EARLY MIOCENE FLORA OF THE MANUHERIKIA GROUP, NEW-ZEALAND .10. PALEOECOLOGY AND STRATIGRAPHY

A stratigraphic sequence of vegetation is recognised from macrofossil assemblages in Lower-Mid Miocene fluvial-lacustrine sediments of the M anuherikia Group, New Zealand. Temperature, water-level, drainage, fir e 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 eas tern Australia, including notophyll vine forest (sometimes with podoca rp conifers), microphyll forest, araucarian notophyll vine forest, tal l open-forest (at times probably closed forest with sclerophyll emerge nts), notophyll feather palm vine forest, and fem fields. The earliest assemblage in the Cromwell region represents Nothofagus forest (micro phyll fem forest or microphyll vine forest), or at least a forest in w hich Nothofagus was probably an important element. Rainfall was high, but the associated presence of Allocasuarina indicates forest edge con ditions, 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 alt ernatively, soil nutrients may have been low. The succeeding Araucaria n zone may indicate lower rainfall (and probably war-mer conditions th an 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. Vegetatio n was araucarian notophyll vine forest. The Eucalyptus zone suggests t hat rainfall continued to decrease, or become more seasonal, to the po int at which the frequency of fires rose to at least once every 350 ye ars, and a tall-open forest developed. The part of this zone in which Allocasuarina was absent may represent the peak frequency of fires, wh ich 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 no tophyll evergreen vine forest, including Elaeocarpaceae, Lauraceae, My rtaceae, Podocarpaceae and, in areas of impeded drainage, palms. A ret urn to drier conditions, or a large fire, heralded the regrowth of Euc alyptus - Allocasuarina woodland or open forest. Rainforest conditions are probably represented in the highest part of the sequence. At vari ous times there were wide expanses of raised peat bog with a generally treeless cover. Climate was microthermal to mesothermial.