This essay was originally written for a botany course at the University of Hawaii at Manoa in 2015
Plants are dispersed across the global, frequently by human hands, from their native homes to new unfamiliar areas. Usually, these plants fail to survive in a foreign environment, but sometimes they survive, thrive, grow, and spread out of control. When this happens, botanists refer to it as an “invasive species.” Invasive species can have long-lasting detrimental effects on the native ecosystem they invade and are often difficult or impossible to eradicate. For this reason, the state of Hawai’i has strict agricultural regulations regarding plants brought to the islands. Unfortunately, many problematic species have already arrived.
One such invasive species found in Hawai’i is Clusia rosea, commonly known as the autograph tree. The name refers to how words and pictures scratched into the thick, leathery leaves which will remain on the leaf indefinitely (Gilman, 1993). Other common names for it include balsam apple, pitch apple, and monkey apple (Starr, 2003). It is native to Central America but has spread to Hawai’i as an ornamental species frequently planted around parking lots and parks (Maguire, 1976). It can have beautiful two to three inch pink or white flowers which bloom at night and sometimes into early morning during summer months (Gilman, 1993). The flowers eventually turn into green, poisonous fruits approximately three inches in diameter which when ripe split open on the tree into a unique star shape. This exposes the bright red seeds, surrounded by a black resin, which attract birds for dispersal (Gilman, 1993). The black resin has historically been used to caulk boats (Gilman, 1993). The tree generally grows to about twenty-five meters tall and forty to fifty centimeters around (Maguire, 1976). These features make it ideal for community decoration. Unfortunately, this exacerbates the spread by humans and should be discouraged. Plantings of native, non-invasive species should be encouraged instead, such as the Koa or Wiliwili tree.
C. rosea spreads naturally by fruit-eating birds. However, a larger concern is the spread by people who plant it ornamentally (Starr, 2003). Moreover, it is able to spread farther without encountering geographic barriers that would stop other species because it is able to survive in a wide range of conditions (Starr, 2003). It is highly resistant to drought and has better than average tolerance for aerosol and soil salt (Gilmari,1993). Light and soil requirements are equally varied. Full-sun, partial sun, and shade conditions all support C. rosea, and acceptable soil types include clay, loam, acidic, and alkaline (Gilman, 1993). This range of adaptability enables C. rosea to spread across the entire state without stopping, causing the most damage to native vegetation.
As of 2003, C. rosea has spread to Kaua’i, O’ahu, Hawai’i, Moloka’i, and Maui (Starr, 2005). On Moloka’i, C. rosea has a medium-sized distribution (Starr, 2005). On the windward side of the Big Island. C. rosea is one of the ten most abundant species in the seedling layer of forests (Mascaro, 2008). This indicates a serious spread of the species which will continue across the islands without immediate intervention.
Additionally, it is potentially the only woody, dicot angiosperm which utilizes Crassulacean Acid Metabolism (CAM) (Ting, 1985). CAM photosynthesis is typically only seen in desert climates, or other environments with sub-standard levels of water availability. CAM is less efficient than the typical C3 or even C4 photosynthetic pathways but allows a plant to conserve more water by only opening stomata at night. In Clusia rosea, CAM allows it to conserve water during its seedling stage when the aerial roots have not yet reached the soil (Starr, 2003). This enables C. rosea to have a high successful germination rate and to grow as an epiphyte. Epiphytes are plants that grow on or around other plants but does not directly derive nutrients from the host like a parasite would. Epiphytes are not inherently bad, but an invasive species like C. rosea can indirectly kill the native trees it grows on by strangling its access to water, sunlight, and nutrients. Furthermore, epiphytes are difficult to remove without killing the desirable native tree as well, so eradication of this species is especially difficult (Starr, 2005).
The control of C. rosea in Hawai’i is vital for the preservation of native forests. Unfortunately, complete eradication of this species on the islands is unlikely due to its abundance and difficulty to remove. Continued, systematic control measures, however, can reduce its impact. Control methods include early removal of seedlings from desirable trees, chemical control on larger trees, and public awareness (Starr, 2003). Care must be taken when implementing these control measures. Removal of epiphytes could damage existing native trees and chemical control always has the possibility of harming other plants or organisms if not applied properly. Overall, a combination of control methods must be employed across the state by not only professionals and organizations, but also by the community as a whole in order to preserve the natural Hawaiian landscape that we love.
REFERENCES
Gilman, E.F., Watson, D.G. (1993). Clusia rosea pitch-apple. Forest Service.
Mascaro, J., Becklund, K. K., Hughes, R. F., & Schnitzer, S. A. (2008). Limited native plant regeneration in novel, exotic-dominated forests on Hawai’i. Forest Ecology and Management, 256(4), 593-606.
Maguire, B. (1976). Apomixis in the genus Clusia (Clusiaceae). A preliminary report. Taxon, 25(3), 241-244.
Starr, F., Starr, K., & Loope, L. (2003). Clusia rosea. United States Geological Survey.
Starr, F., Starr, K., & Loope, L. L. (2005). Roadside survey and expert interviews for selected plant species on Molokai, Hawaii.
Ting, I. P., Lord, E. M., Sternberg, L. D. S., & Deniro, M. J. (1985). Crassulacean Acid Metabolism in the Strangler Clusia rosea Jacq. Science, 229(4717), 969-971.
