plant succession

Vegetation on 5 km (c. 100 ha) of the braided bed of the Orongorongo River, Wellington, was sampled in March from 1973 to 1990. The riverbed has become aggraded since an earthquake in 1855. Surface water covered little of the riverbed; Callitriche stagnalis was the only common vascular aquatic plant. Most grasses and dicot herbs were adventive. The scabweed Raoulia tenuicaulis was the commonest dicot. The extent of plant cover was measured on 300 circular plots (radius 1.5 m); it ranged between years from 5% to 22%, depending on the severity of floods.

The flora and vegetation of the four high domes of Mt Tarawera; Ruawahia, Tarawera, Wahanga and Plateau. are described, and successional rates and trends determined at some sites by comparing 1964 and present-day photographs and records. Although below the regional tree limit, the dome tops are dominated by scattered low shrubs, herbs, grasses, mosses and lichens. With increasing distance from the 1886 eruption craters vegetation com- plexity and rates of succession increase. Plateau dome vegetation is successionally the most advanced.

Vegetation succession on Mt Tarawera, Rotorua, New Zealand, a recently erupted volcano, was studied. Field data from 70 plots were collected and analysed by a clustering algorithm. The plots formed an altitudinal and successional series which included bare scoria, herbfield, mixed hardwood, scrub, kamahi forest and tawa forest. Because of the differential effects of the 1886 eruption, different successional trends are being followed on the north-west and south-east faces of the mountain.

Approximately 50,000 ha of native wet Metrosideros forest on the island of Hawai'i experienced a drastic reduction (dieback) of the tree canopy between 1954 and 1977. Two general hypotheses have previously been suggested to explain this phenomenon: 1) Metrosideros dieback has resulted from recently introduced pathogens, and 2) the dieback has naturally occurred previously in Hawai'i, and is related to plant succession under periodic conditions of climatic instability which effect the soil moisture regime.