Auckland's green volcanic heart: groundcover vegetation and soils of the Auckland volcanic cone reserve network

Abstract

The volcanic cones of Auckland are distinctive landmarks of that city, currently realized as an ‘archipelago’ of grassy reserved hills. Most of these cones were modified as hill forts during former occupation by Māori. Here, we review the Auckland volcanic reserve network and survey the current groundcover vegetation and soils of four large representative examples. Forty-eight reserves currently occur on remnant volcanic features. As well as two inshore island reserves (2372 ha), the other 46 reserves total 1241 ha. Most of the volcanic reserves support exotic grassland with rare forest patches. Groundcover vegetation follows a topographic sequence from flatter, fertile sites dominated by Lolium perenne, Trifolium repens and Pennisetum clandestinum, to moderately steep sites dominated by a diverse range of exotic grasses, to steep, lower fertility sites dominated by Microlaena stipoides. Pennisetum clandestinum is a major component of groundcover vegetation on many sites across all slope classes, its abundance varying in relation to soil N levels rather than to changes in slope. Soil analyses indicate that soils are nutrient-rich with high soil carbon and phosphorus. Future management should restore some native forest and introduce more herbaceous native species where low groundcover vegetation is desirable. Decreasing soil nutrient levels through strategies such as cessation of fertilizer application may assist these processes.