Agronomic qualities of Pennisetum clandestinum in association with Acacia koa for the mitigation of the effects of ranching and promotion of conservation in a silvopastoral context on the island of Maui, Hawaii

Abstract

ABSTRACT Upper-elevation mesic Hawaiian forests have been transformed by cattle ranching and invasive ungulates within the past 200 years. Although the flora and fauna of the Hawaiian Islands have been radically altered by the introduction of invasive species, there is a movement of agriculturalists within the islands interested in reintroducing native species and reforestation in agricultural spaces. One of the keystone species in the Hawaiian Islands is the endemic tree species Acacia koa, known for its ecosystem services and cultural importance. This species is valuable for nitrogen fixation, aquifer recharge, habitat for biodiversity and usefulness in agroforestry systems. This investigation is focused on the relationship between the tree species A. koa and the forage grass species Pennisetum clandestinum (kikuyu) in a silvopastoral context, at an elevation of 5,200 feet above sea level (1585 masl) on the island of Maui. The site of investigation is called Pu’u Makua in Hawaiian and is situated on the southwestern flank of Haleakalā (10,023 feet above sea level–3055 masl) and is part of Ulupalakua Ranch. A total of n = 28 forage samples of P. clandestinum were taken in dry season, 21 of which were taken from underneath the canopies of koa trees (which had been planted in densities of 3, 4, and 6 m apart). The other seven samples were taken in full sun (where no koa trees were growing). This same protocol (sampling n = 28) was repeated in the wet season. Forage samples were then sent to the Agricultural Diagnostics Laboratory at the University of Hawai`i at Manoa for forage analysis to compare attributes of kikuyu under different canopy densities of A. koa. To better understand the leaf area index (LAI) of the canopy of various densities of A. koa, hemispheric (180 degree) photos were taken and then input into Gap Light Analyzer Software for LAI. LAI data of A. koa as well as forage attributes of P. clandestinum were then input into the statistical software InfoStat to investigate relationships between canopy cover and forage quality at each specific location. It was found through Pearson correlation analysis that there are in fact significant relationships between koa and the quality of kikuyu. Measurements of koa trees were also taken to estimate biomass and the amount of carbon sequestered within each tree. These measurements were input into allometric equations specific to koa growing on the Island of Maui that were created by the United States Forest Service. Lastly, to better understand the knowledge of ranchers on Maui, we conducted surveys regarding various ranchers’ perceptions of native and non-native trees and their usefulness for silvopastoral ranching systems. Ranchers also identified various tree species that were of use (or harmful) to their operations. Since A. koa was on this list of helpful trees and conferred various ecosystem services and agroecological benefits, the incorporation of koa into ranching systems on the island of Maui is recommended in zones appropriate for its growth.