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September 16, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Where alien grazers and grasses invade, wildfires follow

Lava and lightning occasionally ignite fires in Hawaiian forests and shrublands where grass or other fine plant fuels are abundant and when the weather is dry and windy. Fires probably burned infrequently and over small areas before Polynesians colonized the islands and began to use fire to open areas for agriculture. Most ecologists believe that fire affected Hawaiian ecosystems only slightly before humans arrived, because native vegetation, especially rain forest, is difficult to burn under normal conditions. For example, fires usually do not burn far from the edges of lava flows or from lightning strikes where the vegetation is dominated by native species. This changed dramatically as introduced grasses spread during the last century.

Today fires are larger, burn more frequently, and have dire consequences for wildland conservation and recreation. Wildfires have become serious problems in ecosystems where fire-promoting alien grasses and other weeds have spread due to damage by introduced browsing and grazing animals, such as cattle, sheep, and goats. Alien grasses were brought to Hawai`i accidentally or intentionally as pasturage for livestock or as ornamentals. They invaded our dry wildlands wherever alien browsers and grazers removed and damaged the native vegetation that protects the soil. Many alien plants carry fire into areas that otherwise would burn with difficulty if at all. Most areas become more heavily infested with alien grasses and other weeds after they have burned; consequently they become increasingly prone to fire.

The most destructive cycle of fire and alien grass occurs in North Kona and South Kohala. Here fountain grass and buffelgrass have invaded lava flows, replacing native plant species over large areas. Both grasses carry fast-moving, hot fires, and their seeds and shoots germinate soon after fire. As fountain grass becomes more common, fires occur more frequently and native shrubs and trees are eventually eliminated.

Once established, alien grasses resistant to fire and grazing are difficult to eliminate. Using cattle, sheep, and goats to reduce alien plant abundance is self-defeating in conservation areas, because these animals graze most alien grasses only after they have devoured the more nutritious native plants, such as mamane and koa. Therefore, maintaining hoofed "lawnmowers" to reduce fire fuel is counterproductive in areas where native plant protection and ecosystem repair are needed.

Recent invasions of alien grasses have escalated fire problems and damaged dry, lowland vegetation in Hawai`i Volcanoes National Park. Beardgrass and broomsedge invaded broad areas of the Park over 35 years ago, when large herds of goats were ravaging the native vegetation. Rangers eradicated goats to protect the remaining native vegetation and to start the slow process of ecosystem repair. Molasses grass and thatching grass have since joined other alien grasses and native pili grass in portions of the recovering coastal vegetation.

In addition to eliminating goats to promote native plant recovery, the National Park staff strives to limit the frequency, size, and consequences of wildfires. They reduce sources of ignition by restricting human access in some areas when weather and fuel conditions warrant. Grasses are mowed along roadsides to prevent ignition from carelessly discarded cigarettes or blazing-hot catalytic converters. Fire patrols monitor the Park to prevent, report, and fight fires, and a rapid-response team is trained and equipped to contain fires before they become large. Recognizing that fire became a serious problem when ecosystems were degraded by alien species, Park workers are vigilant against the return of goats and the further spread of introduced grasses.

Protecting native vegetation from disturbance by fire, grazing mammals, and invasive alien plants is essential to restoring the health of our island ecosystems. Hawai`i Volcanoes National Park is setting a good example of how to do this. Please do your part to prevent wildfires.

Eruption Update

A swarm of shallow earthquakes in the upper east rift zone of Kilauea Volcano between Pauahi Crater and Mauna Ulu commenced at 01:31 a.m. on Sunday morning, September 12. Many of the earthquakes were felt by residents of Volcano and the surrounding subdivisions. The earthquake activity was accompanied by rapid subsidence of the summit region, and this indicated that an intrusion was occurring in the area outlined by the loci of earthquakes in the rift zone. The crater floor of Pu`u `O`o collapsed about 30 m (100 ft) and was covered by rubble with only a tiny pool of molten lava remaining. Lava stopped flowing into the tube system, and the ocean entry was reduced to a dribble by early Sunday afternoon. A collapse of the bench started on Sunday morning and continued into the evening. Two hectares (Five acres) of new land slid beneath the waves.

Reinflation of the summit region started about noon on September 12 and continue as this is written on September 16. The pool of lava within Pu`u `O`o has increased in size, and it would not be a surprise if lava is flowing in the tube system when this article is printed on September 19.

A magnitude-3.7 earthquake from the south flank of Kilauea Volcano was the largest of the many earthquakes felt on Sunday morning. In addition to the tremors felt during the intrusion on September 12, two other earthquakes were reported felt during the past week. A resident of Pahala felt an earthquake at 6:04 a.m. on Tuesday, September 14. The magnitude-2.3 earthquake was located 3 km (1.8 mi) west of Pahala at a depth of 5.2 km (3.1 mi). A resident of Leilani Estates felt an earthquake 34 minutes later. The magnitude-2.5 temblor was located 4 km (2.4 mi) southeast of Pu`ulena Crater at a very shallow depth.

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Updated: 4 Oct 1999