Dynamics of upland podocarp/broadleaved forest on Mamaku Plateau, central North Island, New Zealand.
Forest composition was examined on one hectare of upland Dacrydium cupressinum/Prumnopitys taxifolia/Weinmannia racemosa/Beilschmiedia tawa forest on south Mamaku Plateau, central North Island, New Zealand, a site of catastrophic volcanic disturbance c. 1900 years ago. The phasic model (gap, building, mature) of the forest growth cycle was used. Mean age of sampled stems differed significantly between gap (13 yr), building-phase (68 yr), and mature forest (252 yr), which comprised 10, 50, and 40%, respectively, of the area and persisted on average for 20 yr (gaps) and 60 yr (building phase). 'Expanded' gaps averaged 0.008 ha in size. Tree ferns - mostly Dicksonia squarrosa and Cyathea smithii - were numerically dominant throughout, and dominated basal area in younger (<80 yr) forest; D. cupressinum dominated basal area in older (>80 yr) forest. Current diameter growth rates among broadleaved species followed the ranking: Griselinia littoralis, Elaeocarpus dentatus > B, tawa > Ixerba brexioides, W. racemosa, and were significantly faster during building (3.9 mm yr-1) than mature (2.5 mm yr-1) phases. Emergent D. cupressinum and P. taxifolia sampled were aged between ∼400 and ∼1000 yr; established seedlings of both species were rare. Much younger Prumnopitys ferruginea occurred commonly but only up to small tree size; Podocarpus totara was present only as long-dead fallen trees. Broadleaved species were all much younger (< approx. 250 yr). Shade-tolerant P. ferruginea, B. tawa, and I. brexioides regenerate continuously in the understorey of high forest, although P. ferruginea requires some canopy opening to develop beyond sapling size. Less tolerant G. littoralis, D. squarrosa, C. smithii, and to some extent W. racemosa are gap invaders; D. squarrosa, C. smithii, and W. racemosa also continue to establish during the building phase. E. dentatus maintains a pool of 'advance growth' which develops rapidly if light levels increase, as in gaps. Despite similar population structures, marked differences in the microsites preferred for establishment are evident between some species. Larger-scale studies over longer time periods and over larger areas are needed to determine definitively the regeneration strategies of the conifers. The unexpectedly high proportion of building-phase forest suggests a period of substantial canopy collapse within the past century. Widespread fallen podocarps on the forest floor indicate the presence of dense conifer forest on the site in the discernible past, lending support to the extension of the 'lozenge' regeneration model suggested initially for Agathis australis to tall podocarps as well.