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Continuous cover forestry feasibility study

Harriet Palmer, New Zealand Tree Grower August 2018.

Continuous cover forestry involves regular harvesting small areas or individual trees which then avoids clear-felling. An NZFFA and Sustainable Farming Fund project to investigate the feasibility of a specific continuous cover forest system in radiata pine woodlots started in March this year, although for this project the term target diameter harvesting is being used.

Continuous cover forestry systems are the norm in managed native forests in New Zealand. They are also commonly found overseas. However, continuous cover forestry is very rare in radiata pine which is by far New Zealand’s most common woodlot species. This project aims to provide information and practical guidance for owners of small forests who are interested in alternatives to clear felling radiata pine.

Target diameter harvesting

Target diameter harvesting involves harvesting individual trees once they have reached a specified diameter at breast height. As more trees are extracted and light penetrates the forest floor, natural regeneration of the crop species should begin. This regeneration can be supplemented with planting if necessary. The eventual result will be an uneven-aged forest comprising roughly equal proportions of trees of different ages which can be harvested in perpetuity using target diameter harvesting.

Target diameter harvesting has been refined over many years by John and Rosalie Wardle at their North Canterbury property, Woodside, in radiata pine and black beech managed for timber production. In 2013 Chris Perry, a University of Canterbury School of Forestry student, undertook a detailed economic analysis of radiata pine target diameter harvesting at Woodside. Chris concluded that target diameter harvesting made a positive return at a seven per cent discount rate, although it would be somewhat less profitable than a hypothetical clear-felling system on the property.

The analysis did not include any evaluation of the potential non-timber benefits of the target diameter harvesting system such as soil conservation, water quality protection, biodiversity enhancement or carbon storage. However, the result was encouraging enough to inspire the current project, initiated by the Wellington and Wairarapa branches of the NZFFA, in conjunction with Chris Perry, Mark Bloomberg and John Wardle.

Project methods

The question to answer is − Can the Woodside target diameter harvesting system be profitably applied to other radiata pine woodlots? The method involves undertaking case studies using four radiata pine woodlots of varying size in Greater Wellington and Hawke’s Bay. We will assess the operational, marketing and economic feasibility of applying target diameter harvesting to each woodlot. In each case, pre-harvest forest inventory will be undertaken, followed by modelling to ‘grow’ the forests forward and simulate harvest using target diameter harvesting or clear felling. Investment analysis techniques will then be applied to generate comparative financial results for the two harvesting systems in each woodlot.

Also being assessed will be the practicalities of target diameter harvesting in terms of harvesting and marketing logistics. Local harvesting contractors, log hauliers and domestic sawmillers will be contacted to assess their opinions of the realities of target diameter harvesting systems.

The aim is to generate a summary of the main resource, operations and market factors determining the feasibility of target diameter harvesting in small-scale radiata pine plantations. The findings will be in at least one article in the Tree Grower and elsewhere. There will also be a field-day on one of the case study woodlots at the end of the project.


This is a small feasibility study, running from March to November but with no actual harvesting involved. Only ground-based harvesting sites are being assessed and the focus is on radiata pine woodlots. The project team are Harriet Palmer, Eric Cairns, Chris Livesey from the Wairarapa and Wellington branches, John Wardle, Chris Perry of PF Olsen and Mark Bloomberg from the University of Canterbury.


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