RECENT VEGETATION CHANGES ON MOUNT-TARAWERA, ROTORUA, NEW-ZEALAND

Vegetation changes over the period 1978-1992 on Mt Tarawera volcano ar e described and analysed in order to identify causes of species replac ement. The most significant change has been an increase of tutu (Coria ria arborea), which has spread from the upper side of Kanakana Dome on to much of the top. Tutu colonisation appears to be limited by margina l habitat conditions, such as soil infertility, low water availability , and harsh climate, and the slow spread of its nitrogen-fixing endoph yte. Habitat conditions may have become more favourable over recent de cades because of increased soil fertility from weathering of the subst rate and addition of organic material, and an overall warming of clima te. Severe and unseasonable frosts cause dieback and periodically chec k tutu success; this facilitates replacement by more frost-tolerant, l ater successional species such as broadleaf (Griselinia littoralis). P reliminary analyses of dome-top soils reveal low levels of all nutrien ts tested, particularly nitrogen and phosphorus. Nutrient levels incre ase with increasing vegetation cover and complexity, and nitrogen incr eases markedly following establishment of tutu. Dome-side kamahi (Wein mannia racemosa) forest soils are much more fertile presumably because of the longer established vegetation, and input of nutrients from bre akdown of buried pre-eruption forest debris and from the buried soil. The developing kamahi-broadleaf dome-top forest is being modified by p referential browsing of kamahi by introduced animals.