PARENT SESSION
Oral Session 81: Forest Ecology IV: Seeds, Growth, and Recruitment.
Presiding: J Kush
Wednesday, August 4, 1:30 PM to 5:00 PM, Meeting Room C 120.
Effects of exotic grasses on seedling establishment in mesic Hawaiian forest.
Denslow, Julie*,, Uowolo, Amanda, Hughes, Flint,
ABSTRACT- In the Neotropics, Oceania, and Australia exotic grasses, introduced largely to improve the carrying capacity of rangeland, have become major pests. In seasonally dry climates, exotic grasses increase fire frequency and intensity, alter patterns of moisture and nutrient supply, and suppress establishment of trees and forbs. In Hawaii, introduced African grasses also invade closed wet and mesic forests, under moisture and nutrient supply conditions most models predict would sustain continuous tree cover with little grass biomass. Our objective was to assess the impact of invasive exotic grasses on resource supply and seedling establishment in mid-elevation closed mesic forest on the island of Hawaii. We compared the effects of grass removal and seed augmentation under different canopy conditions to assess the effects of grass cover and dispersal limitation under differences of canopy composition, light availability and elevation. We found that, with the possible exception of Acacia koa, seedling establishment was strongly dispersal limited in these forests and that grass removal significantly improved seedling establishment under most conditions. Seedling regeneration was substantially greater for two species, Sophora crysophilla and Dodonea viscosa, under higher temperature and moisture levels at low elevation forest. Canopy openness and elevation, but not grass removal, increased soil moisture and resin-extracted NO3-N (but not PO4-P). While it is clear that grass cover depresses seedling establishment in these forests, it is likely due to the smothering effect of thatch rather than reduced moisture and nutrient supply. Multiple assaults on these forests from logging and ungulate grazing as well as the invasion of exotic grasses have reduced the regeneration potential of the stand.
Key words: invasive species, Acacia koa, Hawaii, grasses