Post-volcanic forest succession on New Zealand's North Island: an appraisal from long-term plot data

Long-term (c. 40 years) tree population and soil data from conifer/angiosperm forests that have developed since the last major volcanic eruption (ad 232) in the central North Island of New Zealand were used to test the hypothesis that the shift from conifer to angiosperm dominance away from the eruption centre represents a chronosequence. The proportion of stand basal area comprised by conifers (12% to 98%) was negatively correlated with distance from eruption centre and available soil phosphorus. The tree recruitment rate of angiosperms (1.41%) was significantly higher than that of conifers (0.14%). Conifer tree density and basal area decreased and angiosperm tree density increased in most plots. Tree mortality rates exceeded recruitment rates in all conifers except Prumnopitys ferruginea; tree recruitment rates exceeded mortality rates in the angiosperms Beilschmiedia tawa and Elaeocarpus dentatus but not Weinmannia racemosa. Although net recruitment rates of major canopy tree species agreed broadly with the predicted species succession, there was little evidence of the predicted community succession occurring and that the plots represent a chronosequence. A wider variety of successional trajectories than originally envisaged are operating, and some appear to be truncated at intermediate stages of the postulated sere. Despite ordination of plot composition showing no compositional convergence over time, most successional trajectories are towards increasing dominance by angiosperms. The actual or potential presence of B. tawa appears to be a critical factor in determining future successions on Taupo Tephra.