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Hawai'i Forestry Extension

The Invasiveness of the Noxious Weed Gorse (Ulex Europaeus L.) Influenced by Symbiosis in Agricultural and Natural Habitats of Hawai‘i

Dulal Borthakur, Department of Molecular Biosciences & BioEngineering, UH-CTAHR

Gorse is a noxious weed species that is threatening natural habitats and agro-ecosystems around the world, including Hawaii. This research will identify soil nutrients and soil microbes that can promote growth infestation. We will conduct a risk assessment for gorse infestation in all major Hawaiian islands by measuring ecological indicators associated with the invasive capacity of gorse in Hawaii.

Dr. Dulal Borthakur (dulal@hawaii.edu)
Phone: 808-956-6600
Fax: 808-956-3542
FUNDING has been provided to CTAHR for this research from the USDA Cooperative State Research, Education, and Extension Service Project HAW00529-04G.


The State of Hawaii needs updated information for developing proactive strategies on the control of noxious weeds. Gorse infestation is not only restricted to the islands of Maui and Hawaii, recent observations of gorse on Molokai give a reasonable indication that all other islands are vulnerable to invasion of this noxious weed. Many legumes are recognized as pioneers of degraded and infertile landscapes due mainly to their capacity of symbiotic nitrogen fixation. This same pioneering trait is also what makes a lot of these legumes noxious and invasive weed species. We will test our hypothesis with 3 separate objectives. These objectives will measure ecological indicators associated with the invasive capacity of gorse in Hawaii. We will measure these ecological indicators within gorse-infested areas, areas of gorse pioneer introduction, areas previously eradicated of gorse, and areas that could be a potential hazard to gorse invasion.

The specific objectives are:
(i) Characterization of a gorse-infested landscape for symbiotic and edaphic properties that contribute to its dominant nature;
(ii) Risk assessment of non-infested locations by identifying symbiotic and edaphic properties conducive to gorse establishment; and
(iii) Determining the impact of gorse eradication methods on the symbiotic and edaphic properties of a recently infested location.


Experimental field sites for this objective will be established in two heavy infestation and two pioneer establishment locations. This will provide an opportunity to repeat our observations from locations on different islands. Three experimental field units will serve as replication for each location. The data collected from the different locations will establish ecological patterns that are conducive to gorse invasion. Experiments will be conducted to determine the Bradyrhizobium population densities in the soil and identify distinct phylotypes specific to gorse in different locations. Bradyrhizobium densities in the soil will be measured using the most probable number (MPN) plant infection technique. Soil samples will be collected within the same experimental field units to determine the influence of gorse in altering the soil properties and the edaphic conditions that favor pioneer establishment. We will perform a risk assessment for the invasion potential of gorse on each of the major islands in the state of Hawaii. All soil samples collected from the survey will be used to identify and measure the indigenous Bradyrhizobium populations that form effective symbiosis on gorse. This research will measure the long-term effects of the previously applied treatments for gorse control, which include fire, herbicide applications, and forestry planting.


2005/10 TO 2006/09

Gorse is a woody leguminous shrub that is endemic to Western Europe and the Mediterranean, but has become a major cosmopolitan weed with infestations existing in at least 10 countries. To determine the effects of gorse infestations in different geographical locations, separate field analyses were conducted on Mauna Kea, Hawaii and Bank's Peninsula, New Zealand. At both sites, three experimental units were identified and consisted of a high-density gorse population (HD) zone and an adjacent low-density gorse population (LD) zone. Furthermore, the LD zones in Hawaii were areas with young pioneer gorse individuals in early succession, while the LD zones of New Zealand were former infestations in late succession. Soil, plant, and bacterial samples were collected from all zones at both locations. Soil nutrient status was the best indicator of gorse impacts on the landscape. Soil pH was significantly lower in the HD zones for both sites. Concentrations of Ca and Mg were also significantly higher in the LD zones for both sites, while Al concentrations were significantly higher in the HD zones. Plant Al concentration was significantly higher only in the LD zone in New Zealand, but not in Hawaii. This was the only significant difference for all tissue nutrients for both sites. This suggests that changes to the soil are dependent on gorse plant densities (i.e. infestation) and reversion of soil nutritional status may be a successional trait of the landscape. This work also suggests that Al accumulation by gorse in late succession may be an indicator of decline. Bradyrhizobium isolates from Hawaii and New Zealand do not share the same Box-PCR marker profiles, while gorse and koa (Acacia koa) Bradyrhizobium isolates do. This indicates that gorse does not require specific host-symbiont relationships for symbiosis and is compatible with a wide range of indigenous Bradyrhizobium for effective nitrogen fixation. The lack of specificity for root mutualists (Bradyrhizobium) may play a major role in the cosmopolitan success of gorse invasion. Regardless of the geographic location, soil nutrition and symbiosis appear to be important factors to consider when developing preventative or responsive control strategies.


2005/10 TO 2006/09

This research initiative will promote the revitalization of degraded gorse-infested lands from an environmental burden to an economic engine that will advance agriculture and watershed conservation in Hawaii.

Last Updated On 5/14/2007
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