Novel ecosystems are most often defined by their community composition. However, ecosystem processes, particularly disturbances such as fire, can have a fundamental role in the creation and persistence of ecosystem novelty. Fire is an especially significant disturbance in New Zealand due to its negligible presence prior to human arrival c. 800 yr BP. In addition, since human arrival, the New Zealand landscape has been stripped of much of its indigenous flora and fauna, with a suite of invasives now ubiquitous across the country’s main islands, forming novel assemblages with which fire now interacts. This study assessed changes in community composition of 116 ha of forest on Great Barrier Island, New Zealand, following a 2013 wildfire. In 2015, 36 permanent vegetation plots were established across the burned area and in unburned forest recording species’ frequency, height, and reproductive status, as well as associated soil chemistry and environmental variables; the plots were remeasured in 2017. Species frequency data were analysed to determine community composition using dissimilarity-based clustering and then compared to environmental conditions.
Results/Conclusions
Five distinct plant communities were identified, representing a gradient of invasion, from predominantly indigenous to wholly invasive species assemblages. Fire-tolerance was a trait common to all emergent tree species, specifically mānuka (Leptospermum scoparium) – New Zealand’s only example of indigenous serotiny – and the invasives Hakea sericea, Pinus spp., and brush wattle (Paraserianthes lophantha). The major controls on community composition were soil moisture, which acted as a physical barrier to invasion, and soil nutrients (C and N), which conferred resilience by facilitating regeneration of the indigenous species that burned in 2013. However, the greatest change between the pre-fire vegetation and the vegetation identified in 2015 and 2017 was an increase in the extent and density of fire-tolerant woody species such as brush wattle and Hakea sericea, resulting in an increase in the quantity and connectivity of fuels. While a range of successional trajectories are possible, the potential for recurrent fires supported by invasive plant species suggests succession toward mature indigenous forest has been suspended, and a novel ecosystem comprised primarily of fire-tolerant non-indigenous vegetation – alongside fire itself – is arising.