Recent vegetation changes on Mount Tarawera, Rotorua, New Zealand

Abstract

Abstract Vegetation changes over the period 1978–1992 on Mt Tarawera volcano are described and analysed in order to identify causes of species replacement. The most significant change has been an increase of tutu (Coriaria arborea), which has spread from the upper side of Kanakana Dome onto much of the top. Tutu colonisation appears to be limited by marginal habitat conditions, such as soil infertility, low water availability, and harsh climate, and the slow spread of its nitrogen-fixing endophyte. Habitat conditions may have become more favourable over recent decades because of increased soil fertility from weathering of the substrate and addition of organic material, and an overall warming of climate. Severe and unseasonable frosts cause dieback and periodically check tutu success; this facilitates replacement by more frost-tolerant, later successional species such as broadleaf (Griselinia littoralis). Preliminary analyses of dome-top soils reveal low levels of all nutrients tested, particularly nitrogen and phosphorus. Nutrient levels increase with increasing vegetation cover and complexity, and nitrogen increases markedly following establishment of tutu. Dome-side kamahi (Weinmannia racemosa) forest soils are much more fertile presumably because of the longer established vegetation, and input of nutrients from breakdown of buried pre-eruption forest debris and from the buried soil. The developing kamahi-broadleaf dome-top forest is being modified by preferential browsing of kamahi by introduced animals.