Changes in structure and composition over fifteen years in a secondary kauri (Agathis australis)-tanekaha (Phyllocladus trichomanoides) forest stand, Coromandel Peninsula, New Zealand

Abstract

Abstract Changes in structure and composition were assessed after an interval of 15 years in an old secondary Agathis australis (kauri) — Phyllocladus trichomanoides (tanekaha) stand on Coromandel Peninsula, North Island, New Zealand. Stand structure was intermediate between previously described young and mature A. australis stands. Agathis australis, mostly 100–200 years old, had fastest diameter increment (4.3 mm yr−1 in trees ≥25 cm d.b.h.) and contributed almost all the stand basal area increment; its seedling population contracted and larger trees were developing mature crowns. Phyllocladus trichomanoides, also mostly 100–200 years old, had slower diameter increment (2.2 mm yr−1) and contributed most of the remaining stand basal area increment; its seedling population also contracted. Weinmannia silvicola (towai), probably of similar age, incurred mortality at all sizes while Kunzea ericoides var. ericoides (kanuka), 40–80 years old, incurred most mortality in large saplings and small trees. Stand basal area increased rapidly (0.6 m2ha−1 yr−1) and total density decreased slightly. Spatial patterns of stems over 5cm d.b.h.largely reflected topographic variation, with A. australis, smaller P. trichomanoides and K. ericoides tending to occur on ridges and upper slopes, and Cyathea dealbata (ponga, silver fern), the main understorey plant, and smaller W. silvicola tending to occur on lower slopes. Larger P. trichomanoides and W. silvicola were randomly distributed. Several successional pathways (replacement patterns) are evident, with P. trichomanoides replacing K. ericoides, and A. australis replacing W. silvicola and likely to replace P. trichomanoides in the future. Agathis australis thus appears set to become dominant over most of the stand, largely at the expense of the sizeable suppressed component of its own population, temporarily prominent P. trichomanoides, Knightia excelsa (rewarewa) and W. silvicola, and ephemeral K. ericoides. Canopy diversity is likely to be enriched slightly by a few podocarps (Dacrydium cupressinum (rimu), Podocarpus hallii (thin-barked totara), Prumnopitys ferruginea (miro)) and hardwoods (Elaeocarpus dentatus (hinau), Dysoxylum spectabile (kohekohe)) with moderately shade-tolerant seedlings which have probably been present for at least 100 years.