Hawai'i conservation alliance

2009 Runner-up Best Student Poster Presentation

Effects of light availability on biomass and reproductive organ production of the invasive rangeland shrub Ulex europaeus L. on Mauna Kea, Hawaii

Cheyenne Perry 1, Cheyenne Perry3, Mike Robinson2, Rebecca Ostertag1, Pat Hart1
1 University of Hawai‘i, United States, 2 Department of Hawaiian Home Lands, United States, 3 US Forest Service, United States

Ulex europaeus (gorse) are thorny shrubs found in degraded pasturelands, an extreme fire hazard, and noxious weeds in Hawaii. They form monotypic stands infesting thousands of acres and alter native ecosystems, produce highly acidic soils, and suppress native plant germination, such as at Mauna Kea, Hawaii. However, gorse is light demanding so the addition of forest canopy over gorse stands may be an effective bio-control.  I seek to determine the percentage light availability provided by forests and plantations that produce significant decreases in gorse biomass accumulation and reproductive organ production. 

I surveyed forests and plantations on the eastern slopes of Mauna Kea to determine light availability under different species’ canopies, then tested whether artificial shading (shade cloth structures) that mimics these forests would limit gorse growth and reproduction. I created and applied five light treatments to gorse individuals/cohorts: ambient light (control), 73%, 80%, 90%, and 98% shade and measured basal area, height and volume of 80 plants for 12 months. Following the final measurement all plants were harvested to determine dry mass. I used allometric modeling (16 individuals of differing size) to estimate initial biomass and actual dry weight of harvested plants for final biomass to compute Relative Growth Rates. Results show that the 73% shade treatment decreased biomass accumulation to 1/3 that of the control and that this trend continued up to 98% where gorse individuals/cohorts died and biomass was reduced to just 1/10 that of the control. These initial results suggest that planting native forests and/or plantations with contiguous canopies in gorse stands will reduce biomass accumulation.