Plant Succession on the Braided Bed of the Orongorongo River, Wellington, New Zealand, 1973-1990

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 number of plots with little vegetation varied widely, independently of plant cover. Plots with a sandy substrate had most plant cover. Much wind- blown sand accumulated beneath established vegetation. Patterns of early succession, which relied on available seed, were largely dictated by floods and droughts. If not disturbed for 20-25 years, succession invariably led to a temporary dominance of kanuka (Kunzea ericoides), with or without manuka (Leptospermum scoparium).