Tawa: growth and productivity responses to selective logging, Whirinaki
Operational Research Project Code: POL 117
Programme Leader: G A Steward and H S Dungey
Project
Selective logging of dense podocarp forest in Whirinaki Forest Park, in 1979/80, provided an opportunity to monitor the changes in growth and productivity of selected tawa. Two hundred unmerchantable tawa (less than 30 centimetres diameter at breast height (DBH)) were tagged and assessed in 1981, 1985 and 1999. A further selection of mature and merchantable tawa stems (greater than 30 centimetres DBH) were assessed in 2005 as a comparison. This data was analysed for the Indigenous Forestry Unit of the Ministry of Agriculture and Forestry.
Objective
To analyse the data, to compare the influence of the various logging treatments on the productivity of tawa, to identify within the diameter class structure where new merchantable volume was being produced, to determine when new volume was potentially available for harvest from which MAF can review and revise its guidelines for sustainable forest management within this forest type.
Methods
The Okurapoto selective logging trial was established in dense podocarp forest in 1979 in Whirinaki Forest Park, and comprised four 10 hectare blocks.
After logging, 200 unmerchantable tawa were located and categorised by logging treatment, DBH, stem form and length, relationship to and size of logging induced clearings.
Stems were reassessed in 1985 and 1999.
In 2005 a selection of tawa with merchantable DBH in 1979 were reassessed.
Results
The creation of canopy openings through the three logging treatments has had a beneficial effect on growth and productivity for only a small percentage of residual tawa adjacent to logging clearings.
Tawa in undisturbed forest and those on the margins of logging clearings have performed similarly.
For small tawa (less than 30 centimetres DBH) diameter increment was 1.49 millimetres per annum.
Volume increment for merchantable tawa (greater than 30 centimetres DBH) was 0.24 cubic metres per hectare per annum, and not correlated to logging disturbance.
Conclusions
Selective logging of dense podocarp forest has not promoted the widespread improvement of tawa growth and productivity, compared to unlogged forest.
For higher rates of tawa logging (6 percent of standing volume) it will take up to 30 years to replace the merchantable volume removed during logging in 1979.
Estimates of growth indicate that pole tawa (20 to 30 centimetres in diameter) will take approximately 50 years after logging to reach merchantable diameter.
The new merchantable volume developing in this stand is largely tawa.
Contact for Enquiries
Manager, Innovation Policy
Ministry of Agriculture and Forestry
MAF Policy
PO Box 2526
Wellington
New Zealand
Tel:+64 4 894 0618
Fax:+64 4 894 0741
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