The Study Site
The Mākaha ahupua‘a is located within the Wai‘anae moku (district) on the Leeward Coast of O‘ahu Island, Hawai‘i. Our study site was a subwatershed within this ahupua‘a.
Vegetation of Mākaha Valley
Using the Hawai‘i vegetation classification described by Gagne and Cuddihy (1999), the main vegetation community types in our study area include Lowland Mesic Shrublands, Forests, and Mixed communities. Lowland communities are found from 15-2,000 meters in elevation (49-6562 ft). The term mesic describes areas receiving 1,200-2,500 mm (47-98 inches) of rain per year.
Alien vegetation dominates Mākaha Valley below 600 m (1969 ft) and is scattered up to the top of Mt. Ka‘ala. The most common alien plants are coffee, Christmas berry, and strawbery guava. Other important nonnative species include Australian red cedar, common yellow guava, Formosan koa, kukui, and silk oak. Most of these nonnative species are classified as invasive by Pacific Island Ecosystems at Risk (PIER). Additionally, the Hawai‘i/Pacific Weed Risk Assessment determined most of them to be pests (PIER 2006). Remaining patches of native forest are dominated by lama and ‘ōhi‘a. Rare Hawaiian plants including flowering maple, māhoe, na‘ena‘e, and ma‘aloa can still be found within Mākaha Valley. However, invasive species continue to aggressively spread into native communities (Joel Lau, personal communication).
*To see a partial list of scientific and common names for plants in Mākaha, please see the Reference section.
Water and invasive species: potential problem in the area?
Located on the dry leeward coast of O‘ahu, most of Mākaha Valley usually receives less than 1800 mm (71 inches) of rain per year (NCDC 2004). Stream flow rates have been declining since the 1960s (Mair et al. 2007). Potential reasons for these rate decreases include decreasing rainfall, groundwater pumping, and changes in water use by vegetation.
On Lana‘i, studies (Stratton and Goldstein 2001, Stratton et al. 2000) concluded that Christmas berry uses more water during the wet season than native Hawaiian plants. Research in Africa has shown that invasive species can decrease steamflow (Le Maitre et al. 2002, Le Maitre et al. 1996) and ground water recharge (van Wilgen et al. 2008). There is little information about the impact of nonnative species on water resources in Hawai‘i.
A partnership to study invasive species in Mākaha Valley
Mohala i ka Wai and the HBWS formed a partnership in September 2000 in order to address community concerns about access to water resources and the preservation of the Mākaha watershed. They are also interested in knowing what native and invasive communities exist in Mākaha Valley and where these communities are located.
The role of remote sensing
It is widely recognized that remote sensing analysis is important in the effort to manage invasive species. Remote sensing has been used to map plant communities throughout the world.
We believed that remote sensing would be a valuable tool which could be used to map Mākaha's native and nonnative plant communities and provide useful information to the partnership.
To learn more about remote sensing, please see our short introduction.
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