Reliability of Data
The New Zealand total area of planted forests in the 2003 NEFD database is estimated to be accurate to plus or minus 5 percent. That is, the New Zealand total net stocked forest area is estimated to lie between 1.74 and 1.92 million hectares. The level of accuracy is reduced as the forest area is broken down into local authority and age class.
The data that make up the 2003 database have come from three sources as shown in Table 3.
Table 3: Total Area Represented by Each Data Source in the 2003 NEFD Database
| Data Source | Net stocked area (000 ha) |
Proportion of total area (%) |
|---|---|---|
| 2003 NEFD Survey | 1 510 | 83 |
| 1995 Statistics New Zealand small forest grower survey1 (owners with 1 to 39 ha) | 76 | 4 |
| New planting imputation (1992 - 2002) | 241 | 13 |
| Total | 1 827 | 100 |
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Note:
- These data were collected in 1995; since then changes will have occurred to these forests. For instance, 7,500 hectares are now older than 30 years; much of this area is likely to have been harvested since 1995.
It should be noted that no information is available on harvesting of the areas obtained from the 1995 Statistics New Zealand small forest grower survey, and the area is still present in the NEFD database. Work is currently underway to update information on forests with less than 40 hectares of forest. A pilot survey of small forest growers has been completed in the Marlborough district. Regional surveys of small-scale forest owners will be completed across the rest of New Zealand over the next year, with work already underway in Wairarapa, Gisborne, Canterbury and Nelson.
The data supplied by respondents to this survey are generally accurate and up-to-date. This reflects the commitment of the forest industry to having detailed forest resource information readily available. However, it is possible that some small-scale forest owners included in the database may change their management objectives. Area currently intended to produce harvestable timber yield may be left standing, for example to provide shelter or for aesthetic appeal.
To minimise possible errors in the NEFD survey data, the following checks were made:
- Numerical data were limited to certain maximum and minimum values for each entry.
- Aggregated area data were checked at the territorial authority level against the 2002 NEFD area database to ensure variation was within acceptable limits.
Forest areas are sometimes mis-allocated between adjacent territorial authorities. One way for users to minimise this problem is to aggregate data from adjacent territorial authorities, particularly where contiguous forest areas straddle territorial authority boundaries.
The inclusion of the imputed new planting into the 2003 NEFD database makes the following assumptions:
- The new planting estimates from the nursery seedling surveys are the actual areas of new planting established between 1992 and 2002.
- The imputed new planting in each territorial authority occurs in the same proportion as the new planting captured in the 2003 NEFD survey.
- Imputed new planting for radiata pine is assigned to the tending regimes in the same proportions as the radiata pine new planting captured in the 2003 NEFD survey.
The radiata pine yield tables contained in the NEFD yield table database were revised during 1995 - 96, principally from data provided in 1995 by owners of extensive forest areas. Data were collected from these owners because their holdings covered most districts in New Zealand and made up much of the forest resource.
For further details on the NEFD yield tables refer to NEFD Regional Yield Tables as at 1 April 1995.
Contact for Enquiries
MAF Information Services
Pastoral House
25 The Terrace
PO Box 2526
Wellington, NEW ZEALAND
Fax: +64 4 894 0721
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