Reliability of the Data
The total net stocked forest area that formed the base data for these forecasts was 1.73 million hectares. This area represented the entire planted production forest estate as at 1 April 1999. This total area is thought to be accurate to plus or minus 5 percent. That is, the New Zealand total net stocked area was considered to lie between 1.64 and 1.82 million hectares as at 1 April 1999.
The detailed forest area by age and crop type data provided by respondents to the 1999 NEFD survey are thought to be accurate and up-to-date. The crop typing is, however, based on the management intentions of forest owners at the time of the NEFD survey and these intentions are likely to change over time.
A comparison between the 1999 NEFD forest area data and other data sources is available in A National Exotic Forest Description as at 1 April 1999 (Ministry of Agriculture and Forestry, 2000).
The radiata pine yield tables used in the wood supply forecast modelling were produced in 1995-96, based on data provided by forest owners in 1995. Prior to the modelling being undertaken the radiata pine yield tables were reviewed by comparing recent harvesting data on actual yields with the 1995 NEFD yield tables. This analysis did not reveal any systematic bias in the existing yield tables. It was concluded that the 1995 NEFD radiata pine yield tables were satisfactory for the wood supply modelling work.
As a part of the yield table review a number of forest owners and consultants were contacted to seek their views on the existing yield tables and to gauge whether new yield table data was available. Following this consultation it was decided to collect additional data to update the West Coast radiata pine yield tables and to attempt to generally revise the yield tables for the non-radiata species groups. Yield data subsequently collected on the non-radiata species groups were patchy. Updated yield tables for the West Coast radiata pine crop types were however used. These updated West Coast yield tables only differed slightly from those published in National Exotic Forest Description Regional Yield Tables as at 1 April 1995 (Ministry of Forestry, 1996).
The yield tables for the non-radiata pine species used in the forecasts continued to be based on data collected in and prior to 1991. These "other species" yield tables are of variable quality. The tables may not be reliable where only a small area of a minor species exists in a region.
The yield tables represent indicative yields of an "average" site and management regime for each NEFD crop type. New Zealand's largest forest owners provided the data used to construct the 1995 NEFD yield tables. The accuracy of these yield tables within a particular region depends upon the proportion of the forest resource owned by those large owners who provided yield data, and the level of similarity (in terms of sites and management) between these owners and the forest owners from whom yield information was not collected.
The long-term effects of genetic improvement are not explicitly included in the yield tables. The yield data used by the forest owners when constructing the company yield tables may have included some effects of improved growth rates as measured during the inventory of existing stands planted with genetically improved stock. Growth predictions into the future were generally made from growth models without the use of any multipliers for genetic gain. Forecasts of volumes using these yield tables are therefore considered to be conservative in the longer-term.
While average levels of mortality and harvesting waste are implicitly built into the yield tables, these forecasts have not allowed for catastrophic losses caused by factors such as cyclones, fire, disease or insect attack.
Further details about the reliability of the yield data are available in National Exotic Forest Description Regional Yield Tables as at 1 April 1995 (Ministry of Forestry, 1996).
The estimates of new planting and restocking used in the first two years of the model (ie, the years ended 31 March 2000 and 2001) are based on provisional results from the Ministry of Agriculture and Forestry's nursery seedling surveys. Details of how this survey is operated are described in Eyre, 1995. These estimates are considered to give a reasonable indication of new planting levels on a national scale. However, the allocation of the national estimates is based on observed planting patterns by region from previous years.
The national estimate of harvesting for the year ended 31 March 2000 are based on the Ministry's estimate of roundwood removals from planted forests as published in the Statistical Release Estimated Roundwood Removals from New Zealand's Forests March 2000 Quarter. The national estimate of harvesting for the year ending 31 March 2001 is based on the Ministry of Agriculture and Forestry's internal economic forecast Forest Products Exports March Year Forecasts as at March 2000. These national estimates of harvesting have been allocated to wood supply regions based on reported harvesting in the 1999 NEFD survey, along with estimates of regional roundwood removals reported in the Statistical Release Estimate of Roundwood Removals from New Zealand Forests Year Ended 31 March 1999.
