Tropical Ash Invasiveness In Hawaii

Tropical ash invasiveness in Hawaii: rate, environmental control, and impact on native biodiversity

J. B. Friday, Department of Natural Resources and Environmental Management, UH-CTAHR
Adrian Ares, Department of Tropical Plant and Soil Science, UH-CTAHR
Paul Scowcroft, Institute of Pacific Islands Forestry, USDA Forest Service
Sean Gleason, Department of Natural Resources and Environmental Management, UH-CTAHR

Tropical ash, a potential timber species introduced into Hawaii, can spread into native forests and possibly displace native trees. In this project, we investigate how quickly tropical ash spreads into native forest, what environmental factors control where ash can grow, and what the effects of invasion by ash are on the native forest.
Contact Dr. J. B. Friday (jbfriday@hawaii.edu)	Large (120 cm) ash planted in 1936 in Laupahoehoe Forest Reserve
OBJECTIVES

To quantify the spread of tropical ash, an introduced timber species, in the native forest of Hawaii, to assess its impact on native plant diversity, and to examine environmental factors and plant traits potentially controlling its spread. Specific objectives include testing the hypothesis that:

1) the spread of the ash was not continuous over time but occurred through pulses following disturbance events,
2) plant species diversity has decreased in the past 28 years in areas where ash occurs, and
3) spread of tropical ash is influenced by environmental factors including disturbance and native forest decline, topography, soil N and water availability, and light levels.

APPROACH

To determine the rate of spread of the tropical ash, areas occupied by the ash at different time intervals will be mapped and analyzed using a GIS system. To analyze changes in the biodiversity of the forest, a vegetation survey in the field will be compared with a previous survey done in 1971 by one of the principle investigators. To investigate environmental controls of establishment and growth of tropical ash, we will examine soil N and moisture status and ash leaf tissue nutrient and carbon isotope contents along transects through the stand. Past forest use will be gleaned from historical records. Reproduction of ash in the field will be monitored periodically in enclosures established in the forest under different light levels. Photosynthetic capacity of ash under different light levels will be determined in a greenhouse study and compared with known values for native Hawaiian plants.

Tropical ash vs. koa seedling

PROGRESS October 2000 to September 2003

Tropical ash (Fraxinus uhdei) was introduced in Hawaii in the 1930s as a timber species and has naturalized in native forests. We re-surveyed 32 plots in the Laupahoehoe forest reserve on the windward side of Mauna Kea, island of Hawaii, which were originally established after selective logging for koa (Acacia koa) in 1973 and included both native forest and ash stands. Basal area of ash increased from an average of 0.8 m2/ha in 1973 to 9.9 m2/ha in 2000. However, ash did not spread more than 500 m from where it was found in 1973. Basal area of native trees also increased slightly, from 18.9 m2/ha to 19.8 m2/ha, with koa showing an increase and Metrosideros polymorpha and other native species decreasing. Abundance of tree ferns (Cibotium spp.) decreased from an average of 47 stems/ha to 9 stems/ha. Basal area of native trees and number of understory species were both inversely related to basal area of ash. Soils were sampled along transects established running from the native forest to the ash plantation. Ash basal area was positively related to soil pH (r2 = 0.31; p = 0.0001 ) but not related to soil nutrient levels. Ash seed rain was heavy, with up to 16,000 seeds/m2 collected in six months. Seedling recruitment was prolific in gaps created in the forest canopy, with an average of 0.86 seedlings/m2 recruited after one year, but negligible under the canopy. No difference was observed in numbers of seedlings inside and outside of fenced exclosures, despite a high degree of activity of feral pigs in the area.

Seedlings of both species were grown in a greenhouse under three shade treatments: Structural and non-structural carbohydrates in roots and stems were measured to examine the carbohydrate reserve strategies of both species. Light compensation points, maximum photosynthesis, and dark respiration differed significantly among light treatments (100% sun, 60% sun and 30% sun) but did not differ between the species. A defoliation experiment indicated that ash is much better suited to survive defoliation, especially under low light conditions. Tropical ash had much higher non-structural carbohydrate concentrations in roots and stems than did koa. A significant carbohydrate species by light interaction suggests that koa may not produce enough carbon reserves to survive prolonged periods of stress particularly under low light conditions.

Litter traps set out under tropical ash canopy

IMPACTS

Our results confirm that tropical ash out-competes Hawaii's native trees but indicate that ash does not move into intact native forests as quickly as previously thought. The dominant native timber tree in the area, Acacia koa, has proved resilient and able to recover after logging except when it has to compete with tropical ash. If future logging operations are planned in areas where tropical ash occurs, foresters will need to find ways to avoid invasion of harvested areas by ash. Long-term growth data from areas of native forest will complement ongoing studies of forest growth and nutrient use in the area.

Page last updated on 2/9/2004