Volcanic disturbance, Pyroclastic flows and Merapi

Merapi 1920 (from Minakami et al. 1969 as cited in Voight et al 2000)

Disturbance is a key process that can affect diversity levels and change the community (Hobbs & Huenneke, 1992). Natural disturbances, such as volcanic eruptions, hurricanes and wildfires, can catastrophically change the ecosystem over a short time scale (Scheffer et al., 2001). Volcanic eruption has become the most enthralling natural phenomenon and yet largely impacting the environment (Marti & Ernst, 2005). Every year, approximately 60 volcanoes erupt on Earth, and despite almost 80% of them occurring under the oceans, terrestrial volcanic eruptions commonly cause great impacts on vegetation over large areas (Dale et al., 2005a).

More than half of the active volcanoes encircle the Pacific Ocean, known as the ‘ring of fire’. About 80% of these volcanoes are located in developing countries and have remained largely unstudied (Marti & Ernst, 2005). Deforestation, including those that caused by volcanic activities become the biggest threat to forest vegetation. Dale et al. (2005a) identifies six volcanic activities that can have complex effects upon the vegetation; lava formation, pyroclastic flows, debris avalanches, mudflows, tephra and blowdown. One of the most devastating types of volcanic disturbance is pyroclastic flows. A pyroclastic flow is a term to define “the fast movement of tremendously hot (more than 700° C), turbulent gases and fragmental material across a land surface from a volcanic vent” (Dale et al., 2005a p. 231).

The extreme heat that is released is one of the characteristic disturbance mechanisms from pyroclastic flows that destroy the vegetation along the course of the flow (Dale et al., 2005a). Plant responses in a different ways to this pyroclastic flow and the initial impact can be devastating. The heat released from pyroclastic flows can ignite wildfires. Vegetation responses to fire vary. Intense fire most likely kills or damages some of the plants, but others are more persistent and depend on fire for their regeneration (Pickett & White 1985; Riswan & Kartawinata, 1991; Bond & Wilgen, 1996). Hence these responses initiate gradual readjustments in species association through time and this process is known as succession (Walker et al., 2007).

Merapi is one of Indonesia's most active volcanoes, with its first eruptions estimated to have happened in 1006 (Bemmelen, 1970). This stratovolcanic mountain lies in the densely populated province of Central Java and Yogyakarta and has a summit that reaches over 2,900 m in altitude. Merapi has periodical ‘fury’ cycles of 5 to 6 years, and the most recent event was in 2006 (1000 years after its first eruption). Merapi also produces more pyroclastic flows than any other volcano on Earth and it has become the reference volcano of the ‘Merapi-type’ pyroclastic flows (Abdurachman et al., 2000). The pyroclastic flow of Merapi is a product from a collapsed lava dome