 15,000 feet, the High-Fidelity imaging spectrometers (HiFIS) are able to capture and analyze light reflected from some 100,000 laser beams a second that are directed at the forest canopy. Part of the light in each beam is bounced back to the spectrometers each time it hits a leaf or a branch, but what isn\u2019t reflected continues down to the next layer of the forest, stopping only when it reaches the underlying terrain.<\/p>\n
\tThe result is a set of data from which one can reconstruct a three-dimensional image of the forest. The information gleaned from the lasers details the physical shape of the canopy and understory, as well as the terrain itself. By analyzing the reflected light, the spectrometers provide detailed information on the chemical makeup of vegetation at every level of the forest. Matching that profile against the unique chemical signatures of common native and invasive plants, the researchers are able to come up with maps that show precisely where Hawai`i forests are succumbing to the invaders.<\/p>\n
\tAfter a year of work, the researchers honed in on 10 forested areas, ranging in size from 125 to 250 acres, representing five \u201cpairs\u201d of native-dominated and nearby alien-dominated forest. The invaded areas were analyzed for the presence of five invasive species: albizia (Falcataria moluccana<\/i>), tropical ash (Fraxinus uhdei<\/i>), kahili ginger (Hedychium gardnerianum<\/i>), faya tree (Morella faya<\/i>), and strawberry guava (Psidium cattleianum<\/i>).<\/p>\n
Theme and Variations<\/i><\/b><\/p>\n
What they found was that the five invading species wrought substantial changes in the physical makeup of the forest in four different ways.<\/p>\n
\tForests invaded by the tropical ash and faya trees had much denser high canopies than those found in the typical native forest, dominated by `ohi`a (Metrosideros polymorpha<\/i>), the study notes. No more than 4 percent of incoming light was able to penetrate the canopies of mature tropical ash or faya tree, the study found, whereas in adjacent native forests, up to 13 percent of light penetrated the canopy. The mid-story species present in typical native forests, consisting of tree ferns and native shrubs, were all but absent in the invaded forests, while ground-covering ferns and native seedlings, present in the native forests, were detected far less frequently in the invaded areas.<\/p>\n
\t\u201cIn sum,\u201d the authors write about the submontane and montane forests invaded by ash and faya tree, \u201cthese tall, high-leaf-volume invaders create a biologically impoverished environment beneath their canopies.\u201d<\/p>\n
\tThey found a second pattern of invasion on the eastern flank of Kilauea volcano, where kahili ginger has become a problem. Compared with native forests, those invaded by the ginger had three times as much volume in the understory, but native plant volume in the mid-canopy area was two thirds of what it was in the native forest. The ginger forms \u201can impenetrable layer of rhizomes, which is likely the cause of the losses\u201d in mid- and ground-level native species, the scientists write. In addition, they note that other research has already shown that the ginger reduces the nutrient concentrations in trees and depresses the regrowth of native canopy species.<\/p>\n
\tThe invasion of lowland forest in the Puna area by albizia represented yet a third type of invasion. Here, they write, even though the trees reach heights of 100 feet or more, the canopies are more open, allowing up to 21 percent of incoming sunlight to pass through. This, in turn, allows strawberry guava to form dense stands in the midcanopy layer, which in turn shut out almost all light to the ground. \u201cOur airborne measurements showed that this combination of invaders change the 3D volume of the forest by +55%, -66%, and -29% at upper, middle, and lower canopy levels, respectively,\u201d they write.<\/p>\n
\tThe results of Asner\u2019s team \u201cstrongly suggested\u201d that the secondary invasion of guava makes it all but impossible for native regeneration in the understory level. In those rare instances where albizia was present without strawberry guava, \u201cwe found both native and non-native understory plants growing in high densities and abundances.\u201d<\/p>\n
\tThe last type of invasion is that of strawberry guava alone. In more than 200 acres, strawberry guava was found to have reached the upper canopy, with mature `ohi`a trees \u201cprotruding very sparsely \u2026 through the alien canopy.\u201d<\/p>\n
\tBoth airborne and on-the-ground surveys showed that the strawberry guava invasions allowed only 5 percent of incoming light to get into the understory. \u201cIn this scenario, the invasive tree grows within a native canopy, closing at the midlevel position, and precluding the establishment and regeneration of other species.\u201d<\/p>\n
Management Implications<\/i><\/b><\/p>\n
Asner finds the results alarming. \u201cBig island reserves are clearly endangered,\u201d he said. \u201cGood grief! It ought to be on the top of everybody\u2019s list in Washington,\u201d he added. At some point, a threshold will be crossed when forest function is permanently impaired by changes in three-dimensional structure, he said \u2013 and forests in Hawai`i are approaching it at a precipitous clip.<\/p>\n
\tDenslow, recently retired from the Forest Service\u2019s Institute of Pacific Islands Studies, agrees. \u201cAlterations in forest structure cascade into impacts on ecosystem processes,\u201d she says. \u201cSeedling establishment, species composition, and resources available to bird and insect communities will be affected as a consequence.\u201d<\/p>\n
\tThe work of Asner and his colleagues \u201cis one of the first illustrations of the impacts of invasive species on native, intact tropical forest at a large scale,\u201d she says. \u201cIt highlights the vulnerability of even protected areas and otherwise healthy forests to invasive species, and the need for aggressive management to maintain conservation values.\u201d<\/p>\n
\tOne of the great values of the technology, in Asner\u2019s view, is that it can point managers in the direction of problems that, if caught early enough, can head off larger consequences. \u201cEvery time we fly over a place, we could change the management plan,\u201d based on what is learned, Asner said. \u201cThis would be an adaptive management strategy, if the state adapted to what we see.\u201d<\/p>\n
\tRhonda Loh, natural resources manager for Hawai`i Volcanoes National Park, was enthusiastic about the implications of Asner\u2019s work. \u201cThis underscores the need for landscape-scale management, using biocontrol,\u201d she said in a phone interview. \u201cBut just as important, it provides a technique for early detection. That\u2019s where we see a lot of promise. In areas where we have only a few small individuals [of invasive plants], we now have a technique that allows us to find them without having to walk every square foot.\u201d<\/p>\n
\tLoh identified yet another benefit: \u201cIt gives us a better understanding of how invasives affect basic ecosystem properties, such as movement of water and nitrogen through the system. So it can give us an idea of how well some of our native canopy species respond after the removal of ginger.\u201d<\/p>\n
\tPaul Conry, head of the Department of Land and Natural Resources\u2019 Division of Forestry and Wildlife, which oversees management of most of the areas surveyed in the study, had not seen the article. In a phone interview, he told Environment Hawai`i he wanted to have similar surveys of Hawai`i\u2019s forests done statewide. \u201cOne of the things we are trying to pursue, both nationally and locally, is to see this technology be adopted more for general survey and inventory uses,\u201d he said. \u201cWe\u2019re working with the Forest Service to see if we can get funding to have a forest inventory analysis done in Hawai`i, one of three states where this hasn\u2019t been done, so it will be possible to get this quality of information\u201d for the entire state, he said.<\/p>\n
\tAsked whether the specific information in the article would be translated into action anytime soon, Conry suggested it would\tbe used in developing management plans. \u201cThis fits into our watershed partnerships and their management programs, the NARS [Natural Area Reserve Systems] management plans\u2026 We\u2019ve got the partnerships that can now begin to take this information.\u201d<\/p>\n
\t\u201cWhere you\u2019ve got it, we should use it. Where we don\u2019t have it, we should see if we can get the resources to provide better coverage.\u201d<\/p>\n
\tIn the meantime, Asner\u2019s group has already begun working with DOFAW to help plan fencing in the Laupahoehoe Natural Area Reserve. By controlling the movement of pigs, the state hopes to reduce the spread of strawberry guava in the area.<\/p>\n
— Patricia Tummons<\/p>\n
Volume 18, Number 10 April 2008<\/p>\n","protected":false},"excerpt":{"rendered":"
Hawai`i’s forests are under siege. The dry forests have long been held up as the poster child for endangered native landscapes, but a recent study suggests the islands’ wetter rain forests face imminent peril as well. The study’s principal author, … Continued<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[164],"tags":[],"class_list":["post-1231","post","type-post","status-publish","format-standard","hentry","category-april-2008"],"_links":{"self":[{"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=\/wp\/v2\/posts\/1231","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=1231"}],"version-history":[{"count":0,"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=\/wp\/v2\/posts\/1231\/revisions"}],"wp:attachment":[{"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1231"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1231"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1231"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}