Plant Succession on Volcanoes in Papua
every tree and spread a deep deposit of hot ash over the blast area. After this first eruption a dome built up and a second eruption took place in March 1951, after which a second dome was formed, and then activity gradually lessened. During these eruptions a lahar, or mud stream, ran from the base of the dome northwards for 9 km., and this left a deep deposit of rubble which has received the name Avalanche ,Valley. No true lava flow was formed although flows from previous eruptions are in evidence.
The only information available on the previo.us vegetation is an oblique aerial photograph of the lower slopes.taken 10 years earlier. This shows a pattern of garden regrowth stages with few stands higher than 35 m.
No detailed account is available of happenings within' the blast area after the eruption but all eye-witness accounts indicate that erosion was widespread and re- generation of the vegetation was extremely rapid.. After a very few months the blast area was a mass of green. One of tlie most conspicuous features of this early vegeta- tion was the vigorous growth of taro, Colocasia antiquorum Schott and bananas, Musa spp., on the sites of former native gardens.
By July 1953 an almost continuous range of vegetation types from woodland, exceeding 12 m. in height, to an edaphic desert of scattered moss plants was found. The ash cover had remained on extensive flats but on slopes most of it had been ,eroded. In many areas the topsoil was exposed and on the steeper slopes only sub-
The depth of ash
large flats were covered by was 15 cm. deep. This ash
60 cm, of consisted
varied considerably; 2 km. from the dome ash and 8 km. from th,e dome the ash cover of fragmented andesite and had the.size and sand.
On the deep lahar deposits of Avalanche Valley the distribution of the vegetation depends directly on the availability of water. At the base of the dome, 1000 m. elevation, the surface deposits are coarse gravel and rock, with little finer material. Although the annual rainfall here is probably in excess of 3600 mm., with over 200 rainy days, the vegetation is an edaphic desert with only scattered moss plants and ferns present. Nowhere in this section does ground cover exceed 5 per cent and the great majority of the area has less than one 1 cent cover. In this desert are scattered patches of more luxuriant growth where plants are growing on a layer of finer material deposited by surface wash after occasional heavy rains. This layer never exceeds 5 cm. in depth and the vegetation present varies with its depth and texture. The thickest layer, consisting predominantly of fine silt, carries a 50 per cent cover of Saccharurn spontaneum L., 1 m. high, the intervening ground being covered by a mat of moss. Further down Avalanche Valley the percentage of finer material gradually increases. At the same time the ground cover becomes denser, the grasses Irnperata cylindrica Beauv, and Saccharurn spontaneurn L, predominating. The ground cover is 50 per cent 3 km. from the dome, while at 8 km. a 3 m. high dense stand of Saccharurn spontaneurn is found.
Elsewhere in the blast area the vegetation is more luxuriant and has been grouped into several associes. It is recognized, however, that these associes are merely con- venient subdivisions for a continuous range of communities. The distribution of each of these associes shows a dkect correlation with the extent of erosion.
The Saccharurn spontaneurn associes is dominated by the tall grass Saccharurn spontaneurn which frequently forms tussocks. It ranges in height from 2 to over 4 m., has from 80 to 100 per cent soil cover and is very similar to the community of