PARENT SESSION
Oral Session #39: Invasions: Effects of invaders -- Aquatic, marsh, riparian. Presiding: C. Kolar.
Wednesday, August 8, 2001. 8:00 AM to 11:45 AM. Madison Ballroom C.
Effects of a rapid expansion of Sphagnum in Hawaiian wet forest ecosystems.
Hotchkiss, Sara1, Vitousek, Peter2, 1 2
ABSTRACT- The addition or loss of species can influence the structure and function of ecosystems dramatically. It is less clear what factors can trigger a change in the abundance of a native species with ecosystem-level effects. We investigated the effects of a rapid increase in abundance of Sphagnum on the composition, structure, and dynamics of Hawaiian wet forest ecosystems. Although traces of Sphagnum occur in 20,000-yr-old peat from the Kohala Mountains, analyses of 210Pb and 137Cs show that the thick layer of Sphagnum peat at the surface has accumulated within just a few decades. Sphagnum biomass estimates from four sites range from 30,000 to 50,000 kg/ha, with 26 to 85% cover. Growth rates are about 3cm/yr at one site. On Mauna Kea where small patches appear to be spreading, biomass is about 3000 kg/ha with 14% cover. Wet forests on 400,000-year old substrates with and without Sphagnum have 50% lower tree canopy height with Sphagnum, and 38% lower N and 29% lower P content in leaves of the dominant tree, Metrosideros polymorpha. Six Kohala sites show a negative correlation between Sphagnum percent cover (3-80%) and maximum tree canopy height, abundance and species richness of bryophytes, and abundance of some ferns and vascular plants. Pollen analysis from three sites implies canopy opening at or just before the onset of major Sphagnum accumulation, suggesting disturbance as the trigger for Sphagnum's increase.
KEY WORDS: Sphagnum, Hawaii, species effects on ecosystems, paleoecology