Early Miocene flora of the Manuherikia Group, New Zealand. 10. Paleoecology and stratigraphy

Abstract

A stratigraphic sequence of vegetation is recognised from macrofossil assemblages in Lower-Mid Miocene fluvial-lacustrine sediments of the Manuherikia Group, New Zealand. Temperature, water-level, drainage, fire and rainfall were probably the factors that divided the distribution of plant taxa into several distinct communities. These communities are compared with structural vegetation types presently recognised in eastern Australia, including notophyll vine forest (sometimes with podocarp conifers), microphyll forest, araucarian notophyll vine forest, tall open-forest (at times probably closed forest with sclerophyll emergents), notophyll feather palm vine forest, and fern fields. The earliest assemblage in the Cromwell region represents Nothofagus forest (microphyll fern forest or microphyll vine forest), or at least a forest in which Nothofagus was probably an important element. Rainfall was high, but the associated presence of Allocasuarina indicates forest edge conditions, or perhaps disturbance by fire, which removed the canopy long enough for this genus to have a temporary advantage. Temperature may have been cooler than that required for subtropical rainforest, or alternatively, soil nutrients may have been low. The succeeding Araucarian zone may indicate lower rainfall (and probably warmer conditions than when Nothofagus dominated the vegetation), allowing the araucarians to compete with the rainforest trees and the Allocasuarina to persist, but not low enough to result in a high frequency of fires. Vegetation was araucarian notophyll vine forest. The Eucalyptus zone suggests that rainfall continued to decrease, or become more seasonal, to the point at which the frequency of fires rose to at least once every 350 years, and a tall-open forest developed. The part of this zone in which Allocasuarina was absent may represent the peak frequency of fires, which were detrimental to Allocasuarina. A dramatic increase in rainfall and possibly soil-nutrients seems to have eliminated fire and caused the local replacement of Eucalyptus and Allocasuarina by a podocarp notophyll evergreen vine forest, including Elaeocarpaceae, Lauraceae, Myrtaceae, Podocarpaceae and, in areas of impeded drainage, palms. A return to drier conditions, or a large fire, heralded the regrowth of Eucalyptus—Allocasuarina woodland or open forest. Rainforest conditions are probably represented in the highest part of the sequence. At various times there were wide expanses of raised peat bog with a generally treeless cover. Climate was microthermal to mesothermial.