The impact of an invasive African bunchgrass (Pennisetum setaceum) on water availability and productivity of canopy trees within a tropical dry forest in Hawaii

Abstract

1. Tropical dry forests are among the Earth's most threatened ecosystems. On the Island of Hawaii the African bunchgrass Pennisetum setaceum (fountain grass) dominates the understorey of the few remaining fragments of native dry forests and is contributing to the degradation of this once diverse ecosystem. In this study, we examined the impacts of Pennisetum on water use and productivity of the dominant native canopy tree, Diospyros sandwicensis. 2. Over a 3-year period, measurements were made on tree growth rates, and physiological and morphological responses of the most common dry forest native tree, D. sandwicensis, growing with an understorey dominated by Pennisetum, and on trees growing in plots maintained free of grasses. 3. Analysis of stable oxygen isotope ratios indicated that trees growing in the absence of Pennisetum used a higher proportion of water from shallow soil sources. They also sustained higher mid-day water potentials, especially during drier periods. At the leaf level, no significant differences were found in gas exchange measurements between Diospyros trees growing with or without Pennisetum. However, trees growing without Pennisetum had 30% lower leaf mass per unit area and 40% higher diameter growth than trees growing with Pennisetum. 4. These results demonstrate that invasion by Pennisetum has a pronounced negative impact on resource acquisition and use by the dominant native tree of this dry forest ecosystem. Although tree death due to these impacts would be much slower than through more immediate processes such as fire and grazing, our findings suggest that long-term conservation of ecosystems threatened by invasions, such those in Hawaiian dry forests, will ultimately require active management of the invading species. © 2008 The Authors.