The pre-Taupo eruption (c.AD 130) forest of the Benneydale-Pureora district, central North Island, New Zealand

Abstract

The composition and structure of three small areas of west Taupo forest, blasted down and buried during the c. AD 130 Taupo tephra eruption, are revealed by analysis of plant macrofossils, including leaves, seeds and wood. Data from these three sites and from the recently described Pureora buried forest were then used to interpret the pre-eruption vegetation along a 20 kilometre transect, from Pureora westward to Benneydale. All the stands were on flat or undulating terrain and dominated by podocarps, ranging from rimu (Dacrydium cupressinum)-tanekaha. (Phyllocladus trichomanoides) at Pureora to rimu at Benneydale. Broadleaved trees increased westward; on lower hillslopes in the Benneydale buried forest, northern rata (Metrosideros robusta) was an important canopy component. An amelioration of climate with decreasing altitude westward was indicated by a gradual change from prominence of horopito (Pseudowintera colorata) at Pureora to P. axillaris at Benneydale, and the presence of rewarewa (Knightia excelsa) and Metrosideros perforata only at the Benneydale site. In the extant forests adjacent to the sites, the broadleaved tree species, tawa (Beilschmiedia tawa), kamahi (Weinmannia racemosa) and mahoe (Melicytus ramiflorus ssp. ramiflorus) are common, but these were not recorded in any buried forest. Classification and ordination techniques were applied to data from three buried forest sites, approximately 10 km apart along the transect. These arranged the plots and species in a spatial sequence from Pureora to Benneydale, and also reflect a gradient from very poor to comparatively better drainage. The volcanic succession hypothesis of McKelvey (1963, 1986) is discussed in light of the macrofossil data. Our evidence suggests that broadleaved, notably tawa, forest is not the endpoint of the forest succcession on flat to undulating sites, and that forests may not converge to the extent suggested by McKelvey. We postulate that, under the present climate regime, and in the absence of volcanic eruptions and logging, the podocarp species which were widespread in the buried forests will eventually dominate terrain of gentle relief in the Pureora-Benneydale district.