Vegetation patterns and trajectories in disturbed landscapes, Great Barrier Island, northern New Zealand

Fire has been a major driver of forest loss in New Zealand. A conceptual model has been proposed in which positive feedbacks between vegetation, fire and soils can arrest regeneration of recurrently burned wet forest landscapes. We used vegetation data collected across three topographically similar landscapes – Awana, Glenfern and Windy Hill – on Great Barrier Island to (1) describe current vegetation composition and structure and predict future change in composition and (2) assess evidence for interactions between fire and soils slowing regeneration in these landscapes. Compositional data were analysed via classification and ordination, and we used transition matrix models to explore how vegetation composition may change in the future. The vegetation in the three landscapes spans repeatedly burned scrubland dominated by mānuka (Leptospermum scoparium) and exotic fire-dependent woody species such as Hakea sericea, to intact mature forest. Scrubland vegetation tends to be found on north-facing upper slopes and ridges – drier sites where fire has been more frequent and rendered soil conditions (e.g. organic matter and moisture) poor for plant growth. There is a slow reinvasion of forest species into the Leptospermum and Kunzea scrubland from gullies and other remnant patches, with wind-dispersed species preceding fleshy-fruited bird-dispersed ones. In the absence of fire in the next few decades the landscapes will continue to move back towards forest. More fires, however, will further degrade these landscapes by removing remaining fertile topsoils from ridges and slopes and by favouring exotic species adapted to recruit from seed and/or resprout vegetatively after fire.