Age structure and dynamics of podocarp/broadleaved forest in Tongariro National Park

Abstract

Regeneration patterns of three emergent conifer species (Dacrydium cupressinum, Prumnopitys ferruginea and Prumnopitys taxifolia [family Podocarpaceae]) and three broadleaved canopy species (Elaeocarpus dentatus, Nestegis cunninghamii, and Weinmannia racemosa) were inferred from population age structures and spatial patterns. The study was carried out on three plots cleared-felled for a railway deviation, in a mixed podocarp/ broadleaved forest in Tongariro National Park, North Island, New Zealand. Age data was obtained from ring counts on cross-sections of stems from the cleared-felled plots. Multivariate analyses and comparison of densities, basal areas and diameter distributions indicated that the clear-felled plots were representative of the 350 ha. of podocarp/ W. racemosa forest remaining on the lahar plateau in the south-western extension of Tongariro National Park. The maximum ages found for D. cupressinum, P. ferruginea and P. taxifolia were approximately 1,160,771 and 1,013 years respectively. The three podocarps all had population age discontinuities ('regeneration gaps'), these being most marked in P. taxifolia, and least so in P. ferruginea. Regeneration of P. ferruginea on the cleared plots was negatively associated with adult trees of the same species, and was mainly clumped beneath large broadleaved trees of >50 cm d.b.h. and >200 years age. Observations in the surrounding forest suggest a similar pattern for D. cupressinum and P. taxifolia, although development of these species requires more opening of the overstorey. The three broadleaved species were shorter-lived. The maximum ages for E. dentatus, N. cunninghamii, and W. racemosa were approximately 669, 516 and 388 years respectively. E. dentatus appears to regenerate mainly by rapid growth in small canopy gaps. This possibly also applies to W. racemosa, but seedlings of this species depend mainly on elevated surfaces, and vegetative reproduction also occurs. Both W. racemosa and N. cunninghamii had all-aged populations, and the latter species appears to regenerate continuously beneath the canopy. The disturbance history of the stands, examined by dating scars and releases on stem cross-sections, suggests that the main podocarp age-classes (500-770 years old) in the present stands established during period of progressive overstorey collapse, rather than as a result of a single massive exogenous disturbance event. The different age ranges of the three podocarp species probably reflect the interaction of different light requirements with the progressive opening and re-closure of the overstorey. A cyclic replacement of podocarps, probably initiated by catastrophic disturbance, but perpetuated by endogenous factors, is proposed for the lahar plateau forest.