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New Zealand Journal of Forestry (1980) 25(1): 44–57
©New Zealand Institute of Forestry

Research article
Structure and growth of dense podocarp forest at Tihoi, Central North Island [New Zealand], and the impact of selective logging.

J. Herbert

In rimu (Dacrydium cupressinum) dominated forest at Tihoi, 61% of a sample of 120 sound rimu trees had between 400 and 500 growth rings, which were assumed to be annual. A further 14% were 500 to 574 years old and 25% were between 200 and 400 years old. On average, matai (Podocarpus spicatus) were older and miro (Podocarpus ferrugineus) younger than rimu.
Cross-sections at stump height often showed a characteristic diameter growth pattern. After an initial period of very slow diameter growth {usually 60 years) was a period (average 72 years) of relatively rapid growth. Then followed a long period of more even and slower diameter growth culminating in a period of very slow growth (average 65 years) for almost half the sample. Although the largest trees tended to be older than the smallest trees, diameter was an unreliable indicator of age.
Older trees tended to have larger and more healthy looking crowns than younger trees. This may reflect reduced growing space available to younger trees.
Estimated stand increment in a 12 ha control block was 1.79 m3/ha/yr, and measured natural losses over the 3-year period were 2.47 m3/ha/yr. Logging has resulted in an increase in mortality with an estimated net loss of 3.44 m3/ha/yr in the 30%o logged block and 7.89 m3/ha/yr in the 55% logged block. If the short-term pattern of logging-induced losses is maintained, the values of the forest may be seriously diminished. However, the 3-year measurement period provides an inadequate base for the extrapolation of long-term mortality patterns, and both the mortality and increment data require further experimental verification. The spread of internal rots also requires study.
The experience gained from this trial has provided a good basis for drawing up criteria for future selective logging in dense podocarp stands of similar structure. These criteria are currently being tested in new trials established in Whirinaki State Forest.

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