Growth of naturally regenerated Beilschmiedia tawa
(A. Cunn.) Kirk (tawa), Dacrydium cupressinum
Lamb, (rimu), and Prumnopitys ferruginea
(D. Don) de Laub. (miro) over 22 years in untagged and selectively logged podocarp/tawa forest at Pureora, central North Island, New Zealand, was studied in relation to crown class, position in relation to canopy gaps, competition, and phase of forest growth cycle.
Height increment of seedlings (defined as <2.5cm dbh) and diameter increment of saplings and poles (5-30 cm dbh) were significantly affected by crown class in all species. Growth rates were similar in all species; height growth averaged 12-13 cm/annum in dominant and co-dominant plants and 3-6 cm/annum in dominated plants, while diameter growth averaged 2-3.5 mm/annum in dominant and co-dominant plants and 1-2 mm/annum in dominated plants. Tawa saplings and poles grew significantly faster in logged forest (c. 2.5 mm/annum) than in untagged forest (c. 1.5 mm/annum), a reflection of the increased light levels resulting from canopy disruption.
Mortality rates of dominated seedlings (c. 1%/annum over 22 years) were similar in tawa, rimu, and miro. These species maintain slowly-turning-over banks of slow-growing established seedlings in the understorey, surviving on average for c. 100 years at Pureora, and capable of responding to opening or thinning of the canopy. Although tawa can develop to maturity in the understorey, miro and especially rimu appear to become increasingly light-demanding after the seedling stage at Pureora, and to be significantly gap-dependent for development to maturity.